Waveguides induced by steady-state gray solitons in biased photorefractive-photovoltaic crystals

We carry out a theoretical investigation of the properties of waveguides induced by photorefractive one-dimensional steady-state gray spatial solitons (i.e., screening solitons, photovoltaic solitons, and screening-photovoltaic solitons). We demonstrate that waveguides induced by photorefractive steady-state gray spatial solitons are only a single guided mode for both all soliton graynesses and all values of ρ, where ρ is the ratio between the soliton peak intensity and the dark irradiance, and moreover, waveguides induced by gray photovoltaic solitons for closed-circuit condition are also only a single guided mode for all electric current densities. We find that the confined energy near the center of a photorefractive steady-state gray spatial soliton increases with ρ and decreases with an increase in the soliton grayness. We also find that the confined energy near the center of a gray photovoltaic soliton for closed-circuit condition increases with the electric current density. On the other hand, waveguides induced by gray screening-photovoltaic solitons are gray screening soliton-induced waveguides when the bulk photovoltaic effect is neglectable and are gray photovoltaic soliton-induced waveguides when the external bias field is absent.     

Current Oscillation and dc-Voltage-Controlled Chaotic Dynamics in Semiconductor Superlattices

We report a detailed theoretical study of current oscillation and dc-voltage-controlled chaotic dynamics in doped GaAs/AlAs resonant tunneling superlattices under crossed electric and magnetic fields. When the superlattice is biased at the negative differential velocity region, current self-oscillation is observed with proper doping concentration. The current oscillation mode and oscillation frequency can be affected by the dc voltage bias, doping density, and magnetic field. When an ac electric field with fixed amplitude and frequency is also applied to the system, different nonlinear properties show up in the external circuit with the change of dc voltage bias. We carefully study these nonlinear properties with different chaos-detecting methods.     

Self-Deflection of Dark Screening Spatial Solitons Based on Higher-Order Space Charge Field

The effects of higher-order space charge field on the self-deflection of dark screening spatial solitons in biased photorefractive crystals are numerically investigated under steady-state conditions. The expression for an induced space-charge electric field including higher-order space-charge field terms is obtained. Numerical results indicate that dark solitons possess a self-deflection process during propagation, and the solitons always bend in the direction of the c axis of the crystal. The self-deflection of dark solitons can experience considerable increase especially in the regime of high bias field strengths.     

Generation of Anti-Stokes Line in Fundamental Mode of Photonic Crystal Fibre

A photonic crystal fibre （PCF） with zero-dispersion wavelength around 800 nm is designed and fabricated. Simulated results show that the zero-dispersion wavelength of fundamental mode for this PCF is at 826nm, and phase-matched four-wave mixing can be achieved in fundamental mode. Using 20ors Ti：sapphire laser with central wavelength at 810nm as pump, the anti-Stokes line around 610hm ＆ generated efficiently. The output signal has a Gaussian-like profile, which indicates that the anti-Stokes signal is in the fundamental mode of the PCF. The energy of anti-Stokes signal is higher than that of residual pump laser and the maximum ratio of the anti-Stokes signal to the pump component in the output spectrum is estimated to be 1.2.     

High Current Operation of a Semi-insulating Gallium Arsenide Photoconductive Semiconductor Switch Triggering a Spark Gap

A transient peak current as high as 5.6 kA is obtained by a GaAs photoconductive semiconductor switch （PCSS） in series with a spark gap. Based on the characteristics of the GaAs PCSS, mechanisms of discharge between the PCSS and the spark gap are discussed. It is implied that a hybrid operation mode of photo-activated charge domain occurs due to the superposition of the bias voltage and the instantaneous radio frequency voltage.     

Stable High Power and High Beam Quality Diode-Side-Pumped Continuous-Wave Intracavity Frequency-Doubled Nd:YAG Laser

We report a stable high power and high beam quality diode-side-pumped cw green laser from intracavity frequency-doubled Nd： YAG laser with KTP. By using a L-shaped concave-convex resonator, designed with two Nd：YAG rods birefringence compensation, a large fundamental mode size in the laser crystal and a tight focus in the nonlinear crystal could be obtained simultaneously. The green laser delivers a maximum 532nm output power of 23.2 W. Under 532nm output power of 20.9 W, the beam quality factor is measured to be 4.1, and the fluctuation of the output power is less than 1.4% in an hour.     

