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Optical behaviour of VTE treated near stoichiometric LiNbO3 crystals
Solid State Communications 137 (2006) 283–287 www.elsevier.com/locate/ssc
Optical behaviour of VTE treated near stoichiometric LiNbO3 crystals S. Kar a, R. Bhatt a, V. Shukla b, R.K. Choubey c, P. Sen c, K.S. Bartwal a,* a
Laser Materials Development and Devices Division, Centre for Advanced Technology, Indore 452013, India b Laser Physics Application Section, Centre for Advanced Technology, Indore 452013, India c Department of Applied Physics, SGSITS, Indore 452003, India Received 13 October 2005; accepted 19 November 2005 by C.N.R. Rao Available online 22 December 2005
Abstract Near stoichiometric LiNbO3 crystal wafers of thickness up to 2 mm were prepared by vapour transport equilibration technique (VTE) at various process temperatures. Crystals were characterised by measurement of the UV absorption edge, refractive index, second harmonic generation (SHG) efficiency, and conoscopy pattern analysis. The comparison of VTE treated crystals show that the blue shift in cut off wavelength occurred with the increasing process temperature (i.e. increasing Li/Nb ratio). The refractive indices were found decreasing with increasing process temperature of VTE samples. The SHG efficiency increases in the range of 1.98–2.3 times for the VTE processed samples with respected to congruent crystals. Conoscopy pattern reveals the optical homogeneity of the VTE treated crystal. q 2005 Elsevier Ltd. All rights reserved. PACS: 81.10Fq; 81.10.h Keywords: A: Lithium niobate; D: VTE, powder SHG; E: Czochralski; E: UV cut off
1. Introduction LiNbO3 crystal is a versatile material that can be used as optical switches, optical modulators, frequency conversion etc. Commercially available LiNbO3 crystals are usually grown from congruent melt (Li/NbZ48.6/51.4) with the conventional Czochralski technique. However, due to Li deficiency, there are many intrinsic defects in congruent LiNbO3 crystal (CLN). It is well known that many properties of LiNbO3 crystal are related with Li/Nb ratio in the crystal. With the increase in Li/Nb ratio in the LiNbO3 crystal many physical properties of the crystal changes such as near stoichiometric LiNbO3 (SLN) crystal shows large electro-optical coefficient [1], shorter absorption edge [2] and lower electric field required for 1808 ferroelectric domain switching [3] compared to congruent LiNbO3 crystal. The incongruent property of SLN makes the growth of SLN crystal difficult by conventional Czochralski technique. Kitamura et al. [4] used the double crucible Czochralski method with a continuous charging system to grow near SLN from the Li rich melt (58–60 mol% Li2O). By * Corresponding author. Tel.: C91 731 2488656; fax: C91 731 2488650. E-mail address: [email protected] (K.S. Bartwal).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.11.042
introducing different fluxes into melt, SLN or near SLN crystals have been grown by different authors. Malovichiko et al. [5] has grown near SLN by adding 6 wt% K2O into the CLN melt. The SLN crystals could also be grown by the top seeded solution growth (TSSG) method by adding K2O into the SLN melt as a flux [6]. Besides the above mentioned growth methods, another method to obtain SLN crystal sample is to convert CLN into SLN by vapour transport equilibration (VTE) process, which has been first reported by Holman et al. [7]. Bordui et al. [8] studied the phase boundary for Li poor; Li rich and single phase raw materials. Some other authors also reported on obtaining SLN by the VTE method [9,10]. Vapour transport equilibration (VTE) process at high temperature is promising, and applied methods for producing near stoichiometric crystals. The aim of this work is to optimise the parameters for preparation of near stoichiometric LiNbO3 wafer by vapour transport equilibration (VTE). To measure their various stoichiometry sensitive properties such as UV absorption edge, homogeneity of the crystal, refractive index, and SHG efficiency. 2. Growth and preparation of near stoichiometric LiNbO3 wafers by VTE LiNbO3 single crystal was grown in air by Czochralski technique from congruently melt composition having 48.6 mol%
284
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Li2O. This material is known to have a fairly large solid solution range from 42 to 52 mol% Li2O. The platinum crucible was used for the growth of crystals surrounded by zirconia granules. The temperature of the system was monitored by a thermocouple attached to the bottom of the crucible. The [001] oriented seed of dimension 2.5!2.5!20 mm3 was used for crystal growth. Pulling rate was in the range 2–4 mm/h in the body part of the boule and the rotation rate was kept at 12–25 rpm. The post growth cooling was maintained at 20–30 8C/h initially up to 1000 8C and thereafter fast cooling to room temperature [11]. VTE is a technique for preparing LiNbO3 crystals of desired composition with in the solid solution phase [7]. VTE consists of annealing crystal samples in close proximity to a relatively much larger mass of LiNbO3 powder of a desired composition. Given sufficient time at sufficiently high temperature, the Li/Nb ratio in the crystal equilibrates to that in the powder via a mechanism involving vapour transport and solid-state diffusion [8]. Crystal plates of c-direction and thickness 1 mm were cut by diamond wheel from the as grown crystal. These plates were polished properly by alumina powder of size 0.03 mm. High purity (99.99%) Li2CO3 and Nb2O5 powder were used to prepare mixture of lithium rich powder. Li3NbO4 and LiNbO3 powder was prepared separately by solid-state reaction at 950 8C for 24 h. These two powders were mixed in 1:1 ratio and 400 g of the above mixture was taken in platinum crucible of diameter 80 mm and height 80 and 1 mm thickness. Lithium niobate crystal of 10!10!1 mm3 was placed on platinum wire mesh inside the covered platinum crucible to maintain lithium vapour pressure. Crucible was then kept inside a resistive heating furnace. The crucible was heated to various temperatures between 1025 and 1125 8C for 100 h. VTE treated samples were than taken out and polished properly for further characterisation. 3. Characterisations 3.1. UV cut off studies The absorption studies and UV absorption edges of the VTE treated samples was carried out using Shimadzu UV 3101 PC.
