Enhanced nonvolatile holographic properties in Zn,Ru and Fe co-doped LiNbO3 crystals

A series of LiNbO₃ crystals doped with various concentrations of ZnO and fixed concentrations of RuO₂ and Fe₂O₃ have been grown by the Czochralski method from the congruent melts. The type of charge carriers was determined by Kr⁺ laser (476 nm) and He–Ne laser (633 nm). The results revealed that the holes were the dominant charge carriers at blue light irradiation. Dual-wavelength and two-color techniques were employed to investigate the nonvolatile holographic storage properties of Ru:Fe:LiNbO₃ and Zn doped Ru:Fe:LiNbO₃ crystals. The essential parameters of blue nonvolatile holographic storage in Zn:Ru:Fe:LiNbO₃ crystals were enhanced greatly with the increase of Zn concentration. This indicates that the damage resistant dopants Zn²⁺ ions enhance the photorefractive properties at 476 nm wavelength instead of suppressing the photorefraction. The different mechanisms of blue photorefractive and nonvolatile holographic storage properties by dual wavelength recording in Zn:Ru:Fe:LiNbO₃ crystals were discussed.     

Optical characterization of Cr3+ centers in LiNbO3

Low-temperature luminescence spectra of stoichiometric Cr:LiNbO₃, congruent Cr:LiNbO₃ and congruent Cr,Mg:LiNbO₃ were studied. Dominant low-field and minor high-crystal-field optical centers are the Cr³⁺ impurity ions that preferentially occupy Li⁺ sites (Cr_Li) in the Cr:LiNbO₃ crystals. Low-field centers related to Cr³⁺ substitution of Nb⁵⁺ (Cr_Nb) occur in addition to Cr_Li in co-doped Cr,Mg:LiNbO₃ samples. Application of high hydrostatic pressure leads to the transformation of dominant Cr³⁺ centers from low- to high-field type due to strong pressure-induced blue shift of the ⁴ T ₂ state, resulting in its crossing with the ² E state of Cr³⁺. This level-crossing effect was observed for the dominant Cr³⁺ _Li and Cr³⁺ _Nb centers at pressures that correlate well with estimations based on the ⁴ T ₂-² Eenergy gap (230 cm^-1 and 1160 cm^-1) and on the rate of their pressure-induced change (14.35 and 11.4 cm^-1/kbar, respectively). We also studied inhomogeneous broadeningof the ² E?⁴ A ₂transitions at ambient pressure for the minor high-field “defect” Cr³⁺ _Li centers in congruent LiNbO₃. A fine structure in the spectral response of these centers was observed. The obtained results are discussed on the basis of a microscopic hierarchic model for perturbed Cr³⁺ ions in the LiNbO₃ lattice. Received: 25 June 2001 / Published online: 2 November 2001     

Optical properties of alkaline earth oxide crystals

Abstract

Single crystals have been obtained of undoped calcium, strontium and barium oxides. Recent work on the F-band in these crystals will be reviewed, quoting values for the spin-orbit interaction constants for the first excited states of F-centres. A zero-phonon line has been found in undoped calcium oxide, and there has been a controversy whether or not this line is due to the F-centre. Another zero-phonon line occurs in bismuth-doped calcium oxide. The properties of both zero-phonon lines have been studied by applying uniaxial stress or a magnetic field to the crystal at liquid helium temperature. Both lines are found to be due to electric dipole transitions at centres of cubic or terahedral symmetry.     

Electro-optic properties in undoped and Cr-doped LiNbO3 crystals

3 along the ferroelectric c axis in pure and chromium-doped crystals. We have studied four series of samples with various crystal compositions and doping concentrations to separate the influence of the intrinsic defects related to the non-stoichiometry and the effect of the introduction of chromium. In pure and slightly doped crystals, the electro-optic coefficient r_c shows a non-monotonous dependence on the controlled crystal composition, which reflects a similar behaviour of the corresponding dielectric permittivity. In the highly Cr-doped crystals r_c displays a large decrease with increasing Cr concentration whatever the crystal composition. The results have been discussed by means of the LiNbO₃ structure and the substitution site dynamics. Received: 14 November 1997/Accepted: 2 January 1998     

