外电场作用下红外光折变GaAs的双光束耦合增强效应

报道了立方晶体GaAs:Cr在外电场作用下的红外光折交增强效应,着重分析双光束耦合过程中空间电荷电场幅度和位相的空间分布变化情况。计算结果很好地解释了实验测量结果。     

CO2激光加热局域热固定LiNbO3光折变全息的温度影响

利用 CO2 激光加热铌酸锂 ( L i Nb O3)光折变晶体可局域固定体全息 ,但形成的温度场空间上是不均匀的。在对固定显影后空间电荷场幅度 Esc 与固定温度 Tf 的关系进行理论分析的基础上 ,通过定义局域性系数和平均空间电荷场幅度两个判据 ,来评价不同的激光加热温度场的固定效果。这种方法也适用于对其他各类边界条件的 CO2 激光加热温度场局域热固定进行分析。     

Short-pulse grating formation in photorefractive materials

Solutions are obtained for the refractive index grating in a photorefractive material in which the writing or erasing beams are short-pulse (less than tens of nanoseconds), high-irradiance (greater than kW/cm²) lasers. The response of the photorefractive material is modeled using the continuity equation for the charge carriers, a rate equation for the ions, a current equation, and Poisson's equation for the space charge field. Two cases are discussed in detail: 1) the time for recombination of carriers with ions is much less than the pulse length of the laser; and 2) the recombination, drift, and diffusion times are all much longer than the pulse length. The energy requirements for short-pulse writing and erasing are greater than or equal to those obtained for typical CW lasers in all cases investigated. Application to previous observations in BaTiO3, Bi12SiO20, and LiNbO3is discussed.     

The photorefractive effect—a review

Photorefractive materials have been assuming ever increasing prominence in the non-linear optics field due to their unique capability for displaying strong non-linear effects at milliwatt power levels. In this review we describe in some detail the two currently accepted models of the photorefractive process. The “band transport” model assumes that photo excited electrons (or holes) are ejected from filled donor (or acceptor) sites to the conduction (or valence) band where they migrate to dark regions in the crystal before recombining into empty traps. The charge separation results in a space charge field which modulates the refractive index via the linear electro-optic effect. The “hopping” model assumes that carrier transport occurs via light induced hopping from a filled donor site to a neighbouring empty trap. In particular, we examine the timeand spatial frequency dependence of grating formation and discuss the validity of the simplified model of a material response linear in modulation ratio (fringe contrast). In addition we show that the “hopping” model although physically distinct from the band transport model is described by equations which can be considered as a special case of the band transport equations. Results of a complete numerical simulation of the band transport model are presented to illustrate the properties of the photorefractive effect and to test the validity of the simplified model.The two models are normally formulated for an isotropic material illuminated by two plane wave beams which are assumed to be unaffected by the material. Photorefractive materials, however, are naturally anisotropic and in applications one is often interested in non-plane wave beams. We present for the first time an extension of the model to non-plane wave beams and explicity take full account of the vectorial nature of light and the crystal anisotropy. Barium titanate is used as an example to illustrate the extended model and graphs are presented which show the dependence of the nonlinearity on beam angles, polarisation and crystal orientation.Finally we discuss some applications is non-linear optics which exploit some of the unique features of the photorefractive effect. The photorefractive effect is illustrated throughout the paper by experimental results performed in our own laboratory.     

双光子光折变介质中屏蔽光伏空间孤子

为了得到更完善的双光子屏蔽光伏孤子理论,采用光束的空间宽度不是远小于晶体宽度的条件,对有偏压的双光子光伏光折变介质中空间光孤子理论进行了修正,得出了明孤子和暗孤子数值积分解。结果表明,双光子屏蔽光伏孤子源于对外加电场的非均匀空间屏蔽和光伏效应,不同于起源于对外电场非均匀屏蔽的屏蔽孤子和起源于光伏效应的光伏孤子。当光伏效应被忽略时,屏蔽光伏孤子退化成屏蔽孤子,空间电荷场就是屏蔽孤子的空间电荷场;当外加电场为0时,屏蔽光伏孤子退化成开路或闭路的光伏孤子,空间电荷场就是晶体中的光伏场。     

Space Charges Effect of Static Induction Transistor

The space charge effect (SCE) of static induction transistor (SIT) that occurs in high current region is systematically studied.The I-V equations are deduced and well agree with experimental results.Two kinds of barriers are presented in SIT,corresponding to channel voltage barrier control (CVBC) mechanism and space charge limited control (SCLC) mechanism respectively.With the increase of drain voltage,the gradual transferring of operational mechanism from CVBC to SCLC is demonstrated.It points out that CVBC mechanism and its contest relationship with space charge barrier makes the SIT distinctly differentiated from JFET and triode devices,etc.The contest relationship of the two potential barriers also results in three different working regions,which are distinctly marked and analyzed.Furthermore,the extreme importance of grid voltage on SCE is illustrated.     