电光调制对外部光注入垂直腔表面发射激光器的偏振转换及其非线性动力学行为的操控性研究

针对主和副垂直腔表面发射激光器构成的外部注入激光器系统的偏振转换及其非线性动力学行为, 利用周期性极化铌酸锂晶体中准相位匹配线性电光调制, 本文提出了一种新的操控方案并且探索了其控制规律. 研究结果发现, 受到平行光注入或正交光注入的副激光器输出偏振度随外加电场成周期性振荡变化, 其振荡波峰轨迹包络曲线为正弦曲线, 而振荡波谷轨迹包络曲线为余弦曲线; 选取一定的主激光器偏置电流, 通过对来自主激光器的光进行电光调制, 受到两种方式注入的副激光器可以输出任意偏振模, 并且其非线性动力行为经历不同的演变. 另外, 副激光器的偏振度仅依赖于外加电场, 与副激光器的偏置电流无关.     

Period-Doubling in 10 GHz Gain-Switched DFB Laser Diode

The distinct period doubling behaviour in a 10 GHz gain-switched （GS） DFB laser is experimentally investigated in frequency domain and in time domain. The period doubling occurs as the frequency of the rf driving signal is close to or higher than the-3 dB cutoff frequency of the DFB laser diode, and the amplitude of the rf driving signal required to achieve period doubling increases linearly with the increasing bias current of the laser diode.     

双信道偏振复用保密通信系统的完全混沌同步的操控性研究

针对双信道偏振复用保密通信系统,利用线性电光效应,提出了一种新的完全混沌同步的控制方案.在该方案中,每一个线性偏振模的完全混沌同步质量随外加电场成准周期性变化.其变化规律:完全混沌同步?剧烈振荡;固定一定的外加电场,电光调制促使其完全混沌同步质量对偏置电流和反馈强度的稳健性获得极大的加强.在较大的偏置电流和反馈强度范围内,每一个线性偏振模能够实现完全混沌同步,并且调制到每一个线性偏振模的加密信号基本上能够恢复.     

Transverse effects and mode couplings in OPOS

00 pump field. The corresponding differential equations for the time evolution of the self-consistent mode amplitudes are derived for typical cavity geometries including the propagation integrals within the crystal. We discuss the new physical effects introduced by these couplings in some typical cases. This includes a change of the OPO threshold and stable simultaneous oscillation of several mode pairs with fixed relative phases as well as oscillating self-pulsing type solutions. Received: 12 January 1998/Accepted: 11 February 1998     

半绝缘GaAs光电导开关的瞬态热效应

实验用波长1064 nm,触发光能为1.0 mJ的激光脉冲触发电极间隙为4 mm的半绝缘GaAs光电导开关,当光电导开关的偏置电压达到3800 V时,开关进入非线性(lock-on)工作模式,在偏置电场和触发光能不变的条件下,开关输出稳定的非线性电脉冲,1500次触发后GaAs开关表面出现因丝状电流引起损伤的痕迹.分析认为:在一定触发光能和电场阈值条件下,开关芯片内存在两种瞬态热效应:热弛豫效应和光激发电荷畴-声子曳引效应.热弛豫时间很短,在皮秒甚至亚皮秒量级,热弛豫过程导致了热传导的弛豫行为;当光激发电荷畴以10⁷ cm/s的速度从阴极向阳极渡越时,在这两种效应的作用下使得开关芯片瞬态温度变化发生了弛豫振荡现象.光激发电荷畴-声子曳引效应在位错运动方向上传播,声子流携带的热能集中在移动的平面内,使得移动区域温度升高,移动轨迹经多次叠加累积呈现出丝状的损伤痕迹. 关键词： 半绝缘GaAs光电导开关 热弛豫效应 光激发电荷畴-声子曳引效应 丝状电流     

Effect of Discharge Voltage on an Ion Sheath Formed at a Grid in a Multi-Dipole Discharge Plasma

It is experimentally demonstrated that a relatively strong ion-rich sheath formed at a fixed negative bias of the grid can be changed to a rather weak ion sheath （sheath potential weakly retards electrons） only by increasing the discharge voltage in the system. At sufficiently high negative grid bias, an increase of discharge voltage enhances the ion collection current at the grid. An explanation is put forward in support of this experimental observation. A slight density enhancement with a fall in plasma electron temperature is also observed with the increasing negative grid bias.     

Ultrafast magnetization reversal dynamics investigated by time domain imaging

Spatiotemporal magnetization reversal dynamics in a Ni(80)Fe(20) microstructure is studied using ps time scale scanning Kerr microscopy. Time domain images reveal a striking change in the reversal associated with the reduction in switching time when a transverse bias field is applied. Magnetization oscillations subsequent to reversal are observed at two resonance frequencies, which sensitively depend on the bias field strength. The oscillation at f = 2 GHz is caused by the damped precession of M, while the lower frequency approximately 0.8 GHz mode is interpreted in terms of domain wall oscillation.     