Optical damage resistance of Ce:Fe:LiNbO3 crystals with various Li/Nb ratios

Ce:Fe:LiNbO₃ crystals were grown by the Czochralski technique with various ratios of Li/Nb = 0.94, 1.05, 1.20 and 1.38 in the melt. Their UV–Vis absorption spectra were measured in order to investigate their defect structures and their optical damage resistance was characterized by the photoinduced birefringence change and transmission facual distortion method. The optical damage resistance of Ce:Fe:LiNbO₃ crystals improves with the Li/Nb ratio increases. The dependence of the optical damage resistance on the defect structure of Ce:Fe:LiNbO₃ crystals is discussed in detail.     

Investigation of optical photorefractive properties of Zr:Fe:LiNbO3 crystals

A series of Zr:Fe:LiNbO₃ crystals with various levels of ZrO₂ doping were grown by Czochraski technique. The optical damage resistance and photorefractive properties were deeply explored. The results showed that the ability optical damage resistance increased remarkably when the concentration of ZrO₂ is over threshold concentration, but which is lower than that of traditional damage resistant additive MgO. While, the holographic storage properties can be greatly enhanced by proper level of ZrO₂ doping in Fe:LiNbO₃. In terms of ions' site occupation model, the photo-damage resistant ability enhancement and the change of the photorefractive properties were discussed.     

OH impurities in co-doped LiNbO3:Cr :ZnO congruent crystals

Infrared optical absorption has been used to study OH⁻impurities into congruent co-doped LiNbO₃:Cr³⁺:ZnO crystals doped with different Zn²⁺ concentration. The OH⁻ IR absorption spectra present three bands that can be associated with different OH⁻ complex centres available in the lattice. For crystals with lower Zn²⁺ concentrations (<4.7%) only one IR absorption band centred at 2867 nm (3490 cm⁻¹) is reported which is associated with the OH⁻ unperturbed vibration. For crystals with higher Zn²⁺ concentrations (>4.7%), two new bands associated with OH⁻vibration in distortion environment are reported. These bands are centred at 2827 nm (3537 cm⁻¹) and 2847 nm (3512 cm⁻¹) and can be associated with OH⁻-Zn²⁺ and Cr³⁺(Li⁺)-OH⁻-Zn²⁺(Int.) complex centres, respectively. Electron paramagnetic resonance (EPR) has been used to identify the Cr³⁺ centres in the lattice of the doped LiNbO₃:ZnO crystals.     

Photorefractive and optical scattering properties of Zr:Fe:LiNbO3 crystals

The LiNbO₃ crystal co-doped with ZrO₄ and Fe₂O₃ has been grown with [Li]/[Nb]=0.85 and 1.20, respectively, by the Czochralski method in air atmosphere. The incident exposure energy flux threshold for the light-induced scattering was characterized to investigate the scattering properties of the crystals. Applying the results of the incident exposure energy flux threshold effect, the photorefractive properties at different laser wavelengths (473 nm and 532 nm) were also measured by using the typical two-wave coupling experiments. The results show that Zr:Fe:LiNbO₃ crystal has a larger refractive index change, higher recording sensitivity and larger two-wave coupling gain coefficient at 473 nm wavelength than those obtained at 532 nm wavelength under the same experimental conditions. Moreover, the photorefractive properties decrease with the increasing [Li]/[Nb] ratios. The material of Zr:Fe:LiNbO₃ crystal is a promising candidate for blue photorefractive holographic recording.     

Optical properties of LiNbO3:Mg(5.21 mol %) and LiNbO3:Fe(0.009 mol %):Mg(5.04 mol %) crystals

Using methods of electronic spectroscopy, laser conoscopy, photoinduced (photoreactive) light scattering, and Raman light-scattering spectroscopy, we have studied the optical homogeneity, optical transmission, and photorefractive properties of single crystals LiNbO₃:Mg(5.21 mol %) and LiNbO₃:Fe(0.009 mol %):Mg(5.04 mol %) that were grown from congruent melts. We have ascertained that doping with “nonphotorefractive” Mg²⁺ cations causes suppression of the photorefractive effect in a lithium-niobate crystal. Upon double doping (Fe:Mg), if the concentration of Mg²⁺ cations exceeds the threshold concentration, the photorefractive effect is almost not observed and the presence of “photorefractive” Fe cations does not affect the photorefractive effect as strongly as in congruent crystals doped with Fe.     

