Light induced charge transfer processes in Cr doped Bi12GeO20 and Bi12SiO20 single crystals

Optical absorption and ESR spectra of Bi₁₂GeO₂₀ doped with Cr were measured before and after illumination with visible light. It was found that Cr⁴⁺ ions in tetrahedral position are responsible for light induced ESR and optical spectra. The g-factor of the center is 1.945 ± 0.002. Crystal field parameters for the Cr⁴⁺ center are D_q = 820 cm^?1 and B = 429cm^?1. The photochromic effect is explained in terms of a Cr⁵⁺?Cr⁴⁺ charge transfer process.     

Crystal chirality and optical rotation sense in isomorphous Bi12SiO20 and Bi12GeO20

Laevorotatory crystals of isomorphous Bi₁₂SiO₂₀ and Bi₁₂GeO₂₀ are shown, by use of the anomalous X-ray scattering component in the structure amplitudes, to have identical chirality (handedness of atomic arrangement) in contrast with the case of NaC10₃ and NaBrO₃, in which crystals of the same chirality rotate plane polarized light in opposite senses.     

Reversible photochromic effects in doped single crystals of bismuth germanium (Bi12GeO20) and bismuth silicon oxide (Bi12SiO20)

Absorption of Bi₁₂GeO₂₀ and Bi₁₂SiO₂₀ single crystals doped with Mn and Cr were investigated before and after illumination with visible light. Pronounced photochromic effects were found. The effects are explained in terms of light induced charge transfer Mn⁴⁺→Mn⁵⁺ and Cr³⁺→Cr²⁺.     

中子非弹性散射对Bi12GeO20和Bi12SiO20旋声性的研究

通过用中子非弹性散射测量旋声晶体高对称方向的横声学支声子色散曲线的劈裂,从三个方面研究了同构异质晶体Bi₁₂GeO₂₀和Bi₁₂SiO₂₀的旋声特性。值得注意的是,沿〈111〉方向传播的左旋、右旋圆偏振声子具有不同的衰减。其中传播速度快的圆偏振声子衰减较大,该结论为这两种晶体所证实,这是在晶体旋声性研究中首次观察到的现象。实验还直接确定了衰减较大的这支声子的寿命约为2×10^-11S。 关键词：     

Leaky photorefractive surface waves in Bi12TiO20 and Bi12SiO20 crystals

Consistent experimental and numerical simulation studies of photorefractive surface wave propagation in Bi₁₂TiO₂₀ and Bi₁₂SiO₂₀ crystals are presented. We induce the photorefractive surface wave with a Gaussian beam of small diameter and show that light leakage from the surface wave takes the form of multiple beam reflection from the crystal surface. The efficiency of surface wave excitation is proved to be much higher for the divergent Gaussian beam than for the convergent one.     

Temperature-dependent far-infrared properties of Bi12GeO20 single crystal

Far-infrared reflectivity spectra of bismuth germanium oxide (Bi12GeO20) single crystals are measured from room temperature down to 10 K. All the reflectivity spectra are fitted to a complex dielectric function ε(ω) in the factorised form. Phonon modes at low frequency are found to develop upon decreasing temperature and gain considerable spectrum weight below 150 K. The temperature-dependent static dielectric constants are obtained from the Lyddane-Sachs-Teller relation based on the obtained oscillator parameters. The dielectric constants are found to increase upon decreasing temperature, which is attributed to the charge transfer among the ions in the unit cell with the temperature varying.     

“Running” holograms in photorefractive Bi12SiO20 crystals

It is shown that under uniform illumination and application of an external electric field, a phase volume hologram in a photorefractive crystal exists in the form of relaxing running wave. Effective recording of such a running hologram is possible only by an interference pattern moving synchronously with the recorded grating. The physical nature of this phenomenon, the possible enhancement of the hologram diffraction efficiency upon recording of a running pattern and the main factors limiting its value are discussed for the case of photorefractive Bi₁₂SiO₂₀ crystals.     

Normal modes in the vicinity of the brillouin zone center of tightly bound units in Bi12GeO20 and Bi12SiO20

From Raman data and a group-theoretical analysis, we deduce that in Bi₁₂GeO₂₀ and Bi₁₂SiO₂₀ there are four weakly bound oxygen atoms and a tightly bound unit MO₄, where M = Ge or Si. Some normal modes of the tightly bound units are identified in Raman and in infrared reflection data. The kinds of interatomic binding forces in these crystals are determined. The results are compared with structure data determined by X-ray scattering.     

Real-time image subtraction in four-wave mixing with photorefractive Bi12GeO20 crystals

Based on a classical two-exposure holographic technique, real-time image subtraction is obtained in four-wave mixing of a reflection hologram type with highly sensitive photorefractive Bi₁₂GeO₂₀ crystals. This method takes advantage of the relatively slow response and erasure times of holographic recording in Bi₁₂GeO₂₀ crystals.     

Real-time image division in four-wave mixing with photorefractive Bi12GeO20 crystals

Real-time image division is obtained by using four-wave mixing of a reflection hologram type with photorefractive Bi₁₂GeO₂₀ crystals. This method utilizes the nonlinear characteristic of four-wave mixing which reverses the phase of the incident wave.     

Light diffraction and nonlinear image processing in electrooptic Bi12SiO20 crystals

Some characteristic peculiarities of the volume phase holograms in a high-sensitive electrooptic Bi₁₂SiO₂₀ crystals such as polarization properties of the diffracted light beam and nonlinear phenomena in the stationary stage of the hologram formation are investigated. Spatial frequency mixing or combinational hologram writing by means of non-linearity of the holographic process is discussed as a new promising technique of the volume holography. In particular two important problems of Bi₁₂SiO₂₀ crystals: nondestructive readout at the wavelength different from that of the writing beams and correlation analysis of complex two-dimensional pictures can be solved by this method.     