静电感应晶体管的空间电荷效应

研究了静电感应晶体管(SIT)在大注入情况下出现的空间电荷效应,分析了空间电荷效应的物理机制.从理论上推导出了SIT工作在沟道势垒调制下的I-V特性的解析表达式,实验结果表明它们符合得较好.在漏压增长过程中,有两种势垒出现(沟道势垒和空间电荷势垒),它们分别对应于沟道势垒调制和空间电荷势垒调制模式.随漏压增加,SIT逐渐从沟道势垒调制模式转向空间电荷势垒调制模式,对此转变物理过程给出合理解释,直观地给出了SIT在空间电荷效应作用下的变化规律.由于SIT小电流区域的沟道势垒调制使SIT区别于其他器件,其中栅压对SIT空间电荷效应有非常重要的作用.     

Space charge effects in a gyrotron employing a solid electron beam

The influence of space charge forces on the performance of a single cacity gyrotron oscillator which uses a solid electron beam was investigated. It was found that space charge effects cause a large efficiency degradation as the beam current is increased, if the other experimental parameters are unchanged. A small increase in the magnetic field, however, can restore the efficiency to higher values.     

Enhanced photoconductivity and trapping rate through control of bulk state in organic triphenylamine-based photorefractive materials

In organic optical semiconductors, it is rather challenging to achieve precise control of photoconductivity and charge trapping, which determines the device performance. This paper reports on enhanced photorefractive response rate through control of the photoconductivity and trapping rate in organic triphenylamine-based photorefractive materials by means of bulk state tuning. The bulk state in organic triphenylamine-based photorefractive composites was controlled through a rapid cooling process from various melting temperatures during sample fabrication. The photoconductivity and trapping rate were determined from photocurrent measurements. Fabrication at lower melting temperatures enhanced the trapping rate for deep traps, whereas it reduced the trapping rate for shallow traps. As a result, a faster photorefractive response was obtained.     

Analysis of the Optical Amplifier Noise Effect on Electrooptically Generated Hyperchaos

A theoretical analysis regarding the determination of the optical amplifier noise effect on the performance of optical chaos communications using semiconductor lasers with electrooptical feedback is carried out. Analytical expressions for the synchronization error and the signal-to-noise ratio of the extracted message are obtained and verified by extensive numerical simulations. Fiber transmission simulations are also carried out to identify under which circumstances the amplifier noise effect is the dominant impairment.     

Compton散射下光折变晶体中的孤子传输特性

应用多光子非线性Compton模型和耦合非线性薛定谔方程,研究了Compton散射 下光折变晶体中的孤子传输特性,研究结果表明,光折变晶体热透镜效应的实质,是晶体内形成的光孤子与空间电荷场中的孤子传输与耦合的过程。其耦合特性不仅与两波导中的孤子的相对相位有关,而且与Compton散射有关。当两波导中的孤子的相对相位相同或相反,散射都会使它们之间发生能量交换和交换能量的周期发生变化,从而使形成的孤子的轮廓变得模糊。即使在光沿着晶体的光轴方向传输,也有发生双折射的可能性。     

可编程光学二值双轨逻辑门

基于双轨逻辑,本文提出一种可级联的并行二值逻辑门。所有十六种二值逻辑运算可以采用偏振半波相延编程来实现。也提出了用电光晶体实现实时编程的方法。本文中给出了实验结果。     

Theoretical and practical investigation of the thermal generation in gate controlled diodes

The different components of thermal generation in a gate controlled diode are studied theoretically and experimentally. Expressions for the generation current in the space charge layer, the diffusion current from the quasi-neutral bulk and the surface generation current are derived for a gated-diode. The width of the generation zone within the space charge layer is calculated as a function of the energy level of the trap and the diode reverse voltage. This leads to a characteristic of the leakage current as a function of the space charge layer width. It is pointed out that the diffusion current can influence the leakage current and cannot be neglected in structures with a low dark current. In the second part the gate controlled diode is used to characterize the thermal generation in structures with a homogeneous and low dark current. A generation lifetime of 5.5 msec and a surface generation velocity at a depleted surface of 1.5 cm/sec is derived. The generation lifetime is found to be constant as a function of depth into the substrate. A considerable diffusion current is measured which is comparable to the generation current in the space charge layer.     

