title research about coherent ultra-violet light sources based on nonlinear conversion with borate

TitleResearch about coherent ultra-violet lightsources based on nonlinear conversion withborate crystal

Author(s) 曲, 晨

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URL https://doi.org/10.18910/67066

DOI 10.18910/67066

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Osaka University Knowledge Archive : OUKAOsaka University Knowledge Archive : OUKA

https://ir.library.osaka-u.ac.jp/repo/ouka/all/

Osaka University

Doctoral Dissertation

Research about coherent ultra-violet light

sources based on nonlinear conversion with

borate crystal

QU chen

March 2017

Graduate School of Engineering,

Osaka University

i

Contents:

Preface 1

Chapter 1.

Introduction 1

1-1 Background 1

1-1-1 Inspection application for all-solid-state UV laser 2

1-1-2 Introduction for UV laser processing 3

1-2 Purpose of dissertation 4

1-3 Structure of dissertation 6

References in Chapter 1 8

Chapter 2.

Nonlinear optics theory and nonlinear optical crystals for UV light generation 9

2-1 Introduction for optical nonlinear frequency conversion 9

2-1-1 Introduction for nonlinear optics 9

2-1-2 Analysis of SHG with Maxwell’s theory 13

2-1-3 Phase-matching condition 18

2-1-4 Optics of uniaxial crystals 19

2-1-5 Optics of biaxial crystals 22

2-1-6 Phase-matching in uniaxial crystals 23

2-1-7 Quasi phase-matching 30

2-1-8 Optical parametric oscillator 32

2-2 Nonlinear optical crystals for UV light generation 33

2-2-1 Properties required for nonlinear crystals in UV light generation 33

2-2-2 Commonly used borate crystals 34

2-3 Summary 41


Chapter 3.

189 nm VUV light generation with borate crystals 43

3-1 Introduction for VUV light generation at 189 nm 43

3-2 Experiment for 189 nm light generation 44

ii

3-2-1 Scheme for 189 nm light generation 45

3-2-2 SHG with LBO 46

3-2-3 IR light generation with OPO 46

3-2-4 Fourth harmonic and fifth harmonic generation with CLBO 50

3-2-5 Summary for 189 nm light generation preparation 55

3-2-6 189 nm light generation with borate crystals 56

3-3 189 nm light generation results and discussions 58

3-3-1 Phase-matching angles for LBO and CLBO 58

3-3-2 Phase-matching angles for CBO 60

3-3-3 189 nm light generation with CLBO and LBO 62

3-4 Perspective 64

3-5 Summary 66


Chapter 4.

179 nm VUV light generation with borate crystals 69

4-1 Introduction 69

4-1-1 Introduction for VUV light generation in 170 nm-180 nm range 69

4-1-2 Scheme for 179 nm light generation 70

4-2 Experimental setup for 179 nm light generation 71

4-2-1 Setup for DUV and IR light generation 71

4-2-2 SHG with improved conversion efficiency 72

4-2-3 KTP OPO and intra-cavity SHG 74

4-2-4 PPLN OPO for IR light generation 76

4-2-5 198.8 nm DUV light generation 77

4-2-6 Summary for 179 nm VUV light generation system 79

4-2-7 SFG for 179 nm VUV light generation 80

4-3 179 nm light generation result 80

4-4 Summary 83


Chapter 5.

Research about 355 nm UV light generation with CLBO 85

5-1 All-solid-state 355 nm laser 85

5-1-1 355 nm UV light generation with borate crystals 85

iii

5-1-2 CLBO’s outlook for 355 nm UV light generation 85

5-2 Method for walk-off compensation 88

5-2-1 Principle for non-collinear phase-matching 88

5-2-2 Method for achieving non-collinear phase-matching 91

5-2-3 Prism-coupled device structure for non-collinear phase-matching 93

5-3 Experiments for 355 nm light generation 95

5-3-1 Setup preparation for 355 nm light generation 95

5-3-2 355 nm light generation results of conventional CLBO 96

5-3-3 355nm light generation results of LBO 99

5-3-4 355 nm light generation results of walk-off compensation device 100

5-4 Summary 102


Chapter 6.

Conclusions 105

List of abbreviations in the dissertation 107

Acknowledgement 109

Achievements 113

Preface:

Ultraviolet (UV) laser source used in industry, medical and research have

drawn great attentions in recent years. Especially, nonlinear frequency

conversion which considered as a good method for providing high power pulsed

UV laser source, has become a hotspot in the fields of laser, photonics and crystal

growth.

In this dissertation, there are detailed introduction about the principal for

nonlinear frequency conversion, 189 nm and 179 nm UV light generation

systems with borate crystals, and the 355 nm light generation (third-harmonic

generation of 1064 nm) demonstrated with CLBO.

The structure of the dissertation

In Chapter 1, there is an introduction about the background of needs for

inspection laser source in semiconductor manufacture and processing laser

source for industry use. Then, the research purpose of this dissertation and the

ideas for the development of all-solid-state UV laser source at 189 nm, 179 nm

and 355 nm are shown.

In Chapter 2, the basic principle involves in the UV light generation of by

means of nonlinear optical effect is discussed. Firstly, the basic theory for

nonlinear optics and fundamental expression for nonlinear (NLO) generation

were given. Then, the optical properties for NLO crystals and the condition for

achieving the phase-matching were introduced. At the second part of the chapter,

NLO crystals used in UV light generation research and their properties are

shown.

In Chapter 3, current progress of the sub-200 nm deep-UV light generation by

borate crystal and the background of the 189 nm system are introduced. After

that, the all-solid-state 189 nm light generation system is presented. The

phase-matching property for LBO, CBO and CLBO are investigated with the

system and the generation results are shown. A new Sellmeier formula for CBO

found by our laboratory, which can make a better prediction to our experimental

results than the former formula, was presented.

In Chapter 4, the progress for deep-UV light generation till 170 nm is

introduced. After that, the all-solid-state 179 nm light generation system is

presented. Among it, the core part of an optical parametric oscillation based on

KTP is introduced. The phase-matching property for LBO is investigated with

the system, and the generated power is verified with fluorescence as it is too

weak to be detected.

In Chapter 5, I demonstrate the 355 nm (third-harmonic generation of 1064 nm)

generated with CLBO crystal for the first time. In order to compensate the

walk-off involved in the generation by Type II (eoe) CLBO to promote the

conversion efficiency, I design a new prism-coupled device which takes the

advantage of non-collinear phase-matching. A sample is made and the generation

result is compared with conventional CLBO and LBO crystal in the 355 nm light

generation.

In Chapter 6, all results established in this work are summarized and

concluded together.

1

Chapter 1. Introduction

1-1 Background

LASER stands for Light Amplification by Stimulated Emission of Radiation. It is

one kind of light with strong spatial coherence, narrow spectrum and high

intensity. The original concept of laser derives from Quantum theory. The first

functional laser was operated by Theodore Maiman in 1960 with ruby crystal as

the gain medium [1]. Since the discovery, it has found applications in versatile

areas such as science, medical, industry and military.

Because of the high light intensity, nonlinear optical (NLO) effect resulted

from the dielectric polarization responds nonlinearly to the electric field, could

be realized by laser. As a typical second order nonlinear effect, second harmonic

generation of ruby laser was observed with quartz crystal in 1961, which is the

first NLO phenomenon discovered [2]. The experiment marked the beginning of

an intense investigation into the realm of the nonlinear optical properties of

matter. After that, nonlinear frequency conversion such as second harmonic

generation (SHG), sum-frequency generation (SFG), and difference-frequency

generation (DFG) began to play a big part in laser technology particularly for

broadening the spectrum of the laser from infrared (IR) to ultraviolet (UV) range

and extending its application field.

Nowadays, UV laser has become useful tool in material processing and

imaging, for it provides high photon energy and high resolution with short

wavelength. Particularly, UV laser source realized by wavelength conversion

based on diode-pumped solid-state laser and fiber laser draw the attention of

researchers [3]. This kind of lasers has many merits expected in industry such as

high beam quality, high running stability, and low maintenance cost.

For the UV light generated with nonlinear optics theory based on solid laser

system, NLO medium is of great importance. Borate crystals with high nonlinear

effect, short absorption edge and high damage threshold, have become the best

http://en.wikipedia.org/wiki/Spatial_coherence


2

choice for this application [4]. Famous members in this big family include but

not limited to: β-BaB2O4 (BBO), LiB3O5 (LBO), CsLiB6O10 (CLBO), CsB3O5

(CBO) and KBe2BO3F2 (KBBF). Among them, BBO, LBO, and CLBO are now

in mass production and have become the powerful tool in the field.

In this dissertation, I make the UV light generation with borate crystals aiming

at two kinds of application.

1-1-1 Inspection application for all-solid-state DUV laser

As semiconductor technology progressed, high performance optical metrology

tool is wanted in advanced photomask manufacture. Direct approach is to use the

lithography laser source, which is now equipped with 193 nm ArF excimer laser,

for achieving enough resolution [5]. Although modern excimer technology can

support kilohertz oscillation that is able to satisfy the demands for application of

mask metrology and review, excimer laser itself is an improper choice for the

application. The most important point is that excimer laser mode quality, which

involves the spatial homogeneity and divergence properties of the laser beam, is

too poor for such high precision application.

As an alternative, sub-200 nm all-solid-state laser source can provide extreme

narrow band bandwidth that fits for jobs like calibration and interferometric

applications. What is more, it can deliver high equality coherent beam with high

standard operation stability and need little maintenance that makes it attractive in

mass production.

To realize deep-ultraviolet (DUV) laser under 200 nm, many researchers gave

their solutions. Some of the systems have become current service equipment in

recent years. For example, Ohtsuki et al. reported a 193 nm all-solid-state laser

system now installed in inspection system for Nikon Corp. [6]. The generation is

realized by the eighth harmonic generation from the output of an Er-doped fiber

amplifier operating at a wavelength of 1547 nm. The fiber amplifier system

provides output pulses of a single frequency with a line-width less than 0.1 nm,

an average power of 40 mW and 1.7 ns pulse width at a 1 kHz repetition rate.

Imai et al. reported their development for a highly reliable 198 nm light source

3

for semiconductor inspection based on dual fiber lasers [7]. As shown in Fig.1.1,

to obtain the robustness and to simplify the configuration, the fundamental lights

are run in the pulsed operation and all wavelength conversions are made in

single-pass scheme. The kHz level pulse repetition frequency (PRF) makes it

equivalent to continuous wave (CW) light for inspection. The laser source is now

equipped in the leading edge photo-mask inspection machines.

Fig. 1.1. System for 198 nm light generation

1-1-2 Introduction for UV laser processing

High-average power UV laser have been in great demand for processing

applications in industry [8, 9]. Particularly, UV light is superior to longer

wavelengths in two ways for material processing. First, the short wavelength

allows the production of smaller feature sizes than what is achieved with visible

and IR light. It is explained as minimum focused spot diameter d, which is a

function of the beam quality factor M2, the wavelength λ, and the numerical

aperture (NA) of the focusing lens:

NAMd /2 . (1.1)

Second, the high energy photons delivered with UV light can directly atomize

material in a process called photo-ablation. Since the surrounding material in the

process is not thermally transformed or damaged, the ability of UV laser light in


4

the producing has a nice evaluation.

In recent years, a big progress for UV laser is in the area of diode-pumped

solid-state (DPSS) lasers, where Nd-doped crystal (such as yttrium aluminum

garnet: YAG) lasers utilize nonlinear crystals to transform the 1064 nm output to

its third-(355 nm), fourth-(266 nm), or fifth-(213 nm) harmonic that has drawn

more and more attention. This kind of UV laser has merits of high power and

high pulse repetition rates that are critical to achieving a high system throughput

and productivity. Coupled with physically compact, high beam quality,

mechanically rugged and recent improvements in laser reliability, it is to be

employed in an expanding range of applications and give greatly impact the

industrial processing market.

Especially, multi-watt, diode-pumped Q-switched 355 nm lasers are the ideal

tools for high-precision micromachining applications in the microelectronics

industry. To my knowledge, commercial 355 nm laser source has been employed

in works including, without limitation, as UV Micro Via Drilling, Sapphire

Scribing, Low-k Grooving, and Thin-Wafer Full-Cut Dicing [10, 11], that makes

magnificent contribution to integrated circuit chip and LED production.

1-2 Purpose of dissertation

As the next generation lithography laser source which utilized extreme ultraviolet

(EUV) of 13.5 nm is ready for the mass market, upgrading for semiconductor

industry comes to be urgent in the future. And there is no doubt that the market is

still to be troubled by the suspicion about the Moore’s law effectiveness. With

half-pitch size getting close to the physics limit of silicon, to develop the UV

laser source that is able to meet the future demands has become our important

subject.

First, to develop next generation inspection laser source used in photomask

manufacture, shorter wavelength vacuum UV (VUV) laser source based on

solid-state laser could be a reliable solution in my opinion. In this dissertation, I

make VUV light generation with wavelength in the range of 170-190 nm.

Moving the VUV light generation range below 190 nm seems a big challenge.

5

The most commonly used medium in this range is KBBF that provides sufficient

birefringence to directly generate 177 nm light [12], which equals to the sixth

harmonic generation (6HG) of 1064 nm laser source, with SHG process from 355

nm light.

In order to make the generation with commercial crystals, SFG method with

DUV and IR is preferred. Borate crystal LBO, CBO, and CLBO are verified with

phase-matching until 185 nm with a generation scheme that based on Nd:YAG

laser. It makes SHG, 4HG (266 nm), and 5HG (213 nm) of the 1064 nm, use an

optical parametric oscillator (OPO) to generate IR (about 1400 nm), and at last

make the SFG. In this dissertation, I am going to build the system with a

high-repetition rate 1064 nm laser to generate VUV light at 189 nm with high

efficiency.

For further VUV light generation in 170 nm-180 nm range, a scheme of SFG

to 179 nm was found phase-matched with LBO and CBO. It is realized with

DUV at 198 nm which was used as an application wavelength and is thought to

be potential in future use. In this dissertation, a 179 nm VUV light generation

system is to be built based on 1064 nm laser. LBO and CBO will be tested in the

generation.

Now, commercial all-solid-state 355 nm lasers are conventionally realized by

LBO [13]. As the third harmonic generation (THG) of 1064 nm laser, they have a

simple structure for the generation as shown in Fig.1.2. Based on 1064 nm laser

source, after SHG LBO, 532 nm and residual 1064 nm light with orthogonal

polarization direction interact in the second LBO to have the Type II SFG to

generate 355 nm light.

Fig.1.2. Scheme for THG generation based on all-solid-state laser system.

With higher effective nonlinear coefficient, CLBO has been considered as a

useful tool in the UV light generation. What is more, CLBO grown by self-flux


6

method is thought to have better quality than LBO. Especially it has superiority

in large size crystal used for nonlinear frequency conversion with large diameter

beam. In this dissertation, 355 nm light generation by CLBO is to be researched.

As walk-off effect is found a limitation in nonlinear frequency conversion, to

compensate the large walk-off in THG by CLBO, a new method is to be

discussed to improve the conversion efficiency.

At last, as shown in Fig.1.3, it is known CLBO has been broadly used in UV

light generation mainly at 4HG (4ω) and 5HG (5ω), and showed potential in 190

nm DUV light generations [14, 15]. With the researches carried out in this

dissertation, the application range for CLBO in UV light generation is to be

further extended. As an important part in UV light generation, CLBO is going to

show us a brighter future.

Fig.1.3. UV light generation range by CLBO. Blue lines stand for harmonic generations; purple lines stand for DUV generated with SFG; dashed line stand for wavelength to be discussed in this dissertation.

1-3 Structure of dissertation

This dissertation mainly includes the principle introduction and 3 parts of

research for UV light generation with borate crystals.

In the principle introduction, I introduce the principle of nonlinear optics.

Nonlinear frequency conversion process is to be described with Maxwell’s

equations, while the condition for achieving phase-matching with NLO crystal is

to be discussed in detail. At last, commonly used borate crystals will be

introduced.

7

In the introduction for the researches, first, a 189 nm light generation system is

built. I test the phase-matching properties for the LBO, CBO and CLBO crystals

in UV light generation around 190 nm. The three crystals also will be challenged

with high efficiency UV light generation.

Second, a 179 nm light generation system is built. It derives from a 198 nm

light generation system based on 1064 nm laser system. I challenge the

generation with LBO and CBO in air ambient with the system.

Third, 355 nm light generation is realized with CLBO. The generation with

CLBO has a problem of large walk-off in the SFG with Type II phase-matching. I

employ non-collinear phase-matching realized by wedged cut structure, and

make a prism-coupled CLBO device used in the generation for compensation.


8

References in Chapter 1:

[1] T. H. Maiman, Nature 187, 493 (1960).

[2] P. A. Franken, A. E. Hill, C. W. Peters, and G. Weinreich, Phys. Rev. Lett. 7,

118 (1961).

[3] J. M. Bovatsek and R. S. Patel, Proc. SPIE 7585, 75850K (2010).

[4] C. Chen, T. Sasaki, R. Li, Y. Wu, Z. Lin, Y. Mori, Z. Hu, J. Wang, S. Uda, M.

Yoshimura, and Y. Kaneda, Nonlinear Optical Borate Crystals: Principals

and Applications (Wiley, Germany, 2012) 1st ed., Chap. 3.

[5] M. Rothschild, A. R. Forte, R. R. Kunz, S. C. Palmateer, and J. H. C.

Sedlacek, IBM Journal of Research and Development 41, 49 (1997).

[6] T. Ohtsuki, H. Kitano, H. Kawai, and S. Owa, Proc. Conference on Lasers

and Electro-Optics, CPD9-1 (2000).

[7] S. Imai, K. Matsuki, N. Kikuiri, K. Takayama, O. Iwase, Y. Urata, T.

Shinozaki, Y. Wada, and S. Wada, Proc. SPIE 7580 75800H (2010).

[8] N. Hodgson, M. W. Li, A. Held, and A. Krueger, Proc. SPIE 4977, 281

(2003).

[9] H. Endert, M. Scaggs, D. Basting, and U. Stamm, J. Laser Appl. 11, 1 (1999).

[10] C. Dunsky, Proc. IEEE 90, 1670 (2002).

[11] W. Wiechmann, L. Eyres, J. Morehead, J. Gregg, D. Richard, and W.

Grossman, JLMN-Journal of Laser Micro/Nanoengineering 2, 64 (2007).

[12] C. T. Chen, J. H. Lu, G. L. Wang, Z. Y. Xu, J. Y. Wang, C. Q. Zhang, and Y.

G. Liu, Chin. Phys. Lett. 18, 1081 (2001).

[13] D. T. Thomas, M. S. Keirstead, and N. Hodgson, Proc. SPIE 4426, 493

(2002).

[14] H. Kawai, A. Tokuhisa, M. Doi, S. Miwa, H. Matsuura, H. Kitano, and S.

Owa, Proc. Conference on Lasers and Electro-Optics, CTuT4 (2003).

[15] Y. Asakawa, J. Sakuma, H. Sekita, and M. Obara, Advanced Solid-State

Photonics, OSA TOPS 94 187, (2004).

http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=5288520

http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=5

http://profiles.spiedigitallibrary.org/summary.aspx?DOI=10.1117%2f12.456832&Name=Dafydd+T.+Thomas

http://profiles.spiedigitallibrary.org/summary.aspx?DOI=10.1117%2f12.456832&Name=Mark+S.+Keirstead

http://profiles.spiedigitallibrary.org/summary.aspx?DOI=10.1117%2f12.456832&Name=Norman+Hodgson

9

Chapter.2 Nonlinear optics theory and nonlinear optical

crystals for UV light generation

2-1 Introduction for optical nonlinear frequency conversion

In this section, the principle for nonlinear optics will be introduced. As a kind of

second order nonlinear optical effect, nonlinear frequency conversion will be

described from the fundamental to numerical analysis with Maxwell’s equations.