Photorefractive properties of LiNbO3:Zn crystals related to the defect structure

LiNbO₃:Zn single crystals and powders were studied by precise X-ray diffraction methods. Structural refinement yielded new models of the intrinsic defect structure valid for different Zn concentration ranges. For concentrations up to 7 at. % in the crystal, Zn ions incorporate onto Li sites; at higher concentrations Zn ions are found on both Li and Nb sites. Photorefractive properties of LiNbO₃:Zn are discussed in the context of the deduced defect models. A smooth increase in the photoconductivity up to 7 at. % Zn is accounted for by a decrease of Nb antisites. Steeper growth of the photoconductivity at higher Zn concentrations is related to vanishing Li vacancies. Received: 17 November 2000 / Revised version: 23 January 2001 / Published online: 20 April 2001     

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)     

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.     

LiNbO3:Fe晶体中光写入平面光波导的导光特性研究

从光折变效应的单中心模型和载流子的光伏迁移机理出发给出了高斯片光在LiNbO3:Fe晶 体中导致的折射率变化分布的解析表达式.利用片光以“三明治”辐照方式在LiNbO3:Fe晶 体中写入了平面光波导结构.用切片干涉法测量了波导区的折射率分布，并对波导进行了简 单的导光测试.根据射线方程以及波导的折射率分布对光写入波导的导光特性进行了模拟计 算分析.研究结果表明，在LiNbO3:Fe晶体中光写入光波导是可行的. 关键词： LiNbO3:Fe晶体 光辐照法 平面光波导 导光特性 高斯片光     

Optical properties of large germanium monocrystals

Optical characteristics of germanium monocrystals with a diameter of 150 and 200 mm grown by the Czochralski and directional crystallization methods were studied. We measured spectral transmittance of the monocrystals in the wavelength range of 2.3–25.0 μm, directional transmission and light scattering in the 2.4–3.0 μm range, and noninhomogeneity of the refractive index of monocrystals by the interference technique at 3.39 μm. The measurements were performed on antimony-doped germanium monocrystals with a diameter of 200 mm and thickness of 15 mm (grown by the directional crystallization method) and 18 mm thick (grown by the Czochralski method). Measurements were also performed on a germanium monocrystal with a diameter of 150 mm and thickness of 15 mm grown by the Czochralski method without adding a ligand. The measurement results revealed different optical quality of monocrystals, expressed in the nature and amount of refractive index inhomogeneity, which imposes restrictions on the use of blanks.     

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     

Optical spectroscopy of Cr3+ ions in LiF single crystals

This paper reports on the luminescence spectrum, its excitation-wavelength dependence and decaytime of Cr³⁺:LiF crystals. In contrast to Al₂O₃ and MgO, where the Cr³⁺ luminescence at low temperature is due to the R-line and its vibronic sideband, luminescence from Cr³⁺:LiF is assigned to the broad-band ⁴ T ₂⁴ A ₂ transition. A periodic structure in the vibronic sidebands of this transition occurs with energy separations of ca. 60 meV. The linear polarisation of the vibronic sidebands shows that their radiative transition takes place from the lowest excited level of the ⁴ T ₂ state, i.e. ⁴ B ₁, into the ⁴ A ₂ ground state of Cr³⁺ ions in orthorhombic symmetry sites. The crystal-field parameters calculated from the experimental data, and luminescence decay-time of the two polarized configurations of the ⁴ B ₁(⁴ T ₂)⁴ A ₂ transition are also reported.     

Optical Anomalies in LiNbO3:Mg Crystals

Optics and Spectroscopy - We have observed that LiNbO3:Mg (0.19–5.91 mol % MgO) crystals experience rearrangements in their defect structure with increasing concentration of Mg and exhibit...