Process temperature (8C)
Refractive index
UV abs. edge (nm)
CLN 1025 1050 1075 1100
2.2004 2.1861 2.1678 2.1530 2.1468
317.6 313.8 312.5 311.0 308.2
Fig. 1 shows the UV cut off of VTE treated LiNbO3 crystals. The blue shift in cut off wavelength was clearly observed in VTE treated LiNbO3 crystals and it is increasing with increasing process temperature. The UV cut off results are shown in Table 1. UV absorption edge is very sensitive indicator of the composition of LiNbO3 samples [12]. The blue shift in absorption edge is a signature of increase in Li/Nb ratio of the crystals or in other words the crystal approaches to near stoichiometric composition [13,14]. The position dependent UV cut off measurement across the crystal wafer show no variation in UV cut off, which is indicative of composition homogeneity of the VTE treated crystals. 3.2. Refractive index The linear optical properties of congruent as well as VTE treated LiNbO3 crystals were examined by Brewster’s angles at 632 nm radiation wavelength. The results are shown in Table 1. He–Ne laser at wavelengths 632 nm was used as the fundamental beam. The Glan–Taylor polarizer (P) was used to set the polarization state. A rotating table with the least count of 0.18 was used for the measurements. The polarized beam is incident on the test crystal (TC) and is reflected. The intensity of reflected beam is measured on photo diode detector. The Brewster’s angles were measured by polynomial fitting the reflected intensities data. The refractive indices were found decreasing with increasing process temperature of the VTE treated LiNbO3 crystals and are shown in the Fig. 2.
1100 1075 1050 1025 CLN
2.22
Measured for HeNe laser 632nm 2.20
Refractive Index
6 Absorbance (arb.unit)
Table 1 Refractive indices and UV absorption edge of congruent and VTE treated samples
4
2.18
CLN
2.16 2.14
near SLN
2 2.12 2.10
300
310
320 330 Wavelength (nm)
340
350
Fig. 1. UV absorption edge of VTE treated LN samples.
0 251000
1025
1050
1075
1100
1125
VTE processing temperature (˚C) Fig. 2. Variation of refractive index with process temperature.
S. Kar et al. / Solid State Communications 137 (2006) 283–287
3.3. Second harmonic generation (SHG) measurements
L
BS
S
L
M
SHG with respect to KDP
44
40
36
32 5
10 15 20 Fundamental beam energy (mJ)
25
Fig. 4. Powder SHG signal of VTE treated sample wrt KDP.
