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.     

“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.     

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.     

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.     

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.     

Influence of the optical activity on hologram formation in photorefractive crystals

It is shown that dynamical hologram formation in photorefractive, optically active crystals is accompanied by polarization and energy exchange. These effects may be used like new effective holographic methods for determinations of crystal parameters and optical signal processing in real time. In self-diffraction a polarization rotation the order of degrees/cm for Bi₁₂TiO₂₀ and Bi₁₂SiO₂₀ is predicted.     

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：     

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.     

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.     

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.     

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⁺.     

Large Size of Real-Time Bi12Si020 Hologram Device Made with Inexpensive Wafer Cutting Method

The large real-time hologram device made by the previous Bi₁₂SiO₂₀ single crystal wafer cutting method poses several problems such as a large residue of material hinders the practical use. A study was carried out to fabricate a real-time hologram device using inexpensive laterally cut circular (1 0 0) Bi₁₂SiO₂₀ single crystal wafers which would be suitable for practical use for 3-dimensional display. An angle incident on a (1 0 0) wafer is applicable to the real-time hologram. An optimal electrode design for a device with uniform diffraction efficiency was considered, and the device properties were experimentally investigated.     

Holography,coherent light amplification and optical phase conjugation with photorefractive materials

This paper presents a review of photorefractive effects in electro-optic materials and their applications. Materials properties giving rise to increased photo-sensitivities are discussed and compared with experimental data for a series of newer materials. The formation of volume holograms in these crystals, and accompanying optical effects such as beam coupling and phase conjugation, are discussed. Theoretical results derived from charge transport parameters are compared with our original data obtained in “dynamic holography” experiments with Bi₁₂GeO₂₀, Bi₁₂SiO ₂₀ and KNbO₃:Fe²⁺. The optimum parameters for applications such as volume hologram storage, coherent light amplification, optical phase conjugation etc. have been determined, and possible applications are discussed.     

Raman scattering in monocrystal Bi3B5O12

Raman spectra of a Bi₃B₅O₁₂ monocrystal are investigated for the first time and compared with the Raman spectra of crystals α-Bi₂O₃ and Bi₁₂SiO₂₀. The most intensive lines observed in the Raman spectrum of Bi₃B₅O₁₂ are identified. Promising prospects of use of Bi₃B₅O₁₂ crystals as SRS converters of radiation are shown.     

Two-wave mixing on reflection dynamic gratings in sillenite crystals under phase modulation of signal beam

The counter mixing of a strong circularly polarized wave of optical pump with a weak phase-modulated signal wave on the reflection grating formed by them in (111)-, (110)-, and (100)-cut Bi₁₂TiO₂₀ and Bi₁₂SiO₂₀ phototrefractive crystals was theoretically and experimentally studied.     

Synthesis and laser patterning of Bi-doped Y3Fe5O12 crystals in germanosilicate glasses

New germanosilicate glasses giving the crystallization of yttrium iron garnet Y₃Fe₅O₁₂ (YIG) and Bi-doped YIG, 23Na₂O-xBi₂O₃-(12−x)Y₂O₃-25Fe₂O₃-20SiO₂-20GeO₂ (mol%), are developed, and the laser-induced crystallization technique is applied to the glasses to pattern YIG and Bi-doped YIG crystals on the glass surface. It is clarified from the Mössbauer effect measurements that iron ions in the glasses are present mainly as Fe³⁺. It is suggested from the X-ray diffraction analyses and magnetization measurements that Si⁴⁺ ions are incorporated into YIG crystals formed in the crystallization of glasses. The irradiations (laser power: 32-60 mW and laser scanning speed: 7 μm/s) of continuous wave Yb:YVO₄ fiber laser (wavelength: 1080 nm) are found to induce YIG and Bi-doped YIG crystals, indicating that Fe³⁺ ions in the glasses act as suitable transition metal ions for the laser-induced crystallization. It is suggested that YIG and Bi-doped YIG crystals in the laser irradiated part might orient. The present study will be a first step for the patterning of magnetic crystals containing iron ions in glasses.     

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.     

Three-Photon Absorption in Photorefractive BSO and BGO Crystals

The possibility of optical limitation of picosecond laser radiation in photorefractive Bi₁₂SiO₂₀ (BSO) and Bi₁₂GeO₂₀ (BGO) crystals at a wavelength of 1064 nm has been demonstrated. It is shown that the mechanism determining the process of optical limitation is three-photon absorption. The coefficients of three-photon absorption have been determined by the Z-scanning 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.     

Dispersion of electro-optic coefficients in sillenite crystals

The photorefractive crystals of the sillenite family (Bi₁₂SiO₂₀ or BSO, Bi₁₂GeO₂₀ or BGO, and Bi₁₂TiO₂₀ or BTO) that belong to the cubic 23 point group are optically active, and exhibit the piezoelectric, elasto-optic, electro-optic and electrogyration effect. In this paper we measure the dispersion of the electro-optic coefficient for all the crystals of the sillenite family in the visible spectral range (500–800 nm). For this we measure by ellipsometry the polarization properties of a beam transmitted in the transverse configuration under the action of an externally applied field. The experimental data are fitted with an analytical expression of the beam polarization to find the electro-optic coefficient. The results show a normal dispersion of the electro-optic coefficient in all three sillenite crystals, similarly to other electro-optic crystals.