Wave mixing analysis in photorefractive quantum wells in the Franz–Keldysh geometry under a moving grating

The photorefractive (PR) properties of semi-insulating GaAs/AlGaAs multiple quantum wells (MQWs) operating in the Franz–Keldysh geometry are modelled by solving the material equations including the nonlinear transport of hot electrons. This work studies the PR response of MQWs in a two-wave mixing geometry under a moving grating. Calculations were made under the small intensity modulation approximation, and the simulation results are compared with experimental data available in the literature. A reasonable qualitative agreement regarding most experimental characteristics was found. The results can be treated as a test of the correctness of the commonly used band transport model of PR behaviour in MQWs. Analytic solutions for the stationary and transient regimes under negligible diffusion are given. In addition, the conditions for the occurrence of a strong resonance predicted by the model are noted.     

强外加电场与大调制度下光折变动力学光栅形成研究

强外加电场与大调制度在光折变效应的研究中已经得到了广泛应用。采用PDECOL算法,严格求解光折变带输运方程,得到外加电场时不同调制度下光折变晶体中随时间变化的空间电荷场、载流子浓度,并讨论了外加电场对它们的影响。通过将物质方程与耦合波方程联立数值求解,可得到光折变光栅形成过程中两波耦合增益系数以及光束条纹相位的变化。模拟结果表明,在强外加电场作用下,两束记录光之间的光强与相位耦合都得到了增强,而原有的解析式忽视了强外加电场与大调制度对空间电荷场相位耦合的影响,此时不再适用。同时发现折射率光栅与记录光束条纹均发生弯曲,并不再保持平行。     

KTa_(1-x)Nb_xO_3:Cu晶体中直流空间电荷场的进一步研究

分别用扩束光和一束写入光对KTa_(1-x)Nb_xO_3:Cu(X=0.32)晶体中的光折变栅进行擦洗,通过不同的擦洗效果,进一步论证了在写入过程中除了形成周期分布的空间电荷场之外,同时还形成了直流空间电荷场.并对直流空间电荷场被擦洗的动态过程进行了详细研究.此外,在写入之前利用一束均匀光照射在外加电场作用下的KTa_(1-x)Nb_xO_3:Cu晶体,研究了直流空间电荷场的形成对衍射信号强度的影响.     

High-sensitivity read-write volume holographic storage in reduced KNbO3 crystals

Reduced KNbO₃ is a photoconductive ferroelectric in which holograms can be recorded by the photorefractive effect. Read-write volume hologram storage and erase sensitivities ofS ⁻¹=100 J/cm² andS ⁻¹=84 J/cm² (S=d(Δn)/d(I₀t)‖_t=0) have been measured at zero applied electric field, where the charge transport is shown to be due to diffusion of photoexcited electrons. By applying an electric field along thec-axis, the migration length of the photoexcited electrons becomes comparable to the holographic grating spacing. This leads to storage sensitivities comparable to high-resolution photographic plates. Experimental data on storage and erase sensitivity as a function of the grating spacing, applied electric field, writing light intensity and temperature are reported and interpreted on the basis of the theoretical results of Young et al. and Amodei. Changes of the intensity ratio of the writing beams by self diffraction (beam coupling), reflections from surfaces and the residual dark conductivity are assumed to cause experimental results which deviate from the theoretical models. It is shown, that in reduced KNbO₃ and other ferroelectric photoconductors having photocarrier transport lengths much larger than the unit cell dimension, photovoltaic currents do not contribute significantly to the build-up of space-charges leading to the photorefractive effect.     

Nonlinear interaction of moving space-charge and photoconductivity gratings in Bi<Subscript>12</Subscript>SiO<Subscript>20</Subscript> crystal

A nonlinear interaction of moving space-charge and photoconductivity gratings is experimentally investigated. In the presence of a dc electric field, a crystal is irradiated with an oscillating interference pattern with a spatial frequency K and an oscillation frequency ω. An ac electric field with a frequency \gM is also applied to the sample. At certain frequencies ω and \gM, the crystal exhibits two types of interacting oscillations: the space-charge grating moving with velocity |ω−Ω|/K and the photoconductivity grating moving with velocity −ω/K. The effect is studied using the method of the nonstationary photoelectromotive force in a photorefractive Bi₁₂SiO₂₀ crystal.     

Standard model of a photorefractive effect and moving grating wave-mixing in semi-insulating GaAs quantum wells

Theoretical analysis of photorefractive GaAs/AlGaAs multiple quantum well structure in a wave-mixing geometry with moving interference pattern is presented. The numerical calculations for arbitrary value of the fringe contrast were carried out by solving material equations within the band transport model with bipolar conductivity. An influence of the modulation depth and the nonlinear transport of hot-electrons on the photorefractive response is examined. Theoretical predictions are compared with the available results of moving grating experiments.     