SHG signal to fundamental signal for congruent as well as VTE treated sample was plotted against average grain size. The results are shown in Fig. 5. 3.4. Conoscopy studies In order to study homogeneity or uniformity of the Li-in diffusion in the VTE modified near stoichiometric wafers, conoscopic study were performed. The Olympus polarising microscope was used for the study. The conoscopy is commonly used for detection of the orientation of optic axes, optical homogeneity and birefringence of the crystal qualitatively. Conoscopic interference fringes are generated when crystals are viewed between two crossed polarisers with a converging light. The light passing through the crystal consisting of ordinary ray (o-ray) and extraordinary ray (e-ray) with refractive indices depending on the orientation of the crystal. It is basically the components of these two rays, which interferes and produce the characteristic fringe pattern. The shape and symmetry of this pattern reveals the information about homogeneity and strain in the crystal. Conoscopy pattern of the VTE treated sample is shown in Fig. 6. The symmetrical fringe patterns for VTE modified near SLN crystals confirm
1000 SHG Signal (arb. unit)
Since the samples used for VTE analysis were not cut along the phase matched directions, hence in order to study the effect of Li-indiffusion on non-linear optical properties, VTE treated samples were subjected to Kurtz powder second harmonic generation studies. The powder samples were prepared in the range of particle size below 90, 90–210 mm and above 210 mm sizes. To make the relevant comparisons with known SHG material, KDP was also ground and sieved into the same particle size ranges. The experimental set up for Kurtz powder technique is shown in Fig. 3. All the particles were placed in a 5 mm hole made in a 0.5 mm thick aluminium foil and was held tightly between two microscopic slides of dimensions 25 mm wide, 75 mm long and 1.3 mm thickness. Care was taken to tightly pack the material in the aluminium foil. This slide was held in the sample holder. A Q-switched 1064 nm nanosecond pulsed Nd:YAG laser was used as the fundamental beam. The incident beam was split into 98:2 ratios. The 98% fundamental was used to irradiate the test sample while 2% intensity of the fundamental was used as a reference signal. The transmitted beam was collected using a lens L. and passed through a monochromator, used as a harmonic separator and then the SH signal was detected by using photo multiplier tube (PMT). The fundamental beam transmitted through the sample was attenuated using filters. The monochromater is set for 532 nm wavelength with a slit opening of 0.4 mm. The photo multiplier output was connected to the two channel digital oscilloscope. The biasing voltage of the PMT was chosen to be 200 V. At this biasing voltage the PMT response was linear. The SHG signal was measured for the sample at various fundamental beam energies. The fundamental beam signal, from the beam splitter was also monitored simultaneously with the SHG signal on the oscilloscope. The same procedure was repeated for the other grain sizes. The fundamental beam signal and the SHG signal were also similarly measured for the pure LN sample at different fundamental beam energies and different grain sizes. The SHG signal was measured for the sample at various fundamental beam energies and different grain sizes. The measurement technique is to study the amount of SHG from the powder sample as a function of particle size in the sample. The required information is obtained from the measurement of SHG power versus particle size, which is compared, for an identical geometry, with the results determined for KDP powder SHG measurements (shown in Fig. 4). The ratio of
285
VTE Near SLN
800 600 400
Congruent LiNbO3
200 PD
LENS CRO
PMT
Fig. 3. Experimental set up for Kurtz method (L, lens; BS, beam splitter; S, sample; M, mirror; PMT, photo multiplier tube; CRO, cathode ray oscilloscope; PD, photodiode).
40
80
120
160
200
Size ( micron) Fig. 5. Powder SHG signal of congruent and VTE treated samples.
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that the crystals are strain free and homogenous in composition.
the band gap of the material, which increases with increasing the process temperature. We have also observed decrease in density with increasing VTE process temperature in the density measurements by Archimedes principle. This further confirms the decrease in the antisite defects NbLi and thereby increasing the Li/Nb ratio in the crystals. This is in good agreement with the results of Learner et al. [16], where they have reported that the density of LiNbO3 crystals increases when the ratio of Li/Nb decreases. The linear optical properties are investigated by measuring the average refractive index of the VTE modified samples using 632 nm laser. The results show that the refractive index was decreasing with increasing the VTE processing temperature. This decrease in refractive index is expected due to decrease in density and blue shift in absorption edges in the near stoichiometric LiNbO3 crystals. This can also be explained on the basis of chemical bonds and their polarizability in the structure. The electro-negativity of Li, Nb, and O atoms are 1.0, 1.2, and 3.5, respectively. According to Pauling the fraction of ionic character of an AB bond is given by
