The effect of Cr doping on optical and photoluminescence

properties of LiNbO3 crystals

Rajeev Bhatt, S. Kar, K.S. Bartwal*, V.K. Wadhawan

Crystal Growth Laboratory, Laser Materials Division, Centre for Advanced Technology, Indore 452 013, India

Received 29 March 2003; accepted 12 May 2003 by C.N.R. Rao

Abstract

Optical and photoluminescence studies on Cr doped LiNbO3 single crystals have been carried out. Three different Cr doped

compositions, with 0.1, 0.25 and 0.5 mol% Cr2O3, of congruently melting LiNbO3 single crystals were grown by the

Czochralski technique. Optical transmission studies showed prominent absorption bands in the visible region of the spectrum at

photon energies of 1.9 and 2.57 eV. A significant red-shift of 60 nm was observed in the UV absorption edges of doped crystals.

This can be explained in terms of the overall decrease in the Li/Nb ratio in the doped crystals, caused by extrinsic defects

generated by the Cr3þ impurity. The absorption coefficient of the material increases significantly with Cr doping concentration.

The band gap decreases with increasing Cr concentration. The excitation-independent broadband luminescence was observed in

the spectral range of 700–1100 nm, with a peak around 900 nm in the photoluminescence (PL) studies. The broadband PL

emission makes the material a potential candidate for diode pumped all solid-state tunable laser.

q 2003 Elsevier Ltd. All rights reserved.

PACS: 81.10Fq; 81.10.h

Keywords: A. Lithium niobate; D. Transmittance; E. Czochralski technique; E. Photoluminescence

1. Introduction

Lithium niobate, LiNbO3 (LN), continues to be a

material of interest for various optical and surface acoustic

wave (SAW) applications due to its unique combination of

piezoelectric and optical properties [1,2]. Unless special

crystal-growth procedures are adopted, crystals of LN have

a non-stoichiometric composition, with a high concentration

of intrinsic defects. The presence of these defects makes it

difficult to incorporate impurities added intentionally for a

desired application: the necessary charge compensation for

the extrinsic defects caused by the impurity atoms can get

balanced or disturbed by the intrinsic defects [3]. Several

types of dopant have been studied in the past for various

applications: Mg, Zn, In and Sc to make the material

damage resistant to optical radiation; Fe, Mn, Rh, Ce and Cu

for high-density holographic data storage; and Nd, Cr, Ho,

Gd and Er for laser-host applications. The present study

deals with the effect of Cr-doping on some of the optical and

photoluminescence properties of LN crystals. Such crystals

have potential for use as laser hosts in compact, diode-

pumped, all-solid-state, tunable laser systems in the 700–

1100 nm wavelength range.

Efforts for the development of planar waveguide lasers

and amplifiers have resulted in the demonstration of

sophisticated rare-earth-doped wave-guide devices based

on LiNbO3, taking advantage of the electro-optic and

acousto-optic properties of the host material [4,5]. However,

RE3þ ions in LiNbO3 typically exhibit a short tunability

range. Transition metal ions such as chromium have, on the

other hand, been used extensively in the past few years to

demonstrate broad tunability in various crystalline laser-

host materials [6].

The absorption spectrum of Cr-doped LiNbO3 shows

two bands with peaks at 480 and 660 nm approximately [7].

The luminescence spectrum consists of a broad band, with a

0038-1098/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0038-1098(03)00450-2

Solid State Communications 127 (2003) 457–462

www.elsevier.com/locate/ssc

* Corresponding author. Tel.: þ91-731-2488656; fax: þ91-731-

2488650.

E-mail address: bartwal@cat.ernet.in (K.S. Bartwal).

peak at around 900 nm and half-width of about 200 nm [6].

