Optical two-beam coupling for a surface-induced photorefractive effect in undoped liquid crystals

We report beam-coupling measurements of undoped nematic planar cells, for which the surface-induced photorefractive effect (SIPRE) has been confirmed to be due to photoelectric interface activation. We show that the energy transfer strongly depends on experimental geometry. Diffraction efficiency and beam-coupling analyses suggest that the unusual local and (or) nonlocal behavior can be ascribed to an additional in-phase-modulated longitudinal electric field component, unaccountable by standard photorefractivity. We consider a simple charge distribution model of the SIPRE, which produces a space-charge field having two orthogonal modulated components, in phase and pi/2 out of phase. Through the liquid crystal's electric response, the exploited geometries verify the presence of the two components.     

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.     

Temperature dependence of two-beam coupling and dark decay in photorefractive BaTiO3

The temperature dependence of two-beam coupling and dark decay in photorefractive BaTiO₃ is reported. We show that the competition between deep and shallow traps depends on temperature and writing intensity, and influences two-beam coupling and dark decay. The dynamics of dark decay, characterized by a fast decay of partial erasure and a subsequent slow decay, is influenced by the presence of deep and shallow traps. Partial erasure, due to thermal excitation of charges from the shallow traps, decreases with temperature and increases with writing intensity. The time constant of the slow decay, due to thermal excitation of charges from the deep traps, depends strongly on temperature, but not on the writing intensity. At room temperature, the existence of deep and shallow trap leads to intensity-dependent photorefractive gains. As temperature increases, the influence from the shallow trap decreases, and the photorefractive gain becomes independent of the intensity. However, at much higher temperatures (100°C), the photorefractive gain resumes its dependence on intensity due to an increase in dark conductivity at elevated temperature.     

-70 dB optical carrier suppression by two-beam coupling in photorefractive media

Suppression of an optical carrier from an RF modulated laser beam of wavelength 532 nm is performed using two-beam coupling in photorefractive barium titanate. A theoretical analysis reveals that perfect suppression can be achieved at a specific modulation strength, which depends on the gain and intensity ratio between the two beams. The experiments achieve a maximum of -72.9±2.4 dB carrier suppression at the theoretically ideal modulation strength, and -61.0±2.4 dB for small modulation strengths. Received: 29 November 2000 / Revised version: 2 February 2001 / Published online: 27 April 2001     

Effect of photoconductivity and dielectric constant of the photorefractive materials on two-beam coupling gain and phase-shifts for a single unidirectional photorefractive ring resonator

Photoconductive dependence of two-beam coupling between the pump beam and the signal beam in photorefractive materials have been analyzed in case of non-degenerate wave mixing under the undepleted pump approximation method. During the two-wave mixing in photorefractive materials, steady state amplification of the signal beam and oscillation characteristics of a single unidirectional ring resonator has been studied. The domination of the two-beam coupling gain over the combined absorption and resonator losses such as Fresnel reflections from the crystal and imperfect mirrors builds up unidirectional oscillation. The buildup of such an oscillation leads to a saturation of the gain, which can be explained in terms of the photorefractive phase-shift. The existence of this phase-shift between the photorefractive index grating and the illumination intensity pattern, which is of characteristic of the photorefractive effect, leads to an energy transfer between the two beams. For a single unidirectional ring resonators, the effects of photoconductivity of the materials, two-beam energy coupling coefficient, dielectric constant, crystal thickness, and material's absorption coefficient on amplification of the two-beam coupling gain and photorefractive phase-shifts of the signal beam have also been studied in detail. It has been found that amplification of the signal beam and phase-shift can be enhanced by taking the photorefractive crystal having higher photoconductivity and lower dielectric constant, which improves performance of the resonators.     

Time-resolved two-beam coupling in silicon

The analytical solution of the optical field equations and the material equations for two-beam coupling in a medium with real excitation of quasiparticles is given. Transient energy transfer is analyzed throughout the range of the coherent overlapping of the excitation and probe pulses for a hypothetical medium with variable relation between the real part and the imaginary part of the nonlinear response. A detailed comparison between the theory and the model experiments for two-beam coupling in silicon is given.     

Influence of the polarization direction of two-beam coupling in photorefractive Bi12SiO20: Diffusion regime

Beam coupling effects in an optically active photorefractive crystal of Bi₁₂SiO₂₀ are very much influenced by the polarization direction of the interacting beams. Analytic expressions for the signal amplification in the diffusion regime have been derived for two commonly used crystallographic configurations,G<001> andG<110>,G being the grating vector. It is shown that only the <001> orientation is suitable for signal amplification. The results obtained for the diffusion regime can be applied, as a first approximation, to the more common and more intricate drift regime in which a bias electric field is applied to the crystal to enhance the coupling effect.     