Transient energy transfer between writing beams during hologram formation in Bi12GeO20

A large transient energy transfer between writing beams during volume phase-hologram formation in Bi₁₂GeO₂₀ has been observed. The variation of the energy transfer by applying an external field and by changing the fringe spacing of the elementary holograms have been measured and described in the framework of Kukhtarevs nonlinear theory of transient self-diffraction. The results indicate that the transient energy transfer in Bi₁₂GeO₂₀ is due to a non-stationary phase mismatch between the intensity- and refractive index grating caused by photoconductivity and that the transient energy transfer increases with increasing fringe spacing and increasing applied electric field. Up to ten-fold amplification of coherent beams has been measured, corresponding to an exponential gain of Г = 2 cm^-1. A continuous energy transfer has been observed when the phase between recording beams was changed linearly with time during recording. It is shown that light is coherently amplified for electric field strengths larger than 2.3 kV/cm and that a peak amplification factor of Г₁ = 2 cm^-1 has been reached for E₀ = 11 kV/cm.     

Mapping of vibration amplitudes by time average holography in Bi12SiO20 crystals

We report the performance of a holographic set-up with a Bi₁₂SiO₂₀ (BSO) photorefractive crystal for measuring vibration amplitudes in a time average holography system. The volume hologram is recorded with the 514.5 nm wavelength of an Ar⁺ laser. In this set-up the reference beam is sinusoidally phase-modulated with the same frequency of the vibrating object. By increasing the value of the phase modulation, the fringes of the time average pattern run along the object surface, so that the vibration amplitude mapping is carried out. This technique allows the improvement of the measurement sensitivity compared with conventional time average holography, even for interferograms with poor visibility.     

Polarization characteristics of volume holograms in Bi12SiO20

The polarization sensitivity of holographic recording in Bi₁₂SiO₂₀ is experimentally studied. The shape of the obtained curves depends on the magnitude of the applied electric field, crystal thickness, spatial frequency of the recorded gratings, and the order of diffraction.     

Coherent signal beam amplification in two-wave mixing experiments with photorefractive Bi12SiO20 crystals

The amplification of the input signal beam in two-wave mixing experiments with photorefractive Bi₁₂SiO₂₀ crystals is achieved when an additional phase shift is established between the photoinduced index modulation (phase volume hologram) and the incident fringe pattern. This stationary phase shift is introduced by either moving the crystal or the interference fringes at a constant speed. The transferred intensity is measured versus the applied electric field, fringe spacing and crystal velocity. The crystallographic orientation and the relative displacement with respect to the applied electric field polarity determine the amplitude of the energy transfer. For the first time in this crystal, signal beam amplification is reached for an applied field E₀ > 8 kV cm^?1 and a crystal or fringe displacement speed around 5 μm s^?1 at the green line (λ = 514 nm) of an argon laser.     

Phase-conjugate wavefront generation via four wave mixing in Bi12GeO20 crystals - reflection hologram type

Phase-conjugate wavefront generation via real-time holography in degenerate four-wave mixing experiments in photorefractive crystals Bi₁₂GeO₂₀ is presented. The phase volume hologram responsible for the phase-conjugate wavefront generation is a reflection hologram. No external electric field is applied and electron diffusion is the main mechanism responsible for the formation of the hologram. The theoretical and experimental results for wavefront reflectivity as a function of the intensity ratio of the writing beams are given.     

Analysis of reflectivity spectrum and band structure of Bi12GeO20

The fundamental reflectivity spectrum in the energy range 1.5–8.5 eV is reported for Bi₁₂GeO₂₀ (BGO) single crystals. From a Kramers-Kronig analysis the optical constants of BGO are computed. An energy level scheme for interband optical transitions is proposed. A value of 0.7 eV for the spin-orbit splitting of the valence band is suggested. Some features of the band structure are discussed, and the correlations between the band structures of BGO and A₂^VB₃^VI-compounds are indicated. The experimental premises for possible simplifications of a theoretical calculation of the band structure are given.     

锗酸铋晶体的超声特性

本文采用超声脉冲回波法在4.2K—300K温区内测试了锗酸铋晶体[110]和[111]晶向的超声纵波衰减与温度的关系,结果在50K附近均出现由于激活弛豫过程所引起的大的异常衰减峰;本文还用脉冲回波重合法精密测定了[100],[110]和[111]晶向的超声纵波和横波的声速,并由此计算出弹性常数C₁₁,C₄₄和C₁₂。     

Polarization properties of phase gratings recorded in a Bi12SiO20 crystal

Due to the co-existence of optical activity and linear birefringence in a Bi₁₂SiO₂₀ crystal, a phase gratting induced in this medium has many interesting and unusual properties. We study the ellipticity and orientation of the directly transmitted and diffracted beams as a function of the read-beam vector orientation. In image processing experiments the signal to noise ratio can be increased significantly by appropriately choosing the read-beam orientation such that the strong direct beam is linearly polarized and can be eliminated with the help of a polarizer.     

Bi12GeO20晶体的位错蚀斑与对称群

用化学浸蚀法系统的研究了Bi₁₂GeO₂₀晶体{100},{100},{111}等晶面独特浸蚀斑规律并用对称群理论进行分析.理论预示的蚀斑形态与实验结果符合很好.显示位错蚀斑是由{112}晶面组成,即晶体的惯态面为{112}.