These results indicate its significant potential for an

integrated, broadband tunable laser in the 700–1100 nm

spectral ranges. The particular attraction of Cr as a laser-

active ion is that it has a broadband emission spectrum and,

combined with the electro-optic properties of LiNbO3, this

offers the possibility of high-speed electro-optic tuning of

the laser to produce a wavelength-tunable laser. Further-

more, with Cr ions it is possible to pump the material at

around 670 nm with a laser diode. The present work

describes the successful growth of undoped and Cr-doped

(0.1, 0.25 and 0.5 mol% Cr2O3) congruent-composition

LiNbO3 single crystals by the Czochralski technique, and

their subsequent optical and photoluminescence character-

isation. Chromium doping was optimised to find the Cr-ion

concentration required for creating significant absorption

bands without deteriorating much the transmittance of the

crystal.

2. Experimental details

2.1. Crystal growth

Lithium niobate single crystals were grown in air

atmosphere by the Czochralski technique from a congru-

ently melting composition. This material is known to have a

fairly large solid–solution range from 42 to 52 mol% Li2O

[8]. It is an incongruently melting material. The melting

point for the congruently composition (48.6 mol% Li2O and

51.4 mol% Nb2O5) is 1250 ^ 5 8C. Crystals grown from the

stoichiometric melt composition usually exhibit poor

compositional homogeneity down the length of the grown

crystal boule. High-quality crystals with uniform compo-

sitional homogeneity can be obtained from the congruent-

melt composition. However, such lithium-deficient crystals

contain intrinsic defects, which influence various properties

like photorefractive properties, domain-switching proper-

ties, etc.

Specially designed growth chamber made of zirconia

refractory backed by zirconia felt inside a quartz tube lining

was used for growing crack-free and colourless undoped LN

crystals [9]. An induction heating system (50 kW, 20 kHz)

was used. The charge for growing the crystals was placed in

a platinum crucible of 50 mm diameter and 50 mm height. A

(00.1) oriented seed of dimensions 2.5 £ 2.5 £ 20 mm3 was

used. The pulling rate employed was in the range of 3–

5 mm/h, with slow pulling for the body part of the boule.

The rotation rate was 12–25 rpm. A platinum cylindrical

screen was used as an after-heater and for providing a low-

axial-gradient environment for ensuring controlled post-

growth cooling of the crystal. The post-growth cooling was

maintained at 20–30 8C/h initially down to 1000 8C, and,

thereafter fast cooling to room temperature was employed.

In a similar set-up, Cr-doped congruently-melting-

composition lithium niobate crystals were also grown

[10]. Three different starting compositions were chosen:

0.1, 0.25 and 0.5 mol% of Cr2O3.

Nearly-uniform-diameter, transparent and crack free

crystals of LN, and light green colour Cr-doped crystals,

were grown with sizes measured up to diameter 20–25 mm

and height 45–50 mm. The colouration increases with

increasing Cr concentration in the crystals. Growth ridges

were observed on the surface of crystal boule. Polished

crystal plates were subjected to optical and photolumines-

cence studies.

2.2. Optical studies

The optical transmission spectra were recorded with

unpolarised light at normal incidence and room temperature

using Shimadzu UV 3101PC spectrophotometer. The

samples, prepared using standard cutting and polishing

tools, were z-cut slices of size 10 £ 10 £ 0.1 mm3. Fig. 1

shows the transmission spectra from undoped and Cr doped

LiNbO3 crystals in the UV-VIS-NIR region. The transmit-

tance obtained is better then 70% for the undoped crystal,

which is in accordance with reported values, whereas Cr-

doped samples show broad absorption bands in the visible

region.

The mechanism responsible for these bands is explained

in the literature in terms of crystal field splitting [11]. The

octahedral crystal field around the Cr3þ ion splits the 4F

level into a ground state orbital singlet 4A2 and excited state

orbital triplets 4T2 and 4T1. Also, due to the crystal field,

splitting of the free-ion excited state 2G gives rise to level2E. The broad absorption bands are due to transitions from

level 4A2 to the level 4T2 (band centred at 657 nm) and 4T1

(band centred at 484 nm). Transitions from the ground state4A2 to the level 2E lead to narrow absorption band

overlapping with the wing of the broad absorption band

due to transition from 4A2 to 2E (band centred at 725 nm).

