半绝缘GaAs光电导开关产生太赫兹波电场屏蔽效应的二维Monte Carlo模拟

利用Ensemble-Monte Carlo模拟方法，对不同实验条件下半绝缘GaAs(SI-GaAs)光电导开关作为偶极辐射天线在辐射太赫兹电磁波(太赫兹波)中体内电场的分布以及空间电荷屏蔽效应对太赫兹波辐射的影响进行了模拟.载流子的时域空间电场分布表明:用高能量激光脉冲触发低压偏置的GaAs开关，空间电荷屏蔽是限制太赫兹波辐射功率的一个重要因素，并且空间电荷屏蔽能够引起太赫兹波呈现双极性.当高能量飞秒激光脉冲以全电极间隙触发大孔径光电导天线时，空间电荷电场屏蔽效应对太赫兹波的影响不大. 关键词： 光电导开关 Ensemble-Monte Carlo模拟 辐射场屏蔽 空间电荷屏蔽     

光电导开关产生太赫兹电磁波双极特性分析

利用光电导体产生太赫兹电磁波(THz波)，THz远场辐射波形与光电导体材料的载流子寿命、偏置电场以及触发光有直接关系.用不同方法对低温GaAs(LT-GaAs)和半绝缘GaAs(SI-GaAs)光电导开关辐射的THz电磁波所呈现的双极特性进行了模拟计算.结果表明，LT-GaAs光电导开关辐射THz波呈现双极性的主要原因是光生载流子寿命小于一个THz波产生时间；而光生载流子寿命大于100ps的SI-GaAs光电导开关，在不同的实验条件（不同偏置电场、不同光脉冲能量）下，产生的THz波呈现双极特性的主要原因分别是载流子发生了谷间散射和空间电荷电场屏蔽. 关键词： 光电导开关 THz电磁波 载流子寿命 空间电荷屏蔽     

GaAs光电导天线辐射太赫兹波功率的计算

在Larmor公式的基础上建立了适合计算光电导天线辐射太赫兹波功率的数学模型,利用此数学模型通过蒙特卡罗方法分别计算了不同实验条件下GaAs光电导天线辐射太赫兹电磁波功率.计算结果表明,增加光电导天线的偏置电场或触发光能量,都能够提高天线辐射太赫兹波功率,大孔径光电导天线能够承载更多的光生载流子,因而可以产生比小孔径光电导天线功率更高的太赫兹波. 关键词： 光电导天线 Larmor公式 太赫兹波功率     

苏氨酸的太赫兹及拉曼光谱研究

采用太赫兹时域光谱和拉曼光谱法对L-苏氨酸多晶粉末进行测试研究,获得了样品在0.2—2.8 THz（6.7—93.2 cm^-1）波段的特征吸收和10—4000 cm^－1波段的拉曼散射谱;分别在6.7—93.2 cm^－1和400—4000 cm^－1两个波段进行了吸收光谱与拉曼散射光谱的对比,根据晶胞分子所属的空间群,对隶属于分子的极性和非极性振动模式（A, B₁, 关键词： 太赫兹光谱 拉曼散射 氨基酸     

量子限制受主远红外电致发光器件的制备与测量

采用分子束外延技术生长GaAs/AlAs三量子阱,并在中间的GaAs阱中δ-掺杂浅受主杂质Be原子,制作出量子限制受主远红外Teraherz原型电致发光器件.实验上测量得到4.5 K时器件的电致发光谱（EL）和电传输特性（I-V曲线）.在EL发射谱中清楚地观察到222 cm^-1处宽的尖峰,这来源于Be受主奇宇称激发态到其基态的辐射跃迁,而非辐射弛豫过程则使发射谱的信号很弱.另外在I-V曲线中072和186 V的位置出现两个共振隧道贯穿现象,分别对应于中间δ-掺杂量子阱受主能级1s_3/2(Γ₆+Γ₇)到左边非掺GaAs量子阱中HH带,及右边非掺杂GaAs量子阱中HH重空穴带到中间掺杂GaAs量子阱中Be受主杂质原子奇宇称激发态2p_5/2(Γ₆+Γ₇)能级的共振隧穿. 关键词： 量子限制效应 电致发光 共振隧穿效应 δ-掺杂GaAs/AlAs三量子阱     