4. Discussion
FðabÞ Z 1Kexp
Transparent, inclusion and crack free LiNbO3 single crystals were grown by the Czochralski technique. X-ray powder diffraction analysis confirmed the desired phase. Fig. 1 shows the absorbance and UV cut off of VTE treated LiNbO3 crystals. The absorption edges were measured at absorbance of 4 or at 2% transmittance. The absorption edges decreases from 317.8 for CLN to 308 for VTE modified samples, details are given in the Table 1. The blue shift in absorption edges can be clearly seen, indicating the improvement in the stoichiometry of the VTE treated crystal wafer. Further, the UV shift is increasing with increasing the VTE processing temperature. This is in good agreement to the reported literatures [12]. The Nb-antisite defects are mainly responsible for this blue shift. The Li ions are highly mobile at such high temperatures, and hence the diffusion kinetics is very high for Li-ions compared to heavier Nb-ions. Since the CLN has a Li defect of about 4% and about 1% of the Nb ions located at Li sites [15], making Li diffusion thermodynamically more favourable process. Therefore, a sample placed in Li-rich environment at sufficient high temperature, becomes Li rich by replacing the pre occupied Nb antisite at Li sites to their regular position and filling corresponding Li vacancies. This decrease in Li vacancies increases the overall Li concentration in the crystal and thereby increase in Li/Nb ratio of the crystal. In other words, intrinsic defects have been checked by VTE modification. Since many optical properties like absorption edge, refractive index, NLO coefficients are highly Li/Nb ratio dependent [12] and remarkable changes has been observed in these properties after VTE modification. The significant blue shift in UV edge towards shorter wavelength confirms the increase in Li/Nb ratio of the crystal making it to near stoichiometric composition from congruent composition. This also modifies
where Xa and Xb are electro-negativities of a and b atom, respectively, [17]. According to this the Li–O bond is more ionic than Nb–O bond and thus the electron in the Li–O bonds are tightly bound to oxygen atom and weekly polarized. Hence, the Li–O bonds are more ionic in character compared Nb–O bond, which is covalent in nature and relatively more polarisable [18–21]. The refractive index of the materials is determined by the response of the materials to the applied optical field in terms of dielectric bond polarizability. Therefore, the Nb–O bond in the LiNbO3 structure mainly governs the linear optical properties and the contribution of Li–O bonds to optical properties is small compare to Nb–O bonds. In VTE treated sample number of Nb–O bonds are decreasing due to decrease in NbLi antisite defects and accordingly the number of Li–O bonds are increasing. In other words, the unit cell is becoming more ionic in character and weakly polarisable. Leading to decrease in linear optical properties and hence, the refractive index of the VTE modified near SLN crystals. The expected decreases in the refractive index have been observed in the VTE treated samples as shown in Fig. 2. The angle was measured with accuracy of the order of 0.18 at Brewster angle, leading to a percentage error of 0.45% in the refractive index measurement as shown in the Fig. 2. The non-linear optical properties of the VTE modified crystals were investigated by Kurtz powder SHG efficiency technique [22]. Fig. 5 shows the enhancement in SHG efficiency of the VTE treated sample with respected to congruent crystal. Significant improvement in SHG efficiency in the range of 1.98–2.3 was observed in the VTE treated sample. Whereas, in Fig. 4 the congruent crystals are showing around 40 times higher SHG efficiency then the reference KDP crystals. Since the SHG efficiency is inversely proportional to
Fig. 6. Conoscopy pattern of VTE treated sample.
KðXa KXb Þ2 4
S. Kar et al. / Solid State Communications 137 (2006) 283–287
the third power of the refractive index, leading to increase in the SHG efficiencies of VTE modified samples. This is in good agreement to the Jeggo et al. [23] findings, where they have reported that the SHG efficiency is a function of refractive index and refractive index is a function of number of bonds per unit volume. At room temperature non-linear optical properties of lithium niobate is proportional to the number of Li–O and Nb–O bonds. The NLO behaviour of lithium niobate is dominated by NbO6 octahedra. As per the Jeggo et al. [23], that SHG efficiency is proportional to number of bonds per unit volume. In VTE treated samples, the Li–O bond density increases due to decrease in the intrinsic defects as replacing one NbLi by Li atom leads to occupying four Li vacancies by Li ions. In other words, the number of bonds per unit volume increases and therefore NLO efficiencies as well as NLO coefficient enhances in VTE treated near stoichiometric sample.