Fig. 1. Transmission spectra for undoped and Cr-doped LiNbO3

crystals.

R. Bhatt et al. / Solid State Communications 127 (2003) 457–462458

The inset in Fig. 1 shows an expanded portion of the

visible region of Cr-doped crystals for the three doped

compositions. The presence of Cr3þ centres in the crystal is

reflected in the presence of broad absorption bands at 484,

657 and 725 nm in the transmission spectrum, and results in

a slight green colour for the material. These bands are

associated with the vibronic transitions of Cr3þ ions in low

crystal-field symmetry, corresponding to transitions from4A2 ground state to 4T1, 4T2 and 2E excited states,

respectively, [12].

It is observed that for the 0.1 mol% Cr-doped crystal, the

absorption peak at 484 nm is absent. On the other hand, for

both 0.25 and 0.5 mol% Cr-doped crystals, all the three

absorption peaks are present. The absorption is prominent in

the later case. However, when we compare the transmittance

of these three crystals, we see that the transmittance is about

60–65% for both 0.1 and 0.25 mol% Cr-doped crystals. But

the transmittance for 0.5 mol% Cr-doped crystal is only

about 50%. This deterioration in transmittance can affect the

performance of any optical device made out of the crystal.

The UV absorption edge is at about 321 nm for the

undoped crystal and increases towards the higher wave-

length side (red-shift) slightly with increasing Cr3þ ion

concentration. However, no shift was observed in the OH12

peak position at 2864 nm with Cr3þ doping.

Fig. 2 shows the effect of Cr doping on the absorption

coefficient of the crystal for different wavelengths. The

absorption coefficient increases with increasing Cr doping;

hence a decrease in transmittance is observed in the optical

transmission studies. The linear transmission in normal

incidence through a transparent material, assuming that no

interference occurs between reflections from front and back

surfaces, is given by

T ¼ ð1 2 RÞ2e2adð1 þ R2e22adÞ;

where a is the absorption coefficient, d the crystal thickness,

and R the normal-incidence reflectance. Here absorption

losses are assumed to include scattering as well as electronic

absorption. If the reflectivity is weak, then the R2e22ad term

can be neglected in the above equation, and it becomes

T < ð1 2 RÞ2e2ad : The values of a plotted in Fig. 2 were

calculated from this equation using transmittance and

absorbance data. The UV absorption edge was measured

at 20 cm21 and the values are summarised in Table 1.

The absorption coefficient has increased abnormally at

the band centred at 660 nm for the 0.5 mol% Cr-doped

sample. The optical absorption edge in LiNbO3 is decided

by the valence electron transition energy from 2p orbitals of

oxygen to 4d orbital of Nb. The red-shift mechanism may be

explained in terms of a further decrease in the Li/Nb ratio

from unity in crystals with Cr3þ doping, and the effect of the

excessive Nb ions placed at Li sites and interstitial positions,

or to the influence exerted by the local electric field set up by

lithium and oxygen vacancies. The UV absorption edge in

LN strongly depends on the Li/Nb ratio [13–16]. Since the

ionic radius of Cr atom ðrCr ¼ 0:68 �AÞ is similar to that of Li

ðrLi ¼ 0:68 �AÞ and Nb ðrNb ¼ 0:69 �AÞ atoms, in principle

the Cr impurity can replace both atoms, but it preferentially

replaces Li atoms. The congruently-melting-composition

LiNbO3 crystal has almost 6% of Li1þ sites occupied by

Nb5þ ions (referred as Nb antisites), and around 4.7% of

Nb5þ sites are vacant to ensure the charge neutrality [13].