Conversion of unpolarized light to polarized light with greater than 50% efficiency by photorefractive two-beam coupling

All known polarizers operate through a separation of orthogonal electric field components, one of which is subsequently discarded. As a result, 50% of the unpolarized incident light is wasted in the process of conversion to polarized light. We demonstrate a new method by which we use the optical power in the ordinarily discarded component as the pump to amplify the retained component through photorefractive two-beam coupling to achieve greater than 50% throughput.     

Phase conjugation, edge detection and image broadcasting using two-beam coupling configurations in photorefractive KNbO3

Two-beam coupling configurations are employed to obtain the phase conjugate image and edge extraction of an object using photorefractive KNbO₃: Fe. Also, the self-organization of a beam in the material into a hexagonal spot array is utilized to broadcast an input object to the location of each of the spots.     

Multiwave coupling in a high-gain photorefractive polymer

The characteristics of a new high-gain photorefractive polymer composite with a PNP chromophore are investigated. Competition between beam fanning and two-wave coupling (TWC) is predicted and verified experimentally. The intensity dependence of TWC gain is studied. Higher diffraction order and forward phase conjugation in a TWC geometry are observed and explained.     

温度对Ce：Fe：LiNbO3晶体双光束耦合效率的影响

实验研究了温度对Ce:Fe:LiNbO_3晶体双光束耦合的响应时间以及对信号放大倍数的影响,并用带导模型和耦合波方程对其作了理论分析.半定量地解释实验结果.     

Study of transient photorefractive beam coupling in high- and low-molar-mass liquid crystal mixtures

We observed transient signals of Bragg grating formation in high-performance photorefractive high- and low-molar-mass liquid crystal mixtures (HL-LCMs). These transient signals including oscillation-like behavior can be explained by a complex time constant in the case of a finite applied dc field. The time constant for the space-charge field to reach its steady state and the oscillation frequency are estimated. Received: 30 October 1998 / Revised version: 14 December 1998 / Published online: 24 February 1999     

Nonunidirectional two-beam interaction in photorefractive bismuth silicate in an alternating electric field

The effective nonunidirectional energy exchange between like-polarized light beams is investigated experimentally on a photorefractive grating formed by diffusion drift in a bismuth silicate crystal placed in an alternating external electric field. Fiz. Tverd. Tela (St. Petersburg) 41, 632–637 (April 1999)     

双光束耦合增益振荡的理论研究

本文从双光束增益振荡的动态方程组出发,给出一般情况下双光束光折变振荡稳态时的普遍结果,解释了文献[1,2]中理论不完全符合实验规律的原因,表明文献[1,2]中的理论只适合于光折变效应为纯扩散机制的特殊情况。     

光擦除双光束耦合的阈值开关特性

基于光擦除折射率光栅和双光束耦合构成了一种新型的非线性光学器件.用带导模型和耦合波方程建立了理论公式.用KNSBN单晶和Ar~+激光进行了实验演示.理论和实验都表明该器件具有明显的阈值开关特性.     

Vanadium-doped photorefractive titanosillenite crystal

Holographic recording in a vanadium-doped B₁₂TiO₂₀ (BTO) photorefractive crystal puts into evidence a large hole–electron competition showing a fast and a slow hologram components. From the fast component evolution, some material parameters for the electron-donor photoactive centers are computed. The wavelength-resolved photoconductivity is shown to be strongly modified by V-doping compared to undoped and doped BTO with other elements. The increase of photoconductivity by green light preexposure is almost negligible here if compared with undoped BTO. Activation energy for dark conductivity measured for BTO:V is similar to that for undoped BTO, as measured close to room temperature, but sensibly lower than the value reported in the literature for a much higher temperature range. Optical absorption and EPR spectra do confirm already published results and suggestions about the possible role of vanadium in the sillenite structure.     

Use of two-photon absorption in a photorefractive crystal for three-dimensional optical memory

We describe the use of two-photon absorption in a photorefractive crystal for recording bit data in multilayered optical memory. A short-pulse near-infrared laser is used for generating the photorefractive effect by two-photon absorption. We succeeded in recording and reading seven layers of data in a LiNbO(3) crystal with a lateral resolution (distance between bits) of 5microm and an axial resolution (distance between layers) of 20 microm .     

有偏压光折变晶体中的高斯孤子

利用变分方法求解了小光强光折变非线性薛定谔方程，得到了高斯光束在外加正偏压光折变晶体中的演化特性，以及高斯光束宽度压缩与展宽的动态振荡规律，给出了高斯光束的宽度、振幅和波前曲率的表达式，研究了势函数与光束宽度的关系.结果表明，小光强高斯光束在正偏压光折变晶体中传播时，其宽度和振幅将经历周期地变化或者展宽发散，在一定条件下高斯光束在晶体中可以形状不变地传播，形成“高斯孤子”，这些特性在工程实践中具有很高的实用价值. 关键词： 高斯孤子 高斯光束 变分方法 光折变非线性方程