In order to generate sufficient output with the nonlinear process, the condition of

phase-matching should be fulfilled. Nonlinear optical crystal with birefringence

property is fit for the generation with the ability of achieving phase-matching.

The phase-matching properties of the crystals will be discussed in detail.

Most of theories introduced in this section are based on the reference from

Nonlinear Optics (R. W. Boyd, Academic Press, 3rd edition, 2010) [1],

Handbook of Nonlinear Optical Crystals (V. G. Dmitriev et al., Springer, 3rd

revised edition, 1999) [2].

2-1-1 Introduction for nonlinear optics

Nonlinear optics is the research about the phenomenon raised as a consequence

of the modification of the material’s properties by presence of light, such as laser

which provides sufficient electrical field strength. The word “nonlinear” means

such phenomenon will occur when the response of a material system to an

applied electrical field depends in a nonlinear manner on the optical field

strength. In a general sense, it presents in terms of extending the frequency range

of laser sources by means of harmonic generations and parametric oscillations

etc.

Nonlinear optical effects are analyzed by considering the response of the

dielectric material at the atomic level to the optical field of an intense light wave

E(t). The light travels through a material produces change in the spatial and

Chapter.2 Nonlinear optics theory and nonlinear optical crystals

10

temporal distribution of electrical charges as the electrons and atoms react to the

electromagnetic fields of the wave. The main effect of the forces exerted by the

fields on the charged particles is a displacement of the valence electrons from

their normal orbits. Such perturbation creates electric dipoles whose macroscopic

manifestation is polarization P(t). The phenomenon described above can be

written as the power series of electrical field strength:

(1) (2) 2 (3) 3

0( ) [ ( ) ( ) ( )...]P t E t E t E t , (2.1)

where ε0 is the permittivity in vacuum, χ(i)

is electric susceptibility. The equation

is also based on the assumption that media is lossless and dispersionless that the

polarization can responds instantaneously to the field.

For small field strength this polarization is proportional to the optical field by

the linear susceptibility χ(1)

. It is related to medium’s refractive index

corresponding to the linear optical properties like reflection and refraction. χ(1)

is

a tensor which has a quantity larger than other high order nonlinear optical

susceptibilities by several orders. That can explain why reflection and refraction

is usually meet while other nonlinear optical effect is hardly achieved without

laser. For example, the second order nonlinear susceptibility χ(2)

is of the order of

about 10-12

m/V. It will be effective only if the incident light wave has so strong

electrical field strength that make χ(2)

E2~ χ

(1)E. Nevertheless, in such cases, the

reradiation comes from dipoles whose amplitudes do not faithfully reproduce the

sinusoidal electric field that generates them. As a result, the distorted reradiated

wave contains frequencies which are different from that of the original light.

Consider an optical field which consists of two distinct angular frequency

components (ω1, ω2) incident upon a second-order nonlinear optical medium, the

nonlinear polarization has the form of:

1 2

1 2( ) i t i tE t E e E e , (2.2)

1 2 1 2 1 22 2 ( ) ( )(2) (2) 2 2 *

0 1 2 1 2 1 2( ) [ 2 2 ]i t i t i t i tP t E e E e E E e E E e , (2.3)

11

χ(2)

gives rise to nonlinear phenomenon expressed by the first and second term in

the bracket of (2.3) stand for SHG, the third term stands for SFG, and the fourth

term stands for DFG. On a higher order, χ(3)

gives rise to nonlinear phenomenon

like third harmonic generation, stimulated Raman, Rayleigh Scattering etc. that

will be not discussed in detail. Phenomenon such as SHG, SFG and DFG resulted

from χ(2)

can be visualized by the consideration in terms of energy level transition

for photons between the various frequency components of the field as shown in

Fig.2.1. For example, in SHG, energy from two photons with angular frequency

of ω combined, and a photon of 2ω is simultaneously created in single

quantum-mechanical process.

(a) (b) (c)

Fig.2.1. Second order nonlinear effect: (a) SHG, (b) SFG, and (c) DFG (ω3>ω2, ω1). The solid line in the picture represents the atomic ground state; and the dashed lines represent what are known as virtual levels which are not energy eigenlevels of the free atom but rather represent the combined energy of one of the energy eigenstates of the atom and other photons of the radiation field.

χ(2)

comes to be effective in only non-centrosymmetric crystal that do not

display inversion symmetry. Since liquids, gases, amorphous solids, and even

many crystals display inversion symmetry, χ(2)

vanishes identically for such

media that not second-order nonlinear optical interaction produced consequently.

The quantity of χ(2)

, which affect the strength of stimulated nonlinear effect, is

determined by the anisotropicity of the molecule, crystal lattice in the material,

and direction of the electrical field of input light.


12

In general case, the vector for spatially slowly varying field of the optical

wave can be represented as the discrete sum of a number of angular frequency

components as:

( ) ( ) ni t

n

n

E t E e

, (2.4)

where k stand for wavevector for each element wave. Similarly, nonlinear

polarization can be expressed as:

( ) ( ) ni t

n

n

P t P e

. (2.5)

As P(t) and E(t) are vectors, χ(2)

is a tensor of third rank which formed with 27

elements that acts as the constants of proportionality relating the amplitude of the

nonlinear polarization to the product of field amplitudes. It could be written as

(2)

ijk , where the indices ijk refer to the Cartesian components according to

dielectric axes xyz in the field. In this manner, for the example of SHG, the

second order nonlinear polarization can also be expressed with the style of:

(2)

0

,

(2 ) ( ) ( )i ijk j k

j k

P E E . (2.6)

Here introduced a tensor called contracted susceptibility within a new

notational device that is often used in engineering. It has quantity as:

(2)1

2ijk ijkd . (2.7)

dijk is symmetric in its last two indices for the two input waves are exchangable,

so it becomes dil that can be generally represented with 3×6 matrix as:

11 12 13 14 15 16

21 22 23 24 25 26

31 32 33 34 35 36

il

d d d d d d

d d d d d d d

d d d d d d

. (2.8)

So the second order nonlinear polarization can be written as:

13

2

0(2 ) 2 ( )i il lP d E . (2.9)

The nonlinear polarization leading to second-harmonic generation can be

generally described in terms of dil by the matrix equation:

2

2

11 12 13 14 15 16 2

0 21 22 23 24 25 26

31 32 33 34 35 36

( )

( )(2 )

( )(2 ) 2

2 ( ) ( )(2 )

2 ( ) ( )

2 ( ) ( )

x

y

x

z

y

y z

z

z x

x y

E

EP d d d d d d

EP d d d d d d

E EP d d d d d d

E E

E E

. (2.10)

For a particular crystal, one way to determine the form of nonlinear optical

susceptibility is to consider about the consequences of all the symmetry

properties. By means of mathematical method known as group theory, it is found

all crystals can be classified as belonging to one of 32 possible classes depending

on what is called the point group symmetry of the crystal. Furthermore, as 21 out

of 32 usually have one or more symmetry elements (axes or planes of different

orders), which considerably decrease the number of independent components of

the tensor dil, the general prescription for each of the crystal classes has been

presented so far. Therefore, for describing the nonlinear effect of a certain kind of

crystal in practical questions, it is convenient to use a scalar deff which usually

contains a few of elements from the tensor dil. The form of deff mainly depends

on the spatial relation between E and P that is commonly used in depicting

refraction properties of crystals. Consequently, the SHG equation (2.9) could be

simplified to such relationship where P and E stand for scalars in a certain

direction:

2

eff022 EdP . (2.11)


14

2-1-2 Analysis of SHG with Maxwell’s theory

To consider SHG in a lossless nonlinear optical medium involving collimated,

monochromatic, continuous wave input beam, analysis should be made with

Maxwell’s equations numerically. In order to save the space, the original four

equations written with SI units are not shown. Solution of these equations in

regions of space that contain no free charges ( 0 , where ρ represents charge

density), no free currents ( 0J , where J represents current density) and the

material is nonmagnetic (0B H , where B represents magnetic flux density, μ0

represents magnetic permeability in vacuum, H represents magnetic field

strength) is interested in. Also, the material is considered to be nonlinear in the

sense that the electric displacement field D and electric field E are related by

0D E P . (2.12)

where in general the polarization vector P depends nonlinearly upon the local

value of the electric field strength E. To take the curl of curl-E Maxwell equation:

BE

t

, (2.13)

interchange the order of space and time derivatives on the right-hand side of the

resulting equation, and replace B by 0 ( )

D

t

to obtain the equation:

2

0 20E D

t

. (2.14)

This is the most general form of the wave equation in nonlinear optics. It could

be simplified by using an identity from vector calculus to change the left term

and isotropic source-free condition.

22

2 2

0

10E D

c t

. (2.15)

15

For the P has linear part and nonlinear parts in this equation, to deal with the

latter one, the P spilt and the nonlinear part is gotten:

(1) NLP P P . (2.16)

Similarly decompose displacement field D as:

(1) NLD D P , (2.17)

where the linear part is given as:

(1) (1) (1)

0 0D E P E . (2.18)

As ε(1)

is the dimensionless, relative permittivity which depends on the material.

So (2.15) will become:

(1) 2 22

2 2 2 2

0

1 NLE PE

c t c t

. (2.19)

The equation has the form of a driven wave equation; the nonlinear response of

the medium acts as a source term which appears on the right-hand side of this

equation. Let’s assume the configuration shown in Fig.2.2, where the applied

waves fall onto the nonlinear medium at normal incidence.

Fig.2.2. Process for SHG by a NLO crystal with a length of L.

The solution to this equation for a plane wave at frequency 2ω propagating in

the +z direction is:

( 2 )( 2 )

2( , ) i k z tE z t A e

, (2.20)


16

where (2 ) (2 ) 2 /k n c stands for wavevector and the amplitude (A) of the

wave is a constant. The amplitude of the nonlinear polarization can then be

written according to (2.11) as:

( )2 2( )

2 0 eff2 i k z tP d A e

. (2.21)

As the elementary field can be written as:

( )( )( ) i k z tE A e

, (2.22)

substitute (2.20), (2.21), and (2.22) into the wave equation (2.19), calculate the

second order differential of time t, simplify the equation on two sides to eliminate

eiωt

, and get:

( ) (2 )2 2

(2 ) 2 (2 )2 2 eff

2 2

4 (2 )2 i k k zd A dA dik A e

dz dz c

, (2.23)

The first term on the left-hand side of this equation is much smaller than the

second term so that it can be neglected. This approximation is known as the

slowly varying amplitude approximation:

222 eff

(2 ) 2

2 (2 ) i kzdA idA e

dz k c

. (2.24)

where the quantity:

( ) (2 )2k k k , (2.25)

is called wavevector phase-mismatch. Equation (2.24) is known as a

coupled-amplitude equation, because it shows how the amplitude of the 2ω

varies as a consequence of its coupling to the ω.

Note that for the special case where Δk=0, the amplitude A of the SHG wave

increases linearly with z, and consequently that its intensity increases

quadratically with z. It is known as the condition of phase matching. When this

17

condition is fulfilled, the generated wave maintains a fixed phase relation with

respect to the nonlinear polarization and is able to extract energy most efficiently

from the incident wave.

When such perfect condition is not fulfilled, the intensity of the emitted

radiation becomes smaller. The amplitude of SHG field at the exit plane of the

nonlinear medium is given in the case by integrating equation (2.24) from z=0 to

z=L, yielding:

2 2 2 2

eff eff2 (2 ) 2 (2 ) 20

2 (2 ) 2 (2 ) 1( ) ( )

i kLL

i kzid A id A eA L e dz

k c k c k

. (2.26)

As the field amplitude is defined by:

2

02I n c A , (2.27)

the equation (2.26) can be transformed to:

22(2 ) 2 4

0 eff

2 (2 ) 2 3

8 (2 ) 1

( )

i kLn d A eI

k c k

. (2.28)

At last, the expression of SHG intensity can be written as the relation with

incident intensity:

2 2 22 2eff

2 ( ) 3 2

0

8 (2 )sinc ( / 2)

( )

d II L kL

n c

, (2.29)

where sinc(ΔkL/2)=sin(ΔkL/2)/(ΔkL/2). It should be noted that the efficiency of

the SHG decreased as L increased for some oscillations occurred in such process.

The reason for the behavior is that if L is greater than 1/Δk, the output wave can

get out of phase with its driving polarization, and power can flow from the 2ω

back into the ω. Therefore, it defines:

coh 2 /L k ; (2.30)

as the coherent buildup length of the interaction.


18

With the phase-matched condition, in a certain generation, it is found crystal

length L, input power intensity I, and effective nonlinear coefficient deff

determine the generated output with equation (2.29). There will also be

discussions about such factors in the following experiments of generation.

2-1-3 Phase-matching condition

It is required the phase-matching condition Δk=0 to get sufficient SHG output as

described in (2.29). Also, the phase-matching condition can be understood as

momentum reservation in the process that two photons interact and form the third

photon as shown in Fig.2.1:

( ) ( ) (2 )k k k , (2.31)

( ) (2 )2k k , (2.32)

where k stands for wave vector which has 2 /k n . The phase-matching

condition which indicates the relation of three waves with wavelengths and

refractive indices is known as:

(2 ) ( )n n . (2.33)

Note that in the optical transparency region in isotropic crystals (and also in

anisotropic crystals for identically polarized waves), the equality for SHG will

never be fulfilled because of normal dispersion (usually n(2ω)

>n(ω)

), and the

phase-matching condition will never be satisfied. Therefore, it is only possible to

achieve phase-matching condition by making use of anomalous dispersion: to use

anisotropic medium, for example NLO crystals, under the interaction of

differently polarized waves.

Consequently, it can be simply concluded that the combination of nonzero

square nonlinearity of an optically transparent anisotropic medium with phase

19

matching is the necessary and sufficient condition for an effective two (for SHG)

or three (for SFG) wave interaction.

For the general case in SFG, the relation of the interacting three waves and

their refractive indices can be written as:

1 2 3( )( ) ( )kk k

, (2.34)

and

31 2 ( )( ) ( )

1 2 3

n n n

. (2.35)

where λ3 <λ1, λ2.

On the other hand, from energy conservation for the photons interaction in

SFG as shown in Fig.2.1(b), the relation for wavelength of the interacting three

waves can also be express as:

1 2 3 , (2.36)

and consequently,

1 2 3

1 1 1

. (2.37)

which is the commonly used instruction for calculation wavelength in SFG.

2-1-4 Optics of uniaxial crystals

To discuss the phase-matching property, crystal birefringence is always discussed

in polar coordinate system as shown in Fig.2.3. In uniaxial crystals there is one

special direction exists called the optic axis. Usually, as the optic axis is defined

parallel to z axis, the phase-matching property of the crystal mainly depends on

polar angle θ and have little connection with azimuth angle φ.

http://en.wikipedia.org/wiki/Azimuth


20

Fig.2.3. Polar coordinate system for description of refraction properties of uniaxial crystal (k is the light propagation direction).

As light wave propagates through the crystal, the plane containing the z axis

and the wave vector k of the light wave is termed the principal plane. For linear

polarized light, it is known as an ordinary beam or o beam (Fig.2.4a) if the light’s

polarization direction is normal to the principal plane. The beam polarized in the

principal plane is known as an extraordinary beam or e beam (Fig.2.4b).

(a) (b)

Fig.2.4. Principal plane inside the uniaxial crystal with (a) ordinary beam and (b) extraordinary beam.

The refractive index of the o beam is a constant for the frequency in the crystal,

and does not depend on the propagation direction, whereas for the e beam it does.

Thus, the refractive index in anisotropic crystals generally depends both on light

polarization and propagating direction.

The refractive indices of the ordinary and extraordinary beams in the direction

normal to the z axis are termed the principal values of the refractive index and are

21

denoted by no and ne, respectively as shown in Fig.2.5. The different between

them is defined as birefringence Δn. The refractive index of the extraordinary

wave on arbitrary direction is, in general, a function of the polar angle θ that

determined by the equation:

2

2 2

1 tan

1 / tano

o

e

e

nn n

n

(2.38)

If no > ne the crystal is called negative, unless it is called positive. The

dependence of the refractive index on light propagation direction inside the

uniaxial crystal (index surface) is a combination of a sphere with radius no (for an

ordinary beam) and an ellipsoid of rotation with semi-axes no and ne (for an

extraordinary beam, the axis of the ellipsoid of rotation is the z axis). In the z axis

direction the sphere and ellipsoid are in contact with each other. In a negative

crystal the ellipsoid is inscribed in the sphere (Fig.2.5a), whereas in a positive

crystal the sphere is inscribed in the ellipsoid (Fig.2.5b).

(a) (b)

Fig.2.5. Dependence of refractive index on light propagation direction and polarization in (a) negative and (b) positive uniaxial crystal and walk-off angle ρ.

When a plane light wave propagated in a uniaxial crystal, the direction of

propagation of the wave vector (vector k) generally does not coincide with that of

the wave energy (vector S) which is also called Poynting vector. The direction of


22

S can be defined as normal to the tangent drawn at the point of intersection of

vector k with the refractive index curve. For an ordinary wave the refractive

index dependence is a sphere with radius no. Therefore, the normal to the tangent

coincides with the wave vector k. For an extraordinary wave the normal to the

tangent (with the exception of the cases θ=0, θ=90) does not coincide with the

wave vector k but is rotated from it by angle called walk-off angle:

2

arctan / taneon n

, (2.39)

where the upper signs refer to a negative crystal and the lower signs to a positive

one. The walk-off angle also can be seen in Fig.2.5. From this structure, it is

found the birefringence and walk off angle are intrinsic co-existence relation. The

NLO crystal which has refractive ellipse with a large eccentricity that means

large birefringence will also obtain a large walk-off angle. That is the reason

NLO with outstanding phase-matching power also suffers from large walk-off

effect.

2-1-5 Optics of biaxial crystals

For biaxial crystals, the dependence of the refractive index on light propagation

direction and its polarization (index surface) corresponds to a much more

complex function than for uniaxial crystals’. Similar to the case of a uniaxial

crystal, the propagation direction of plane light wave is defined by two angles:

polar θ and azimuthal φ except the optical axis is no longer parallel to z axis.

Note that the use of terms ordinary (o) and extraordinary (e) waves for the

general case of light propagation inside a biaxial crystal is senseless except in the

principal planes of a biaxial crystal.

In the plane xy the refractive index of the wave polarized normally to this

plane is constant and equals nz, and that of the wave polarized in this plane

changes from ny to nx with varying from 0 to 90. Hence, a biaxial crystal with

nx<ny<nz in the plane x-y is similar to a positive uniaxial crystal with no=nz and:

23

1/22

1/22

2

1 tan

1 / tane y

y x

nn

n n

. (2.40)

In the plane y-z the refractive index of the wave polarized normally to this

plane is constant and equals nx, whereas for the wave polarizaed in this plane the

refractive index changes from ny to nz with θ varying from 0 to 90. Hence, a

biaxial crystal with nx<ny<nz in the plane y-z is similar to a negative uniaxial

crystal with no=nx and:

1/22

1/22

2

1 tan

1 / tane y

y z

nn

n n

. (2.41)

It is easy to deal with practical question with the expression for birefringence in

biaxial crystals, , for example, to make sure the phase-matching angle or walk-off

angle in principle plane with the similar method as in uniaxial crystals. Vz is an

angle formed by one of the optic axes with the axis z which has different form in

the two cases. It can help to estimate the spatial relation of the two optical axes.