Cr-doped crystals show decrease in transmittance with

increasing Cr ion concentration. This decrease in transmit-

tance can be explained in terms of increase in absorption

coefficient ðaÞ of the material with Cr doping (impurity).

The UV absorption depends on composition and impurity

concentration in the material.

Fig. 3 shows the dependence of absorption coefficient on

incident photon energy. The absorption coefficient slowly

increases with photon energy. The absorption bands centred

at 1.88 and 2.5 eV are due to the above-said vibronic

Fig. 2. Absorption spectra for undoped and Cr-doped LiNbO3

crystals.

Fig. 3. Absorption spectra vs. incident photon energy for undoped

and Cr-doped LiNbO3 crystals.

R. Bhatt et al. / Solid State Communications 127 (2003) 457–462 459

transitions. The gradual increase in absorption coefficient at

higher photon energies is due to interband electronic

transitions associated with bound electrons. The exponential

shape of the curves near the absorption edge is commonly

known as the Urbach tail [17]. This is a well-known feature

in ferroelectric materials like LiNbO3 [18]. At room

temperature it may be due to effect of micro fields arising

from charged point defects (Li and O deficiencies).

The interband electronic transitions near the funda-

mental absorption edges is given by a/ ðhn2 EgÞn;

where Eg is the band gap of the material and hn photon

energy. The exponent n may have different values like

non-integer 1/2 for direct band gap materials and integer

for indirect band gap materials. The absorption coefficient

for an indirect band gap material near the fundamental

absorption edge can be written as a/ ðhn2 Eindg ^ hVÞ2;

where hV is the photon energy associated with the

transition. The straight-line fitting near the absorption

edge in a1=2 vs. hn plot indicates indirect allowed

transitions, as shown in Fig. 4.

From Fig. 4 the optical band gap obtained for pure

lithium niobate sample is 3.79 eV, which is in close

agreement with values reported in the literature (measured

by different techniques). In the present studies, it was

observed that the band gap energies decrease with increas-

ing Cr concentration and are listed in Table 1.

2.3. Photoluminescence studies

Cr3þ ions in LiNbO3 exhibit broadband luminescence

when excited with visible light, and this observation has

stimulated many recent spectroscopic studies of Cr3þ:

LiNbO3 [19,20]. We performed PL studies on doped LN

samples. A c-cut polished sample of size 15 £ 15 £ 1 mm3

was prepared for this purpose. The excitation source

wavelength was chosen corresponding to the absorption

bands in the transmission spectra. He–Ne (632.8 nm) and

diode laser (532 nm) were used as excitation sources. The

sample was subjected to chop excitation at glancing angle,

and the PL signal emerging from the sample was focused on

a computer-controlled monochromator followed by a silicon

detector. The detector output was given to a lock-in

amplifier locked at the chopped frequency.

The same broadband PL emission in the range 700–

1100 nm was observed, with a slight red-shift in the peak

position, when excited by both the above-said sources.

However, the signal intensities were quite high in the case of

excitation with He – Ne laser, in keeping with the

pronounced absorption at this band.

The nature of the PL signal reveals that its range is

almost independent of the excitation wavelength, in

agreement with an earlier report [20].

Fig. 5 shows the single broadband photoluminescence

spectra for Cr-doped LiNbO3 crystals with different doping

concentrations. Broadband radiation from 700 to 1100 nm,

centred at 900 nm with 184 nm of FWHM, was observed in

0.5 mol% Cr doped crystal. The FWHM increases with

increasing Cr concentration (see Table 1). This PL emission

originates from the vibronic transitions from levels 4T2 and4T1 to 4A2. A 240 nm Raman shift is observed in the

excitation and emission bands. This shift is due to non-

radiative internal conversion associated with the vibronic

transitions. The photoluminescence in 0.5 mol% Cr-doped

crystals is almost double than those of 0.25 mol% Cr-doped

crystals. This is what is expected, as more Cr3þ ions are

present in the former case. However, when we see the

absorption characteristics of these crystals, particularly the

Table 1

The UV absorption edge for undoped and doped crystals. Corresponding bandgap is calculated