铁磁/反铁磁双层膜中冷却场对交换偏置场的影响

用铁磁畴壁模型研究了非补偿界面铁磁/反铁磁双层膜中冷却场（包括大小及其方向）对交换偏置场h_E的影响.结果表明：当冷却场的方向与反铁磁层磁易轴一致时,h_E大小与冷却场大小无关.当冷却场的方向偏离磁易轴时,h_E的大小随偏离角度的增大有缓慢的改变,但当冷却场的方向偏离到临界角度γ_c处,h_E的大小发生突变,其γ_c的大小随冷却场的增大而增大.特别是当冷却场的偏离角度大于γ_c后,h_E出现由负转正的现象,其转变点还与冷却场的大小有关.另外,h_E与铁磁层原子层数N_F的关系会发生由h_E∝N^-1_F向h_E∝N^-λ_F的转变,其中λ>1.其发生转变的条件与N_F、冷却场大小和方向密切相关. 关键词： 铁磁/反铁磁双层膜 交换偏置 冷却场     

飞秒激光泵浦LaAlO3/SrTiO3异质结产生太赫兹波辐射

自铁磁金属在飞秒激光泵浦下的超快退磁效应发现以来,电子的自旋属性逐渐被应用于太赫兹电磁波的产生.利用逆Rashba-Edelstein效应产生太赫兹辐射首先在Ag/Bi界面得到证实,而LaAlO₃/SrTiO₃界面通过该效应产生直流的自旋-电荷转换效率要高于Ag/Bi界面约一个数量级,但利用该结构转化自旋流来产生太赫兹的有效性尚待系统的研究.本文制备了NiFe/LaAlO₃//SrTiO₃(001)系列样品,在飞秒激光泵浦下观察到了太赫兹辐射的产生及其对磁场方向的依赖效应,并通过改变LaAlO₃层的厚度验证了超扩散模型与光学传输模型的有效性,观察到了在LaAlO₃/SrTiO₃界面由于多次反射导致太赫兹波的减弱,为进一步优化太赫兹波的产生提供了实验和理论支持.     

电子磁矩对同步辐射频谱的修正

本文从偶极子辐射场的Heaviside-Feynman表达式出发, 用经典的电动力学方法推导了考虑内禀磁矩影响后的相对论电子辐射频谱分布的表达式, 并对做匀速圆周运动的极端相对论性电子的同步辐射, 计算了两个偏振方向上的考虑磁矩修正后的辐射谱. 计算结果表明对于特征频率为ω_c的同步辐射, 如果ħω_c≥10 keV, 内禀磁矩对辐射的修正是可观的. 通过同步辐射的内禀磁矩修正, 本文讨论了电子束极化度与辐射场偏振度的依赖关系, 并基于此关系提出一种测量电子束极化度的新方法. 关键词： 同步辐射 电子内禀磁矩 同步辐射偏振度 束流极化度     

磁场微重力效应的研究

将导电的液体置于磁场中,使导电液体中浮力驱动的自然对流减弱甚至消失,在导电液体中制造出了二级微重力效应,将这种情况称为磁场微重力效应.通过研究导电液体自然对流驱动力的无量纲Grashof数的变化,发现微重力效应的水平可以用公式g_m＝（β₀/β_m）（ν₀ /ν_m）²g₀ 计算,如果略去一次项,则可用g_m＝（ν₀ /ν_m）²g₀来估算.测量研究了不同磁场条件下硅熔体的磁黏度,估算不同磁场强度对应的磁场微重力水平后,发现估算结果与实验结果基本符合. 关键词： 磁场 二级微重力效应 Grashof数 微重力水平     

