Multiline coherent oscillation in photorefractive crystals with two species of movable carriers

The coherent oscillation, because of nearly degenerate four-wave mixing in photorefractive crystals with two types of movable charge carriers, occurs at two spectral lines symmetrically shifted with respect to the pump frequency. Consequently the output oscillation exhibits the high contrast intensity modulation. The frequency separation of two oscillation modes (and modulation frequency of the output intensity) depend on the incident light intensity and spatial frequency of the developing grating. A model is presented explaining this type of oscillation by the two-maxima shape of the gain spectrum in crystals with sufficiently different relaxation times of two space-charge gratings, one formed by movable electrons and the other one by movable holes. The experimental data for coherent oscillator with tin hypothiodiphosphate (Sn₂P₂S₆) are in reasonable quantitative agreement with the calculations. Received: 12 November 1998 / Revised version: 11 January 1999 / Published online: 7 April 1999     

Transient gain in photonic crystals

Using the optical response formula for photonic crystals and the Bloch equations, we analyze coherent emission in arbitrary two-dimensional photonic crystals. The transient emission depends not only on the photon density of the states, but also on the intensity and duration of the pump laser pulses. A transient gain exists if the pump pulse area is a certain value and the transition frequency is tuned to the band edge. The sharp structure of the density of states near the band edge leads to a dramatic enhancement of the transient emission intensity and an abrupt change in the oscillation of the emission field. PACS 42.70.Qs; 32.80.-t; 42.25.Bs     

Parameters of light-induced charge transfer processes in photorefractive crystals

A method is outlined by which the parameters governing light-induced charge transfer processes in photorefractive crystals can be determined. The system BaTiO₃:Rh is treated as an example. EPR is used to obtain information on the EPR-active defect charge states. Through simultaneous observation of light-induced EPR-and optical absorption changes, the corresponding optical absorption bands are established as fingerprints of the defect charge states. Consistency arguments make it possible to label EPR-silent absorption bands in this way also, significantly extending the scope of EPR-based defect studies. On this basis, the charge transfer paths taking place under illumination are identified. Quantitatively, the defect concentrations are directly or indirectly derived from the available EPR signals. In addition, the kinetics of the light-induced changes of the densities of the occurring defect charge states are studied. In conjunction with the defect concentrations, this makes it possible to deduce the responsible transfer parameters.     

Critical enhancement of nonlinear response in fast photorefractive crystals

It is shown that when approaching the spatial subharmonics generation threshold in fast photorefractive crystals (the sillenites, CdTe), a practically unlimited (singular) amplification of the nonlinear photorefractive response is possible, which results in a drastic increase in the spatial amplification of weak signals. A theory of critical spatial amplification is developed. This theory takes into account real attributes of fast photorefractive crystals such as the vectorial nature of wave coupling and nonuniform broadening of resonances owing to light absorption. The theory is applied to the analysis of the observable characteristics of critical enhancement and to optimization of the conditions of experiments aimed at the detection and investigation of this phenomenon.     

Nonstationary gain processes of holographic gratings by two-wave interaction in photorefractive crystals

The transformation processes of holograph gratings in photorefractive crystals from two-wave interaction with detuning have been studied. For weak interactions an analytic solution has been obtained. Analysis of the gain, both for a purely nonlocal response, and for phase-shifted responses has been performed. Gain thresholds have been determined in terms of their dependence on materials parameters and the conditions of interaction. It is shown that the main effects in the gain process are a rotation of the grating vector towards its Bragg orientation and the appearance of a strong nonuniformity in the grating in proportion to the gain. As a result, for significant increases in the thresholds, the growth of the diffraction field of a non-Bragg-matched grating is larger than for diffraction of a Bragg-matched grating.Tomsk State Academy of Linear Systems and Radioelectronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 77–83, July, 1995.     

Space charge field limitations in photorefractive LiNbO3:Fe crystals

We use holographic techniques for the investigation of strongly oxidized LiNbO₃:Fe crystals with small Fe²⁺ concentrations and compare the results with theoretical predictions. Experimental evidence is presented for enhanced phase shifts between light intensity pattern and refractive index grating and for limitations of optically induced space charge fields in photovoltaic crystals due to the low concentration of filled traps. Our findings do not support the model of a nonlocal photovoltaic effect in LiNbO₃.     

Light-induced charge transport processes in photorefractive crystals II: Materials

3 , LiTaO₃, BaTiO₃, Ba_1-xSr_xTiO₃ (, BST), Ba_1-xCa_xTiO₃ (, BCT), KNbO₃, KTa_1-xNb_xO₃ (, KTN), Sr_1-xBa_xNb₂O₆ (, SBN) and Bi₁₂(Si,Ti,Ge)O₂₀ (BSO, BTO, BGO) are discussed. Utilizing the knowledge on the charge transport processes, consequences for applications are deduced; improved techniques for nondestructive readout of holograms with light of the recording wavelength are described. Received: 14. October 1996     

Method for determining the two-beam coupling gain coefficients of photorefractive crystals

A new method is proposed for determining the two-beam coupling gain coefficients of photorefractive crystals with both o- and e-polarized lights. This method enables one to determine simultaneously and precisely the gain coefficients of a crystal for o- and e-polarized lights while the fanning effect is diminished. Experimental demonstrations are presented.     