Quantum-Confined Stark Effects in a Single GaN Quantum Dot

Using analytical expressions for the polarization field in GaN quantum dot, and an approximation by separating the potential into a radial and an axial, we investigate theoretically the quantum-confined Stark effects. The electron and hole energy levels and optical transition energies are calculated in the presence of an electric field in different directions. The results show that the electron and hole energy levels and the optical transition energies can cause redshifts for the lateral electric field and blueshifts for the vertical field. The rotational direction of electric field can also change the energy shift.     

激光二极管触发光导开关实验研究

介绍了利用大功率半导体激光二极管触发3 mm间隙GaAs光导开关、产生非线性电脉冲输出的实验,激光二极管输出功率为70 W,上升前沿约20 ns,脉冲半高宽（FWHM）约40 ns。随着开关两端偏置场强增加,输出电压也线性增加,当偏置场强超过一定阈值,增至约2.53 kV/mm时,经过一个较小的电压峰值和时间延迟后,输出电压急剧增加,产生雪崩现象。实验结果表明:GaAs开关非线性输出的产生与载流子聚集和碰撞电离有关,偏置电场的提高增加了开关芯片中载流子聚集数量,加剧了碰撞离化程度,从而使开关从线性模式进入雪崩模式。     

The effect of a high external electric field on a photorefractive ring phase conjugator

It is shown that a high external electric field often used to enhance the efficiency of photorefractive beam coupling and to decrease the photorefractive response time, may cause mode switching in photorefractive ring phase conjugator leading to a significant drop in the phase conjugate reflectivity.     

Current oscillation and chaotic dynamics in superlattices driven by crossed electric and magnetic fields

We have theoretically studied current oscillation and chaotic dynamics in doped GaAsAlAs superlattices driven by crossed electric and magnetic fields. When the superlattice system is driven by a dc voltage, a stationary or dynamic electric-field domain can be obtained. We carefully studied the electric-field-domain dynamics and current self-oscillation which both display different modes with the change of magnetic field. When an ac electric field is also applied to the superlattice, a typical nonlinear dynamic system is constructed with the ac amplitude, ac frequency, and magnetic field as the control parameters. Different nonlinear behaviors show up when we tune the control parameters.     

Laser induced self-phase modulation in nematic liquid crystals and effects of applied dc electric field

We investigated the ring-pattern characteristic of self-phase modulation in a homeotropic nematic liquid crystal cell induced by a focused Ar-ion laser beam. Laser induced self-phase retardation versus incident angle was experimentally studied, and good agreement with theory was found. Effects of applied dc electric field on the ring-pattern were studied, to our knowledge, for the first time. When the electric field was below a critical value, laser induced self-phase retardation could be varied by changing the electric field strength. Above this value, irregular shaped diffraction pattern accompanied by intense light scattering was observed, which is attributable to honeycomb-like domain formation.     

光电导开关工作模式的蒙特卡罗模拟

利用二维ensemble-Monte Carlo方法模拟了直流偏置的半绝缘GaAs(SI-GaAs)光电导开关在飞秒激光脉冲触发下的两种工作模式.结果表明：当偏置电场低于耿氏电场(GaAs为4.2kV/cm)时，开关输出电脉冲呈线性模式；偏置电场超过耿氏电场但触发光脉冲能量低于光能阈值，光电导开关仍然表现出线性工作模式；当偏置电场和触发光脉冲能量都超过非线性模式(“lock-on”模式)所需阈值，开关呈现非线性模式；在光子能量较高的激光脉冲触发下，开关非线性模式所需光能阈值降低.非线性工作模式源于光生载流子发生谷间散射，并在开关体内局部区域形成高场区，电场变化大，弛豫时间长，电脉冲呈现非线性.     

强电场作用下反铁电锆钛酸铅的介电行为研究

采用电滞回线方法和偏置直流电场中叠加小交变电场方法研究了锆钛酸铅反铁电陶瓷材料在强电场作用下的介电行为.测量结果显示，锆钛酸铅反铁电材料的介电常数随外加电场强度呈非线性变化，在反铁电 铁电转变的电场区间形成介电峰.表征极化强度随电场强度变化率的微分介电常数εd峰值出现在反铁电 铁电转换电场强度处，最高达到41000.随着偏置电场增加反铁电向铁电体转变过程中，小信号介电常数εc减小；在电场降低铁电回复成反铁电过程中，小信号介电常数εc增大，小信号介电常数εc峰先于微分介电常数εd峰出现.根据电场作用下反铁电 关键词： 锆钛酸铅反铁电陶瓷 介电行为 强电场条件