Fig.2.6. Dependence of refractive index on light propagation direction in biaxial crystals under nx<ny<nz condition

Vz


24

2-1-6 Phase-matching in uniaxial crystals

Generally, in order to achieve phase-matching through the use of birefringence

crystal, the waves involve the interaction should have orthogonal polarization

directions that they can be termed o beam and e beam as described in 2-1-4.

Fig.2.7 shows the dispersion of refractive indices in a negative crystal to

illustrate the SHG (from λω to λ2ω). Since the difference between no and ne gives

the maximum value of the birefringence of the crystal, when the birefringence of

λω and λ2ω light have a part coincided, the phase-matching for SHG is possible.

(a) (b)

Fig.2.7. Dispersion of the refractive indices in a negative crystal. The phase-matching depends on the birefringence property: (a) phase-matching possible, (b) phase-matching impossible.

If the input waves with frequency ω have the same polarization direction, while

the radiation at 2ω has polarization in the perpendicular direction, Type I phase

matching is realized, which has:

( ) (2 )2 o ekk . (2.42)

This is called ooe interaction of phase-matching of Type I. Similarly, in positive

crystals, there is also:

(2 )( )2 e ok k , (2.43)

which is called eeo interaction of Type I phase-matching.

25

Typically, phase-matching is accomplished by tuning the direction of

wavevector with respect to optical axis. Fig.2.8 illustrates how to find the

direction of collinear (scalar) phase matching. For the ooe interaction, it has:

( ) (2 )

PM2 ( )o ek k , (2.44)

or corresponding refractive indices relation as:

( ) (2 )

PM( )o en n . (2.45)

Therefore, the phase-matching direction k for this case is formed when the circle

2ko(ω)

intersects the ellipse ke(2ω)

(θ) or when the circle of the ordinary refractive

index at frequency ω crosses the ellipse of the extraordinary refractive index at

frequency 2ω.

(a) (b)

Fig.2.8. Collinear (scalar) type I phase matching for SHG in uniaxial negative crystal in coordinates described by (a) refractive index function for o beam shown with dash and dot line curve, for e beam shown with dashed curve and (b) wave vector shown with different line for o beam and e beam.

Also there is phase-matching occurred with input lights of different frequency.

In the equation of SFG, the first symbol in the expressions refers to the wave

with the lowest frequency, the third symbol to the wave with the highest


26

frequency (λ3>λ2>λ1 or ω3>ω2>ω1). For example, in negative crystal, it has ooe

interaction like:

31 2 ( )( ) ( )

oo ek k k . (2.46)

While in positive crystal, there is eeo interaction like:

31 2 ( )( ) ( )

e oekk k . (2.47)

Fig.2.9. Collinear (scalar) type II phase-matching for SHG in uniaxial negative crystal in coordinates described by wavevector. Dash and dot line stands for o beam, while dash line stands for e beam.

If the inputted ω waves have orthogonal polarizations, Type II phase matching

takes place and the 2ω wave corresponds to an extraordinary wave in negative

crystals:

( ) ( (2 )) ( )o e ek k k . (2.48)

There is also Type II phase-matching in positive crystal shown as:

( ( ) () 2 )

o e ok k k . (2.49)

Accordingly, such phase-matching can be also written with refractive index

relation as:

27

( ) ( (2 )) ( )o e en n n , (2.50)

and:

( ( ) () 2 )

o e on n n . (2.51)

For the case of SFG, it has interactions for Type II phase-matching like (oee):

31 2 ( )( ) ( )

eo ek kk , (2.52)

and (eoe)

31 2 ( )( ) ( )

ee okk k , (2.53)

in negative crystals. While, there are oeo interaction:

31 2 ( )( ) ( )

eo ok k k , (2.54)

and eoo interaction:

31 2 ( )( ) ( )

oe okk k , (2.55)

for SFG in positive crystals.

Collinear phase-matching described above is usually used in SHG or SFG

because of its simplicity in realization. However, it also has a limitation of

walk-off effect occurred in the generation with phase-matching angle θPM other

than 0 or 90. For example, in a Type II (eoe) SHG realized with a negative

uniaxial crystal (such as CLBO) as shown in Fig.2.10, there are walk-off angles

(ρ(ω)

, ρ(2ω)

) between the Poynting vectors (Se(ω)

, Se(2ω)

) and wavevectors (ke(ω)

,

ke(2ω)

) of the e beams. These walk-off angles will consequently exist between the

Poynting vectors of the three interacting waves and cause the overlap reduction

between the waves. As the result, the generated field intensity will get decreased

and not fully comply with equation (2.29).


28

Fig.2.10. Scheme for Type II phase-matching of SHG. Orange line and dashed line stand for wavevector and Poynting vector for the ω (o beam); while red and green lines stand for the ω and 2ω waves. Walk-off angle in the generation is given as the separation between the wave vector and Poynting vector.

There is a factor called aperture length (l a ) that describes the impact of the

walk-off effect. The conversion to the 2ω wave will cease to grow proportionally

to the square of the crystal length beyond the aperture length. In Type I

phase-matching, it is defined as:

(I)

0 /al w ; (2.56)

while in Type II phase-matching, it has a form as:

(II)

01.16 /al w . (2.57)

where w0 stands for radius of the beam. From the equations it is found that the

walk-off effect should be evaluated with beam’s aperture involved in the

interaction.

On the other hand, the walk-off effect will disappear when phase-matching

angle becomes θPM=90 that Poynting vector collimates with wave vector. This

condition is often called non-critical phase-matching that is used to increase in

the conversion efficiency.

29

Fig.2.11. Configuration of twin-crystal mothed for walk-off compensation in a Type II second-harmonic generation. The figure is devoted to show the direction of each vector, so the length of line does not represent the real quantity of the vector.

Since there is no way to eliminate the walk-off effect; several methods have

been proposed to reduce the impact. One of most utilized method is to use a

twin-crystal configuration [3], where two identically cut crystals are mounted

with their optical axes symmetrically crossed. An example based on Type II SHG

is shown in Fig.2.11, Poynying vector of e beams separate from their wavevector

in opposite direction in the two cascaded crystals. Such configuration will

improve the coverage area between the two ω waves and lift the conversion

efficiency. It is also can be considered as an improvement in the aperture length

of the generation. The method is effective for either Type I or Type II

phase-matching and generations either the walk-off angle is occurred in input

wave or generated wave.

Another effort aimed at compensating the walk-off angle only within Type II

phase-matching, involves the use of non-collinear phase-matching [4]. As the

phase-matching conditions means the space resonance of the propagating waves

physically, there is also one kind of non-collinear (vector) phase-matching that

the interacting waves are not on the same direction. Fig.2.12 demonstrates the

positions of scalar (angle θPM) and vector phase-matching of Type II for SHG in a

negative uniaxial crystal. The phase-matching direction in the non-collinear case

is determined by intersection of the ellipse ke(2ω)

(θ) with the quasi-ellipse

ko(ω)

+ke(ω)

(θ). Type II vector phase-matching is possible in the region

θPM≤θ’PM≤π-θPM.


30

As a Type II phase-matching SHG shown in Fig.2.13, the method for walk-off

compensation takes the advantage of the separated angle between the

wavevectors of the two input waves to compensate the deviation between the

Poynting vectors of the waves. The practical application of the method in Type

355 nm light generation with CLBO will be one of the themes of the dissertation

introduced in Chapter 5.

Fig.2.12. Scheme for collinear and non-collinear phase-matching for Type II SHG in a negative uniaxial crystal. k and θPM stand for collinear phase-matching, while k' and θ’PM stand for non-collinear phase-matching.

Phase-matching angles calculation for Type II SHG given above is:

( ) ( ) ( ) ( ) ( ) ( ) (2 ) (2 ) (2 )( )sin( ) ( )sin( ) 2 ( )sin( )e e e o o o en n n , (2.58)

( ) ( ) ( ) ( ) ( ) ( ) (2 ) (2 ) (2 )( )cos( ) ( )cos( ) 2 ( )cos( )e e e o o o en n n , (2.59)

( ) ( )( ) ( ) 2

( )

( )Tan( ) Tan( )[ ]e e

e o

o

n

n

. (2.60)

Equations (2.58) and (2.59) are for the non-collinear phase-matching condition in

Type II SHG, while (2.60) is for the collinear energy flow of o beam and e beam

of ω.

31

Fig2.13. Walk-off compensation for Type II SHG with non-collinear phase-matching configuration.

2-1-7 Quasi phase-matching

The efforts to meet phase-matching condition also face difficulty when NLO

material posses insufficient birefringence to compensate for the dispersion of the

linear refractive indices over the wavelength range of interest. Particularly, in

generation in UV range, the problem of insufficient birefringence becomes

increasingly acute.

Fig.2.14. Comparsion of the field strength variation of the generated wave along the nonlinear medium in nonlinear optical interaction. Red curve stands for the phase-matching condition. Green curve stands for the condition that wavevector mismatch is nonzero. Blue curve stands for quasi phase-matching condition.

There is a method known as quasi phase-matching that can be used when

normal phase-matching cannot be implemented [5]. It is realized with a structure


32

called periodically poled material which has been fabricated in such a manner

that the orientation of one of the crystalline axis is inverted periodically as a

function of position within the material. That means a periodic alternation of the

sign of deff, which can compensate for nonzero wavevector mismatch Δk, is

realized.

As shown in Fig.2.14, if the mismatch of interaction is nonzero, the

consequent field strength of the generated wave will oscillate periodically with

propagation distance. After the compensation for the influence of wavevector

mismatching with quasi phase-matching, the generated field strength grows with

propagation distance and approaches phase-matched condition. The period ( )

of the alternation of the crystalline axis has been set equal to twice the coherent

length (Lcoh) as defined in equation (2.30), of the nonlinear interaction as shown

in equation (2.61) and Fig.2.15. Each time the field strength of the generated

wave grew to the maximum, it should have begun to decrease as a consequence

of the wavevector mismatch. Thanks to a reversal of the sign of deff, the field

amplitude is able to continue to grow monotonically.

coh2 2 /L k . (2.61)

Fig.2.15. A periodically poled material used to realize a SHG from ω to 2ω. Arrows stand for crystalline axis which alternates in orientation with period Λ.

2-1-8 Optical parametric oscillator

Optical parametric oscillation (OPO) is a useful configuration for nonlinear

frequency conversion to generate wavelength that cannot be achieved with other

methods. OPO requires phase matching condition to be achieved and is also

considered as the opposite process of SFG. Similar as the wavelength relation

given as (2.37), for OPO it has (λ3<λ2<λ1):

https://www.rp-photonics.com/phase_matching.html

33

3 2 1

1 1 1

. (2.62)

The shortest wavelength λ3 act as pump light, while in generated waves, shorter

wavelength λ2 is called signal; longer wavelength λ1 is called idler. So the relation

turns to be:

pump signal idler

1 1 1

. (2.63)

OPO is practically based on NLO crystal that provides gain at both signal and

idler when get pumped, and is schematically represented by an optical cavity

which is built with dichroic mirrors. In this research, two singly resonant

oscillators will be employed.

The usage of OPO has a merit of a wide potential wavelength tuning range.

The tuning is in most cases achieved by influencing the phase-matching

conditions, e.g. by changing the crystal temperature, or the angular orientation of

the crystal.

Since high spatial coherence and high power intensity pump is preferred in

OPO, diode laser cannot be directly used as the pump, which means the OPO

system needs a complicated layout. Pump wave with pulsed operation typically

generated by Q-switched laser is always used to get high gain at generated

waves.

2-2 Nonlinear optical crystals for UV light generation

Nonlinear optical crystals are considered as a kind of key material for solid

laser system development because of their ability to change frequency of laser

beam and modulate it in field strength and phase. In the following part of this

chapter, important NLO properties called for nonlinear frequency conversion are

to be discussed; borate crystals as one kind of useful NLO crystal for UV light

generation are to be introduced.

https://www.rp-photonics.com/dichroic_mirrors.html


34

2-2-1 Properties required for nonlinear crystals in UV light generation

There are several important properties of the nonlinear crystals should be

concerned before start to design a frequency converter in UV range, especially

for sub-200 nm usage [6].

Firstly, a relatively large effective nonlinear coefficient (deff) should be

considered. It depends strongly on the geometric symmetry of the crystals, and

the phase-matching type achieved in the practical generation. From the

expression of generated intensity in equation (2.29), it is obviously found the

nonlinear coefficient plays an important part in generation.

Wide transparency range either on IR side or UV side is in demand. Because

the generation under 190 nm is generally realized by SFG with UV and IR light,

it requires the crystal work in either UV range around 200 nm or IR range around

2000 nm.

As a useful NLO crystal, it is necessary for the crystal to possess an

appropriate birefringence spanning UV and IR, which provides sufficient ability

for phase-matching.

What is more, since high peak power density is needed in nonlinear

wavelength conversion, enough high damage threshold (bulk damage above 100

MW/cm2 for ns pulse duration) is important for the crystal to sustain high

intensity radiation. Otherwise the NLO effect has not chance to be realized in

practical use.

At last, it is obvious that as a useful NLO material, good physical properties

such as, high optical quality, high mechanical properties and stable chemical

property are demanded. Also, reliable growth method for achieving bulk crystal

is required.

2-2-2 Useful borate crystals

As coherent UV laser is found more and more applications in industry, NLO

crystals suited for UV light generation have drawn great attentions. Especially,

borate crystals, such as β-BaB2O4 (BBO) belongs to anionic group (B3O6)3-

;

35

LiB3O5 (LBO), CsB3O5 (CBO), CsLiB6O10 (CLBO) belong to anionic group

(B3O7)3-

; and KBe2BO3F2 (KBBF) belongs to anionic group (BO3)3-

, can provide

large effective nonlinear coefficient and phase-matching ability that are

considered effective choice in UV light generation.

Beta-barium borate (BBO)

BBO is a negative uniaxial crystal belonging to point group 3m, which is

grown by top-seeded solution growth (TSSG) technique with flux method. The

crystal belongs to anionic group (B3O6)3-

that makes the large nonlinear

coefficient d22 theoretically possible [2].

The cutoff wavelength on UV side is locates at 189 nm while on IR side

locates at 2500 nm. The crystal has a moderately large nonlinear coefficient,

large birefringence and relatively small dispersion. It also features a large

temperature tolerance in phase-matching and good physical and chemical

properties. BBO has a wide phase-matching range for SHG from 205 to 1500 nm.

It is also widely used in tunable optical parameter oscillators and amplifiers

besides SFG.

However, BBO has too small angular acceptance and large walk-off angle,

which limit its application for laser systems possessing larger divergence and for

focusing to increase the power density. Also the low damage threshold restricts

its application in high power UV light generation such as fourth and fifth

harmonic generation of Nd-doped laser.

Lithium triborate (LBO)

LBO belongs to point group mm2, is grown with flux method with TSSG

technique. The crystal belongs to anionic group (B3O7)3-

which is an ideal

structure from the point view of absorption edge, SHG coefficient and damage

threshold. The cutoff wavelength on UV side is locates at 160 nm while on IR

side locates at 2600 nm. It has birefringence of about Δn=0.04 (532 nm). LBO is

a kind of negative biaxial crystal whose Sellmeier’s equations are given as [7]:

2 2

2

0.0112492.45414 0.014591

0.01135xn

,


36

2 2

2

0.0127112.53907 0.01854

0.012523yn

,

2 2

2

0.0130992.586179 0.017968

0.011893zn

. (2.64)

Nonlinear optical coefficients are d31=0.94 pm/V, and d32=1.12 pm/V. The

effective nonlinear coefficient can be simply expressed as [8]:

xy plane: deff=d32cosφ (Type I),

yz plane: deff=d31cosθ (Type II),

xz plane: θ>Vz deff= d32sin2θ+d31cos

2θ (Type I),

θ<Vz deff= d32sin2θ+d31cos

2θ (Type II). (2.65)

The major advantage of LBO are summarized as follows: (1) it possesses

exceptionally low angular sensitivities, wide acceptance bandwidth, small

walk-off angles for SHG and THG of Nd-doped laser; (2) it has a widest

temperature-tuned NCPM range to SHG from 0.9 to 1.9 μm, and larger effective

nonlinear coefficient in these generations than critical phase-matchings; (3) it is

also featured by relatively high optical-damage threshold, mechanical hardness,

chemical stability, and non-hygroscopicity.

The major weakness of LBO comes from small birefringence that hinders the

crystal being used from achieving SHG shorter than about 277 nm.

Cesium triborate (CBO)

CBO belongs to point group 222 and anionic group (B3O7)3-

, is grown with

flux method or stoichiometric melts with TSSG technique. The cutoff wavelength

on UV side is locates at 167 nm while on IR side locates at 3400 nm. It has

birefringence of about Δn=0.06 (532 nm). CBO is a negative biaxial crystal

whose Sellmeier’s equations are given as [9]:

2 2

2

0.013782.3035 0.00612

0.01498xn

,

2 2

2

0.015282.3704 0.00939

0.01581yn

,

37

2 2

2

0.018062.4753 0.01654

0.01752zn

. (2. 66)

Nonlinear coefficient is d14=0.86 pm/V. Expressions for the effective nonlinear

coefficient in the principal planes of CBO are [10]:

xy plane: deff=d14sin2φ (Tpye II),

yz plane: deff=d14sin2θ (Type I),

xz plane: θ<Vz deff=-d14sin2θ (Type II),

θ>Vz deff=-d14sin2θ (Type I). (2.67)

Though the nonlinear coefficients for CBO and LBO are approximately equal,

their effective NLO coefficients are quite different due to different crystal

symmetry. The CBO can get nearly maximum effect nonlinear coefficient in

some UV light generations such as Type II 355 nm (1064 nm +532 nm 355

nm) generation and Type I 266 nm (1064 nm + 355 nm 266 nm) generation.

This suggests CBO a more effective NLO material for frequency conversion into

UV region than LBO. Unfortunately, its application in these generation is not so

popular as LBO because of limitation of the high hygroscopy.

Cesium lithium borate (CLBO)

CLBO is first developed by Osaka University and is also the key research

object in this dissertation. LBO belongs to point group -42m and anionic group

(B3O7)3-

, which is grown with flux method or stoichiometric melts with TSSG

technique. The cutoff wavelength on UV side is locates at 180 nm while on IR

side locates at 2750 nm. It has birefringence of about Δn=0.05(532 nm). CLBO is

a negative uniaxial crystal with Sellmeier’s equations as [11]:

2

2

2 01258.001424.0

01018.02104.2

on ,

2

2

2 00607.001363.0

00838.00588.2

en . (2.68)


38

Nonlinear coefficient is d36=0.95 pm/V. The effective NLO coefficients for Type

I and Type II processes are given by[12]:

deff=d36sinθsin2φ (Type I),

deff= d36sin2θcos2φ (Type II). (2.69)

CLBO has relatively large effective NLO coefficients in SFG for DUV light

especially under 200 nm. CLBO features large angular and temperature

bandwidths favorable for stable DUV operation. Compared with BBO, CLBO

also has smaller walk-off angles, which make it produce better spatial profile and

overlapping of the mixing beams. The second, fourth, and fifth harmonic

generations of 1064 nm laser with CLBO have been proved effective, the data of

these generations with THG are concluded in Table 2.1. CLBO has remarkable

properties particularly in fourth and fifth harmonic generations.

Table 2.1. Properties for CLBO in harmonic generation with 1064 nm laser source.

