Light-induced absorption instability in weakly reduced congruent Er:LiNbO3 crystal

Light-induced absorption (LIA) characteristics in weakly reduced (or strongly annealed) congruent and/or vapor transport equilibration (VTE)-treated Er-doped LiNbO₃ crystals have been investigated in comparison with their corresponding as-grown ones and undoped crystals by using a polarized 632.8-nm beam as probe light and another polarized 632.8- or 488-nm beam as pump light. Under He-Ne pump, the LIA was observed only in strongly reduced pure VTE LiNbO₃ crystal. Under 488-nm pump, LIA is still not observed in the doped or undoped as-grown crystals. The weakly reduced VTE-treated Er(0.2 mol %):LiNbO₃ crystal displays weak and stable LIA. On the other hand, the corresponding weakly reduced congruent crystal displays a rather unpredictable light-induced absorption instability phenomenon. The instability was shown by the random competition of the LIA and light-induced transparency (LIT). When both crystals were further reduced, the VTE sample still shows stable LIA but with increased LIA, while the LIA in the congruent sample also becomes stable enough. The instability was experimentally proved to be associated with the presence of the Er³⁺ ion that performs the role of an extrinsic defect instead of photoluminescence. A three-level model is suggested that consists of a deep level (the bipolaron) and two shallow levels: the small polaron level and the level with respect to the Er³⁺ ion. The model has been employed to qualitatively explain the LIA characteristics of the weakly reduced congruent Er:LiNbO₃ crystal, including the the instability, the effect of the state of reduction, the pump intensity and the pump–probe polarization dependences. The inhomogeneity of the defects caused by the weak reduction and the simultaneous participation of the two shallow levels in the light-induced electron-transport process result in the random competition between LIA and LIT, and consequently result in the LIA instability. PACS 42.70.Hj; 42.70.Ln; 42.70.Mp; 42.65.Hw; 81.05.-t     

Fast photorefractive response in strongly reduced near-stoichiometric LiNbO(3) crystals

The photorefractive response time of LiNbO(3) crystal is of the order of minutes, and such a long response time limits the crystal's practical applications. We report the photorefractive properties of nominally pure near-stoichiometric LiNbO(3) crystal that is strongly reduced in vacuum. A short photorefractive response time of the order of 100 ms is measured at a wavelength of 514.5 nm, with incident light intensity of 1.6 W/cm (2) , and possible corresponding mechanisms are discussed. To our knowledge this is the first experimental evidence of a subsecond photorefractive response in pure LiNbO(3) crystals. The diffraction efficiency of a holographic grating written in this reduced crystal is low but can be enhanced by an externally applied electric field.     

Raman scattering study of reduced Er:LiNbO3 crystals

Raman spectra of as-grown and reduced (or annealed) Er:LiNbO₃ crystals, which have different cut orientations, varied Li/Nb ratio, and different Er-doping levels of 0.2, 0.4, 0.6, 1.0, and 2.0 mol%, were recorded at room temperature over a wavenumber range of 50–1000 cm^-1 by use of backward scattering geometries. The spectra are assigned on the basis of their Raman scattering features and previous relevant work. A weak but well-resolved peak around 633 cm^-1 appears in the E(TO) spectra that were recorded under the configuration of X(ZY)X̄(for an X-cut sample) or Y(ZX)Ȳ(for a Y-cut sample) for all crystals studied. The appearance of this peak in the E(TO) spectrum provides further evidence for a previous attribution of this peak to E(TO₉) mode. Some additional peaks distributed in the low wavenumber region ranging from 101–137 cm^-1 are attributed to Er³⁺ fluorescence with a wavelength range of 490.41–491.3 nm. The reduction effects include a significant drop of the Raman scattering intensity and a slight narrowing instead of a broadening in the linewidth. The reduction procedure hardly affects the spectral shape and the wavenumber of most of the phonons. The anneal effect is similar to the reduction effect and both effects are not as obvious as the vapor transport equilibration (VTE) effect. In addition, the present Raman scattering result provides evidence for our earlier reported individual result on light-induced diffraction from strongly reduced Er:LiNbO₃ crystals. PACS 42.70.Hj; 81.05.-t; 63.20.-e; 78.30.-j.     

化学计量比LiNbO3晶体的激光显微拉曼光谱研究

对用助熔剂提拉法生长的近化学计量比LiNbO3晶体进行了激光显微拉曼光谱测试分析,与同成分LiNbO3晶体相比较,A1(TO)与E(TO)模的谱线数目、频率基本不变,验证了LiNbO3晶体属置换式固溶体的结论.738cm-1处拉曼振动峰（A1(LO)模）的相对强度随晶体中Li2O含量的增加发生了明显的变化,在两种掺杂晶体中,此振动峰已经消失.根据Li空位模型,从占位和结构上进行了分析讨论.在152和872cm-1处拉曼振动峰半高宽随晶体中Li/Nb的增大而变窄,对二者关系进行了线性拟合,利用拟合公式可根据拉曼振动峰半高宽计算晶体中的Li/Nb值. 关键词： 化学计量比 拉曼光谱 线性拟合 半高宽     

