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1.
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.  相似文献   

2.
The optical absorption and ESR spectra of Bi12GeO20 and B12SiO20 doped with Mn have been measured before and after illumination with visible light. Uniaxial stress measurements on a sharp line observed at 8026 cm?1 were performed. The observed ESR spectrum is a superposition of six lines resulting from the hyperfine interaction of manganese ions in tetrahedral positions. The g-factor and hyperfine constant are g = 1.999 ± 0.003 and A = 78 Gs. Analysis of the light-induced absorption spectrum leads to the conclusion that a small hole polaron bound to an Mn impurity at a tetrahedral site is responsible for the very broad absorption band which appears after illumination. The sharp line is interpreted as due to a transition inside the Mn+ center in tetrahedral coordination. Bands in the region 10,000–16,000 cm?1 are due to Mn3+ centers in interstitial positions, whose symmetry can be treated to a first approximation as tetragonal. The following crystal field parameters for this center were found: B = 565 cm?1, Dq = 1400 cm?1, Dt = ?330 cm?1, Ds = 4170 cm?1 and C = 2260 cm?1. The illumination conditions which are needed for homogeneous coloration of the sample are also discussed.  相似文献   

3.
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 Dq = 820 cm?1 and B = 429cm?1. The photochromic effect is explained in terms of a Cr5+?Cr4+ charge transfer process.  相似文献   

4.
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.  相似文献   

5.
We present a recent development in holography with multimode, large free-spectral range (FSR) diode lasers in photorefractive sillenite crystals. A novel refractometry method based on this type of holographic recording in Bi12TiO20 (BTO) crystals is proposed. The holographic image of a prism-shaped transparent sample appears covered of interference fringes, and as the sample is properly translated, the fringes run along the holographic image. An expression providing the refractive index of the medium as a function of the sample displacement and the correspondent number of running fringes was derived. The refractive indexes of optical (BK7) glass, ethanol, hexan, cumene and aqueous solution of NaCl with different concentrations were measured in order to test the method. The obtained results are in good agreement with the ones reported in literature or measured by us using a commercial Abbe refractometre.  相似文献   

6.
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.  相似文献   

7.
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.  相似文献   

8.
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.  相似文献   

9.
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.  相似文献   

10.
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:  相似文献   

11.
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+.  相似文献   

12.
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.  相似文献   

13.
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.  相似文献   

14.
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.  相似文献   

15.
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.  相似文献   

16.
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.  相似文献   

17.
A simple method for accurate measurement of Faraday rotation in materials with low rotation is developed using photorefractive Bi12TiO20. High optical activity of Bi12TiO20 is made use for this measurement. Detailed theoretical and experimental analysis is given.  相似文献   

18.
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.  相似文献   

19.
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.  相似文献   

20.
A photorefractive Bi12TiO20 (BTO) crystal is exposed with high intensity pulsed-laser beams 532 nm (the average intensity of the laser was 110 mW/cm2, the average intensity per pulse was 2 MW/cm2), and the photo-induced dynamics of the absorption between 480 and 900 nm are studied and explained by two long-lived energy levels in the forbidden band. The relaxation times of the long-lived energy levels are experimentally found to be 104 and 105 s. The hysteresis character of the absorption coefficient is discussed. PACS  42.70.Mp; 42.70.Gi  相似文献   

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