Wavelength

(nm)

(PM Type)

Phase-

matching

angle θ

(deg.)

deff

(pm/V)

Δθl

(mrad·cm)

Δλl

(nm·cm)

ΔTl

(°C·cm)

Walk-off

angle

(mrad)

1064+1064

532

(Type II)

42.6 0.68 1.91 5.6 57.0 33.81(ω)

35.72(2ω)

1064+532

355

(Type II)

48.8 0.71 0.92 0.84 19.9 33.4(ω)

36.8(3ω)

532+532

266

(Type I)

62.0 0.79 0.55 0.13 6.7 32.2

1064+266

213

(Type I)

68.4 0.95 0.48 0.16 4.1 28.9

39

Potassium beryllium fluoroborate (KBBF)

KBBF belongs to point group 32, is grown with flux or hydrothermal method.

The crystal belongs to (BO3)3-

group, which make it powerful in phase-matching

with large birefringence and achieve transparency down to VUV range. The

cutoff wavelength on UV side is locates at 147 nm while on IR side locates at

3400 nm [6].

KBBF is the only kind of NLO crystal that can directly generate UV light

under 200 nm with SHG process. For instance, it can be used to realize the sixth

harmonic light at 177 nm of Nd:YAG laser with the input third harmonic light at

355 nm. However, because of limitation in the growth technology, KBBF crystal

cannot be grown to enough thickness other than a few millimeters. Therefore, it

cannot be cut according to phase-matching angles as wishes just like other NLO

crystals.

In order to achieve phase-matching with KBBF, a prism-coupled device (PCD)

[13] should be used to increase the transmittance of the incident light at the

surface of the crystal. As shown in Fig.2.16, 177 nm VUV has been successfully

demonstrated with the device, which makes the crystal a potential in future

application use.

Fig.2.16. Prism coupled device used with KBBF in VUV light generation. Prism is cut with wedge angle which equals to the phase-matching angle of θPM=68.6°. Normal incident light will get through the glass-crystal boundary without obvious refraction, because the refractive indices on the two sides are almost equaled. So the light inputted in the KBBF can meet the phase-matching condition. Generated light will separate with original light due to dispersion in the glass.


40

There is a summary of main parameters of the borate crystals listed in Table 2.2.

Table 2.2 Borate crystals to be discuss in this work

Crystals Point

group

Transparent

range (nm)

Nonlinear

coefficient

(pm/V)

Birefringence

Δn

Shortest

SHG

(nm)

Anionic

group

BBO 3m 189-2500 d11=1.844 0.12 205 (B3O6)3-

LBO mm2 160-2600 d31=0.94

d32=1.13

d33=0.256

0.05 277 (B3O7)5-

CBO 222 167-3400 d14=0.863 0.06 273 (B3O7)5-

CLBO -42m 180-2750 d36=0.95 0.05 237 (B3O7)5-

KBBF 32 147-3400 d11=0.8 0.08 162 (BO3)3-

There are two other kinds of crystal used in this research. Here are brief

introductions for both of them.

Potassium titanyl phosphate (KTP)

KTP belongs to point group 32, which is grown with flux or hydrothermal

method. The cutoff wavelength on UV side is locates at 350 nm while on IR side

locates at 4500 nm [6].

Due to large effect nonlinearity and excellent optical properties, KTP suits for

the nonlinear wavelength conversion material in various applications. Especially,

SHG can be realized with KTP ranges from CO2 lasers to Nd:YAG lasers. With

large temperature bandwidth in phase-matching process, KTP is employed to

achieve QPM by temperature tuning to realize perfect phase-matching.

Although KTP is very attractive for various SFG, DFG, and optical parametric

applications over its entire transparency range, high power operation with KTP is

limited by its low damage threshold.

Periodically poled lithium niobate (PPLN)

PPLN is made by periodical layout of nonlinear material that utilizes quasi

phase-matching in nonlinear wavelength conversion processes. The frequency

41

conversions with PPLN use nonlinear coefficient d33= 25pm/V which is much

larger than the off-diagonal coefficients [14].

With absorption cutoff at about 400 nm, PPLN becomes an efficient choice for

SHG of light ranges from 1000 nm to about 2000 nm. Of course, the actual

conversion efficiency depend on the properties of the laser beam used (e.g. pulse

length, repetition rate, beam quality, and line width).

Another common use for PPLN is for building an optical parametric oscillator

(OPO) that generate IR light at about 1500-2000 nm. The operation wavelengths

in this process can be regulated by changing the PPLN temperature or the poled

period of PPLN. Because of photorefractive effect occurred can damage the

crystal and cause the output beam to become distorted, 5% MgO doping lithium

niobate is widely used to make the device for it can significantly increases the

optical and photorefractive resistance of the crystal while preserving its high

nonlinear coefficient.

2-3 Summary

In this chapter, the principle of nonlinear optics is introduced. As the second

order nonlinear effect, nonlinear frequency conversion is described from the

fundamental to numerical analysis with Maxwell’s equations. For generating

sufficient output with the nonlinear process, the condition of phase-matching

should be fulfilled. Nonlinear optical crystal with birefringence property is fit for

the generation, whose phase-matching properties are discussed in detail.

As key material for generating new frequencies with solid laser system, NLO

crystals are introduced and their important properties that required for UV light

generation are discussed. Moreover, as the most popular material for UV light

generation in this field, borate crystals used in the dissertation are briefly

introduced.


42


[1] R. W. Boyd, Nonlinear Optics (Academic Press, 2010) 3rd ed., Chap. 2.

[2] V. G. Dmitriev, G. G. Gurzadyan, and D. N. Nikogosyan, Handbook of

Nonlinear Optical Crystals (Springer, 3rd revised edition, 1999).

[3] J. Zondy, M. Abed, and S. Khodja, J. Opt. Soc. Am. B 11, 2368 (1994).

[4] K. Asaumi, Appl. Opt. 37, 555 (1998).

[5] J. A. Armstrong, N. Bloembergen, J. Ducuing, and P. S. Pershan, Phys. Rev.

127, 1918 (1962).




[7] K. Kato, IEEE J. Quantum Electron. 26, 1173 (1990).

[8] D. A. Roberts, IEEE J. Quantum Electron. 28, 2057 (1992).


[10] Y. Wu, T. Sasaki, S. Nakai, A. Yokotani, H. Tang, and C. Chen, Appl. Phys.

Lett. 62, 2614 (1993).

[11] N. Umemura, K. Yoshida, T. Kamimura, Y. Mori, T. Sasaki, and K. Kato,

Advanced Solid-State Lasers, OSA TOPS 26, 715 (1999).

[12] N. Umemura and K. Kato, Appl. Opt. 36, 6794 (1997).

[13] C. Chen, J. Lu, T. Togashi, T. Suganuma, T. Sekikawa, S. Watanabe, Z. Xu,

and J. Wang, Opt. Lett. 27, 637 (2002).

[14] L.E. Myers, R. C. Eckardt, M. M. Fejer, and R. I. Byer, Opt. Lett. 21, 8

(1996).

javascript:searchAuthor('Lu,%20J')

javascript:searchAuthor('Togashi,%20T')

javascript:searchAuthor('Suganuma,%20T')

javascript:searchAuthor('Sekikawa,%20T')

javascript:searchAuthor('Watanabe,%20S')

javascript:searchAuthor('Xu,%20Z')

javascript:searchAuthor('Wang,%20J')

43

Chapter 3. 189 nm VUV light generation with borate crystals

3-1 Introduction for VUV light generation at 189 nm

Although lots of researches have reported the UV generation in the range of

190 nm-200 nm [1-4], however, there is very few record about the generation in

180 nm-190 nm range as far as I know. For example, Kouta et al. reported a

generation of 186 nm light based on Ti:sapphire laser with the SFG with the

fundamental (774 nm) and the third-harmonic (248 nm) light. The generation was

confirmed with a BBO crystal which is cooled to 91K so that the cutoff

wavelength shortened from 189 nm to 180 nm [5].

Another 180 nm-190 nm range generation is realized by SFG with fifth

harmonic light of 1064 nm laser and IR light of about 1400 nm generated by

OPO. Because a large birefringence is not required in such “UV+IR” form SFG,

borate crystals LBO, CBO, and CLBO were all proved to be feasible for the

generation until 185 nm [6-8]. However, conversion efficiency of the SFG with

these borate crystals has not been investigated, because the experiments in these

reports were based on Q-switch laser sources with a pulse repetition of 10 Hz for

phase-matching angles measurement use. In order to comply with laser source

equipped in practical applications, repetition rates in the tens of kilohertz range

are preferable for the generation [9]. Under such circumstance, to ensure

sufficient conversion efficiency, the high-repetition rate laser beam should be

focused in nonlinear optical crystal to make the peak power density reach the

order of 100 MW/cm2 which is similar to unfocused 10 Hz laser beam.

In this research, as shown in Fig.3.1, a laser system of 189 nm light generation

by SFG with 213 nm and IR light is built based on a high repetition rate 1064 nm

laser source. PPLN, which can provide high conversion efficiency, is used as the

OPO material. The generated VUV wavelength is till about 189 nm due to the

limitation of the generated range of the PPLN OPO used. Table 3.1 lists the

phase-matching properties at 189 nm light generation for the potential borate

Chapter 3. 189 nm VUV light generation by borate crystals

44

crystals. Among them, CBO is expected to give largest output power according to

the effective nonlinear coefficient. With the system, I attempt to make highly

efficient VUV generation and unveil the phase-matching property under 190 nm

of these borate crystals with a high-repetition rate laser source.

Fig.3.1. Scheme for 189 nm light generation.

Table 3.1. Property of borate crystals for SFG at 189 nm.

*Phase-matching angles for SFG at 189.1 nm were calculated with the interacting wavelengths of 212.8 and 1697.9 nm.

3-2 Experiment for 189 nm light generation

In this section, the 189 nm VUV light generation realized by SFG with 213 nm

and IR light by borate crystal will be introduced. The description of the whole

system will be divided into several parts which contains the demonstration of

SHG, OPO, 4HG, 5HG and the final generation setup. The generation results will

be summarized in the last part of the section.

Candidate

crystal

Transmission

range (nm)

Phase-matching

type

Phase-matching

angle* (θ, φ)

Effective nonlinear

coefficient deff

(pm/V)

LBO [6] 160-2600 Type I

in xy plane

(90.0, 71.0) 0.28

CBO [7] 167-3400 Type I

in yz plane

(54.5, 90.0) 1.04

CLBO [8] 180-2750 Type I (59.5, 45.0) 0.64

45

3-2-1 Scheme for 189 nm light generation

The experimental setup of the 189 nm light generation system is depicted in

Fig.3.2. A commercial Q-switched Nd:YAG laser of 1064 nm operating at a 10

kHz repetition rate (COHERENT MATRIX 1064) was employed as the

fundamental laser source, with a pulse width of 60 ns and a maximum average

output of 7 W.

The system is built with five stages of nonlinear frequency conversions. First,

it is a harmonic generator ranges from the SHG at 532 nm to the fifth-harmonic

generation at 213 nm. SHG is realized with LBO, while 266 nm and 213 nm light

generations are realized by CLBO1 with SHG2 and by CLBO2 with the SFG1.

The crystals selected are best suited for these generations to our knowledge. On

the other hand, IR is produced by a PPLN OPO, which pumped with residual

1064 nm light after SHG LBO. At last, the SFG for producing VUV light at

around 190 nm is realized by borate crystals. Delay lines are set in light paths of

213 nm and 189 nm light generation to improve the conversion efficiency of

SFGs.

Fig.3.2. Experimental setup for 189 nm light generation. Cs stand for cylinder lenses while Ls stand for spherical lenses. There are delay lines set in 1064 nm and IR light path for 213 nm and 189 nm light generations.


46

3-2-2 SHG with LBO

The second-harmonic generation (SHG) is performed using a non-critical

phase-matching (NCPM) LBO crystal. It is cut with angles: θ=90, φ=0 and a

dimension of 5×5×10 mm3. The oven mounted LBO is heated to 151C to meet

the phase-matching condition along x-axis. The fundamental beam is focused to a

radius of about 36 µm. As shown in Fig.3.3, SHG is got with a maximum

conversion efficiency of 36.8% with input of 6.39 W.

Fig.3.3. SHG measured by the LBO. Green spots stand for output, grey spots stand for conversion efficiency.

3-2-3 IR light generation with OPO

After the SHG part, a PPLN OPO pumped by residual 1064 nm light is employed

to obtain IR light generation [10]. As shown in Fig.3.2, the pump light is

polarized vertical to the paper tuned by a half-wave plate and focused by a

spherical lens (L2). The OPO is operated in quasi-phase-matching mode with

three interacting waves polarized parallel for using a large d33 nonlinear

coefficient. The PPLN device used named “OPO1-20” which has nine periods

range from 29.5 to 31.5 µm, manufactured by Covesion Ltd. It has a length of 20

mm and an aperture of 0.5×0.5 mm2 for the single period.

47

The OPO built here can directly produce IR light with a bandwidth of

moderate range that is suited for efficiently interact with common frequency

conversion crystals. Further, the IR can be frequency mixed with an harmonic of

the pump laser, since the build-up time of the OPO is short enough.

The process determining the parameters for resonate cavity shown in Fig.3.4 is

critical for OPO’s construction. First, the length of linear resonance cavity is set

to be 40 mm which is a little longer than the oven mounted PPLN, to make sure

the oscillation is stable. The second step is to decide the beam waist and radius of

curvature for the concave mirrors (M1 and M2) which formed the resonate cavity.

The device “OPO1” has specification for generation of signal light from 1410 nm

to 2100 nm corresponding to the idler branch from 2100 nm to 4300 nm. To

generate VUV around 190 nm, the OPO need to operate at SRO mode to generate

IR at about 1677 nm of signal, according to idler of about 2900 nm, where is the

center of the OPO oscillation range.

The oscillation range is mostly ruled by the transparent and reflection range of

the concave mirrors. In this system, it used concave mirrors customized by

SIGMAKOKI CO. The mirrors are ordered with high-reflection coating from

2500 to 3200 nm with a center wavelength at about 2850 nm, which corresponds

to signal light range of about 1600 to 1850 nm. Otherwise, the coat is transparent

at the signal range and also at 1064 nm.

Fig.3.4. Sketch for the PPLN OPO operating at SRO mode. It is built by a PPLN and two concave mirrors.

Parameters related to resonate cavity is defined in (3.1). R(z) stands for the

radius of curvature for the resonate cavity that equals the radius of curvature for


48

the two concave mirrors. w0 is the beam waist size usually located in the center of

the cavity. z stands for the position inside the cavity that the center is defined as 0

point. As the center oscillation wavelength is chosen around 2850 to 2900 nm,

given a value for R(z), beam waist size could be calculated. Given the waist size,

the laser intensity at the waist can be obtained to evaluate the whether the

damage threshold of OPO medium is exceeded. After proper value of w0 and R(z)

is get, the next question becomes how to make the waist needed.

22

0R( ) 1w

z zz

(3.1)

Inside the OPO cavity, as the pump light and idler light have same confocal

parameter during oscillation, the waist size of 1064 nm light can be obtained with

w0 of the idler light (which is also called the eigenmode of the resonate cavity).

Therefore, the next step becomes to focus the 1064 nm light with the waist

needed. There is also a complicated calculation about q factor for laser beam and

will not be discussed here.

Fig.3.5. Waist of 1064 nm beam measured in the cavity. It has the same confocal parameter with idler light which make the oscillation of OPO.

In our experiment, 100 mm-radius curvature concave mirror is chosen which

49

can make the focused 1064 nm beam to a waist needed in the center of the

resonate cavity. Fig.3.5 shows the waist size of 1064 nm light measured in the

experiment, which is about 130 μm. During the oscillation, the output IR light is

tuned from 1610 nm to 1890 nm in the signal branch and corresponding idler

branch is from 3140 nm to 2430 nm.

The center output wavelength depends on the grating period of PPLN and

temperature tuning will change the period slightly. For PPLN, temperatures in the

100°C-200°C range are used in order to minimize the photorefractive effect that

can damage the crystal and causes the output beam to be distorted. Since the

photorefractive effect is more severe in PPLN when higher energy photons in the

visible part of the spectrum are present in the crystal, it is especially important to

use the crystal only in the suitable temperature range.

(a) (b)

Fig.3.6. Signal light wavelength measured by optical spectrum analyzer. The spectral bandwidth for 1474 nm is read as about 4 ns, for 1659 nm is read as about 9ns.

In the multi-grating section, the grating period increases from 2950 nm to 3150

nm every other 0.25 μm. OPO exhibits a free-running optical bandwidth without

injection seeding. I measured the wavelength of the signal with optical spectrum

analyzer (Advantest Q8381A) as shown in Fig.3.6. It is found the spread-width

of the spectrum is changed at different signal wavelength according to the

property of PPLN. As the optical spectrum analyzer used has a measurement

range spreads from about 800 to 1600 nm, the longer part of the signal


50

wavelength from OPO in the experiment cannot be directly measured. Nonlinear

frequency conversion occurred in the OPO contains not only the OPO process

1/1064=1/signal+1/idler, but also SFG processes such as 1/1064+1/signal=1/red1,

1/1064+1/idler=1/red2. When the power is enough for the oscillation, it will be an

infinite process. Thanks to the generated by-product red light, measurement of

the signal light wavelength that is out of the measurement range of the optical

spectrum analyzer becomes possible.

For 189 nm light generation, an IR wavelength of 1697.9 nm corresponding to

189.1 nm light generation is chosen, at the grating period of 30.75 μm operated at

the temperature of 120C. The free-running spectral bandwidth (FWHM) at the

center wavelength was about 2.6 nm. The maximum output power of 1697.9 nm

was 450 mW with a maximum conversion efficiency of 17.8% from 1064 nm

pump power.

3-2-4 Fourth harmonic and fifth harmonic generation with CLBO

The crystals used in 266 nm and 213 nm light generation are kept at 150C and

used argon gas flow to reduce water impurity [11]. As CLBO is one kind of

hygroscopic material, the method is effective for improve the degradation

resistance of the crystal in UV generation and extend its life time [12].

Fig.3.7. Conversion efficiency of 4HG by CLBO. Blue spots stand for output while black spots stand for conversion efficiency.

http://dict.youdao.com/w/hygroscopic/

51

The 532 nm beam was focused to a waist of 34 μm in radius. The conversion

efficiency of the generation is shown monotonically increased with the input

power as shown in Fig.3.7. The fourth-harmonic generation is obtained via a

5×5×10 mm type I CLBO crystal (CLBO1 in Fig.3.2) with a maximum

conversion efficiency of 28.9% at 2.35 W input power.

Then, the fifth-harmonic generation was obtained by SFG with the

fourth-harmonic light and depleted pump light after the OPO in a 5×5×15 mm3

type I CLBO crystal (CLBO2). The Type I SFG was achieved with both beams

polarized horizontally and gave the output vertically polarized. Three cylindrical

lenses (fC1=130 mm, fC2=100 mm, and fC3=100 mm) were used in the

fourth-harmonic branch to correct the beam shape which deformed due to

walk-off effect. The beam pattern involved in the generation approximates round,

which is typical configuration for nonlinear frequency generation.

Fig. 3.8. Setup for 213 nm light generation. Ls stand for lenses; Cs stand for cylinder lenses. Orange line stand for 1064 nm light path that has a delay path of about 3.9 m arranged into it. L is used in delay path for collimating the beam to a radius under 500 μm in case it diverges.

In this step, for the effect of OPO resonance, the 1064 nm pulse’s shape got

asymmetrically depleted in temporal range. The power from its “latter” part

transferred to OPO and became oscillating power. As the result, the 1064 nm

pulse could not “cover” the 266 nm pulse that the conversion efficiency dived.

How to effectively utilize the undepleted leading edge of the 1064 nm pulse has


52

become an issue for lifting the conversion efficiency in this generation.