5. Conclusion Transparent, crack free congruent LiNbO3 crystal were grown. VTE treated LiNbO3 crystal wafer of thickness 1 and 2 mm at various process temperature were prepared. These wafers are characterise by UV absorption edge, refractive index, density, SHG efficiency analysis. We found blue shift in absorption edge with increasing process temperature of VTE treated LiNbO3 crystals and the maximum blue shift in UV spectra occurs at 1100 8C. The blue shift in absorption edge shows the crystal approaches to near stoichiometric composition. The NLO efficiency as well as coefficient were found enhanced in VTE treated sample.
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References [1] F. Abdi, M. Aillerie, P. Bourson, M.D. Fontana, K. Polgar, J. Appl. Phys. 84 (1998) 2251. [2] D.L. Sun, Y. Hang, L.H. Zhang, J. Synth. Cryst. 31 (2002) 314. [3] A. Grisard, E. Lallier, K. Polgar, A. Peter, Electron Lett. 36 (2000) 1043. [4] K. Kitamura, J.K. Yamamot, N. Iyi, S. Kimura, T. Hayashi, J. Cryst. Growth 116 (1992) 327. [5] G. Malovichiko, V. Grachev, O. Schirmer, Appl. Phys. B 68 (1999) 785. [6] Y. Furukawa, K. Kitamura, S. Takehkawa, K. Niwa, Y. Yajima, N. Iyi, I. Mnushkina, P. Guggenheim, J.M. Martin, J. Cryst. Growth 211 (2000) 230. [7] R.L. Holman, P.J. Cressmann, J.F. Revelli, Appl. Phys. Lett. 32 (1978) 280. [8] P.F. Bordui, R.G. Norwood, D.H. Jundt, M.M. Fejer, J. Appl. Phys. 71 (1992) 875. [9] N. Cheng, W.X. Hua, X.Q. Feng, H.R. Tan, Mater Sci. Eng. B33 (1995) 91. [10] Y.L. Chen, J.P. Wen, Y.F. Kong, S.L. Chen, W.L. Zhang, J.J. Xu, G.Y. Zhang, J. Cryst. Growth 242 (2002) 400. [11] R.V.A. Murthy, K.S. Bartwal, K. Lal, Mater. Sci. Eng. B18 (1993) L4. [12] L. Kovacs, G. Ruschhapt, K. Polgar, G. Corradi, M. Wohlecke, Appl. Phys. Lett. 70 (21) (1997) 2801. [13] P. Sen, P.K. Sen, R. Bhatt, S. Kar, V. Shukla, K.S. Bartwal, Solid State Commun. 129 (11) (2004) 747. [14] R.K. Choubey, P. Sen, P.K. Sen, R. Bhatt, S. Kar, V. Shukla, K.S. Bartwal, Opt. Mater. (in press). [15] D. Berber, K. Buse, S. Wevering, J. Appl. Phys. 87 (2000) 1034. [16] P. Lerner, C. Legras, J.P. Dumas, J. Cryst. Growth 3 (1968) 231. [17] J.C. Philips, Rev. Mod. Phys. 42 (3) (1970) 317. [18] B.F. Levine, Phys. Rev. B 7 (6) (1973) 2591. [19] I.V. Kityk, M.M. Janusik, M.D. Fontana, M. Alleric, F. Abdi, Cryst. Res. Technol. 36 (2001) 577. [20] I.V. Kityk, M.M. Janusik, M.D. Fontana, M. Alleric, F. Abdi, J. Appl. Phys. 90 (2001) 5542. [21] P. Sen, N. Sisodia, K.S. Bartwal, Opt. Mater. (in press). [22] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798. [23] C.R. Jeggo, G.D. Boyd, J. Appl. Phys. 41 (1970) 2741.
Optical behaviour of VTE treated near stoichiometric LiNbO3 crystals S. Kar a, R. Bhatt a, V. Shukla b, R.K. Choubey c, P. Sen c, K.S. Bartwal a,* a
Laser Materials Development and Devices Division, Centre for Advanced Technology, Indore 452013, India b Laser Physics Application Section, Centre for Advanced Technology, Indore 452013, India c Department of Applied Physics, SGSITS, Indore 452003, India Received 13 October 2005; accepted 19 November 2005 by C.N.R. Rao Available online 22 December 2005
Abstract Near stoichiometric LiNbO3 crystal wafers of thickness up to 2 mm were prepared by vapour transport equilibration technique (VTE) at various process temperatures. Crystals were characterised by measurement of the UV absorption edge, refractive index, second harmonic generation (SHG) efficiency, and conoscopy pattern analysis. The comparison of VTE treated crystals show that the blue shift in cut off wavelength occurred with the increasing process temperature (i.e. increasing Li/Nb ratio). The refractive indices were found decreasing with increasing process temperature of VTE samples. The SHG efficiency increases in the range of 1.98–2.3 times for the VTE processed samples with respected to congruent crystals. Conoscopy pattern reveals the optical homogeneity of the VTE treated crystal. q 2005 Elsevier Ltd. All rights reserved. PACS: 81.10Fq; 81.10.h Keywords: A: Lithium niobate; D: VTE, powder SHG; E: Czochralski; E: UV cut off
1. Introduction LiNbO3 crystal is a versatile material that can be used as optical switches, optical modulators, frequency conversion etc. Commercially available LiNbO3 crystals are usually grown from congruent melt (Li/NbZ48.6/51.4) with the conventional Czochralski technique. However, due to Li deficiency, there are many intrinsic defects in congruent LiNbO3 crystal (CLN). It is well known that many properties of LiNbO3 crystal are related with Li/Nb ratio in the crystal. With the increase in Li/Nb ratio in the LiNbO3 crystal many physical properties of the crystal changes such as near stoichiometric LiNbO3 (SLN) crystal shows large electro-optical coefficient [1], shorter absorption edge [2] and lower electric field required for 1808 ferroelectric domain switching [3] compared to congruent LiNbO3 crystal. The incongruent property of SLN makes the growth of SLN crystal difficult by conventional Czochralski technique. Kitamura et al. [4] used the double crucible Czochralski method with a continuous charging system to grow near SLN from the Li rich melt (58–60 mol% Li2O). By * Corresponding author. Tel.: C91 731 2488656; fax: C91 731 2488650. E-mail address: [email protected] (K.S. Bartwal).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.11.042