Effects of applied DC/AC fields in electrical fixing process with SBN:60

We present experimental investigations of electrical fixing techniques in Ce:SBN:60. The effect of crystal fatigue on the diffraction efficiency of electrically fixed photorefractive gratings is studied. We observed that applying an ac field to the crystal eliminated crystal fatigue and improved diffraction efficiency. A controllable diffraction efficiency of a photorefractive grating is presented. A reproducible diffraction efficiency of up to 75% is obtained using a write-reveal grating technique with high-voltage pulses of opposite polarity. We also show that the diffraction efficiency can be controlled by varying the intensity of the writing beams incident to the crystal during the recording process. A method of determining the hysteresis loop using the domain-fixing technique is proposed. The dependence of the diffraction efficiency on the direction of the applied electric field while writing the hologram is studied. Our experiments show that, during writing, when the electric field is applied opposite to the c axis the grating can be successfully revealed with both positive and negative dc voltages. However, when the grating is written with a field parallel to the c axis, the grating can be revealed only with a field applied in the opposite direction. Received: 28 October 2002 / Revised version: 28 February 2003 / Published online: 14 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-812/872-6167, E-mail: azad.siahmakoun@rose-hulman.edu     

Diffraction efficiency and energy transfer during hologram formation in reduced KNbO3

Volume phase-hologram formation by the photorefractive effect in KNbO₃ is accompanied by a stationary energy transfer between writing beams. The change in energy transfer by applying an electric field on the reduced crystals is shown to be due to an efficient increase in migration length which can reach values comparable or larger than the fringe spacing. It is demonstrated that photovoltaic contribution to the diffraction efficiency and energy transfer is rather small in reduced KNbO₃ and that diffusion of photogenerated free holes is the dominant charge transport for the photorefractive effect in unbiased crystals. The experimental results for diffraction efficiency and energy transfer as a function of grating spacing, electric field, light intensity and temperature is well described by a recent theory of Kukhtarev and Vinetskii.     

Detection of small phase modulation using two-wave mixing in photorefractive crystals

Two-wave mixing of phase-modulated beams in photorefractive crystals under a dc electric field is studied both theoretically and experimentally. Based on the vectorial theory of light diffraction in crystals of cubic symmetry, we derived an analytical expression that describes the phase demodulation in crystals with an arbitrarily oriented electric field. The phase-demodulation technique was used for estimation of the space-charge field created in photorefractive crystals. It is shown that the space-charge-field grating created in GaP and Bi₁₂TiO₂₀ crystals is much smaller than that predicted from the one-level band-transport model of the photorefractive effect. Received: 12 December 2000 / Revised version: 8 February 2001 / Published online: 27 April 2001     

Nonlinear frequency response of a moving grating with an applied field in bismuth silicon oxide

A fully nonlinear frequency response of a moving grating in bismuth silicon oxide, including the effects of an applied electric field, is modeled by solution of the time-dependent Kukhtarev equations for photorefractive materials. The numerical results are used to define fully the nonlinear response function F(m)=a(-1)[1-exp(-am)], where m is the modulation index in the intensity pattern, to yield the unknown quantity a over a broad range of detuning frequencies f . For low f, the response is superlinear with a<0, and for relatively large f it is sublinear with a>0 . In the midrange we predict, for the first time to our knowledge, a characteristic frequency f(l) at which a=0 and the response is linear, that is, F(m) approximately m, despite the presence of nonlinearly generated higher harmonics of the fundamental grating wave number. In view of this linear behavior, writing a hologram at the linear-response frequency f(l) might permit a more faithful reproduction of an object than that which is possible by writing at the frequency of maximum response at the resonance.     

Processes limiting the rate of response in photorefractive composites

The processes limiting the rate of response of highly diffractive, reorientationally enhanced photorefractive polymer composite materials are identified from a series of degenerate four wave mixing and Mach–Zehnder interferometric measurements. In the regime of low intensity writing beams charge generation limits the rate of holographic grating formation, but at higher intensities charge transport or reorientation of dye molecules can restrict the rate of grating formation more strongly. A grating risetime of 540 ms is observed in a composite of high dye content with high reorientational mobility of the dye molecules. In this case it is proposed that the charge carrier mobility of the doped poly(N-vinylcarbazole):2,4,7-trinitro-9-fluorenone (PVK:TNF) matrix is the principal limiting factor in grating response rates.     