MgF2单晶的THz光谱研究

利用THz时域光谱技术对MgF₂晶体(样品1)和MgF₂：Co晶体(样品2)在0.5—2.5 THz的吸收特性进行了研究.在0.5—2.5 THz波段，样品1吸收系数α(ν)随频率ν增加而增大，最大值为24 cm^-1.样品2的吸收系数比样品1大得多，Co掺杂使晶格吸收带边向低频移动，而且样品2在1.9 THz有吸收峰，吸收系数达到70 cm^-1，由此求出F^--Co²⁺离子键伸缩振动的键力常数K为3.40×10^-2　N/cm.这一结果表明，THz光谱分析有可能成为研究晶体化学键的一种重要手段.利用光学常数之间的关系计算了两个样品在0.5—2.5 THz的介电函数的实部ε₁(ν)，得到样品1的ε₁(ν)值在4.67至4.73之间，样品2的ε₁(ν)值在4.62至5.01之间. 关键词： THz辐射 光谱 2晶体')" href="#">MgF₂晶体     

X-ray structural and optical studies of PbZrO0.598Ti0.042O3 single crystals in electric fields up to 4×107 V/m

Field dependences of the rhombohedral unit cell parameter a and birefringence of crystals of lead titanate zirconate with 4.2 mol % additions of Ti are found to be linear in the range from zero to 400 kV/cm. The linear and bulk expansions of the crystal in a field of 400 kV/cm are close to 0.25 and 0.7% respectively. The coefficient of the longitudinal piezoeffect f ₃₃ computed based on data from Fx-ray structural analysis exhibit a strong nonlinearity: it increases monotonically by a factor of 7–8 as the electric field increases from zero to 200 kV/cm, and reaches saturation at stronger fields. Fiz. Tverd. Tela (St. Petersburg) 40, 327–329 (February 1998)     

Effect of electric field on neutron scattering in lead magnoniobate

The temperature dependence of the intensity of the Bragg and the transverse component of quasi-elastic neutron scattering from the single-crystal model relaxor PbMg_1/3Nb_2/3O₃ (PMN) has been studied for various applied electric fields. It is shown that application of a field E>E _th ≈1.6 kV/cm increases the elastic scattering intensity and reduces the intensity of the transverse diffuse-scattering component and that, below 230 K and for E>6 kV/cm, the elastic-scattering intensity saturates while the temperature-dependent part of the transverse diffuse-scattering component becomes practically suppressed. The measured temperature and field dependences of the intensity of type (h00) and (hh0) Bragg reflections provide supportive evidence both for the presence of considerable lead-ion displacements relative to the ideal perovskite sites and for the existence in strong electric fields of an induced transition to the ferroelectric phase below 250 K. Fiz. Tverd. Tela (St. Petersburg) 40, 1905–1910 (October 1998)     

Enhanced light emission in Si-nanoclusters arrays

An array of silicon nanoclusters aimed at producing light emission upon injection of electrons and holes from external sources is studied by Monte Carlo simulations. The conditions for obtaining a significant charge accumulation in the emitting nanoclusters are investigated as a function of array geometry and applied electric fields. It is found that if a stationary state, reached for an applied field F₀, is suddenly perturbed by a field F₁≫F₀, a significant increase in electron-hole pairs population can be obtained with respect to the case of a single field of constant intensity F₁, leading to enhanced light emission when the conductivity of the array is above 6×10^-10 [ Ω cm] ^-1. The excess population thus created gets fully recombined on the time scale of milliseconds, suggesting a device that can produce enhanced light emission in the range of kilohertz.     