Light–induced charge transport processes in photorefractive barium titanate crystals doped with iron

3 ) are annealed in a hydrogen atmosphere at various temperatures. After these reducing treatments, absorption, light–induced absorption changes, two–beam coupling direction, photo electron paramagnetic resonance (photo EPR), dark and photoconductivity as well as bulk photovoltaic current density are investigated. The samples are electron conductive and the charge transport is governed by only one level, which is identified by photo EPR as Fe²⁺/Fe³⁺. The photoconductivity exceeds the dark conductivity for intensities above 1 kWm^-2. A relation between the absorption constant and the Fe²⁺ concentration is derived. From the known charge transport parameters the advantageous photorefractive properties of optimized reduced BaTiO₃:Fe are deduced; possible response times in the millisecond range at an intensity of 10 kWm^-2 are estimated. Received: 22 January 1997/Accepted: 23 January 1997     

Optical bistability by two laser beams in photorefractive crystals

For a nonlinear medium, a given incident laser beam may produce different transmitted light waves. This phenomenon (the so-called optical bistability) corresponds to multiple solutions of the boundary value problems of the nonlinear Helmholtz equation. Optical bistability can be useful in the design of optical switches. Devices that display this behavior could potentially play a major role in the development of optical communication systems and computing. In this article we present experimental results concerning the optical bistability in photorefractive BaTiO₃:Fe crystal. Two laser beams were used to interact with the photorefractive crystal which resulted in the bistability of the intensity of transmitted wave. This was achieved without the application of any optical resonator. High contrast optical bistability is found experimentally in the pump-ratio dependence of the output intensity.     

利用两种响应时间不同的光折变晶体实现仅对物体变化部分成像

用两种响应时间相差很大的光折变晶体Ce:SBN和Fe:LiNbO_3作为相位共轭麦克尔逊干涉仪的两个相位共轭镜,得到同一物体的两个有时间差的相位共轭像,再对这两个像做实时“异或”运算,就能实现仅对物体随时间变化部分的成像.这种方法可以大大增加两相位共轭镜的响应时间差,具有最小可探测速度低,输出信号强、输出保持时间长的优点.文中对此作了理论分析,并提供了该系统对真实物体成像的实验结果.     

Dynamic light beam deflection caused by space charge waves in photorefractive crystals

During holographic recording in photorefractive crystals (BSO, BGO, and BTO) by an oscillating interference pattern we observe a strong dynamic deflection of the laser beams reflected from the crystal’s surface. The theoretical treatment shows that this new effect is associated with a nonlinear interaction of space charge gratings resulting in a quasi-homogeneous oscillating space charge field which provides deformations of the crystal via the piezoelectric effect. Received: 3 August 1999 / Accepted 5 August 1999 / Published online: 8 September 1999     

Electron-phonon interaction and charge carrier mass enhancement in SrTiO3

We report a comprehensive THz, infrared and optical study of Nb-doped SrTiO3 as well as dc conductivity and Hall effect measurements. Our THz spectra at 7 K show the presence of an unusually narrow (<2 meV) Drude peak. For all carrier concentrations the Drude spectral weight shows a factor of three mass enhancement relative to the effective mass in the local density approximation, whereas the spectral weight contained in the incoherent midinfrared response indicates that the mass enhancement is at least a factor two. We find no evidence of a particularly large electron-phonon coupling that would result in small polaron formation.     

Dynamic study of the gain in two beam coupling in photorefractive materials

Wave equations describing the non-linear two-beam coupling have been solved and expressions for the gain of the two beams in the photorefractive crystals have been derived. In case of the degenerate two-beam coupling, the gain depends upon the crystal thickness, coupling coefficient, absorption coefficient and the input intensity ratio. The effect of these parameters on the gain has been studied in details. In case of non-degenerate two-beam coupling the gain not only depends upon crystal thickness, coupling coefficient, absorption coefficient and the input intensity ratio but also on the response time of the photorefractive medium. This response time is the function of concentration ratio. The influence of oscillation frequency shift, concentration ratio on the gain for the non-degenerate two-beam coupling has also been studied.     

Light-induced charge transport processes in photorefractive crystals I: Models and experimental methods

Received: 14. October 1996     

On the dependence of the free carrier concentration on light intensity in photorefractive crystals

The dependence of the free charge carrier concentration on light intensity is analyzed in two types of crystals where photorefractive centers are either traps or donors. Conditions under which the dependence becomes sublinear at relatively low light intensities are established.     

Discrepancy between the times of space charge grating formation and relaxation in photorefractive crystals

A discrepancy between the times of formation and relaxation of space charge gratings is found. This effect is observed when the uniform and nonuniform parts of the optical excitation of the medium appear and disappear simultaneously. It is shown that the formation time of these gratings is always shorter than the relaxation time and the higher the light intensity, the greater the discrepancy.     

Competition between subharmonic and signal beams for photorefractive gain in BSO with two pump beams

It is shown that two pump beams incident upon a bismuth silicon oxide crystal give rise to a subharmonic beam bisecting the pump beam. Simultaneously a signal beam injected between the pump beams may be strongly amplified at the expense of the pump beams. It is shown that the amplified signal beam grows with a faster time constant than the subharmonic beam and that there is competition for gain in the steady state regime.     

Dependence of space charge field and gain coefficient on the applied electric field in photorefractive materials

Intensity dependent space charge field and gain coefficient in the photorefractive medium due to the two interfering beams have been calculated by solving the material rate equations in presence of externally applied dc electric field. The gain coefficient has been studied with respect to variations in the input intensity, modulation depth, concentration ratio and normalized diffusion field in the absence and presence of the externally applied dc electric field. Space charge field has also been computed by varying the intensity ratio in the presence and absence of the externally applied dc electric field. It has been found that the rate of change of the space charge field with the normalized dc field decreases with the increasing intensity ratio for different values of the normalized diffusion field. It has also been found that the externally applied dc electric field has appreciable effect only when it is larger than the diffusion field.