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Applied Surface Science 253 (2006) 2674–2677

The optical properties of planar waveguides in LiB3O5

crystals formed by Cu+ implantation

Yi Jiang a,*, Chuan-Lei Jia a, Lei Wang a, Xue-Lin Wang a, Feng Chen a,Ke-Ming Wang a, Qing-Ming Lu b, Hong-Ji Ma c, Ding-Yu Shen c

a School of Physics and Microelectronics, Shandong University, Ji’nan 250100, Shandong, Chinab School of Chemistry and Chemical Engineering, Shandong University, Ji’nan 250100, Shandong, China

c The Key Laboratory of Heavy Ion Physics (Peking University), Ministry of Education, Beijing 100871, China

Received 6 March 2006; received in revised form 10 May 2006; accepted 18 May 2006

Available online 7 July 2006

Abstract

A planar optical waveguide has been formed in a LiB3O5 crystal using 6.0 MeV Cu+-ions with a dose of 1 � 1015 ions/cm2 at room temperature.

Possible propagating modes were measured at a wavelength of 633 nm using the prism-coupling method. The refractive index profiles of the

waveguide were reconstructed by an effective refractive index method and the beam propagation method was used to investigate the properties of

the propagation modes in the formed waveguide. The results suggest that the fundamental TE0 and TM0 modes may be well-confined and propagate

a longer distance inside the waveguide. The implantation process was also simulated using the transport of ions in matter code (TRIM), which

indicates that the nuclear energy deposition may be the main factor for the refractive index change.

# 2006 Elsevier B.V. All rights reserved.

PACS: 61.72.Ww; 61.72.Dd; 42.82.Et

Keywords: Ion-implantation; Optical waveguide; LiB3O5 crystal

1. Introduction

Lithium triborate (LiB3O5 or LBO) is an attractive material

for frequency conversion applications due to its high UV

transmission, large acceptance angle, small walk-off angle,

good chemical stability and non-hygroscopicity [1]. However,

the relatively moderate effective non-linear coefficient of LBO

has so far limited the application of this material to harmonic

generation of high power laser beams. Waveguide structures

have the advantage over bulk geometries of maintaining high

intensities over long interaction lengths, which may make it

possible to extend harmonic generation use of LBO to low

power lasers, such as diode lasers.

Several techniques have been developed to produce

waveguide structures in materials, such as metal diffusion,

ion exchange, thin film deposition and ion implantation [2–6].

Because of its ability to modify the surface of materials, ion

implantation has become a universal method for fabricating

* Corresponding author. Tel.: +86 531 88564655, fax: +86 531 88565167.

E-mail address: sdujy@mail.sdu.edu.cn (Y. Jiang).

0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2006.05.117

waveguide structures in most optical materials because it has

superior controllability and reproducibility [7–9]. Implanta-

tion of light ions, such as H and He, has proven to be a

successful way to fabricate optical waveguides in LBO

crystals [10,11]. Recently, the implantation of heavy ions has

attracted much attention for waveguide fabrication because it

usually requires much lower doses and hence offers better

confinement of light in the waveguide [12]. However, to our

knowledge use of heavy ions for fabrication of optical

waveguides in LBO crystals has not been reported. In this

paper, we report on the fabrication of optical waveguides in

LBO crystals by 6.0 MeV Cu+-ion implantation.

2. Experiments in details

Samples of x-cut LBO (2 mm � 5 mm � 6 mm) were

provided by the School of Chemistry and Chemical Engineering,

Shandong University. The samples were optically polished and

cleaned before implantation. The 6.0 MeV Cu+-ion implantation

was performed in a 1.7 mV tandem accelerator at Peking

University at room temperature, and the dose of the ions was

Y. Jiang et al. / Applied Surface Science 253 (2006) 2674–2677 2675

Fig. 1. Measured relative intensity of the light reflected from the prism versus the effective refractive index of the incident (a) TE and (b) TM polarized light at

wavelength 633 nm in the LBO waveguide formed by 6.0 MeV Cu+-ion implantation with a dose of 1 � 1015 ions/cm2 at room temperature.