As measure with oscilloscope, the distance between the pulses of 1064 nm and

fourth-harmonic is about 13 ns. According to the relation: l=c‧t (l stands for

delay length, c stands for light speed of 3.0×108

m/s, t stands for delay time), an

optical delay path of 3.9 m was set in 1064 nm light to optimize the temporal

overlap of the two pulses. As shown in Fig.3.9, the pulses of the two lights

mostly coincide in temporal range when the delay line has been set. The dashed

line outlines the original 1064 nm pulse, which implies the origin location of the

two pulses before the OPO oscillation and the delay line.

Fig.3.9. Adjustment for depleted 1064 nm pulse in temporal range for 5HG by CLBO. Blue line stands for asymmetrically depleted 1064 nm pulse; blue dashed line outlines the original pulse of 1064 nm before it pumps the PPLN OPO, they both depict the 1064 pulse after a 3.9 m delay line is set. Cyan line stands for 266 nm pulse.

Obviously, the residual power of 1064 nm contributed for 213 nm light

generation is affected by the conversion efficiency of the OPO. As IR power

generated from OPO and the residual 1064 nm power left after OPO both share

the 1064 nm pump power, how to make the balance of the two parts of power

became another issue in the generation. It will be lack of power for either 213 nm

or 1697.9 nm light if the balance is poor, that there will be not enough power in

the final 189 nm light generation.

Rel

ativ

e lig

ht

inte

nsi

ty

53

(a) (b)

(c) (d)

Fig.3.10. Temporal profile of 1064 nm pulse changed in 213 nm light generation. Yellow profile stands for 1064 nm pulse; blue profile stands for 266 nm pulse. The power for 266 nm pulse was fixed at 520 mW; the power for 1064 nm pulse and generated 213 nm are (a) 860 mW, 45 mW; (b) 1020 mW, 62 mW; (c) 1440mW, 100mW; (d) 1800mW, 125mW. (The delay adjustment is not optimal condition in these pictures.)

As seen from Fig.3.10, the temporal profile of the 1064 nm pulse increases as

its power increased with the conversion efficiency of OPO reduced. The

adjustment is achieved by tilting of the concave lens of the OPO to break its

optimal oscillation condition during the 5HG. As known from Fig.3.10 (a), the

1064 nm pulse is got depleted in the center part that formed a dip. In order to

make the best use of the 266 nm power, which is generated from fundamental

laser source with a small total conversion efficiency, the temporal profile of the

1064 nm pulse should be enlarged by reduce the conversion efficiency of OPO,

to a great extent to cover the 266 nm pulse (as shown in Fig.3.10(d)).

Re

lati

ve li

ght

inte

nsi

ty

Inte

nsi

ty

Re

lati

ve li

ght

inte

nsi

ty

Re

lati

ve li

ght

inte

nsi

ty

Re

lati

ve li

ght

inte

nsi

ty


54

At last, the power of 1697.9 nm light is decided to be set at 240 mW, with

about 1.6 W power left in residual 1064 nm. The 213 nm output reached, at most,

155 mW with 22.8% SFG efficiency, from input power of the fourth-harmonic

light.

Also in this generation, for achieving higher conversion efficiency in SFG, it is

also need to consider the focusing condition of 266 nm and 1064 nm beams.

Fig.3.11 compares two focusing conditions, one is to focus the two beam to the

same radius; the other is to focus the 1064 nm beam to the same confocal

parameter as 266 nm beam according to (3.2) [13]. The latter configuration is

thought to give better performance as the wavefront of the two beams coincide.

2 2

1 1 2 2

1 2

2 2n w n w

(3.2)

(a) (b)

Fig.3.11. Sketch for two kinds of focusing condition in SFG to 213 nm. (a) Two beams are focused to the same waist size; (b) 1064 nm beam is focused to the same confocal parameter as the 266 nm beam.

I have tried the generation with several focusing conditions for the 1064 nm

beam. The results of 213 nm output power shown in Fig.3.12 is measured as the

input 266 nm fixed at 680 mW with radius of 44 μm. When the confocal

parameter of the 1064 nm beam was set same to 266 nm beam at about 7 cm, the

SFG showed the best performance.

55

Fig.3.12. 213 nm output with different focusing conditions for 1064 nm. 266 nm beam was focused at 44 μm in radius while 1064 nm beam was changing. Each group of data are acquired with 266 nm power fixed at 680 mW and the 1064 nm input power changed.

3-2-5 Summary for 189 nm light generation preparation

A summary for the stages prepared for 189 nm light generation, which are

adjusted to optimal condition is shown as Fig.3.13.

Fig.3.13. Summary for 189 nm light generation system.

For the DUV light generation, 2.35 W output for 532 nm light is generated


56

with conversion efficiency of 36.8% from fundamental laser power. 680 mW

output for 266 nm light is generated with conversion efficiency of 28.9% from

532 nm power. At last, 213 nm light was generated by SFG of residual 1064 nm

light and 266 nm light, with 22.8% conversion efficiency from 4HG and has an

output power of 155 mW. IR light of 1698 nm is generated from OPO with 240

mW output by 9.5% from 1064 nm pump power.

3-2-6 189 nm light generation with borate crystals

At the final stage, VUV light at around 190 nm was generated by SFG with the

fifth-harmonic light and IR in the last borate crystal. Before the generation, a

temporal distance between 213 nm pulse and IR pulse after they are generated is

measured by oscilloscope with silicon detector. The measurement is similar to the

last stage generation so the figure is omitted here. The quantity of temporal

distance reads about 10 ns that corresponding to a distance of 3 m in light path.

As the same arrangement in 213 nm light generation, a delay line was set in the

path of IR.

As shown in Fig.3.14, IR is focused by a lens with fL6=200 mm after the delay

line. Two cylinder lenses with fC4=100 mm and fC5=130 mm to used focus 213

nm beam which is deformed due to walk-off effect. Both of the beams have

vertical polarization direction for the Type I phase-matching plane and the

generated light polarized horizontally. After the SFG to 189 nm light generation

borate crystal, a 60 dispersion prism was used to disperse VUV light from other

lights.

Fig.3.14. Setup for 189 nm light generation. C4 and C5 stand for cylinder lens and L6 stands for lens used for focusing. The borate crystal is set on a rotating stage for angle tuning. Generated light is dispersed by a 60 dispersion prism after the crystal.

57

The prism used here is made of fused quartz which was chosen to prevent

absorption of the VUV. The distance between the output face of the borate

crystals and power meter was about 50 mm. Power loss of VUV included

reflection at the surface of the crystals, and absorption in air was not taken into

account. The generation was conducted in air ambient with no cell and no

temperature tuning for the crystal sample. The oxygen in the air is thought to

present an elevating absorption rate as VUV wavelength is below 190 nm. The

setup used in the experiment is designed in the view of simplicity that no vacuum

chamber or nitrogen gas flowing is allocated to keep oxygen away.

During the generation, turn the rotating stage mounted crystal carefully and to

observe the glass sheet placed beside. Fluorescence will be produced when

illuminated by the VUV light to confirm the generation. The measure method for

phase-matching angle of the crystal is shown in Fig.3.15. First to measured the

external angle and calculate the internal angles according to (3.3), where nDUV

and nIR stand for refractive index in the crystal. The average of internal angle for

DUV and IR light is termed as deviation angle (ϕ) that the phase-matching angle

is set to be the sum of cut angle of crystal sample and deviation angle in this

dissertation.

Fig.3.15. Measurement for calculating phase-matching angle inside the crystal. The DUV and IR light have the same external angle; internal angle of them is calculated with Snell’s equation.


58

sinarcsin

DUVDUV internalexternal

DUVn

,

sinarcsin

IRIR internalexternal

IRn

,

( ) / 2DUV IR

external external . (3.3)

3-3 189 nm light generation results and discussions

In this section, the 189 nm VUV light generation results with borate crystal of

CBO, LBO and CLBO that listed as candidates in Table 3.1 are shown. High

efficiency generation is also challenged with these crystals.

3-3-1 Phase-matching angles for LBO and CLBO

In the measurement for phase-matching angles, a CLBO crystal cut at θ=60.2

and φ=45 is used, which has a size of 5×5×10 mm3. For LBO crystal, two

samples with size of 5×5×15 mm3 cut at θ=70, 80 and φ=90 and two samples

with size of 6×7×8 mm3 cut at θ=65, 75 and φ=90 are prepared. LBOs used in

the experiment are purchased, while the CLBOs used are grown in Mori

laboratory from Osaka University. Each sample used is polished and not coated.

The 189 nm light generation is realized with both LBO and CLBO that is

justified with fluorescence which is marked with red circle in Fig.3.16. The

generation results with each LBO sample and CLBO are concluded in Table 3.2.

Fig.3.17 shows the experimental results that indicating the relationship of the

phase-matching angles for LBO and CLBO and the wavelengths generated with

this system. Phase-matching angles are measured by changing the wavelength of

IR while 213 nm is fixed. The measured phase-matching angles of 189.1 nm by

SFG with 212.8 nm and 1697.9 nm were φ=72.7 for LBO and θ=60.2 for

CLBO. It is found that the measured Type I phase-matching angles of θ for

CLBO are in good agreement with the theoretical curve. On the other hand, the

59

measured φs for LBO in the x-y plane are slightly larger than the theoretical

curve and the difference is growing greater towards shorter wavelength side. This

can be explained as the phase-matching angles in the process of approaching 90,

deviation between larger wavelength occurred in Sellmeier formula and is getting

larger and larger that resulted in such difference.

Fig.3.16. 189 nm VUV light generation by CLBO judged with fluorescence. The picture captures the beam spots on glass sheet after the 189 nm, 213 nm, 1698 nm, and other residual light are dispersed by the prism. 189 nm spot is the on the first place of the left side marked with red circle. The brightest one in the picture is the spot of 213 nm light.

Table 3.2. Phase-matching measurement with LBO and CLBO.

LBO samples: a: φ=65, b: φ=70, c: φ=75, d: φ=80 and θ=90.

Wavelength (nm)

IR VUV

Phase-matching angle of

LBO (deg.)

Calculated Measured

Phase-matching angle of

CLBO (deg.)

Calculated Measured

1893.4 191.3 61.4 65.1 a 54.8 55.0

1808.8 190.4 65.0 62.4 a 56.6 57.0

1764.7 189.9 67.2 68.1 a, b

57.7 58.1

1714.2 189.3 70.1 70.9 b 59.0 59.5

1697.9 189.1 71.1 72.7 b, c

59.5 60.2

1643.1 188.4 75.3 77.1 c, d

61.1 61.7

1605.8 187.9 79.2 81.9 d 62.4 63.1

189 nm 213 nm


60

Fig.3.17. Phase-matching angles for LBO and CLBO at around 190 nm. Red line stands for calculated curve for phase-matching angles of CLBO with Sellmeier formula from Ref. 8; red dots stand for measured phase-matching angles of CLBO. Green line stands for calculated curve for phase-matching angles of LBO with Sellmeier formula from Ref. 6; green dots stand for measured phase-matching angles of LBO. The wavelength of VUV is calculated with wavelength of IR which is measured by optical spectrum analyzer.

3-3-2 Phase-matching angles for CBO

Two samples with size of 3×3×15 mm3 cut at φ=70, 77.5 and θ=90 for CBO

are used in the measurement. The CBO is grown from a self-flux solution with a

composition of 74 mol% B2O3 [17]. Fig.3.18 shows the measured

phase-matching angles with CBO at around 190 nm SFG. It was predicted to

obtain the highest output power among candidate crystals according to effective

nonlinear coefficient as the calculated result is shown in Table 3.1. However,

while phase-matchings are realized at about 190 nm range, there is no SFG

obtained at 189 nm. The measured phase-matching angles are quite different

from the theoretical curve calculated with the reference [7].

61

Table 3.3 CBO samples used in measurement for phase-matching angles.

Wavelength (nm)

IR VUV

Phase-matching angle (deg.)

Calculated Measured

1891.9 191.3 70.9 71.2 a

1808.8 190.4 75.5 75.6 b

1766.2 189.9 80.1 79.9 b

LBO samples: a: φ=70, b: φ=77.5, and θ=90.

Fig.3.18. Phase-matching angles for CBO at around 190 nm. Red dashed line stands for theory value calculated with Sellmeier formula from Ref. 7; purple line stands for theory value calculated base on Ref. 15. Blue dots stands for measured values. The wavelength of VUV is calculated with wavelength of IR which is measured by optical spectrum analyzer.

However, such result is found to comply with the discovery made by

Kagebayashi [14] that there are difference between the calculated values and

theoretical curve about phase-matching angles of CBO in VUV light generation

at 193 nm by the SFG with DUV and IR. It seems that Sellmeier formula of CBO

[7] have a deviation in UV range concluded from these results. On the other hand,

in Fig.3.18, there is another calculated curve of CBO closed to the experimental

results of the SFG with 213 nm and 1698 nm light.

[7]

[15]


62

In this measurement, each sample used is polished and not coated. The results

of measurement with each sample are concluded in Table 3.3 where the

calculated phase-matching angle is based on equations (3.4). It is calculated from

Sellmeier formula developed by H. Shimatani from Mori laboratory, Osaka

University [15]. The equations are given as

2

2

2 00683.00124.0

01172.034403.2

xn ,

2

2

2 00926.001387.0

01198.040077.2

yn , (3.4)

,01335.001369.0

0129.048737.2 2

2

2

xn

where λ is in micrometers.

Otherwise, phase-matching angles calculated from the two sets of Sellmeier

formula are found almost identical in the visible range and begin to separate in

the near-UV range. It could be confirmed that the latter Sellmeier formula used

have better accuracy in depicting the CBO crystals grown Mori laboratory, Osaka

University.

It can be assumed that the differences in phase-matching angle caused by the

refractive index change of the crystal resulted from different growth conditions.

The CBO utilized in previous researches was grown from stoichiometric melt

[16]; in recent researches, CBO used is grown from a self-flux solution, because

it is a better way to obtain large-sized and inclusion-free crystals [17].

3-3-3 189 nm light generation with CLBO and LBO

The generation of 189 nm light is shown as in Fig.3.19. In this generation, a

minimum beam waist of w=32 μm is set for 213 nm, which located in the center

of the crystal. At the same time, the IR of 1697.9 nm is collinearly focusing into

the crystal to the beam waist of w=105 μm, approximately corresponding to the

same confocal parameter of about 70 mm as the DUV light. During the

generation, the IR power was maintained at 220 mW while the input of the 213

63

nm changed from 0 to 155 mW.

Fig.3.19. Parameters for 189 nm SFG.

As a result of the generation, a notable output of VUV was achieved by a

CLBO with size of 5×5×15 mm3. Fig.3.20 shows the output power as a function

of the input fifth-harmonic power. The 189 nm light output power increased

monotonically as the input power increased. A maximum output power of 11.4

mW was achieved at an input power of 155 mW. The corresponding SFG

efficiency was 7.3% from the fifth-harmonic power, and the generation efficiency

from the source laser was 0.16%. Near the maximum input power, saturation

behavior was observed. This could be caused by water impurity in CLBO [11]

for the generation was in air ambient and a promotion is expected with a

dehydration process before and during the generation.

On the other hand, under the same generation condition with CLBO as shown

in Fig.3.19, LBO showed output of less than 1 mW even by using the longest

sample with a length of 20 mm. The great difference of the two crystals could be

explained according the information given by Table 3.1 and Fig 3.17. The

calculated value for effective nonlinear coefficient of LBO is about 44% of

CLBO. According to equation (2.29), the output of SFG is directly proportional

to the effective nonlinear coefficient. Furthermore, as the measured

phase-matching angles for LBO are larger than the calculated values, its effective

nonlinear coefficient will get further smaller. Therefore, it is estimated that the

output of LBO is about 15% of CLBO, which has a good accordance with the


64

experimental result.

Fig.3.20.189 nm output generated by CLBO.

3-4 Perspective

From the phase-matching property of CLBO shown in Fig.3.21, as the SFG

achieved shorter wavelength to about 182.4 nm, the effective nonlinear

coefficient increases to the maximum value d36 as the phase-matching angle rises

toward θ=90 as given in equation (2.69). Consider the actual phase-matching

angles approach 90 faster than calculated values and UV absorption edge of the

crystal becomes larger and larger under 190 nm, it is expected the generation

range could possibly reach 185 nm with high conversion efficiency.

To achieve VUV light generation with shorter wavelength, the output of OPO

should be tuned to about 1400 nm which is the shortest edge for the signal that

can be generated by MgO:PPLN as the specification. Furthermore, the according

idler wavelength which should be reflected during the OPO oscillation has

beyond the reflection range of mirrors coated with normal material. My

experiments with customized mirrors have failed to produce the short wavelength

IR and the reason is not well understood.

65

Fig.3.21. Perspective for VUV light generation with CLBO. The generation is expected to be extended to the range in the blue round.

Therefore, new scheme for achieving 185 nm VUV light generation with other

types of OPO is considered. A reasonable alternative is to use KTP OPO which

can produce shorter wavelength IR than PPLN with phase-matching in xz plane.

With the method, pump light is changed to 532 nm light, and the scheme for the

generation is shown as Fig.3.22. However, because the OPO and fourth harmonic

generation stage share the power of 532 nm light, such scheme will need large

conversion efficiency in the first SHG stage.

Fig.3.22. Scheme for VUV light generation based on KTP OPO.


66

3-4 Summary

A high-repetition-rate, all-solid-state laser system for 189 nm VUV light

generation is built based on a 1064 nm Nd:YAG laser. The generation is realized

by SFG with 213 nm light (5HG) and IR in a borate crystal. In the system, 266

nm and 213 nm UV light generation is produced by CLBO crystals while IR light

is produced by PPLN OPO.

As the result, with this laser system, 189 nm VUV was demonstrated by LBO

and CLBO successfully. The phase-matching property for LBO and CLBO

around 190 nm was found have good accordance with the theory value. On the

other hand, the VUV light generation with CBO was not realized until the

generated wavelength extended to 190 nm, which is inconsistent with the theory

prediction. The new phase-matching property for CBO around 190 nm was found

to have better agreement with new Sellmeier formula. It could be explained by

the different growth method of the CBO crystals.

An output of 11.4 mW at 189 nm was generated with CLBO of 15 mm length.

CLBO is considered suitable for VUV light generation until 185 nm given its

phase-matching property and short absorption range. With high power laser

source, a practical level output of 189 nm light could be expected with the

scheme.

67


[1] J. Sakuma, K. Moriizumi, and H. Kusunose, Opt. Express 19, 15020 (2011).

[2] K. Deki, J. Sakuma, Y. Ohsako, N. Kitatochi, T. Yokota, M. Horiguchi, Y.

Mori, and T. Sasaki, Proc. Conference on Lasers and Electro-Optics, CPD4

(1998).

[3] Y. Urata, T. Shinozaki, Y. Wada, Y. Kaneda, S. Wada, and S. Imai, Appl. Opt.

48, 1668 (2009).

[4] H. Masuda, K. Kimura, N. Eguchi, and S. Kubota, Advanced Solid-State

Lasers, OSA TOPS 50, 490 (2001).

[5] H. Kouta and Y. Kuwano, Opt. Lett. 24, 1230 (1999).





[9] J. J. Jacob and A. J. Merriam, Proc. SPIE 5567, 1099 (2004).

[10] L. E. Mayers, R. C. Eckardt, M. M. Fejer, R. L. Byer, W. R. Bosenberg, and

J. W. Pierce, J. Opt. Soc. Am. B 12, 2102 (1995).

[11] T. Kawamura, M. Yoshimura, Y. Honda, M. Nishioka, Y. Shimizu, Y.

Kitaoka, Y. Mori, and T. Sasaki, Appl. Opt. 48, 1658 (2009).

[12] K. Takachiho, M. Yoshimura, Y. Takahashi, M, Imade, T, Sasaki, and Y.

Mori, Opt. Mater. Express 4, 559 (2014).

