Holographic storage properties of In:Ce:Cu:LiNbO 3 crystals

Transcription

1 Holographic storage properties of In:Ce:Cu:LiNbO 3 crystals Wusheng Xu, Yuheng Xu, Jian Zhang Department of Applied Chemistry, Harbin Institute of Technology, Harbin , China 1. ABSTRACT The optical properties of the Ce:Cu:LiNbO 3 co-doped with different concentrations of In were measured, including absorption spectra, infrared transmittance spectra and holographic storage properties. The doped threshold of In was determined via the results of spectrum measurement. When the doped concentration of In is over its threshold, the photo-damage resistance ability of In:Ce:Cu:LiNbO 3 is improved dramatically. In the measurement of holographic storage properties, the response time of 34s, diffraction efficiency of 33%, and exponential gain coefficient of 18cm -1 were obtained for In(3mol%):Ce(0.20wt%):Cu(0.015wt%):LiNbO 3 crystal. Keywords: In:Ce:Cu:LiNbO 3 crystals, holographic storage, photorefractive 2. INOTRODUCTION Photorefractive materials are being widely investigated for their applications in holographic data storage. Lithium niobate is one of the most promising crystal materials used in this aspect 1, but it has the serious problem of volatility. The double-photon method is a new way to realize the nonvolatile real-time storage 2,3. Liu et al reported that Ce:Cu:LiNbO 3 crystal had the largest nonvolatile diffraction efficiency 4. However, Ce:Cu:LiNbO 3 has the bad photodamage resistance ability, which limits its application in the holographic storage. This deficiency can be overcome by doping the photo-damage resistance impurities (be abbreviated to PR), such as magnesium, zinc, indium 5 and scandium 6. Not only the photo-damage resistance ability but also the photoconduction can be improved by these impurities 5, i.e. doped LiNbO 3 co-doped with Mg, Zn, In or Sc have the shorter response time. The threshold concentration of the impurities can be determined by the location of OH - peak 7 and the absorption edge 8. When several impurities are codoped in PR:LiNbO 3, the threshold concentration will be influence by the co-doped ions 9,10. In this paper, we investigated the influence of indium in Ce:Cu:LiNbO 3, and determined the doped threshold of In via the spectrum measurement. When the doped concentration of In was over its threshold, the photo-damage resistance ability of In:Ce:Cu:LiNbO 3 improved dramatically. The photorefractive properties and the photo-damage resistance ability of In:Ce:Cu:LiNbO 3 crystals were measured. 3. EXPERIMENTS 3.1. Crystal growth and sample preparation We doped 0.015wt% CuO, 0.10wt% Ce 2 O 3 and In 2 O 3 with different concentrations from 1mol% to 3mol% in LiNbO 3 to grow In(1mol%, 2mol% and 3mol%)Ce(0.20wt%):Cu(0.015wt%):LiNbO 3 crystals in congruent ratio along the c-axis by the Czochralski method. The starting materials (Nb 2 O 5, Li 2 CO 3, CeO 2, CuO and In 2 O 3 with the purity of 99.9% or higher) were weighed, mixed carefully and then filled into a Pt crucible. The technological parameters of crystal growth were different with the variation of In concentration. The temperature gradient in axial direction was 40 C/cm above the liquid surface and 15 C/cm under the surface, respectively. The rotation rate of 20 rpm and pulling rate of 2mm/h are adopted for the doped concentration of In with 1mol%, and the rotation rate of 25rpm and pulling rate of 0.5mm/h are adopted for the doped concentration of In with 2mol% and 3mol%. After poled, the as-grown crystals are cut in the wafers with the dimension of mm 3 (c a b) as the experimental samples. Only y-faces of the samples were polished. wushengx@hit.edu.cn; phone Advances in Optical Data Storage Technology, edited by Duanyi Xu, Kees A. Schouhamer Immink, Keiji Shono, Proceedings of SPIE Vol (SPIE, Bellingham, WA, 2005) X/05/$15 doi: /