Optical, nonlinear optical, and electrooptical properties of4'-nitrobenzylidene-3-acetamino-4-methoxyaniline (MNBA) crystals

Optical, nonlinear optical, and electrooptical pg-properties of 4'-nitrobenzylidene-3-acetamino-4-methoxyaniline (MNBA) single crystals have been investigated. The refractive indices were determined in the wavelength range of 514 to 1064 nm using an interferometric method. The electrooptical coefficients r₁₁, r₁₃, r₃₁ , and r₃₃ were measured using a phase modulation technique. The dispersion of the largest electrooptical coefficient, r ₁₁ (=29 pm/V at λ=633 nm), was seen to follow the theoretical two-level model. Using the standard Maker fringe technique, we determined the nonlinear optical susceptibilities. The largest coefficient was found to be d₁₁=175 pm/V at λ=1064 nm. Electric-field-induced second-harmonic generation was used for the determination of the molecular second-order nonlinear optical susceptibility yielding a good quantitative agreement when compared with the largest nonlinear optical and electrooptical coefficients in the framework of the molecular gas model with charge transfer along the polar molecular axis. Electroabsorption measurements near the band edge were performed that allowed us to calculate the field-induced frequency shift of the dominant oscillator. This field-induced frequency shift is of the same order of magnitude as the frequency shift calculated from the electric field-induced changes of the refractive indices (electrooptical effect)     

磁控管中的空间电荷效应研究

对磁控管中由于外加磁场存在而导致的空间电荷效应进行了研究.通过求解正交场中的粒子运动方程,给出了阴极表面电位分布与外加磁场和阴极发射电流的关系,并指出磁控管中合适的发射电流是其最佳工作的前提.通过全电磁粒子仿真和实验测试证实了该效应对磁控管工作性能的影响,为高效率低噪声磁控管的研究提供了新思路.     

用两次曝光相移全息干涉实现光学幅相转换

提出一种实现光学图像由纯振幅型转换为相应的钝相位型的新方法。该方法基于2次曝光光折变相移全息干涉,在光学4-f系统中,用振幅型空间光调制器(SLM)输入振幅(或强度)图像,用光折变晶体作为全息记录介质,经过2次曝光相移全息干涉记录,再由原参考光再现读出,可在输出面上得到与输入图像相应的空间相位分布。该方法可在光学信息处理相关领域应用。阐述了该方法的基本思想,进行了理论分析、计算机模拟和光学实验,实验结果与理论分析一致,表明该方法是可行的。     

Active Pixel Concept Combined With Organic Photodiode for Imaging Devices

The active pixel concept is a promising architecture for imaging systems. We report on the electrooptical characterization of a hybrid organic active pixel sensor (APS) where an organic photodiode is integrated on top of an amorphous silicon thin-film transistor circuitry, which drives the image sensor and performs the signal processing. The active pixel approach provides an on-pixel amplification of the signal with a charge gain of up to 10. A fill factor that is close to 100% is obtained by embedding all transistors underneath the organic photodetector. We show that, as compared with organic passive pixels, the organic APS shows a higher sensitivity, making the detection of smaller signals possible.     

双掺杂LiNbO3晶体光折变效应研究

研究了系列不同掺杂、不同掺杂浓度、单掺杂和双掺杂以及不同后处理态(生长态、还原态和氧化态)铌酸锂晶体的透过率光谱和光折变二波耦合效应。实验结果表明，高掺杂铌酸锂样品的透过率光谱范围较窄；双掺杂样品比单掺铁样品的抗光折变效应增强，光折变效应降低。氧化态样品具有较大的透过率光谱范围；还原态样品具有较大的光折变二波耦合增益特性。     

利用铁电晶体四波混频的光学无损检测

本文描述了一种利用铁电晶体简并四波混频的实时二次曝光法。该方法利用这种晶体的介电弛豫时间检测相位物体的不均匀性。文中给出了较详细的有关理论分析和公式推导,并且对实际玻璃样品进行了测量,验证了该方法的可行性及实用性。