LiNbO3中光折变全息的热固定特性

带输运模型方程和耦合波方程结合起来研究ＬｉＮｂＯ３晶体中体全息记录和热固定的特性。发现在一边记录一边固定时,同时存在的离子光栅通过减小耦合增益而影响了电子光栅的写入,且得到的固定后折射率光栅随深度的改变较小,这与记录完固定有非常大的区别,两种热固定方法得到的折射率光栅的衍射效率大小将主要决定于入射记录的实始光强比,该模型同时显示对于高掺杂浓度的ＬｉＮｂＯ３晶体的热固定,在其显影过程中,由于导带电子     

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)     

Temporal response of photorefractive multiple quantum wells in Franz-Keldysh geometry

Transient behavior of the photorefractive grating generated in multiple quantum well structure working in the transverse Franz-Kieldysh geometry is considered. Analysis was performed in the framework of the band transport model with material parameters of GaAs/AlGaAs system. The influence of carrier transport nonlinearity caused by the intervalley scattering of electrons was taken into account. An analytical expression for the grating formation time is derived and compared with numerical solutions as well as with the available experimental results. The results show that in the case of stationary interference pattern the nonlinear transport of electrons does not considerably improve the response time of the structure. This conclusion, checked for parameters used in majority of experiments, does not confirm the statements of earlier papers.     

掺杂CdS纳米粒子聚合物的光折变效应研究

研究了掺杂CdS纳米粒子的有机聚合物体系的光折变效应,体系中以CdS纳米粒子为光敏剂,聚乙烯咔唑(PVK)为载流子输运剂,4-(4-硝基苯偶氮)苯胺染料(DO3)为非线性生色团,9-乙基咔唑(ECZ)为增塑剂。反胶束法合成的CdS胶束颗粒采用紫外可见吸收光谱和透射电子显微镜(TEM)表征,结果表明,具有纳米尺寸和量子限制效应。研究了PVK-CdS薄膜的光电导特性,实验结果表明,CdS纳米粒子与PVK之间的电荷转移可以有效的提高PVK的光电导率。双光束耦合实验证明了该体系的光折变特性,在无外加电场下,获得二波耦合增益系数78.4 cm-1,分析表明聚合物薄膜具有强的光致取向增强效应;样品在两相干光束作用下,可建立折射率光栅,其衍射效率达到4.4%。     

在局域响应的光折变晶体中的相干光放大

理论和实验表明，在由光生伏打电荷迁移为主的局域非线性响应的薄光折变晶体（LiNbO₃∶Fe）中实现弱信号光放大是可能的．光能流从抽运光向信号光不可逆转移的起因是两束入射光与背面反射光之间的四波作用形成互倾斜、彼此有一定空间相移的两组相位栅．当入射抽运光在非布喇格条件下读出由反射光和其衍射光所写入的新光栅时，弱信号便获得相干放大．对理论计算与实验结果进行了比较． 关键词：     

Photorefractive effects in long, narrow BSO crystals with applied electric field

In a photorefractive Bi₁₂SiO₂₀ crystal with high applied electric ac field of square-wave shape a fast two-wave coupling response (less than 1 s) and a slow hologram readout decay (minutes) was found for a wavelength of 633 nm. This can be explained by electron–hole transport with two trap levels. An intensity dependence of the slower complementary grating was found. Illuminating with the readout wave without applied electric field leads to a very slow grating decay (many hours). Received: 18 November 1998 / Revised version: 3 February 1999 / Published online: 7 April 1999     

Theory of 2×2 switch based on electro-optic and piezoelectric effects in photorefractive LiNbO3 crystal

Based on the electro-optic and piezoelectric effects, the principle of a 2×2 bypass-exchange switch in photorefractive LiNbO₃ crystal is discussed. In photorefractive volume grating, the Bragg condition can be changed by applying a field during readout. That is, by applying a specific field E_s or zero, the diffraction efficiency will be 0 or 1, respectively and this property can be used to realize the bypass or exchange operation of the switch. In this paper, considering both the piezoelectric and electro-optic effects caused by the applied electric field, we analyzed relations of both the specific field and the incident intensity ratio with respect to the writing angles and polarizations of writing beams.     

Ce:KNSBN晶体衍射特性研究

在非同时读出条件下测量了Ce:KNSNB晶体两波耦合体光栅衍射效率随写入光强比、写入光偏振态和写入光夹角的变化关系,并与同时读出条件下衍射效率变化规律的测量结果进行了比较,发现二者基本一致.利用耦合波理论对实验结果进行了理论分析和拟合,拟合结果和实验数据较好的吻合.     

Electric field effect on the second-order nonlinear optical properties in semiparabolic quantum wells

Electric field effect on the second-order nonlinear optical properties in semiparabolic quantum wells are studied theoretically. Both the second-harmonic generation susceptibility and nonlinear optical rectification depend dramatically on the direction and the strength of the electric field. Numerical results show that both the second-harmonic generation susceptibility and nonlinear optical rectification are always weakened as the electric field increases where the direction of the electric field is along the growth direction of the quantum wells, which is in contrast to the conventional case. However, the second-harmonic generation susceptibility is weakened, but the nonlinear optical rectification is strengthened as the electric field increases where the direction of the electric field is against the growth direction of the quantum wells. Also it is the blue (or red) shift of the resonance that is induced by increasing of the electric field when the direction of the electric field is along (or against) the growth direction of the quantum wells. Finally, the resonant peak and its corresponding to the resonant energy are also taken into account.