2 3.2. Measurements of Spectrum properties Both absorption spectra and infrared transmittance spectra were measured under room temperature. The absorption spectra were carried out within the range between 300~900nm with a CARY UV-Visible Spectrophotometer. Unpolarized light propagating along the y-direction of samples was used, and the scan step was 1nm. The infrared transmittance spectra were obtained in the range of wavenumber from 3600 to 3400cm -1 with a Nicolet-710 FT-IR Spectrometer under an unpolarized light source. 3.3 Measurements of Photorefractive properties The schematic diagram of two-wave coupled is shown in Fig. 1. A He-Ne laser operating at 632.8nm was used as the light source and the polarization direction of the beam is in the incident plane, and the c-axis of the samples was in the incident plane too, i.e. extraordinary light radiation. The beam splitter divided the output beam into the pump I 1 the signal beam I 2. M 1 He-Ne laser BS M 3 I 2 2θ I 1 c-axis I 2 ' I 1 ' In:Ce:Cu:LiNbO 3 D M 2 Fig. 1 Schematic diagram of two-wave coupled. M 1, M 2 and M 3 are mirrors, BS is a beam splitter, D is a detector. The exponential gain coefficient Γ can be experimental measured through the relation 1 I 2 I ln 1 Γ = (1) l I 2I 1 where l is interaction length of the two beams in crystal, in the experiment, take as the thickness of the sample, I 2 (I 2 ) is the transmitted signal beam intensity with (without) coupling, and I 1 (I 1 ) is the transmitted pump beam intensity with (without) coupling. When the intensity of pump beam is much larger than that of signal beam, i.e. I 2 >>I 1, the two-beam coupling gain coefficient G becomes independent of the pump beam intensity because of I 1 I 1 1 I ln 2 Γ = (2) l I2 The modulation depth of 230 was taken to measure the exponential gain coefficient of two-beam coupling, and the pump intensity was 800mW/cm 2. The diffraction efficiency η is defined as the ratio of the diffraction light intensity I D to the transmitted intensity I 0 for the signal light, i.e. I D η = 100% (3) I0 During the recording of the gratings, the diffraction light intensity increases as following expression (1 t I ) D = I0 e τ (4) where τ is defined as the response time, which can be obtained through fitting the curve of diffraction light intensity during the building of the gratings. The same modulation depth of 1 was taken to measure the diffraction efficiency and the response time, and the pump intensity was 82mW/cm 2. The directly observing facula distortion method, a quasi-quantity method, was used to investigate the photo-damage resistance ability of In:Ce:Cu:LiNbO 3. The experimental setup is shown in Fig. 2. An Ar + laser operating at 514.5nm was used as light source. The output beam was focused on a sample through the light shed and the convex lens. The 374 Proc. of SPIE Vol. 5643