introducing different fluxes into melt, SLN or near SLN crystals have been grown by different authors. Malovichiko et al. [5] has grown near SLN by adding 6 wt% K2O into the CLN melt. The SLN crystals could also be grown by the top seeded solution growth (TSSG) method by adding K2O into the SLN melt as a flux [6]. Besides the above mentioned growth methods, another method to obtain SLN crystal sample is to convert CLN into SLN by vapour transport equilibration (VTE) process, which has been first reported by Holman et al. [7]. Bordui et al. [8] studied the phase boundary for Li poor; Li rich and single phase raw materials. Some other authors also reported on obtaining SLN by the VTE method [9,10]. Vapour transport equilibration (VTE) process at high temperature is promising, and applied methods for producing near stoichiometric crystals. The aim of this work is to optimise the parameters for preparation of near stoichiometric LiNbO3 wafer by vapour transport equilibration (VTE). To measure their various stoichiometry sensitive properties such as UV absorption edge, homogeneity of the crystal, refractive index, and SHG efficiency. 2. Growth and preparation of near stoichiometric LiNbO3 wafers by VTE LiNbO3 single crystal was grown in air by Czochralski technique from congruently melt composition having 48.6 mol%
284
S. Kar et al. / Solid State Communications 137 (2006) 283–287
Li2O. This material is known to have a fairly large solid solution range from 42 to 52 mol% Li2O. The platinum crucible was used for the growth of crystals surrounded by zirconia granules. The temperature of the system was monitored by a thermocouple attached to the bottom of the crucible. The [001] oriented seed of dimension 2.5!2.5!20 mm3 was used for crystal growth. Pulling rate was in the range 2–4 mm/h in the body part of the boule and the rotation rate was kept at 12–25 rpm. The post growth cooling was maintained at 20–30 8C/h initially up to 1000 8C and thereafter fast cooling to room temperature [11]. VTE is a technique for preparing LiNbO3 crystals of desired composition with in the solid solution phase [7]. VTE consists of annealing crystal samples in close proximity to a relatively much larger mass of LiNbO3 powder of a desired composition. Given sufficient time at sufficiently high temperature, the Li/Nb ratio in the crystal equilibrates to that in the powder via a mechanism involving vapour transport and solid-state diffusion [8]. Crystal plates of c-direction and thickness 1 mm were cut by diamond wheel from the as grown crystal. These plates were polished properly by alumina powder of size 0.03 mm. High purity (99.99%) Li2CO3 and Nb2O5 powder were used to prepare mixture of lithium rich powder. Li3NbO4 and LiNbO3 powder was prepared separately by solid-state reaction at 950 8C for 24 h. These two powders were mixed in 1:1 ratio and 400 g of the above mixture was taken in platinum crucible of diameter 80 mm and height 80 and 1 mm thickness. Lithium niobate crystal of 10!10!1 mm3 was placed on platinum wire mesh inside the covered platinum crucible to maintain lithium vapour pressure. Crucible was then kept inside a resistive heating furnace. The crucible was heated to various temperatures between 1025 and 1125 8C for 100 h. VTE treated samples were than taken out and polished properly for further characterisation. 3. Characterisations 3.1. UV cut off studies The absorption studies and UV absorption edges of the VTE treated samples was carried out using Shimadzu UV 3101 PC.
Process temperature (8C)
Refractive index
UV abs. edge (nm)
CLN 1025 1050 1075 1100
2.2004 2.1861 2.1678 2.1530 2.1468
317.6 313.8 312.5 311.0 308.2
Fig. 1 shows the UV cut off of VTE treated LiNbO3 crystals. The blue shift in cut off wavelength was clearly observed in VTE treated LiNbO3 crystals and it is increasing with increasing process temperature. The UV cut off results are shown in Table 1. UV absorption edge is very sensitive indicator of the composition of LiNbO3 samples [12]. The blue shift in absorption edge is a signature of increase in Li/Nb ratio of the crystals or in other words the crystal approaches to near stoichiometric composition [13,14]. The position dependent UV cut off measurement across the crystal wafer show no variation in UV cut off, which is indicative of composition homogeneity of the VTE treated crystals. 3.2. Refractive index The linear optical properties of congruent as well as VTE treated LiNbO3 crystals were examined by Brewster’s angles at 632 nm radiation wavelength. The results are shown in Table 1. He–Ne laser at wavelengths 632 nm was used as the fundamental beam. The Glan–Taylor polarizer (P) was used to set the polarization state. A rotating table with the least count of 0.18 was used for the measurements. The polarized beam is incident on the test crystal (TC) and is reflected. The intensity of reflected beam is measured on photo diode detector. The Brewster’s angles were measured by polynomial fitting the reflected intensities data. The refractive indices were found decreasing with increasing process temperature of the VTE treated LiNbO3 crystals and are shown in the Fig. 2.
1100 1075 1050 1025 CLN
2.22
Measured for HeNe laser 632nm 2.20
Refractive Index
6 Absorbance (arb.unit)
Table 1 Refractive indices and UV absorption edge of congruent and VTE treated samples
4
2.18
CLN
2.16 2.14
near SLN
2 2.12 2.10
300
310
320 330 Wavelength (nm)
340
350
Fig. 1. UV absorption edge of VTE treated LN samples.
0 251000
1025
1050
1075
1100
1125
VTE processing temperature (˚C) Fig. 2. Variation of refractive index with process temperature.