Selectively dopedn-AlxGa1−xAs/GaAs heterostructures with high-mobility two-dimensional electron gas for field effect transistors

The transport properties of warm and hot electrons in selectively dopedn-Al _x Ga_1–x As/GaAs heterostructures created by electric fields up to 500 V/cm were studied by Hall effect, conductivity, and Shubnikov-de Haas measurements at lattice temperatures from 4.2 to 300 K. Hall measurements revealed a substantial decrease of electron mobility and also of sheet electron concentration at 77 K with enhanced electric field. The accelerated 2D electrons are partly scattered into the low-mobility first excited (E ₁) subband, and they are partly trapped in immobile states located in the Al_xGa_1–xAs near the interface. Consequently, two differentv(E) characteristics were obtained at 77 K. The 2D electrons populating only the lowest (E ₀) subband exhibit a velocity of v-2×10⁷ cm/s at 500 V/cm, while the averaged velocity due to all electrons reaches a value of v-1.5×10⁷cm/s at 500 V/cm. The analysis of the Shubnikov-de Haas oscillations and Fast Fourier transformation of the data manifested that the 2D electrons are very rapidly accelerated at 4.2 K and achieve electron temperatures much higher than the lattice temperature at electric fields as low as 1 V/cm. The major cooling process for these electrons is scattering into the low-mobilityE ₁ subband.     

Amplification of light during the scattering of a relativistic electron by a nucleus in a moderately strong field of a circularly polarized light wave

The amplification (attenuation) factor of an electromagnetic wave during the scattering of a relativistic electron by a nucleus in a moderately strong field of a circularly polarized electromagnetic wave is studied theoretically. The effect of amplification of an electromagnetic field is discovered in a certain interval of polar angles of the incident electron; this interval of angles essentially depends on the electron energy and the field intensity. It is shown that the amplification of a field attains its maximum for nonrelativistic electrons in the range of medium fields. As the electron energy increases, the amplification decreases and vanishes for ultrarelativistic electrons. An increase in the field intensity for a given electron energy also leads to a slow decrease in the amplification of a field. At high intensities of the wave, the effect of amplification vanishes. It is shown that, in the range of optical frequencies for medium fields (F ～ 10⁶V/cm), the amplification factor of laser light may amount to about μ ～ 10^?1 cm^?1 for sufficiently high-power electron beams.     

Investigation of the incident light intensity effect on the internal electric fields of GaAs single junction solar cell using bright electroreflectance spectroscopy

The incident light intensity (I_ex) effects on a GaAs single junction solar cell (SC) was investigated using bright electroreflectance spectroscopy (BER) and current-voltage (J-V) measurements at room temperature. The p-n junction electric field (F_pn) of the SC was evaluated by analyzing the Franz Keyldesh oscillation (FKO) in the BER spectra.The I_ex effect on F_pn was investigated at various incident light intensities from 0.03 to 25 suns. The F_pn decreased gradually with increasing I_ex due to the photovoltaic effect. For the forward bias voltage, some part of the electrons and holes drifted to the p and n sides, respectively, and produced the induced electric field in the same direction of the F_pn. Therefore, the F_pn increased up to 2.5 suns. At more than 2.5 suns, most of the electrons and holes moved to the n and p sides and decreased the F_pn due to the photovoltaic effect.In addition, the F_pn was examined under light illumination as a function of different DC bias voltages (−0.2–0.4 V). The F_pn decreased with increasing bias voltage due to the decrease in potential barrier. The F_pn increased with increasing bias voltage due to the decrease in the photogenerated carrier-induced electric field for high I_ex.     