3 facula could be observed on the screen. When the laser beam power density achieves a certain value, the facula is distorted along c-axis of crystal. This value is defined as the photo-damage resistance ability of the crystal. Beam splitter Ar + Laser Light shed Convex Lens Crystal Power detecter Fig. 2 Set-up for measurement of photorefractive resistance ability 4. RESULTS AND DISCUSSIONS The growth striations are easily appeared in LiNbO 3 doped with In, and with the doped concentration increasing, the striations appear more easily. So we varied the technological parameters to grow In:Ce:Cu:LiNbO 3, and the slow pulling rate was employed when the doped concentration of In was high. Under the conditions above mentioned, the integrated and no macro defect crystals were gained, and the color of all as-grown crystals was nearly the same, in reddish brown. Fig. 3. is the Infrared transmittance spectra of In:Ce:Cu:LiNbO 3. Transmittance (%) Wavenumber (cm -1 ) Fig. 3 Infrared transmittance spectra of In:Ce:Cu:LiNbO 3. The solid line, dash line, dot line and dash-dot line signifies Ce:Cu:LiNbO 3, In(1mol%):Ce:Cu:LiNbO 3, In(2mol%):Ce:Cu:LiNbO 3 and In(3mol%):Ce:Cu:LiNbO 3, respectively. In the figure, the absorption peak is generated by OH - that enters the crystal due to the aqueous vapor of ambient environment [ 11 ]. In congruent LiNbO 3 the proton locates on the longest O-O bond in the oxygen triangle nearest to the normal Li site and substitutes the Li + ion in the meantime [ 12 ]. It can be seen from Fig. 3 that the location of the OH - absorption peak is at about 3584cm -1 for In:Ce:Cu:LiNbO 3 doped with low In concentration (below 2mol%), and that shifts to 3508cm -1 for In(3mol%):Ce:Cu:LiNbO 3. Generally, the OH - absorption peak reflects the influence of the O-H vibration by its ambient ions. According the Li vacancy mode 13, there exist Li vacancy and anti-site Nb (Nb at Li site, Nb 4+ Li ) in LiNbO 3. When the doped concentration of In is below its threshold value, In will take anti-site Nb (Nb at Li site, Nb 4+ Li ) to form In 2+ Li that exclude H +, so H + will not get together in the bargain. After the concentration of In achieves its threshold, Nb 4+ Li is replaced by In completely. Furthermore, as doped In is over its threshold concentration, In begin to replace Nb at normal site to form In 2- Nb that attract H + to aggregate here, so the more energy is needed to make O-H vibrate and the OH - absorption peak shifts to violet for In(3mol%):Ce:Cu:LiNbO 3, i.e. the doped concentration of 3mol% In in Ce:Cu:LiNbO 3 has achieved its threshold value. The absorption spectra of In:Ce:Cu:LiNbO 3 is shown in Fig. 4. It can be seen in the figure that with the content of In increasing in In:Ce:Cu:LiNbO 3, the absorption edge shift to violet more, and the most violet shift level occurs when the doped concentration of In is 2mol%. According Polgar et al s result that the absorption edge shifts most to violet when Abs (a. u.) Screen Ce:Cu:LN In(1mol%):Ce:Cu:LN In(2mol%):Ce:Cu:LN In(3mol%):Ce:Cu:LN Wavelength (nm) Fig. 4 Absorption spectra of In:Ce:Cu:LiNbO 3 Proc. of SPIE Vol

4 doped quantity of Mg achieved the threshold concentration and when over the concentration, the absorption edge shift to red, it can be deduced that the threshold concentration of In in Ce:Cu:LiNbO 3 is between 2mol% and 3mol% in the melt. From above coincident results of spectrum experiments, it can be concluded that the threshold concentration of In in Ce:Cu:LiNbO 3 is between 2mol% and 3mol% in the melt. The conclusion is the same with LiNbO 3 singly doped with In [ 14 ] because the co-doped concentration of Ce and Cu is too little to influence the threshold concentration of In. The experimental results of the photo-damage resistance ability R, diffraction efficiency η and response time τ of In:Ce:Cu:LiNbO 3 are shown in Table 1. As the doped concentration of In is over the threshold value, the photo-damage resistance ability of In:Ce:Cu:LiNbO 3 is two orders of magnitude higher than that of In( 2mol%):Ce:Cu:LiNbO 3 crystals. The Nb Li 4+ is substituted by In, and the intrinsic defect and one of the