1 � 1015 ions/cm2. The beam current was restricted to less than

200 nA to avoid surface charging and thermal effects. The ion

beam was electrically scanned to ensure a uniform implantation

over the samples. In order to avoid channeling, the samples were

tilted 78 from the beam direction. The prism coupling method

was used to measure waveguide modes with a Model 2010 Prism

Coupler (Metricon, USA). In this method, a laser beam strikes the

base of a prism, and hence the laser beam is coupled into the

waveguide region. A photodetector was used to detect the

reflected beam. The prism, waveguide and photodetector were

mounted on a rotary table so that the incident angle of the laser

beam could be varied. The intensity of the reflected light was

plotted as a function of incident angle, where a sharp drop in the

intensity profile corresponds to a possible mode. A laser with a

wavelength of 633 nm was used in the measurement.

3. Results and discussions

During the prism-coupling measurement, the incident angle

a and the effective refractive index neff in the planar waveguide

Fig. 2. Reconstructed refractive index profiles of nz (a) and nx (b) in the waveguide

room temperature. The refractive indices of the virgin LBO crystal are also given

have the relationship [13],

neff ¼b

k¼ np � sin a (1)

where np is the refractive index of the prism and b � kni sin ai is

defined as the propagation constant. Fig. 1 shows the relative

intensity at 633 nm reflected from the prism for the transverse

electric (TE) polarized light (a) and transverse magnetic (TM)

polarized light (b) at 633 nm versus the effective refractive

index of incident light in the LBO planar waveguide formed by

6.0 MeV Cu+-ion implantation. As can be seen in the figure,

nine and six dips are observed in the cases of TE and TM

polarized light, respectively, which may correspond to propa-

gating modes.

The refractive index profiles of formed waveguides are

reconstructed by the reflectivity calculation method (RCM)

[14]. RCM has proven to be successful in characterizing non-

stationary waveguides, particularly ion-implanted waveguides.

A least-squares fitting program based on RCM was used to

calculate the refractive index profile by adjusting certain

formed by 6.0 MeV Cu+-ion implantation with a dose of 1 � 1015 ions/cm2 at

(dashed line).

Y. Jiang et al. / Applied Surface Science 253 (2006) 2674–26772676

Table 1

Comparison of measured and calculated effective refractive indices of TE modes and TM modes at the wavelength 633 nm for the LBO waveguide formed by

6.0 MeV Cu+-ion implantation with a dose of 1 � 1015 ions/cm2 at room temperature

Mode number TE mode TM mode

Measured Calculated Difference Measured Calculated Difference

0 1.5357 1.5355 �0.0002 1.5358 1.5358 �0.0000

1 1.5285 1.5285 0.0000 1.5286 1.5291 �0.0005

2 1.5164 1.5166 0.0002 1.5164 1.5161 �0.0003

3 1.4994 1.4990 �0.0004 1.4993 1.4992 �0.0001

4 1.4770 1.4775 0.0005 1.4765 1.4760 �0.0005

5 1.4493 1.4486 �0.0007 1.4484 1.4479 �0.0005

6 1.4157 1.4152 �0.0005

7 1.3764 1.3771 0.0007

8 1.3287 1.3293 0.0006

parameters until the theoretical mode indices matched the

experimental ones within a satisfactory error. Fig. 2 shows the

reconstructed refractive index profiles for refractive indices nz

(a) and nx (b) of the formed waveguide. The profiles have a

typical barrier shape. The optical barrier with an index

reduction of 0.15 (a) and 0.09 (b) acting as a cladding layer,

together with air, may confine the light propagation to relevant

modes. Table 1 lists the comparison of the measured mode

indices with fitted values of the indices of the TE and TM

modes. The measured effective refractive index is in agreement

with the calculated values better than 10�3.

A regular planar waveguide (with refractive index n1) is

generally confined between two cladding layers, which have

relatively lower values of refractive indices (n0 and n2).

Normally, for an asymmetric dielectric waveguide, one

cladding is air, with an index of n0 = 1 and the other cladding

(substrate) has an index n2 (usually n2 > n0). If n2 > neff > n0

holds true, the modal field (electronic or magnetic) of the

relative modes will extend into the substrate, resulting in

substrate modes. The barrier-type waveguide is irregular, and

the two claddings are air (n0) and the barrier (nb). Since the

index of the barrier nb is lower than that of the crystal substrate

n2 and the barrier has a finite thickness compared with the

dimension of the waveguide, the field of some modes (even for

Fig. 3. Field intensity distribution of (a) TE:TE0 (solid line), TE1 (dashed line) and TE

line) modes for the waveguide formed by 6.0 MeV Cu+-ion implantation with a d

some of those with neff > nb > n1) may extend into the

substrate region (called the tunneling effect [6]), acting as

substrate modes.