Crystal UV absorption edge

(nm) at 20 cm21

Absorption coefficient

(cm21) band-centred at

Band gap

(eV)

FWHM of PL

signal (nm)

Colour of crystal

658 nm 484 nm

LN 321 3.5 – 3.79 – Colourless

CLN0.1 355 5 – 3.21 178 Light green

CLN0.25 376 17 10.7 3.06 182 Mild green

CLN0.5 381 23 12.3 3.03 184 Green

Fig. 4. Square root of absorption coefficient against photon energy

for undoped and Cr-doped LiNbO3 crystals.

R. Bhatt et al. / Solid State Communications 127 (2003) 457–462460

transmittance in that spectral range, 0.25 mol% Cr doping is

sufficient to produce the desired photoluminescence. In

reflection geometry, the transmittance of the host crystal

(medium) does not affect the gain of the laser very much.

Therefore, 0.5 mol% Cr doped LiNbO3 crystals should

result in higher gain in this geometry. Also, for waveguide

amplifier geometry this doping level would give higher gain.

On the other hand, for transmission geometry 0.25 mol% Cr

doped crystals are most suitable. So, depending upon the

application one can choose the appropriate doping level in

the crystal, compromising between the transmittance and

the laser gain.

Depending upon whether the level 2E lies below or

above 4T2, the luminescence spectrum shows sharp R

lines or a broad luminescence band. In the case of

LiNbO3, these levels are almost degenerate and the

luminescence spectrum shows both features. The broad

luminescence band is due to the transition from excited

level 4T2 to ground state 4A2. Transitions from excited

level 2E to ground state 4A2 give rise to sharp R lines

near 725 nm. These lines are seen only at low

temperatures because the transition from level 4T2 is

vibronically broadened [19]. These sharp R lines

(2E ! 4A2 transitions) point to the important feature

that in LiNbO3 there are three distinct Cr3þ environ-

ments, producing three distinct R line systems.

This broadband indicates that a tunable laser in the 700–

1100 nm spectral range can be made from Cr3þ:LiNbO3

crystals. However, LiNbO3 crystals have low optical

damage threshold on the lower wavelength side required

for excitation. To overcome this problem of optical damage,

MgO (4.5–7 mol%) is added to these crystals. This

MgO:Cr3þ:LiNbO3 crystal can be exploited for making a

broadband tunable laser in the 700–1100 nm spectral range.

Further studies on Cr doping in bulk crystals for the

realisation of laser amplification are in progress.

3. Conclusions

Fairly large sized undoped and Cr-doped LiNbO3

crystals were grown successfully. Transmittance better

than 70% was obtained for the undoped crystals. The

decrease in transmittance of doped crystals is due to Cr-

impurity dependent absorption in the material. UV cut-off

increases with increasing Cr concentration. The band gap

decreases with increasing Cr concentration. The red-shift in

UV absorption can be explained in terms of overall decrease

in the Li/Nb ratio, or, in other words, an increase in extrinsic

defects of the material with Cr doping.

Broadband luminescence spectra in the 700–1100 nm

range, with a peak around 900 nm, was obtained for both

0.25 and 0.5 mol% Cr:LiNbO3 crystals. PL signal range was

found to be excitation-independent, albeit with a red-shift in

the peak position. The PL signal is Raman-shifted by

240 nm from the excitation absorption band. It is concluded

from the present study that the 0.25 mol% Cr-doping is

optimum: it gives all the three absorption peaks responsible

for luminescence, without affecting much the transmittance

of the crystal. The doping levels of 0.25 and 0.5 mol% Cr

can be used for bulk doping to produce laser amplification

depending upon the geometry.

Acknowledgements

The authors are thankful to Mr Sanjay Porwal and Mr

Ravi Kumar of Laser Physics Division for the PL

measurements.

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