Raman study on vapor-phase equilibrated Er:LiNbO3 and Er:Ti:LiNbO3 crystals

Raman spectra of Er:LiNbO₃ crystal and Ti-diffusedEr:LiNbO₃ strip waveguide, in which the Li/Nb ratio was altered using a vapor-phase equilibration (VPE) technique, were measured at room temperature in the wave-number range 50–3500 cm^-1. Both 488 and 514.5 nm radiations were used to excite Raman scattering, A₁(TO) and E(TO) modes were recorded at backward scattering geometry. The results indicated that the lattice vibrational spectra of the as-grown Er:LiNbO₃ are almost the same as those of pure LiNbO₃ except for the little shift of the peak position and the change of relative intensity of some peaks. In comparison with the spectra of as-grown Er:LiNbO₃ crystal the vapor-phase equilibrated Er:LiNbO₃ and Er:Ti:LiNbO₃ crystals in the lattice vibrational region exhibit the following features: firstly, Raman peaks become narrow, indicating that the VPE process has brought Er:LiNbO₃ and Er:Ti:LiNbO₃ crystals closer to a stoichiometric composition; secondly, relative intensity of some peaks varies with the VPE time; and finally, slight blue shifting in peak position was observed. Some of these features were correlated with the NbO₆ octahedra and with the site distribution of the doped Er ions. In addition, green fluorescence peaks and/or bands associated with the electron transitions ² H _11/2?⁴ I _15/2 and ⁴ S _3/2?⁴ I _15/2 of the doped Er³⁺ were also observed. For 488 nm excitation they appear in the wavenumber range of 1200–3000 cm^-1 and are well separated from lattice vibrational region; for 514.5 nm excitation, however, these fluorescence peaks shift towards the low wavenumber region and overlap partially with the lattice vibrational spectra. Received: 24 May 2000 / Accepted: 29 May 2000 / Published online: 13 September 2000     

Domain-inversion characteristics of congruent and stoichiometric LiNbO3 crystals

The characteristics of the ferroelectxic domain inversion structures fabricated by applying voltage at room temperatture in Z-cut congruent LiNbO3 and stoichiometric LiNbO3 crystals were examined. The voltage required for domain reversal in congrueat crystals is about 2.8 times larger than the voltage for stoichiometric crystals. The periodicity of the sstoichiometric crystals are more uniforn than that of the congruent crystals.     

Lattice site of Er in LiNbO3:Mg,Er crystals

Abstract

The lattice site of Er has been determined in LiNbO₃:Mg, Er crystals by the Rutherford backscattering channeling technique. It has been shown unambiguously that Er occupies Li site in the crystal lattice. OH absorption band has not been found in the 3500–3525 cm^?1 region. The RBS result confirms the assumption that OH^? ions may form defect complexes only with those trivalent impurities which replace niobium.     

近化学计量比铌酸锂晶体周期极化畴反转特性研究

采用K2O作助熔剂直接拉晶法和气相输运平衡技术制备出了高质量近化学计量比铌酸锂晶体,研究了铌酸锂晶体中的[Li][Nb]比含量对其畴反转结构和极化电场的影响.实验结果表明:随着晶体中[Li][Nb]比的提高,畴极化反转电场呈明显下降趋势,使用近化学计量比铌酸锂晶体,在4.0±0.5kVmm大小极化电场条件下,成功地实现了1.0mm厚度的周期极化畴反转.并用铌酸锂晶体的Li空位缺陷模型对上述实验结果给出了合理的解释. 关键词： 近化学计量比铌酸锂晶体 周期极化 畴反转     

Light-induced thermal gratings in LiNbO3: Fe

Holographic gratings are recorded in LiNbO³: Fe using frequency-doubled pulses of aQ-switched Nd : YAG laser (light wavelength = 532 nm, pulse durationt _p = 20ns). We monitor diffraction efficiencies of the holograms during and several milliseconds after exposure. Variations of Fe²⁺ concentration, light intensity, and fringe spacing show that different types of gratings are involved: photochromic, thermooptic, pyroelectric, and space-charge gratings. The influence of the internal pyroelectric field on the light-induced charge transport is discussed in terms of a two-center model. Pyroelectric and bulk photovoltaic effect are shown to partially compensate each other. In LiNbO₃: Fe, the photovoltaic effect is the dominant charge-driving force for our recording conditions.     

Electrical properties of LiNbO3 crystals reduced in a hydrogen atmosphere

The temperature dependence of the electrical conductivity and pyroelectric coefficient of lithium niobate crystals reduced in a hydrogen atmosphere has been studied. It has been established that the activation energy of dark electrical conduction in these crystals in the temperature range 288?C350 K differs from the corresponding values for crystals reduced in vacuum and is equal to 0.68 ± 0.02 eV. It has been shown that the annealing of LiNbO₃ crystals in a hydrogen atmosphere hardly affects their pyroelectric properties. The mechanism of electrical conduction of LiNbO₃ crystals reduced in a hydrogen-containing atmosphere has been discussed.     