For a planar waveguide, the refractive index in the

waveguide region is characterized by n = n(x), which does

not depend on y or z. The wave vector lies in the (y, z) plane, and

we choose the z direction as the propagation direction without

loss of generality. All components of the electromagnetic field

are shaped according to

Fðt; x; y; zÞ ¼ FðxÞe�ivteibz (2)

for a TE mode, the electric field strength E = E(x) has to obey,

1

k20

E00 þ n2E ¼ n2effE (3)

for a TM mode, the magnetic field strength has to obey the

following mode equation,

1

k20

n2 d

dxn�2 d

dxH þ n2H ¼ n2

effH (4)

By performing a modal analysis [15] on the reconstructed

refractive index profiles of observed TE and TM modes, we can

get the electromagnetic field strength as a function of the depth.

2 (dotted line); and (b) TM:TM0 (solid line), TM1 (dashed line) and TM2 (dotted

ose of 1 � 1015 ions/cm2 at room temperature.

Y. Jiang et al. / Applied Surface Science 253 (2006) 2674–2677 2677

Fig. 4. Comparison of reconstructed refractive index profiles (nz) of the formed

waveguide (a) and the normalized nuclear energy loss (vacancy distribution) (b)

as a function of penetration depth of 6.0 MeV Cu+ ions implanted into LBO

crystal based on TRIM’98.

Fig. 3 shows the field intensity distribution of electric field

strength for the (a) TE (electric field strength versus depth) and

(b) TM (magnetic field strength versus depth) modes for the

waveguide formed by 6.0 MeV Cu+-ion implantation. Fig. 3(a)

shows that the TE2 mode has a wide extension into the

substrate, although its effective refractive index is larger than

the barrier index (neff > nb > n1). The reason for this is that the

barrier with low refractive index is not sufficiently thick, and

significant tunneling can occur. The TE1 mode may be regarded

as a guided mode, but its electronic field has a slight extension

into the substrate, which leads to an inevitable increase of the

propagation loss of the guided light. As can be seen in the

figure, the electronic field for the fundamental TE0 mode is

well-confined and no tunneling effect is observed. This results

in a low propagation loss and therefore light may propagate a

longer distance inside it. Similar results are obtained for the TM

modes (see Fig. 3(b)).

We use the transport of ions in matter (TRIM’98) code [16]

to simulate the process of the 6.0 MeV Cu+-ion implantation

into LBO. Fig. 4 shows both the reconstructed refractive index

profiles (nz) of the formed waveguide (a) and the normalized

nuclear energy loss (vacancy distribution) (b) as a function of

penetration depth of 6.0 MeV Cu+ ions implanted into the LBO

crystal based on TRIM’98. The shapes of the vacancy

distribution and index profile are similar to a certain extent.

The vacancies usually accompany a physical density reduction,

which causes a reduction of the refractive index and forms an

optical barrier [6]. It also should be noted that the peak

positions of the refractive index profiles are deeper than that of

the vacancy distribution. Similar phenomena have been

reported in MeV Ni+-ion implanted Tm:NaY(WO4)2 wave-

guides [17]. This may be due to the radiation-enhanced

diffusion (RED) of defects occurring during the process of the

implantation. The collision cascades followed by localized

energy deposition cause a high concentration of vacancies,

which sometimes result in greater defect movement. This

suggests that the RED of the defects may be on the scale of the

collision cascade itself and the ion track [18]. Nevertheless, a

detailed understanding of such phenomena still needs further

investigation.

4. Summary

The LBO waveguide has been formed by 6.0 MeV Cu+-ion

implantation with a dose of 1 � 1015 ions/cm2 at room

temperature. The refractive index profiles of the formed

waveguide were reconstructed by RCM. By performing a

modal analysis on the observed TE and TM modes, the

fundamental TE0 and TM0 modes may be well-confined and

propagate a longer distance inside the waveguide. The

TRIM’98 simulation suggests that the nuclear energy deposi-

tion may be the main factor for the refractive index change of

the waveguide structure.

Acknowledgement

This work is supported by the National Natural Science

Foundation of China (Grant No. 10475052).

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