S. Kar et al. / Solid State Communications 137 (2006) 283–287
3.3. Second harmonic generation (SHG) measurements
L
BS
S
L
M
SHG with respect to KDP
44
40
36
32 5
10 15 20 Fundamental beam energy (mJ)
25
Fig. 4. Powder SHG signal of VTE treated sample wrt KDP.
SHG signal to fundamental signal for congruent as well as VTE treated sample was plotted against average grain size. The results are shown in Fig. 5. 3.4. Conoscopy studies In order to study homogeneity or uniformity of the Li-in diffusion in the VTE modified near stoichiometric wafers, conoscopic study were performed. The Olympus polarising microscope was used for the study. The conoscopy is commonly used for detection of the orientation of optic axes, optical homogeneity and birefringence of the crystal qualitatively. Conoscopic interference fringes are generated when crystals are viewed between two crossed polarisers with a converging light. The light passing through the crystal consisting of ordinary ray (o-ray) and extraordinary ray (e-ray) with refractive indices depending on the orientation of the crystal. It is basically the components of these two rays, which interferes and produce the characteristic fringe pattern. The shape and symmetry of this pattern reveals the information about homogeneity and strain in the crystal. Conoscopy pattern of the VTE treated sample is shown in Fig. 6. The symmetrical fringe patterns for VTE modified near SLN crystals confirm
1000 SHG Signal (arb. unit)
Since the samples used for VTE analysis were not cut along the phase matched directions, hence in order to study the effect of Li-indiffusion on non-linear optical properties, VTE treated samples were subjected to Kurtz powder second harmonic generation studies. The powder samples were prepared in the range of particle size below 90, 90–210 mm and above 210 mm sizes. To make the relevant comparisons with known SHG material, KDP was also ground and sieved into the same particle size ranges. The experimental set up for Kurtz powder technique is shown in Fig. 3. All the particles were placed in a 5 mm hole made in a 0.5 mm thick aluminium foil and was held tightly between two microscopic slides of dimensions 25 mm wide, 75 mm long and 1.3 mm thickness. Care was taken to tightly pack the material in the aluminium foil. This slide was held in the sample holder. A Q-switched 1064 nm nanosecond pulsed Nd:YAG laser was used as the fundamental beam. The incident beam was split into 98:2 ratios. The 98% fundamental was used to irradiate the test sample while 2% intensity of the fundamental was used as a reference signal. The transmitted beam was collected using a lens L. and passed through a monochromator, used as a harmonic separator and then the SH signal was detected by using photo multiplier tube (PMT). The fundamental beam transmitted through the sample was attenuated using filters. The monochromater is set for 532 nm wavelength with a slit opening of 0.4 mm. The photo multiplier output was connected to the two channel digital oscilloscope. The biasing voltage of the PMT was chosen to be 200 V. At this biasing voltage the PMT response was linear. The SHG signal was measured for the sample at various fundamental beam energies. The fundamental beam signal, from the beam splitter was also monitored simultaneously with the SHG signal on the oscilloscope. The same procedure was repeated for the other grain sizes. The fundamental beam signal and the SHG signal were also similarly measured for the pure LN sample at different fundamental beam energies and different grain sizes. The SHG signal was measured for the sample at various fundamental beam energies and different grain sizes. The measurement technique is to study the amount of SHG from the powder sample as a function of particle size in the sample. The required information is obtained from the measurement of SHG power versus particle size, which is compared, for an identical geometry, with the results determined for KDP powder SHG measurements (shown in Fig. 4). The ratio of
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VTE Near SLN
800 600 400
Congruent LiNbO3
200 PD
LENS CRO
PMT
Fig. 3. Experimental set up for Kurtz method (L, lens; BS, beam splitter; S, sample; M, mirror; PMT, photo multiplier tube; CRO, cathode ray oscilloscope; PD, photodiode).
40
80
120
160
200
Size ( micron) Fig. 5. Powder SHG signal of congruent and VTE treated samples.
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that the crystals are strain free and homogenous in composition.