[13] G. D. Boyd and and D. A. Kleinman, J. Appl. Phys. 39, 3597 (1968).

[14] Y. Kagebayashi, K. Deki, Y. Morimoto, S. Miyazawa and T. Sasaki, Jpn. J.

Appl. Phys. 39, 1224 (2000).

[15] H. Shimatani, Master’s Thesis, Graduate School of Engineering, Osaka

University, Osaka (2009) [in Japanese].

[16] T. Saji, N. Hisaminato, M. Nishioka, M. Yoshimura, Y. Mori, and T. Sasaki,

J. Cryst. Growth 274, 183 (2005).

[17] Y. C. Wu, T. Sasaki, S. Nakai, A. Yokotani, H. G. Tang, and C. T. Chen, Appl.

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69

Chapter 4. 179 nm VUV light generation with borate crystals

4-1 Introduction

In order to develop the next generation inspection laser source with solid-state

system, it is focused on the generation of VUV light in 170 nm-180 nm range. In

this section, examples for the VUV light generation are to be introduced and

difficulties in generation of the range are to be discussed. As our plan for

generating VUV light at 179 nm, a new system based on two OPOs is designed.

4-1-1 Introduction for VUV light generation in 170 nm-180 nm range

There are lots of difficulties in sub-180 nm VUV light generation with

nonlinear frequency conversion. One of the biggest of them is lacks of NLO

crystals which have enough birefringence and transparency in the range.

KBe2BO3F2 (KBBF) and RbBe2(BO3)F2 (RBBF) crystals which have extensive

transmission range and large birefringence are considered suitable NLO medium

for the generation. The 177.3 nm light generation by direct SHG from the third

harmonic light of 1064 nm at 354.7 nm has been demonstrated with these

crystals [1, 2]. Because KBBF belongs to (BO3)3-

anionic group, it tends to be

grown in a layer structure that cannot be cut with the phase-matching angle. The

VUV generation by KBBF is now realized using an optical contacted prism

coupled device (PCD), showing potential for future use [3].

Besides, as reported several years ago, a UV generation until 172.7 nm light

has been realized based on a 200-fs system pumped by Ti:sapphire amplifier [4].

It is generated with SFG using fourth harmonic generation and parametrically

generated IR by LBO. However, the phase-matching is made based on

femtosecond system that is not as practical as nanosecond laser source. There has

been no other report in this range so far to my knowledge.

Therefore, in order to make VUV light generation in the range of 170 nm-180

nm, proper NLO crystal and original system design are of the same importance.


70

As commercially used nonlinear optical crystals, CLBO and BBO do not have

enough transparency under 180 nm for UV generations. LBO and CBO are

featured with absorption edges below 170 nm, however, their birefringence do

not allow simple generation like KBBF to get direct SHG from 354.7 nm [5, 6].

4-1-2 Scheme for 179 nm light generation

As an alternative method to approach the wavelength wanted, another group of

SFG with DUV and IR light making the generation to 179 nm (198.8 nm +1797.2

nm ~179 nm) is found [7, 8]. CBO and LBO are considered fit for the generation.

The design of the new scheme for 179 nm light generation is as shown in

Fig.4.1. It uses a 198 nm DUV light [9] that coincide with the inspection laser

source in practical service. The merit of the design is that utilization of OPO

makes DUV wavelength adjustable to accommodate the phase-matching

properties of the crystals in the last stage. By tuning the output wavelength of

KTP OPO, the generated DUV ranges from 197.8 nm to 198.8 nm.

Fig.4.1. Scheme for 179 nm VUV light generation with LBO. The generation is realized from SFG of 198.8 nm DUV and 1797 nm IR light based on two OPOs.

In the final SFG process of the generation, LBO and CBO showed

phase-matching possibility and CBO seems to give larger output as effective

71

nonlinear coefficient shown in Table 4.1. In this research, the VUV generation at

179 nm is tried to unveil the phase-matching property of the two borate crystals

in the range.

Table 4.1. Property of borate crystals for the generation.

*Phase-matching angles for SFG at 179 nm were calculated with the interaction of 198.8 nm and 1797.2 nm light.

4-2 Experimental setup for 179 nm light generation

This section will introduce the stages in the system for the 179 nm light

generation, including SHG part, KTP OPO and intracavity LBO SHG part, PPLN

OPO part, and 198 nm SFG part. At last, there will be a brief summary of the

generation results of all the stages.

4-2-1 Setup for DUV and IR light generation

As shown in Fig.4.2, a commercial Q-switched neodymium doped yttrium

orthovanadate (Nd:YVO4) 1064 nm laser (HIPPO H10-106QW, Spectra Physics)

is employed as the fundamental laser source. It delivers linear polarized beam

with M2<1.2, up to 11.5 W output at repetition rate of 15 kHz with a pulse width

of 8 ns. The system is composed of 7 stages of nonlinear wavelength conversion,

divided in tunable DUV light generation part, tunable IR light generation part

and the final VUV light generation part. The IR light is realized with a PPLN

OPO. The DUV part is based on a compact implementation of the KTiOPO4

(KTP) OPO with intra-cavity LBO SHG [10]. As a popular material in UV light

generation, CLBO crystal is used to generate 244 nm by SHG and 199 nm by

Candidate

crystal

Transmission

range (nm)

Phase-matching

angle (300K)

Effective nonlinear

coefficient

deff (pm/V)

Ref.

LBO 160~2600 φ=74.1, Type I,

xy plane

0.23 7

CBO 167~3400 θ=50.1, Type I,

yz plane

1.02 8


72

SFG that also plays important role in the generation. There are delay lines set in

light path of 1064 nm and IR for temporal adjustment.

Fig.4.2. Experimental setup for 179 nm light generation. Ls stand for spherical lenses; Ms stand for mirrors in KTP OPO and intracavity LBO SHG setup; CMs stand for mirrors in PPLN OPO; and Cs stand for cylindrical lenses. There are delay lines set in light paths of 1064 nm and IR.

4-2-2 SHG with improved conversion efficiency

In the first step, the SHG of 1064 nm was performed by a type I noncritical

phase-matching (NCPM) LBO with AR coating for 1064 and 532 nm. The LBO

is housed in oven, kept at 151C with a stability of ±0.1C, to satisfy the

non-critical phase-matching along x axis.

At first, a LBO with size of 5×5×10 mm3 was used for the SHG in this system.

After that, the ratio between SHG power and residual 1064 nm power is set about

2:1. Consider about the whole system, the SHG power will then be generated to

DUV light while the residual 1064 nm power will be generated to IR light. For

there are two more steps of nonlinear frequency conversion in the SHG branch,

and each step has about 40% conversion efficiency, it will lead to too small DUV

power compared with IR power for the last step SFG.

73

The direct method to improve the balance of the power of DUV and IR to lift

the efficient of the system is to increase the conversion efficiency of the SHG

process. As the SHG output given by equation (2.29), a simple method is to use a

longer crystal to realize the goal, because the conversion efficiency is

proportional to crystal length’s square, compared with focusing the fundamental

beam to a thinner radius that may cause damage in the crystal. The theory for

determining the focusing condition is given by Boyd and Kleinman [11]. For

there is no walk-off in this NCPM SHG, the optimal confocal parameter

(2nπw02/λ) of the beam has a simple relation with the length of the crystal.

Fig.4.3. Compared the conversion efficiency of two SHG LBOs with different optimal focusing conditions using the same Nd:YVO4 laser source. LBO with length of 25 mm is used with 1064 nm focused to radius of 85 μm while for 10 mm-length LBO is 54 μm.

In the experiment, the length of LBO is changed from 10 mm to 25 mm as

long as it can be covered with the oven. The corresponding proper waist radius of

1064 nm has changed from 54 μm to 85 μm. The SHG was generated with a

maximum conversion efficiency of 62.5% as shown in Fig.4.3 compared with 10

mm-long LBO. The improvement was not as large as the square of the crystal

length’s ratio may be resulted from the loss occurred as the laser propagates

LBO 25 mm

LBO 10 mm


74

through the crystal. It is noted that all the data presented in this experiment are

not corrected for the absorption losses of the crystals and the reflection losses at

the crystal surfaces.

4-2-3 KTP OPO and intra-cavity SHG

After that, a singly-resonant OPO and intra-cavity SHG to access 489 nm with

532 nm pulse is constructed as shown in Fig.4.4. The resonator is a L-shaped

standing wave cavity formed by three mirrors, contains a 3×3×15 mm3 KTP

serving as OPO material, as well as a 3×3×10 mm3 LBO for intra-cavity SHG of

OPO signal. The physical length of the cavity is about 30 mm that slightly

exceeds the sum length of the two crystals. The Type II KTP was cut at θ=90

and φ=59 with AR coat at 978 nm. While the Type I LBO was cut at θ=90 and

φ=17.6 with 978/489 AR coating. The waist of the pump light of 145 μm, is

positioned on the surface of M3.

(a) (b)

Fig.4.4. Details for KTP OPO and intra-cavity SHG LBO. (a) shows the setup for the cavity which is formed by concave mirror M1; two dichroic mirrors M2 and M3; reflection mirror M4. (b) shows pulses involved in the generation observed with oscilloscope. Cyan curve stands for pump pulse; pink curve stands for depleted pump pulse; blue curve stands for generated 489 nm pulse.

489 nm pulse

Depleted pump pulse

Pump pulse

75

The OPO is built on the wavelength relation: 532 nm (pump) 978 nm

(signal) + 1167 nm (idler). The process for the generation is like: 532 nm pump

pulse passes through curved end mirror M1 (radius of curvature of 500mm); the

signal light at 978 nm of OPO is reflected by dichroic flat folding mirror M2; the

signal and its SHG are retroreflected by end flat mirror M3. As seen from Fig.4.4,

depleted pump pulse still has considerable power left after the oscillation of the

OPO. For utilizing the residual power of the pump pulses passed through M2, a

double-pass pumping retroreflected structure is set with M5, making it pump the

OPO for another time. Also there is distance observed between 489 nm pulse and

depleted pump pulse which means the buildup time of OPO. To make the pulses

coincide, an optical delay about 400 mm that is of the order of the buildup time

of the OPO was set [11]. A lens with f=400 mm is used in delay line for

collimating the depleted beam to make sure it can return with origin beam shape.

The difference of 489 nm output between single pass and double pass with delay

configuration is show in Fig.4.5. The oscillation threshold decreased and the

output increased after the improvement. As the result, generation efficiency of

the 489 nm from the 532 nm pump power of 6 W reached 9.3%.

Fig.4.5. Compare the output generated with single pass configuration and double pass with delay line configuration. Black squares stand for double pump; red circles stand for double pump with delay line.

Double pass Single pass


76

The whole conversion process is in the air condition at room temperature,

which brings a problem that the generation becoming unstable. It is mainly

because of the poor temperature tolerance, which means birefringence of LBO is

easily changed by thermal effect. As LBO heated with beams, the oscillation

mode of the cavity changed and output power dived. TEM00 mode turning into

TEM10 or TEM20 mode can also be observed from 489 nm beam pattern. For the

resonate cavity built has limited space for cover an oven to keep the crystal

constant temperature, the adjustment of the system should be done with low

power to keep stable of generation made by the LBO.

Then, 244.5 nm was generated as the SHG of 489 nm via a 3×3×10 mm3 type I

CLBO (CLBO1) cut at θ=78.1. In order to correct the walk-off in 489 nm light,

a pair of cylinder lens was employed to focus it to a 38 μm radius circular waist

in the CLBO crystal. 244.5 nm was generated with a maximum conversion

efficiency of 35.7%.

4-2-4 PPLN OPO for IR light generation

To generate tunable IR light at around 1800 nm, a PPLN OPO pumped by

residual 1064 nm light was employed. The 20-mm-long PPLN with a

multi-grating section is set in a 40-mm-long linear resonance cavity. The PPLN

used here is same to the Chapter 3 as “OPO1-20” that produced by Covesion Ltd.

The cavity is formed by two concave mirrors (M1 and M2 with radius of

curvature of 100 mm) with high-reflection coating for the wavelength from 2300

to 2800 nm, roughly corresponding to the idler branch of the singly resonant

OPO. Consequently, the signal wavelength of the OPO spreads from 1770 to

1970 nm.

The wavelength is tuned by the grating period of PPLN and its temperature,

exhibits broad free-running optical bandwidth of about 3.5 nm without injection

seeding. The grating period is chosen to be 29.5 μm and certain temperature is set

for generating the IR wavelength from 1797 nm to 1903 nm corresponding to

VUV light generation from 179 nm to 180 nm. With the 1064 nm input focused

to 160 μm radius, PPLN OPO gave the maximum conversion efficiency of 23.7%

77

with 2.9 W input as shown in Fig.4.6. After conversion efficiency of SHG raised

and for adjustment of balance between IR light and residual 1064 nm light after

the OPO, the output power of 1797.2 nm is set at 350 mW with a conversion

efficiency of 16.5%.

Fig.4.6. IR output property of OPO in 179 nm light generation system. Black points stand for output power; Red points stand for conversion efficiency.

In the experiment, compared with the PPLN OPO built in 189 nm light

generation system, this OPO seems more easily to start oscillation. There are two

reasons concluded: One is the laser source used in 179 nm has a shorter

pulse-width that can produce larger peak power to promote the oscillation; the

other is the oscillating idler wavelength here is about 2600 nm that has the best

reflection property of the AR coat of the concave mirror building the OPO, so the

oscillation in this case is promoted.

4-2-5 198.8 nm DUV light generation

Then, DUV light generation of 198.8 nm was obtained by SFG with the 224.5

nm light and depleted pump light at 1064 nm after the OPO in a 5×5×15 mm3

type I CLBO (CLBO2) cut at θ=78.1. The 244.5 nm was focused by a

cylindrical lens pair to a waist with radius of 36 μm and collinearly combined

1797 output

Conversion efficiency


78

with 136 μm radius waist 1064 nm beam. Such sizes make two beams the same

confocal parameters of about 60 mm, which leads to optimal nonlinear

conversion efficiency. In this stage, from the effect of parametric oscillation, the

1064 nm temporal pulse shape was asymmetrically depleted, so it looks like

faster than the 244.5 nm pulse as shown in Fig.4.7. To effectively utilize the

undepleted leading edge of 1064 nm pulse, an optical delay path of 1.9 m was set

to optimizing the temporal overlap with 244.5 nm pulse. With the output

properties shown in Fig.4.7, SFG efficiency for the DUV light generation was, at

most, 25.3% from input power of the 244.5 nm.

(a) (b) Fig.4.7 Adjustment of 224.5 nm and 1064 nm pulses in temporal range observed by oscilloscope. Red line stands for 244.5 nm pulse; blue line stands for 1064 nm pulse. (a) The two pulses do not coincide; (b) The two pulses got coincide after the delay line set.

Fig.4.8. 198.8 nm DUV light generated with SFG by CLBO. Purple spots stand for output, black spots stand for conversion efficiency.

Rel

ativ

e li

ght

inte

nsi

ty

Inte

nsi

ty 6 ns

Rel

ativ

e li

ght

inte

nsi

ty

198.8 nm output

Conversion efficiency

79

CLBO crystals used in this system were grown in our laboratory by solution

stirring top-seeded solution growth [12]. They are polished on two sides without

coating. While with excellent optical properties and crystal quality, they suffered

from hygroscopic issues that hinder long-term operation potential. In our

experiment, operating at 150C with argon gas flow is used as a solution to

reduce water impurity inside the crystal [13].

4-2-6 Summary for 179 nm VUV light generation system

A summary for the system is shown as Fig.4.9. All of the stages were adjusted to

optimal condition for the final 179 nm light generation. 6 W output of 532 nm

light with was generated with conversion efficiency of 62.5% from fundamental

laser power. 560 mW output of 489 nm light was generated with KTP OPO and

intra-cavity LBO SHG with total conversion efficiency of 11.2% from 532 nm

pump. 244.5 nm light with output of 200mW was generated from 489 nm by

35.7%. IR of 1797 nm was generated from OPO with 350 mW output by 16.5%

from 1064 nm pump power. 198 nm DUV light was generated by SFG of

residual 1064 nm light and 244.5 nm light, with 30% conversion efficiency from

244.5 nm and has an output power of 60 mW.

Fig.4.9. Summary for 179 nm light generation system.


80

4-2-7 SFG for 179 nm VUV light generation

In the final stage of conversion, a minimum beam waist of w=29 μm is set for

198.8 nm DUV, while collinearly focused IR is set with a beam waist of w=114

μm, approximately corresponding to the same confocal parameter of the DUV

light of about 60 mm. The input power of DUV was kept at 60 mW and IR at 350

mW. VUV light at around 179 nm was generated by SFG with the DUV light

and IR in the last borate crystal. There was also an optical delay path of 2.2 m set

to synchronize the arrival time of both pulses. A 60 dispersion prism made from

fused quartz was used to disperse VUV light from the DUV light after the SFG.

The distance between the output face of the final borate crystals and power meter

was about 8 cm. As absorption of 179 nm in air was not predicted large in such

short distance, the final generation was kept in air ambient at room temperature

for the scheme investigation.

For the VUV light generation, two 5×5×15 mm3 samples cut at φ=80, 85 and

θ=90 and three 6×7×8 mm3 samples cut at φ=65, 70, 75 and θ=90 for LBO,

two 3×3×15 mm3 samples cut at θ=70, 77.5 and φ=90 for CBO are prepared.

The CBO sample was grown from a self-flux solution with a composition of 74

mol% B2O3 [14], while the LBO sample was purchased commercially. All of the

samples are polished on both sides without coating. As the theoretical

phase-matching angles listed in Table 4.1, CBO seemed to be a promising choice

for the generation with a larger effective nonlinear coefficient [8].

4-3 179 nm light generation result

As the result of our experiment, as a candidate crystal, CBO did not satisfy

phase-matching for the final SFG at 179 nm, despite the theoretical calculation

[8]. Furthermore, there was no SFG obtained at around 180 nm with the sample

that is against the calculation based on another reference [15]. In Chapter 3, the

measurement in 190 nm range light generation with CBO has a deviation from

calculated value, which is still an evidence for a published Sellmeier formula

[15]. In this generation, as the wavelength became shorter, the birefringence

81

property is considered changed even larger that make the deviation become too

large to predict phase-matching.

In the case of LBO, 179 nm light is confirmed by fluorescence as shown in

Fig.4.10. The output was estimated to be less than 0.1 mW which could not be

measured by our detector in air ambient. The main reason attributed for the low

output power is the small effective nonlinear coefficient in the generation, which

is the same as the case in 189 nm light generation with LBO. The absorption of

oxygen and other optical units is also considered to have impact for such result.

In further research, to generate detectable output power in VUV, chamber could

be employed to contain the final stage and make an environment of vacuum or

Nitrogen gas. Also optical units without much loss in VUV range should be

choose more carefully.

Fig.4.10. 179 nm light generation observed with fluorescence on glass sheet. 179 nm spot is in the dashed circle on the left while 198.8 nm spot is bright and on the right.

As shown in Fig.4.11, phase-matching angles of φ for LBO around 180 nm

were taken while tuning IR wavelength with DUV fixed at 198.8 nm. The

observed slope is roughly the same as that of the calculated curve. The data for

measurement is also concluded in Table 4.2. However, the observed values in phi

are approximately 5 more than those of the calculated curve. As wavelength of

IR used in SFG and generated VUV are at edge of the transmission curve of the

crystal, it is suspected that there is also a deviation occurred in birefringence

179 nm 198.8 nm


82

property of LBO. Therefore such deviation in the experiment is still reasonable.

Table 4.2 LBO samples used in measurement for phase-matching angles.

Wavelength (nm)

IR VUV

Phase-matching angle (deg.)