photorefractive center decrease, which is the reason that the photo-damage resistance ability of Ce:Cu:LiNbO 3 doped with indium is enhanced. The response time of In(3mol%):Ce:Cu:LiNbO 3 is about a quarter of Ce:Cu:LiNbO 3 because indium doped in Ce:Cu:LiNbO 3 increase the photoconduction of the crystal. Table 1 Holographic properties of In:Ce:Cu:LiNbO 3 crystals Crystals Ce:Cu:LiNbO 3 In(1mol%):Ce:Cu:LiNbO 3 In(3mol%):Ce:Cu:LiNbO 3 R (W/cm 2 ) η (%) τ (s) The dependence curves of diffraction efficiency η and exponential gain coefficient Γ of In(3mol%):Ce:Cu:LiNbO 3 crystal on the included angle 2θ between the signal light and pump light are shown in Fig. 5 and Fig. 6, respectively. The maximum diffraction efficiency of 33% is obtained at the 16 and the maximum exponential gain coefficient of 18cm -1 achieves at η (%) θ (deg.) Fig. 5 Experimental curve Of η~2θ Γ (cm -1 ) θ (deg.) Fig. 6 Experimental curve of Γ~2θ 5. CONCLUSIONS Doping CuO, Ce 2 O 3 and In 2 O 3 in LiNbO 3, In:Ce:Cu:LiNbO 3 crystals without micro defects were obtain by the Czochralski method. The doped threshold value concentration of In was determined between 2mol% and 3mol% by measuring the spectrum properties of In:Ce:Cu:LiNbO 3 crystal. The holographic storage property measurement results show that the photo-damage resistance ability of In(3mol%):Ce:Cu:LiNbO 3 is two orders of magnitude higher than that of In( 2mol%):Ce:Cu:LiNbO 3 crystals, and the response time of In(3mol%):Ce:Cu:LiNbO 3 is shorter than that of Ce:Cu:LiNbO 3. The dependences of diffraction efficiency and exponential gain coefficient on the included angle between the signal light and pump light were measured. 376 Proc. of SPIE Vol. 5643

5 Because in comparison with Ce:Cu:LiNbO 3, In(3mol%):Ce:Cu:LiNbO 3 possesses the higher photo-damage resistance ability, rapider response rate and nearly the same diffraction efficiency, it can be concluded that he crystal is more promising candidate than Ce:Cu:LiNbO 3 for application in the holographic storage. ACKNOWLEDGEMENTS This paper was sponsored by the National Research Fund for Fundamental Key Projects No.973 (G ) (China), the High Technology Research and Development Program of China (2001AA313040) and the Nature Science Foundation of Heilongjiang. REFERENCES 1 J. Ashley, M.-P. Bernal, G. W. Burr, H. Coufal, H. Guenther, J. A. Hoffnagle, C. M. Jefferson, B. Marcus, R. M. Macfarlane, R. M. Shelby, G. T. Sincerbox, IBM J. Res. Develop. 44 (2000) K. Buse, A. Adibi, D. Psaltis, Nature 393 (1998) Lambertus Hesselink, Sergei S. Orlov, Alice Liu, Annapoorna Akella, David Lande, Ratnakar R. Neurgaonkar, Science 292 (1998) Youwen Liu, Liren Liu, De an Liu, Liangying Xu, Changhe Zhou, Opt. Commun. 190 (2001) T. Volk, N. Rubinina, M. Wöhlecke, Opt. Soc. Am. B 11 (1994) Joyce K. Yamamoto, Kenji Kitamura, Nobuo Iyi, Shigeyki Kimura, Yasunori Furukawa, Macayoshi Sato, Appl. Phys. Lett. 61 (1992) T. R. Volk, N. M. Rubinina, V. I. Pryalkin, V. V. Krasnikov, V. V. Volkov, Ferroelectrics 109 (1990) K. Polgar, L. Kovacs, I. Foldrari, I. Crarero, Sol. State Commun. 59 (1986) Jianjun Liu, Wanlin Zhang, Guangyin Zhang, Phys. Lett. A 212 (1996) So Ik Bae, Junichiro Ichikawa, Kiyoshi Shimamura, Hideya Onodera, Tsuguo Fukuda, J. Crystal Growth 180 (1997) K. Kasemir, K. Betzler, B. Matzas, B. Tiegel, T. Wahlbrink, M. Wohlecke, B. Gather, N. Rubinina, T. Volk, J. Appl. Phys. 84 (9) (1998) Yongfa Kong, Jingjun Xu, Wanlin Zhang, Guangyin Zhang, Phys. Lett. A 250 (1998) P. Lerner, C. Legras, J. P. Dumas, J. Crystal Growth 3/4 (1968) Yongfa Kong, Jinke Wen, Huafu Wang, Appl. Phys. Lett. 66 (1995) 280 Proc. of SPIE Vol