Generation of THz Radiation from Laser Beam Filamentation in a Magnetized Plasma

The terahertz (THz) frequency radiation production as a result of nonlinear interaction of high intense laser beam with low density ripple in a magnetized plasma has been studied. If the appropriate phase matching conditions are satisfied and the frequency of the ripple is appropriate then this difference frequency can be brought in the THz range. Self focusing (filamentation) of a circularly polarized beam propagating along the direction of static magnetic field in plasma is first investigated within extended‐paraxial ray approximation. The beam gets focused when the initial power of the laser beam is greater than its critical power. Resulting localized beam couples with the pre‐existing density ripple to produce a nonlinear current driving the THz radiation. By changing the strength of the magnetic field, one can enhance or suppress the THz emission. The expressions for the laser beam width parameter, the electric field vector of the THz wave have been obtained. For typical laser beam and plasma parameters with the incident laser intensity ≈ 10¹⁴ W/cm², laser beam radius (r₀) = 50 μm, laser frequency (ω₀) = 1.8848 × 10¹⁴rad/s, electron plasma (low density rippled) wave frequency (ω₀) = 1.2848 × 10¹⁴ rad/s, plasma density (n₀) = 5.025 × 10¹⁷cm^–3, normalized ripple density amplitude (μ)=0.1, the produced THz emission can be at the level of Giga watt (GW) in power (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of the electric field on structural transformations and phase boundary for PMg1/3Nb2/3O3-xPbTiO3 single crystals

The influence of different regimes of application of a dc electric field (0 < E < 4 kV/cm) on structural phase transformations and behavior of the phase boundary in [001]-oriented single crystals PMg_1/3Nb_2/3O₃-xPbTiO₃ (PMN-xPT) with compositions near the morphotropic phase boundary (x = 28 and 32%) and far from it (x = 13%) has been investigated. The studies have been performed using optical methods, namely, optical transmittance and small-angle light scattering. It has been revealed that the number, symmetry, and stability of field-induced phases depend on the regime of electric field application. During cooling of the PMN-28PT crystals from the cubic phase in the field (FC regime), the tetragonal phase is induced already in weak fields of ∼0.5 kV/cm, whereas even a field of 3 kV/cm is insufficient for inducing this phase by applying the field in the ferroelectric phase. In the PMN-32PT crystal (which has a composition closer to the morphotropic phase boundary as compared to that of the PMN-28PT crystal) in the FC regime and when the field is applied in the ferroelectric phase, the tetragonal phase is induced already in weak electric fields, even though this phase in the latter case appears to be unstable. The E-T phase diagrams have been constructed for different regimes of field application. The possible factors responsible for the observed differences have been discussed.     

Response of Granular La1??xCaxMnO3 Film to 10?GHz and 35?GHz Frequency Electromagnetic Radiation

The nonbolometric response of La_1 − xCa_xMnO₃ film to 10 GHz and 35 GHz frequency electromagnetic radiation is investigated in the case when, in addition to the strong electric field of the wave, the film is subjected to a stationary electric bias field. Dependences of responses on the radiation power P at temperature T = 80 K are presented. In the low power region, a linear dependence of the response on P is observed at both frequencies whereas for high powers the dependence behaves as ~P ^1/2. The obtained results are explained taking into account that the nonbolometric response originates from the intergranular junctions that operate in the reverse current regime. There two effects take place: (i) at low powers the detection resistance decreases with increasing power P, and (ii) at higher powers in addition to that the film resistance decreases as P ^1/2 due to the avalanche of charge carriers in the electric field of the electromagnetic wave.     

MC20F20（M=Li，Na，Be和Mg）几何结构和电子性质的密度泛函计算研究

采用密度泛函理论（density functional theory, DFT）中的广义梯度近似（generalized gradient approximation, GGA）对MC₂₀F₂₀（M=Li,Na,Be和Mg）的几何结构和电子性质进行了计算研究.几何结构研发现：随着内掺原子序数的增加,金属原子M对C₂₀F₂₀中的C—C键的影响越来越大,而对C—F键的影响甚微.掺杂能计算表明：MC₂₀F₂₀的掺杂能均为负值,需要在一定的实验条件下才能被合成.内掺碱金属和碱土金属分别产生了两类截然不同的能隙和磁性.其中,内掺碱金属的能隙非常小,且带有1μ_B的净磁矩,表现出磁性;而内掺碱土金属的能隙比C₆₀的能隙还大,净自旋为0,表现出非磁性. 关键词： 富勒烯 几何结构 电子结构 密度泛函