Calculated Measured

1972.7 180.6 64.6 a 68.0

1949.1 180.4 65.6 a 68.6

1926.0 180.2 66.6 a 69.1

1903.4 180.0 67.7 a, b

70.2

1881.3 179.8 68.9 a, b

70.8

1870.4 179.7 69.5 b 72.9

1838.4 179.4 71.3 b, c

74.1

1817.6 179.2 72.7 b, c

75.8

1797.2 179.0 74.1 c 77.5

1777.3 178.8 75.7 c, d

80.1

LBO samples: a: φ=65, b: φ=70, c: φ=75, d: φ=80 and θ=90.

Fig.4.11. Phase-matching property of LBO measured at around 180 nm. The measurement was taken with scanning of IR with DUV fixed at 198.8 nm.

83

4-4 Summary

I built a high-repetition-rate, all-solid-state laser system for 179 nm VUV light

generation based on a 1064 nm Nd:YVO4 laser. The generation is realized from

SFG with 198.8 nm DUV and 1797 nm IR light in a borate crystal. In the system,

198.8 nm DUV light generation is based on a KTP OPO and intracavity SHG

with LBO.

As the result, 179 nm VUV light is demonstrated by LBO successfully. The

output is hardly be detected except observation with a fluorescence.

Phase-matching properties for LBO around 180 nm were investigated with the

system. The weak output can be explained with small nonlinear coefficient of

LBO in the generation.

By using CBO, generation at 179 nm and 180 nm is not achieved with the

system, which is not accordance to the theory prediction. It could be attribute to

the birefringence of CBO has changed so much in VUV range that the deviation

is not predictable.


84


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and Electro-Optics, CThW4 (2008).

[10] Y. Kaneda, N. Peyghambarian, K. Miyazono, H. Shimatani, Y. Honda, M.

Yoshimura, Y. Mori, Y. Kitaoka, and T. Sasaki, Opt. Lett. 33, 231 (2008).

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85

Chapter 5. Research about 355 nm UV light generation with

CLBO

5-1 All-solid-state 355 nm laser

In this section, 355 nm UV (as the third-harmonic generation, THG) light

realized SFG of 1064 nm and 532 nm light by different kinds of borate crystals is

introduced. Although considered as a useful tool in UV light generation, CLBO

has not been used in 355 nm light generation so far. I try to explain the merit of

the utilization of CLBO in the generation.

5-1-1 355 nm UV light generation with borate crystals

As 355 nm UV is achieved through sum-frequency generation (SFG) with

1064 nm and 532 nm light, the material used as the nonlinear frequency

conversion media is of significant importance. As one type of nonlinear optical

(NLO) crystal suitable in this field, borate crystals are featured with relatively

large nonlinear coefficients, short absorption edges, and high laser-induced

damage thresholds [1, 2]. With borate crystals LBO, BBO, and CBO, highly

efficient 355 nm light generation has been discussed in a number of previous

reports. In the example for CBO, Kitano et al. showed 3 W output with a large

total conversion efficiency of 30% [3]. 355 nm light generation by BBO was also

verified in ps system many years ago [4]. LBO is most commonly used for the

application for its high quality and small walk-off angle in the generation [5, 6].

It has been reported employed in commercial lasers [7]. In these generations,

Type II phase-matching is used for the simplicity in the setup of laser system.

5-1-2 CLBO’s outlook for 355 nm UV light generation

Since developed in the 1990s, CLBO has drawn significant attention due to its

potential uses in high average power harmonic generation, particularly in UV

Chapter 5. Research about 355 nm UV light generation by CLBO

86

range [8-12]. Kojima et al. showed a 20 W 266 nm light generation by CLBO [8].

After that, 42 W output of 266 nm UV by CLBO was reported by Nishioka et al.

[9]. Katsura et al. reported 10.2 W output of 213 nm UV achieved with SFG by

CLBO [10]. Sakuma et al. has implemented research about 266 nm and 213 nm

CW laser generations, they used Brewster-cut CLBO with external resonant

cavity for promoting generation efficiency [11, 12]. Furthermore, in longer

wavelength range, there is also a report about a 12.5 J second-harmonic

generation by Type II CLBO based on a diode-pumped Nd:glass laser [13].

Table 5.1 compares the THG properties of LBO and CLBO in Type I and

Type II phase-matching. Phase-matching angles are calculated by SNLO [14]

based on Ref. 15 (LBO), 16 (CLBO), while effective nonlinear coefficients are

calculated based on Ref. 17 (LBO), 18 (CLBO). As can be seen in this table,

CLBO appears to be a promising THG media because of its superiority to LBO

in terms of effective nonlinear coefficient in Type II phase-matching.

Table 5.1. THG Phase-matching properties for LBO and CLBO.

PM angle

423 K

(θ, φ) (deg.)

Effective

nonlinear

coefficient

deff (pm/V)

Temperature

range (K·cm)

Walk-off

(mrad)

LBO Type I (ooe)

(90, 36.1)

0.73 5.3 16.9

Type II (oeo)

(49.7, 90)

0.47 3.3 9.9

CLBO Type I (ooe)

(39.0, 0)

0.52 20.0 36.8

Type II (eoe)

(48.8, 45)

0.71 19.9 33.4(ω),

36.8(3ω)

Phase-matching angles are calculated by SNLO based on data

come from Ref. 15 (LBO), 16 (CLBO). Effective nonlinear coefficients

are calculated based on data come from Ref. 17 (LBO), 18 (CLBO).

According to equation (2.29), as the output intensity is in direct proportion to

http://dict.youdao.com/w/in/

http://dict.youdao.com/w/direct/

http://dict.youdao.com/w/proportion/

http://dict.youdao.com/w/to/

87

the square of effective nonlinear coefficient, larger conversion efficiency could

be expected with CLBO. Also, it has a larger temperature range, which means

the generation will not be affected by non-uniform temperature distribution

significantly during the generation. However, 355 nm light generation by CLBO

has not been previously demonstrated due to its large walk-off angle (in both

Type I and Type II phase-matching) and its result of a separation between the

interaction beams. If compensation for this walk-off issue were to be achieved, a

substantial increase of the output power could be expected.

There have been many previous reports exploring ways to circumvent the

conversion efficiency limitation that results from walk-off in Type II

phase-matching [19, 20]. One conventional scheme involves using a twin-crystal

configuration, where two identically cut crystals are mounted with their optical

axes symmetrically crossed [19]. Here, partial walk-off compensation is used to

enhance output by increasing the effective interaction length in comparison to

that of a single crystal. For example, Zondy et al. presented a report on second

harmonic generation (SHG) of 1.3 μm and 2.53 μm light by twin-KTP at, in

which a single-pass increase of 3.2-3.5 times the amount of single-crystal output

was achieved [19]. This method can be utilized in both Type I and Type II

phase-matching.

Another effort aimed at Type II phase-matching walk-off compensation

involves the use of a non-collinear phase-matching configuration [21-23]. A

wedged cut structure is considered an especially effective solution for achieving

this walk-off compensation method [23]. It makes oblique incident beams on

wedged cut surface refract to meet the non-collinear phase-matching condition.

Asaumi geometrically analyzed compensated walk-off in Type II SHG of 1064

nm by KTP and LBO, and demonstrated significant power enhancement [21].

Furthermore, Yan et al. recently investigated a wedged cut LBO designed for

Type II 355 nm light generation and provided an algorithm for use in

determining the optimal wedge angle [23].

In this research, in an attempt to achieve highly efficient 355 nm light

generation and inspired by the non-collinear phase-matching method, I designed


88

a walk-off compensated prism-coupled device based on Type II CLBO. To

accomplish this, I performed 355 nm light generation with collinear and

non-collinear phase-matching and then tested the output enhancement made

possible by the new device.

5-2 Method for walk-off compensation

In this section, I will introduce the method realizing non-collinear

phase-matching for walk-off compensation in 355 nm light generation by CLBO.

Similar to the example given section 2-1-6 of a Type II SHG, principle for

collinear and non-collinear phase-matching for the THG (THG will be used as a

term in this section for simplicity) will be briefly discussed, and it will become

easy to understand why non-collinear phase-matching can overcome the

shortcoming of collinear phase-matching. Then, for achieving non-collinear

phase-matching that is not so usual in UV light generation, a wedged-cut

structure is needed for it has the ability to combine the beams’ Poynting vector

while keeping their wave vector separated. At last, for the walk-off angle for

CLBO is too large for achieving non-collinear phase-matching, a prism-coupled

structure is designed as the solution.

5-2-1 Principle for non-collinear phase-matching

Fig.5.1 (a) shows a commonly used collinear phase-matching for Type II (eoe)

THG in a negative uniaxial crystal. The phase-matching condition is expressed

with wave vector relations as:

( ) (2 ) (3 )

PM PM( ) ( )e o ek k k . (5.1)

It also can be written with a refractive index as:

( ) (2 ) (3 )

PM PM( ) 2 3 ( )e o en n n . (5.2)

In the plane determined by the z and x (or y) axis of the crystal, the wave vector

89

(k) of second-harmonic light (2ω) is shown as a quarter circle (ordinary wave)

with a solid line, while fundamental light (ω) and third-harmonic light (3ω) are

shown as a quarter ellipse (extraordinary wave) with a dished line. The

phase-matching angle θPM is decided in the manner of k when the ellipse of the ω

plus the circle of the 2ω crosses the ellipse of the 3ω. The wave vectors of the

incident waves, as well as the resultant wave, dispose in the same direction. As ω

and 3ω are extraordinary waves, their associated Poynting vectors (S) are offset

by walk-off angles of ρ(ω)

, ρ(3ω)

clockwise from the respective wave vectors. The

walk-off angles in the figures are shown larger than actual size so that this can be

seen clearly. In such situations, all Poynting vectors in the generation are pointed

in different directions, thereby resulting in a progressively reduced overlap

between the beams as they traverse the crystal. This leads to a shorter effective

interaction length and, ultimately, lower conversion efficiency.

(a) (b)

Fig.5.1. Collinear (a) and non-collinear (b) phase-matching configurations illustrated in x(or y)-z wave vector plane for type II (eoe) THG by a negative uniaxial crystal.

Improving the conversion efficiency of this process requires walk-off

compensation in order to extend the effective interaction length of the incident

beams. For this purpose, consideration may be given to aligning the Poynting

vector directions. As shown in Fig.5.1 (b), by rotating the wave vector of ω

counterclockwise at a certain angle and rotating the wave vector of 2ω clockwise

slightly, the Poynting vectors of ω and 2ω become parallel, thus allowing


90

optimal overlap of the two energy flows to be maintained throughout the length

of the crystal, while the deviation between them and 3ω decreases as well.

When the vector sum of ω and 2ω is equal to 3ω, the non-collinear

phase-matching configuration for walk-off compensation is established. The

mathematical expression of refractive index for the configuration is shown as:

( ) ( ) ( ) (2 ) (2 ) ( ) (3 ) (3 ) (3 )( )sin( ) 2 ( )sin( ) 3 ( )sin( )e e en n n , (5.3)

( ) ( ) ( ) (2 ) (2 ) (2 ) (3 ) (3 ) (3 )( )cos( ) 2 ( )cos( ) 3 ( )cos( )e o en n n , (5.4)

( ) ( )( ) (2 ) 2

( )

( )Tan( ) Tan( )[ ]e

o

n

n

. (5.5)

θ(ω)

, θ(2ω)

and θ(3ω)

are the phase-matching angles for the three waves measured

from the z axis, which are shown in Fig.5.2. Equations (5.3) and (5.4) are for

non-collinear phase-matching condition in Type II THG, while (5.5) is for the

collinear energy flow of ω and 2ω.

Fig.5.2. Phase-matching and walk-off angles of non-collinear phase-matching configuration for type II (eoe) THG by a uniaxial crystal. Effective walk-off for THG after the compensation will get smaller than the walk-off angle for the collinear phase-matching.

Table II compares phase-matching and walk-off angles for the waves of Type

II THG in collinear and non-collinear phase-matching. It is found that the θ(2ω)

for the two conditions were almost the same. As shown in Fig.5.2, the Poynting

vector direction of ω is θ(ω)

+ρ(ω)

, which is equal to 2ω, thus making them parallel.

The effective walk-off for 3ω in non-collinear phase-matching equals

θ(3ω)

+ρ(3ω)

-θ(2ω)

=1.4°(26.2 mrad, the value counted with mrad has a higher

accuracy here), which is smaller than ρ(3ω)

in collinear phase-matching.

91

Table 5.2. Calculated phase-matching and walk-off angles for collinear and non-collinear THG by Type II CLBO (at 150°C).

θ(2ω)

(deg.)

θ(ω)

(deg.)

ρ(ω)

(deg.) (mrad)

θ(3ω)

(deg.)

ρ(3ω)

(deg.) (mrad)

collinear 48.8 48.8 1.9 33.4 48.8 2.1 36.6

non-collinear 48.9 46.9 2.0 34.0 48.2 2.1 37.3

Values for non-collinear phase-matching are calculated based on data come from Ref. 16.

5-2-2 Method for achieving non-collinear phase-matching

Theoretically, the non-collinear configuration is available with usual device by

input the lasers into the crystal with a small angle. However, as the laser beam is

commonly adjusted with µm level, it is hard for adjusting even with

high-precision equipment for the experiment. Therefore, look for special

technique seem to be the only way for the research.

Similar to the case of LBO discussed in [23], an effective approach for

achieving such non-collinear phase-matching is adopted which involves using a

crystal with its input surface cut at a wedge angle α as shown in Fig.5.3. Here,

the crystal is cut with its z-axis oriented at angle θ. 2ω with an ordinary

polarization is selected to propagate at a direction parallel to the crystal’s edge

for ease of alignment. Therefore, θ is chosen to be equal to θ(2ω)

in order to meet

the phase-matching condition for THG. The ω and 2ω beams enter the crystal’s

wedged cut surface at an oblique angle of ψ0 relative to the line perpendicular to

the surface. After occurrence of the refraction effect, they propagate at angles

ψ(ω)

and ψ(2ω)

inside the crystal relative to the line perpendicular to the surface.

The angles on either side of the interface satisfy Snell’s law, so a smaller amount

of ω will be refracted than 2ω because it has a smaller refractive index. As the

amount of separation between refractive beams (ψ(ω)

-ψ(2ω)

) equals the walk-off

angle ρ(ω)

, the compensation condition will be fulfilled.


92

Fig.5.3. Scheme of wedged cut structure for achieving non-collinear phase-matching in type II (eoe) THG by negative uniaxial crystal.

Fig.5.4. Relation of compensated walk-off angle and wedged cut angle. Blue line stands for the range that the wedge angle is small, light can be directly input on the wedged-cut surface; red line stands for the range that total reflection is happened, so there is no direct way to input the laser. The optimal wedge angle for compensation for CLBO is in the dashed line circle which is not available.

The wedge angle is calculated to be α=50.2° for CLBO based on data given in

Table II. In Fig.5.4, the relationship between the wedge angle and, accordingly

amount for the compensated angle is shown. For fully walk-off compensation,

blue line expresses range that incident angle at wedged cut surface is smaller than

90 degrees, while red line expressed the opposite range.

Unfortunately, the α desired is in red line range, which means the incident

93

angle ψ0 necessary for achieving the desired refractive angle will be over 90° on

a wedged cut surface. This is caused by the CLBO’s large walk-off angle, which

is about 4 times of LBO’s. It is easy for LBO to use the configuration to

compensate the walk-off as described in [23], while the incident angle is not

going to be achieved in the case of CLBO. In order to make the refraction with a

relative small incident angle for achieving the same compensation effect,

consideration may be given to inputting the beam from the media with a

refractive index closer to the CLBO.

5-2-3 Prism-coupled device structure for non-collinear phase-matching

In order to solve this problem, I adopted a structure with a prism attached to the

wedged cut surface of crystal, as shown in Fig.5.5. This structure is derived from

an optical contact prism-coupled device based on the KBe2BO3F2 (KBBF) crystal

used for achieving phase-matching in 177.3 nm UV light generation [24].

The prism used in this design is made of fused-quartz, which has a similar

refractive index to CLBO and high transparency from 1064 to 355 nm. As a

result, the incident beam only needs to be slightly refracted on the surface in

order to achieve the non-collinear phase-matching desired. (The deviation

between ω and 2ω in Fig.5.5 equals the actual angle, which differs from the

figures above.) Cut angle β of the prism also should be selected so that it is equal

to incident angle ψ0’ while making sure a normal incident angle is present on the

surface between the prism and outside. In this condition, no separation will occur

between ω and 2ω as they propagate through the prism collinearly. How to attach

the prism to the crystal tightly is a task for the configuration. Thanks to the

optical-contact technique, the two materials can be attached each without any

other assistant. The risk of laser-induced damage on combined surface seems to

be an issue in high power practical use. Also, cares should be given to make sure

the prism not separate from the crystal during the generation.


94

Fig.5.5. Scheme of prism-coupled device on one side. The device is fabricated by attaching the fused quartz prism to the CLBO crystal with optical-contact technology.

When the chosen CLBO cut with θ equals the phase-matching angle of 2ω at

θ(2ω)

=49.0°, given the refractive index for ω, 2ω in fused-quartz (n(ω)

FQ=1.450,

n(2ω)

FQ=1.461), α=ψ(2ω)

=58.6° is calculated as the wedge angle and β=ψ0’=61.0°

as the prism cut angle. The sample prism-coupled device created is shown in

Fig.5.6. The device is based on a CLBO crystal with dimensions of

h×w×l=5×5×10 mm3, and is cut with θ=49.0° and φ=0°, and then fabricated with

a wedge angle and prism cut angle of α=β=58.6°. For fabrication convenience,

the device is made into a cuboid, and there is a small deviation in the prism cut

angle from the calculated result. The fused quartz prisms are attached to the

CLBO crystal via the optical-contact technique provided by Kogakugiken.

Fig.5.6. A prism-coupled device sample fabricated with CLBO and fused quartz. CLBO crystal used has a dimension of h×w×l=5×5×10 mm

3. CLBO is cut with θ=49°, φ=0°; the

device is fabricated with α=β=58.6°.

95

5-3 Experiments for 355 nm light generation

In this section, 355 nm light generation is tested with LBO, CLBO and the

new-designed device. From the generation result, walk-off compensation effect

of non-collinear phase-matching configuration is to be discussed.

5-3-1 Setup preparation for 355 nm light generation

The experimental setup for THG is shown schematically in Fig.5.7. As can be

seen in this figure, a Q-switched neodymium-doped yttrium orthovanadate

(Nd:YVO4) laser (HIPPO H10-106QW, Spectra Physics) is employed as the

fundamental laser source. This laser delivers linear polarized beam with M2<1.2,

maximum output of 11.5 W at repetition rate of 15 kHz with a pulse width of 8

ns. After the laser source, a polarizer and half wavelength plate set is installed for

adjusting the power and polarization direction of the output.

Fig.5.7. Experimental setup for 355 nm light generation. The system is based on a commercial Nd:YVO4 laser. Ls stand for spherical lenses.

In the first step, SHG was performed by a 5×5×25 mm3 x-cut Type I

non-critical phase-matching (NCPM) LBO at 151C with anti-reflection (AR)

coating for 1064 and 532 nm. Fundamental light is focused to a 170 μm radius

waist into the LBO. The maximum conversion efficiency of SHG is 55% at an

input of 9 W. In the second step, residual fundamental light and second-harmonic

light with orthogonal polarization directions are separately focused by lens pairs


96

L1, L2 and L3, L4 into the 355 nm light generation borate crystal in order to

create the SFG. By adjusting these lens pairs, the waist diameter of both beams is

set at 73 μm. Generated 355 nm light will then be reflected by two dichroic

mirrors and detected by a power meter. The loss on the samples’ surface will be

considered in the generation.