Photoluminescence of LiNbO3 crystals doped with Er3+ and Yb3+ ions

Results of cooperative phenomena investigations in the impurity subsystem of lithium niobate crystals doped with Er³⁺ and co-doped with Yb³⁺ impurity ions under continuous wave and pulsed excitation at 975 nm and 1064 nm wavelengths are presented. Dependences of some spectroscopic characteristics on the intensity of laser pumping are studied. Based on the pair centers model the analysis of the cooperative luminescence behavior in LiNbO₃:Yb³⁺+Er³⁺ crystals is performed.     

Light-induced refractive index changes in LiNbO3:Ti waveguides

Holographic measurements of light-induced refractive index changes in LiNbO₃Ti waveguides are reported. The results demonstrate that Fe²⁺ centers are stabilized against oxidation in the region of large Ti concentration near the surface thus increasing the sensitivity to optical damage considerably. This undesired effect may be largely abolished by additional in-diffusion of protons.     

Studies of absorption spectra and the photovoltaic effect in LiNbO3:Mg:Fe crystals

The absorption spectra, photoconductivities and photovoltaic currents of LiNbO₃:Fe crystals with different Mg doping levels and Li/Nb ratios in the oxidized state have been investigated at room temperature. The Fe²⁺ ions in LiNbO₃:Mg:Fe with Mg content above a critical value are more easily oxidized than in crystals with Mg content below a critical value. The photoconductivity of LiNbO₃:Mg:Fe crystals with Mg content above a critical value is one order of magnitude higher than those with Mg content below a critical value, however, the photovoltaic current of the former is one order of magnitude lower than the latter. The differences are postulated to be due to different sites of Fe in these two classes of crystals.     

化学计量比LiNbO_3晶体的激光显微拉曼光谱研究

对用助熔剂提拉法生长的近化学计量比LiNbO3晶体进行了激光显微拉曼光谱测试分析 ,与同成分LiNbO3晶体相比较 ,A1 (TO)与E(TO)模的谱线数目、频率基本不变 ,验证了LiNbO3晶体属置换式固溶体的结论 .738cm- 1 处拉曼振动峰 (A1 (LO)模 )的相对强度随晶体中Li2 O含量的增加发生了明显的变化 ,在两种掺杂晶体中 ,此振动峰已经消失 .根据Li空位模型 ,从占位和结构上进行了分析讨论 .在 15 2和 872cm- 1 处拉曼振动峰半高宽随晶体中Li Nb的增大而变窄 ,对二者关系进行了线性拟合 ,利用拟合公式可根据拉曼振动峰半高宽计算晶体中的Li Nb值 .     

Light-induced absorption of cerium-doped lead barium niobate crystals

Tetragonal lead barium niobate (Pb(1-x)Ba(x)Nb(2)O(6);1-x<0.63) crystals are promising photorefractive materials for applications such as holographic data storage. Measurements of light-induced absorption in Ce-doped lead barium niobate (Pb(0.5)Ba(0.5) Nb(2)O(6)) were performed. The induced absorption that was due to the presence of shallow traps was as large as 0.3cm(-1) at 633 nm and exhibited saturation behavior at green-pump intensity greater than 2W/cm(2) . The temporal profile of the dark decay was a double exponential, and the activation energies of the slow- and the fast-decaying components were determined to be 0.16 and 0.3 eV, respectively.     

Q-switched Ti:Er:LiNbO3 waveguide laser

Q-switched operation of a Ti:Er:LiNbO3 waveguide laser has been achieved with up to 2.4 W pulse peak power (0.18J) at 2kHz repetition frequency with an intracavity Mach–Zehnder-type amplitude modulator. A theoretical model has been developed to describe the laser operation. Good agreement between numerical and experimental results has been achieved. Significant potential for improvement is predicted.     

Effect of external electric field on R line absorption spectra in LiNbO3:Cr3+ crystals

The effect of an external electric field on the R absorption lines of LiNbO₃:Cr³⁺ crystals has been studied by a high-sensitivity differential technique at 77 K. Linear Stark effect has been observed in a field directed along the trigonal crystal axis. No effect of the field was observed when directed perpendicular to this axis. These characteristics of the Stark effect provide a convincing argument for the electric dipole moments of Cr³⁺ centers being oriented along the trigonal C ₃ axis of the crystal and indicate that the chromium centers in LiNbO₃ have C ₃ symmetry. Fiz. Tverd. Tela (St. Petersburg) 39, 2053–2056 (November 1997)     

Photorefractive light scattering in LiNbO3: Cu crystals

The time dependence of the photorefractive light scattering in lithium niobate single crystals of the congruent composition (LiNbO₃) doped by “photorefractive” Cu cations (0.015 wt %) is studied for a radiation power of 160 mW. The data for the photorefractive scattering are confirmed by changes in Raman scattering spectra.