the band gap of the material, which increases with increasing the process temperature. We have also observed decrease in density with increasing VTE process temperature in the density measurements by Archimedes principle. This further confirms the decrease in the antisite defects NbLi and thereby increasing the Li/Nb ratio in the crystals. This is in good agreement with the results of Learner et al. [16], where they have reported that the density of LiNbO3 crystals increases when the ratio of Li/Nb decreases. The linear optical properties are investigated by measuring the average refractive index of the VTE modified samples using 632 nm laser. The results show that the refractive index was decreasing with increasing the VTE processing temperature. This decrease in refractive index is expected due to decrease in density and blue shift in absorption edges in the near stoichiometric LiNbO3 crystals. This can also be explained on the basis of chemical bonds and their polarizability in the structure. The electro-negativity of Li, Nb, and O atoms are 1.0, 1.2, and 3.5, respectively. According to Pauling the fraction of ionic character of an AB bond is given by
4. Discussion
FðabÞ Z 1Kexp
Transparent, inclusion and crack free LiNbO3 single crystals were grown by the Czochralski technique. X-ray powder diffraction analysis confirmed the desired phase. Fig. 1 shows the absorbance and UV cut off of VTE treated LiNbO3 crystals. The absorption edges were measured at absorbance of 4 or at 2% transmittance. The absorption edges decreases from 317.8 for CLN to 308 for VTE modified samples, details are given in the Table 1. The blue shift in absorption edges can be clearly seen, indicating the improvement in the stoichiometry of the VTE treated crystal wafer. Further, the UV shift is increasing with increasing the VTE processing temperature. This is in good agreement to the reported literatures [12]. The Nb-antisite defects are mainly responsible for this blue shift. The Li ions are highly mobile at such high temperatures, and hence the diffusion kinetics is very high for Li-ions compared to heavier Nb-ions. Since the CLN has a Li defect of about 4% and about 1% of the Nb ions located at Li sites [15], making Li diffusion thermodynamically more favourable process. Therefore, a sample placed in Li-rich environment at sufficient high temperature, becomes Li rich by replacing the pre occupied Nb antisite at Li sites to their regular position and filling corresponding Li vacancies. This decrease in Li vacancies increases the overall Li concentration in the crystal and thereby increase in Li/Nb ratio of the crystal. In other words, intrinsic defects have been checked by VTE modification. Since many optical properties like absorption edge, refractive index, NLO coefficients are highly Li/Nb ratio dependent [12] and remarkable changes has been observed in these properties after VTE modification. The significant blue shift in UV edge towards shorter wavelength confirms the increase in Li/Nb ratio of the crystal making it to near stoichiometric composition from congruent composition. This also modifies
where Xa and Xb are electro-negativities of a and b atom, respectively, [17]. According to this the Li–O bond is more ionic than Nb–O bond and thus the electron in the Li–O bonds are tightly bound to oxygen atom and weekly polarized. Hence, the Li–O bonds are more ionic in character compared Nb–O bond, which is covalent in nature and relatively more polarisable [18–21]. The refractive index of the materials is determined by the response of the materials to the applied optical field in terms of dielectric bond polarizability. Therefore, the Nb–O bond in the LiNbO3 structure mainly governs the linear optical properties and the contribution of Li–O bonds to optical properties is small compare to Nb–O bonds. In VTE treated sample number of Nb–O bonds are decreasing due to decrease in NbLi antisite defects and accordingly the number of Li–O bonds are increasing. In other words, the unit cell is becoming more ionic in character and weakly polarisable. Leading to decrease in linear optical properties and hence, the refractive index of the VTE modified near SLN crystals. The expected decreases in the refractive index have been observed in the VTE treated samples as shown in Fig. 2. The angle was measured with accuracy of the order of 0.18 at Brewster angle, leading to a percentage error of 0.45% in the refractive index measurement as shown in the Fig. 2. The non-linear optical properties of the VTE modified crystals were investigated by Kurtz powder SHG efficiency technique [22]. Fig. 5 shows the enhancement in SHG efficiency of the VTE treated sample with respected to congruent crystal. Significant improvement in SHG efficiency in the range of 1.98–2.3 was observed in the VTE treated sample. Whereas, in Fig. 4 the congruent crystals are showing around 40 times higher SHG efficiency then the reference KDP crystals. Since the SHG efficiency is inversely proportional to
Fig. 6. Conoscopy pattern of VTE treated sample.
KðXa KXb Þ2 4
S. Kar et al. / Solid State Communications 137 (2006) 283–287
the third power of the refractive index, leading to increase in the SHG efficiencies of VTE modified samples. This is in good agreement to the Jeggo et al. [23] findings, where they have reported that the SHG efficiency is a function of refractive index and refractive index is a function of number of bonds per unit volume. At room temperature non-linear optical properties of lithium niobate is proportional to the number of Li–O and Nb–O bonds. The NLO behaviour of lithium niobate is dominated by NbO6 octahedra. As per the Jeggo et al. [23], that SHG efficiency is proportional to number of bonds per unit volume. In VTE treated samples, the Li–O bond density increases due to decrease in the intrinsic defects as replacing one NbLi by Li atom leads to occupying four Li vacancies by Li ions. In other words, the number of bonds per unit volume increases and therefore NLO efficiencies as well as NLO coefficient enhances in VTE treated near stoichiometric sample.
5. Conclusion Transparent, crack free congruent LiNbO3 crystal were grown. VTE treated LiNbO3 crystal wafer of thickness 1 and 2 mm at various process temperature were prepared. These wafers are characterise by UV absorption edge, refractive index, density, SHG efficiency analysis. We found blue shift in absorption edge with increasing process temperature of VTE treated LiNbO3 crystals and the maximum blue shift in UV spectra occurs at 1100 8C. The blue shift in absorption edge shows the crystal approaches to near stoichiometric composition. The NLO efficiency as well as coefficient were found enhanced in VTE treated sample.
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