I tested the borate crystals for 355 nm light generation. One is a conventional

CLBO crystal which was cut for Type II phase-matching at θ=49.0° and φ=0°.

This crystal, which has a size of 5×5×10 mm3, is optically polished on both sides,

and does not have an AR coat. As a comparative experiment, a LBO crystal cut

with for Type II phase-matching of θ=49.0° and φ=0° is also prepared. It is

fabricated without an AR coated and a size of 5×5×10 mm3. The crystals are both

maintained in a cell heated to 150C with control accuracy of 0.1C. The cell is

purged with AR coated windows and flowed with argon gas with 70 ml/min for

dehydrate of CLBO [25]. The CLBO crystal was grown in our laboratory using a

self-flux method; the LBO crystal was purchased.

5-3-2 355 nm light generation results of conventional CLBO

In the case of conventional CLBO, phase-matching angle for Type II CLBO is

measured with temperature changed as shown in Fig.5.8.

Fig.5.8. Phase-matching angle tuned by temperature for THG by CLBO.

40 60 80 100 120 140 160 180

49.20

49.25

49.30

49.35

49.40

Ph

ase-

mat

chin

g a

ng

le (

deg

.)

Temperature (oC)

97

Particularly, the phase-matching condition is meet at θ=49.4° under 150C.

The angle increases mildly with the temperature increases. Also from this figure,

CLBO is thought to have a broad temperature range. The value given by SNLO

is about 19.9 K‧cm that is larger than other crystals in the generation, which is

also a merit for the robustness of the crystal in generation.

As shown in Fig.5.9, I compared the focusing conditions for 1064 nm and 532

nm beams with A: two beams with the same diameter, B two beams with the

same confocal parameter. It is found that A has a better performance than B

where the ω beam was focused to the same confocal parameter as 2ω [26] that is

different from the SFG discussed in Chapter 3 and 4. It can be explained as the

difference between the value of refractive index/wavelength (n/λ) for 1064 nm

and 532 nm is much smaller than the difference between DUV and IR. So the

two conditions (2πw02n/λ and w0

2) are approaching to each other that the optimal

condition is deviated from our image based on experience.

Fig.5.9. THG outputs with different 1064 nm beam focusing condition. 532 nm is focused to 102 µm diameter beam for A while 72 µm for B.

The power ratios of 1064 nm and 532 nm, which were regulated by changing

the SHG LBO crystal temperature, were optimized to yield the maximum THG

output at about 3:4 as shown in Fig.5.10. The ratio under the ideal conditions is


98

thought to be 1:2, which was unable to meet under the actual conditions because

of non-ideal beam quality factor (M2>1), different sectional areas of 1064 nm and

532 nm beams, different distribution of the two lights, walk-off effect,

group-velocity mismatch and so on.

Fig.5.10. Relation between total conversion efficiency for THG and ratio of fundamental and SHG input.

Fig.5.11. 355 nm output and total conversion efficiency by conventional CLBO. Blue spots stand for output while grey spots stand for total conversion efficiency.

99

After the generation condition is fixed as the discussion above, the 355 nm

light generation results are shown in Fig.5.11. A maximum output of 1.2 W was

obtained with under a fundamental laser power of 9 W, which corresponds to

29.0% conversion efficiency from SHG and a 13.6% total conversion efficiency

level. This is the first output measurement of 355 nm light generated by CLBO.

5-3-3 355nm light generation results of LBO

For contrast experiment, a 5×5×10 mm3 LBO is to be tested. It is cut for θ=49.7,

φ=90 to meet the phase-matching condition for 355 nm light generation. The

generation condition with LBO is same to CLBO as the discussing in last section.

As an experiment result, 355 nm output power generated by LBO and

corresponding conversion efficiency form SHG input power are shown in

Fig.5.12. Maximum 355 nm output of was 1.6 W at 9 W fundamental power

where the total conversion efficiency was about 18% W. The output of LBO

exceeded the results got with conventional CLBO.

Fig.5.12. 355 nm light generation result by LBO. Purple spots stand for output while black spots stand for total conversion efficiency.


100

Also, the temperature range of 355 nm light generation by LBO is measured

with a fixed input angle. The center temperature shown at about 177C. Fig.5.13

read about 4°C at the full width at half maximum, which complies with the value

given in Table I. Such temperature range was measured with two output power

that will lead to different thermal affect in the crystal. The results were same,

which means the temperature tuning of the oven is thought effective. What is

noteworthy is that the small dip appears on the left shoulder of the curve, it

showed two times in the measurement, which is considered as the secondary

peak of the measurement that hidden in envelope curve.

Fig.5.13. Temperature dependence of 355 nm light generation by LBO.

5-3-4 355 nm light generation results of walk-off compensation device

To test the effectiveness of walk-off compensation, I implemented THG using

the prism-coupled device. This device is maintained in an argon gas environment

and the lower temperature was set at 70°C to avoid prism separation.

Comparative THG output results are shown in Fig.5.14. For the conventional

CLBO, a maximum THG output of 1.3 W was obtained, which corresponds to

33.3% conversion efficiency from SHG and a 14.6% total conversion efficiency

level. In comparison, a maximum THG output of 1.6 W was obtained via LBO.

0

50

100

150

200

250

300

350

172 174 176 178 180 182

Ou

tpu

t p

ow

er (

mW

)

Temperature (ºC)

101

By using new device, with 9 W of fundamental laser power and 1.9 W of THG

output was obtained at 47.6% conversion efficiency from the SHG input. This is

a 1.43-fold improvement over a conventional cut CLBO and is 1.15 times the

generation obtained via LBO. At maximum output, the input incident angle for

the ω and 2ω beams was 1.8°. The output was stable to temperature tuning and

no damage was found on the optical-contact surface following the experiment.

These data have been corrected to compensate for the reflection losses at the

uncoated facets of the THG crystals while the reflection loss at the wedged cut

surface is not taken into the consideration.

Fig.5.14. Generation results of 355 nm light with SFG by conventional CLBO, LBO, and prism-coupled device based on CLBO. THG conversion efficiency from SHG input increased as fundamental laser power increased.

This is the first prism-coupled device design reported for walk-off

compensation in Type II THG. Moreover, AR coatings on both sides of the

device will improve the generation in future test.

There are also some questions left for the generation with new device. ω, 2ω

beams’ incident angle on the surface of the device, due to deviation in prism cut


102

angle β in this demonstration, resulted separation between the two refracted

beams. It will grow continuously when they propagating through the prism, and

affect the generation obviously. Moreover, measured phase-matching angle is

different form cut angle θ of the CLBO crystal, which caused mismatch in SFG

bringing output down. Also as seen from Fig.5.14, output improvement from the

new device is larger at lower fundamental power input than at maximum input. It

could be described as self-heating effect during high-power SFG making

temperature distribution not uniform and the mismatch increased.

More investigation should be given to optimize the cut angle and wedge angle

for device fabrication and to find the optimal compensation condition from now

on.

5-4 Summary

In this research, I realized 355 nm light generation (THG) by CLBO. The

phase-matching and output properties of CLBO are investigated in the

generation.

In order to compensate for the walk-off angle of CLBO occurred in the Type II

phase-matching for 355 nm light generation, a non-collinear phase-matching

configuration is employed and the new phase-matching properties are calculated.

To realize non-collinear phase-matching configuration, I adopt a wedged cut

structure for the CLBO and design a prism-coupled device for achieving

phase-matching. The device is made by contacting fused quartz prisms to the

CLBO sample to make the non-collinear phase-matching achieved with a nearly

normal incident of the input beams.

The 355 nm light generation by the new device is tested comparing with

conventional LBO and CLBO. It made the generation with 47.6% conversion

efficiency from second-harmonic input. This amounts to 1.43 times the output of

a conventional CLBO crystal and exceeds the output by LBO. The design

provides an effective method for the walk-off compensation in Type II

phase-matching.

103


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105

Chapter 6. Conclusions

In this dissertation, for developing UV laser sources aiming at the applications of

processing and inspection, UV light generation of three wavelengths are

researched.

189 nm VUV light generation:

In order to generate 189 nm VUV light, a laser system based on

high-repetition-rate, all-solid-state 1064 nm Nd:YAG laser is built. The

generation is realized by SFG with 213 nm light (5HG) and IR light in a borate

crystal.

As candidate NLO crystals for 189 nm light generation, LBO and CLBO make

the demonstration successfully. The phase-matching property for LBO and

CLBO around 190 nm is found to have good accordance with the theory value.

On the other hand, by using CBO, the VUV light generation is not realized

until the wavelength extended to 190 nm and highly efficient output is not

available. The new phase-matching property for CBO around 190 nm is found to

have better agreement with a Sellmeier’s formula provided by Osaka University.

The result can be explained with the difference in growth method for the CBO.

An output of 11.4 mW at 189 nm was generated with a 15 mm-length CLBO.

CLBO is considered suitable for VUV light generation until 185 nm given its

phase-matching property and absorption range.

179 nm VUV light generation:

In order to generate 179 nm VUV light, a laser system based on

high-repetition-rate all-solid-state 1064 nm Nd:YVO4 laser is built. The

generation is realized by SFG with 198.8 nm DUV and 1797 nm IR light in a

borate crystal. In the system, 198.8 nm DUV light generation is based on a KTP

OPO and intracavity SHG with LBO.

Chapter 6. Conclusions

106

As the result, 179 nm VUV light is demonstrated successfully by LBO. The

output is smaller than 1 mW that cannot be detected except observation with

fluorescence. Phase-matching property of LBO around 180 nm is investigated

with the system. The weak output can be explained with small nonlinear

coefficient of LBO in the generation.

By using CBO, generation at neither 179 nm nor 180 nm is achieved by the

system, which is not accordance to the theory prediction. As the generation range

is too short, the birefringence of CBO has changed so much that the deviation is

not predictable.

355 nm light generation with CLBO and prism-coupled device:

355 nm light generation (THG) by CLBO with SFG is demonstrated for the first

time. The phase-matching and output properties are investigated.

In order to compensate for the walk-off angle of CLBO occurred in the Type II

phase-matching for 355 nm light generation, a non-collinear phase-matching

configuration is employed and the new phase-matching properties are calculated.

To realize non-collinear phase-matching configuration, a wedged cut structure is

adopted for the CLBO and a prism-coupled device is designed for achieving

phase-matching.

The 355 nm light generation by the new device is tested comparing with

conventional LBO and CLBO. It made the generation with 47.6% conversion

efficiency from second-harmonic input. This amounts to 1.43 times the output of

a conventional CLBO crystal and exceeds the output by LBO. The design

provides an effective method for the walk-off compensation in Type II

phase-matching.

107

List of abbreviations in the dissertation

UV ultraviolet

DUV deep ultraviolet

VUV vacuum ultraviolet

IR infrared

SHG second harmonic generation

SFG sum-frequency generation

DFG difference-frequency generation

OPO optical parametric oscillation

PPLN periodically poled lithium niobate

CBO cesium triborate CsB3O5

LBO lithium triborate LiB3O5

CLBO cesium lithium borate CsLiB6O10

BBO beta-barium borate β-BaB2O4

KTP potassiumtitanyl phosphate KTiOPO4

KBBF potassium beryllium fluoroborate KBe2BO3F2

AR anti-reflection

THG third harmonic generation

PRF pulse repetition frequency

109

Acknowledgement

This dissertation was conducted at the Division of Electrical Electronic and

Information Engineering, Graduate School of Engineering, Osaka University. By

that time, I have worked with a great number of people whose contribution in

assorted ways to the research and the making of the dissertation deserved special

mention. It is a pleasure to convey my gratitude to them all in my humble

acknowledgement.

In the first place I would like to express my special thanks of gratitude to my

mentor Professor Yusuke Mori who gave me considerable help by means of

suggestion, comments and criticism. His truly scientist intuition has made him as

a constant oasis of idea and passions in science, which exceptionally inspire and

enrich my growth as a student, a researcher and a scientist want to be.

I would like to record my gratitude to Professor Ryuji Katayama and Professor

Masashi Yoshimura (Institute of Laser Engineering) for their help in the

completion of this dissertation. Without their consistent and illuminating

instruction, this dissertation could not have reached its present form. I would also

like to express thanks to Professor Masanori Ozaki and Professor Masayoshi

Tonouchi for giving their kind suggestion for this dissertation.

Quantum Electronic Device Course of Division of Electrical, Electonic and

Information Engineering at Osaka University is a very stimulating intellectual

environment. My thinking benefited a lot from the inspiring and creative

atmosphere as well as the professional excellence of the scientists. I gratefully

acknowledge Professor Toshimichi Ito, Professor Mitsuhiro Katayama, Professor

Emeritus Toshiaki Suhara, Professor Masahiko Kondow, Professor Emeritus

Yutaka Ohmori, Professor Nobuya Mori, Professor Emeritus Kenji Taniguchi,

110

Professor Emeritus Sezo Morita, Professor Tetsuya Yagi, Professor Akira Oiwa,

Emeritus Professor Hajime Asahi, Professor Noriaki Miyanaga, Professor

Hiroaki Nishimura, for their kindly supervision and guidance.

I gratefully acknowledge Professor Emeritus Takatomo Sasaki, Associate

Professor Mamoru Imade, Assistant Professor Masayuki Imanishi, Specially

Appointed Professor Hiroaki Adachi (SOSHO, Inc.), Research Professor Yushi

Kaneda (The University of Arizona), Specially Appointed Associate Professor

Mihoko Maruyama (Kyoto Prefectural University) and Dr. Yoshinori Takahashi

for their supervision, advice, and guidance from the very early stage of this

research as well as giving me extraordinary experiences throughout the work.

I gratefully acknowledge Yukikatsu Okada and Mitsuhiro Tanaka from

Kogakugiken for the support to my research. I am also grateful to Assistant

Professor Takahiro Kawamura (Mie University) for giving me advice about the

research.

I am also very grateful to Dr. Daisuke Matsuo, Dr. Hiroki Imabayashi, Hideo

Takazawa, Yuma Todoroki, Dr. Kosuke Murakami, and Keiko Hino, for their

support and encouragement. Their unlimited willingness to invest themselves

into the guidance and support of young scientists and their ability to stimulate

thinking and to encourage love for your work are unrivaled.

I would like to acknowledge the invaluable discussions with my team members,

Kazuki Masuda, Hiroshi Tootake, Yuichi Oeki, and Kentaro Ueda. This

dissertation could not have been finished without the help and support from them.

I also gratefully acknowledge Yusuke Mizobe, Jun Tsunoda, Zhiming Lu, Kai

Zhang, Yuji Fukushima, Takashi Moritani and Kei Takachiho for their kindly

advices.

Besides mentors and team members, this dissertation is influenced by the

111

contribution of peers: Dr. Keiko Masumoto, Dr. Masatomo Honjo, Dr. Yuan Bu,

Dr. Tomoaki Sumi, Dr. Yoichiro Mori, Koshi Nakamura, Hitoshi Iga, Satoshi

Nakayama, Toshihiko Yamada, Masami Juta, Kenji Ikeda, Tomohiro Takizawa,

Taro Sato, Yusuke Tominaga, Shogo Ogawa, Suguru Fukukita, Yuuki Taniyama,

Takumi Yamada, Shin Yamagata, and Shin Fujiwara. Their collegiality and

friendship have helped me in numerous times.

I am very grateful to the secretaries, Sachiko Okamoto, Saori Kataoka, Yurie Ishii,

Ryoko Outsuki, and Yumi Kubo for their kindly help and friendly assistant on my

daily life.

I am gratefully thanking Summer Research Internship Program (RIP 2010) from

Global COE program for giving me chance to visit Osaka University and become

a formal student. Thank Professor Emeritus Kenji Taniguchi and Professor

Masanori Ozaki and all professors for the help given in the program. Thank

Mariko Okamoto for the help in this program.

My sincere thanks should also go to my parents, and all of my friends for their

generous help and valuable suggestions.

Finally, I would like to thank everybody who was important to the successful

realization of dissertation, as well as expressing my apology that I could not

mention personally one by one.

May 2017

Chen Qu

113

Achievements:

List of publications:

1. Chen Qu, Masashi Yoshimura, Jun Tsunoda, Kai Zhang, Yushi Kaneda,

Mamoru Imade, Takatomo Sasaki, and Yusuke Mori

“Phase matching properties around 190 nm in various borate crystals”

Applied Physics Express 5, 062601-01-03 (2012).

2. Chen Qu, Masashi Yoshimura, Jun Tsunoda, Kai Zhang, Yushi Kaneda,


“Sub-180 nm generation with borate crystal”

Optical Materials 36, 1970-1972 (2014).

3. Chen Qu, Masashi Yoshimura, Yoshinori Takahashi, and Yusuke Mori

“Highly efficient 355 nm UV generation with non-collinear phase-matching by a

prism-coupled device based on CsLiB6O10”

Applied Physics Express 8, 052601-01-04 (2015).

International conference presentations:

1 Chen Qu, Kai Zhang, Jun Tsunoda, Yusuke Mizobe, Masashi Yoshimura,

and Yusuke Mori

“Long-term reliability of CsLiB6O10 crystal in UV generation”

2nd Global COE Student Conference on Innovative Electronic Topics 2010

2010. 7 Osaka, Japan. (Poster)

2 Chen Qu, Masashi Yoshimura, Jun Tsunoda, Yushi Kaneda, Mamoru Imade,

Takatomo Sasaki, and Yusuke Mori

“189 nm generation with borate crystals”

Eco-materials and Eco-innovation for Global Sustainability 2011

2011. 11 Osaka, Japan. (Poster)

114



“11.4 mW power generation of 189-nm wavelength by CLBO”

The 1st Advanced Lasers and Photon Sources (ALPS’12)

2012. 4 Yokohama, Japan. (Poster)



“189 nm generation with borate crystals”

Conference on Lasers and Electro-Optics (CLEO)

2012. 5 San Jose, USA. (Oral)

5 Chen Qu, Kei Takachiho, Masashi Yoshimura, Jun Tsunoda, Yushi Kaneda,


“Research about CsLiB6O10 in VUV and DUV generation”

7th Photonics Center Symposium "Nanophotonics in Asia 2012"

2012. 9 Kanazawa, Japan. (Poster)



“VUV light generation with borate crystals”

The 2nd Advanced Lasers and Photon Sources (ALPS’13)

2013. 4 Yokohama, Japan. (Oral)



“179 nm generation with borate crystal”

The 10th Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR)

2013. 7 Kyoto, Japan. (Oral)

115



“VUV light generation with CsLiB6O10 and LiB3O5”

JSAP-OSA Joint Symposia 2013

2013. 9 Kyoto, Japan. (Oral)



“VUV generation below 200 nm with borate crystals”

The 6th International Symposium on Lasers, Scintillators and Non-Linear Optical

Materials (ISLNOM-6)

2013.11 Shanghai, China. (Oral)

10 Masashi Yoshimura, Chen Qu, Yoshinori Takahashi, and Yusuke Mori

“Highly efficient 355 nm UV generation with non-collinear phase-matching by a

prism-coupled device based on CsLiB6O10”

The 4th Advanced Lasers and Photon Sources (ALPS’15)

2015. 4 Yokohama, Japan. (Oral)

11 Kentaro Ueda, Masashi Yoshimura, Chen Qu, Yoshinori Takahashi, and

Yusuke Mori

“Non-collinear phase-matched 355 nm UV generation in a prism-coupled

CsLiB6O10 crystal”

Advanced Solid State Lasers (ASSL) 2015

2015. 10 Berlin, Germany. (Oral)

title research about coherent ultra-violet light sources based on nonlinear conversion with borate

Documents