High transparency of a two-photon scattering channel in triple-barrier structures

An analytical solution of the Schrödinger equation with open boundary conditions in all scattering channels has been found for asymmetric triple-barrier resonance-tunneling structures with thin high barriers. This solution describes resonance transitions between three quantum levels in a high rf electric field. It is found that, under certain conditions, most electrons incident on the upper resonance level can emit two photons and leave a structure through the lower level without intermediate interaction with phonons. The structure appears to be almost absolutely transparent in a wide range of the rf field amplitude. This behavior fundamentally distinguishes the multiphoton scattering process from previously considered single-photon scattering processes and the quantum efficiency of such transitions can be twice as high as the maximum quantum efficiency of the transitions between neighboring levels and can reach a value of 160% in the limiting case.     

Optical Bistability and Multistability in an EIT Medium with Two-Photon Resonance Transitions

Intensity threshold of optical bistability (OB) and optical multistability (OM) can be controlled by amplitude and phase control of microwave driven field in the two-photon resonance transitions in a parametric region. It is found that in two-photon resonance case, the weak microwave field can reduce the threshold of optical bistability and strong microwave field can lead to optical multistability. The effect of relative phase between applied fields is also discussed. It is shown that for weak and strong microwave field, intensity threshold of OB and OM can be modified. Moreover, it is found that for strong microwave field, relative phase has an essential role for switching between OB and OM or vice versa.     

Circular-polarization-dependent study of the microwave photoconductivity in a two-dimensional electron system

The polarization dependence of the low field microwave photoconductivity and absorption of a two-dimensional electron system has been investigated in a quasioptical setup in which linear and any circular polarization can be produced in situ. The microwave induced resistance oscillations and the zero resistance regions are notably immune to the sense of circular polarization. This observation is discrepant with a number of proposed theories. Deviations between different polarizations occur only near the cyclotron resonance where an unprecedented large resistance response is observed.     

Rydberg原子的微波电磁感应透明-Autler-Townes光谱

主要研究了室温下微波场缀饰的铯Rydberg原子的电磁感应透明-Autler-Townes(EIT-AT)光谱.首先,以铯原子6S_(1/2)→6P_(3/2)→50S_(1/2)形成阶梯型三能级系统,利用强耦合光作用于6P_(3/2)→50S_(1/2)的Rydberg跃迁,弱探测光耦合基态跃迁6S_(1/2)→6P_(3/2)并探测由耦合光形成的电磁感应透明(EIT)效应.然后,以频率为30.582 GHz的微波电场耦合相邻的Rydberg能级50S_(1/2)→50P_(1/2)产生微波AT分裂.利用Rydberg EIT探测微波耦合相邻Rydberg能级产生的AT分裂,形成EIT-AT光谱,进而实现微波电场的测量.当微波场的强度增加到一定值时,EIT-AT光谱表现为多峰光谱结构.分析EIT-AT多峰光谱的成因,发现这主要是由场的不均匀性导致的,一定的EIT-AT光谱特征对应于特定的非均匀场分布.研究表明,利用Rydberg EIT-AT光谱可以实现微波电场的测量,利用其光谱特征可实现微波场的实时监测,进而提出了一种提高微波场空间分辨率的测量方法.     

Optical bistability via amplitude and phase control of a microwave field

We investigate the steady-state optical bistability behavior in a three-level Λ-type atomic system closed by a microwave field under the condition that the applied fields are in resonance with corresponding atomic transitions. It is shown that the bistable hysteresis cycles can be controlled by both the amplitude and the phase of the microwave field.     

Novel magneto-oscillatory behavior of confined electrons at a twin boundary in ZnSe and at an interface in GaAs/AlGaAs heterostructures

Electronic properties of confined electrons at naturally formed twin boundaries in ZeSe have been investigated by microwave absorption measurements. Cyclotron resonance signal of the confined electrons was observed. On the cyclotron resonance peak, novel oscillation of the absorption intensity was observed. Similar oscillatory behavior was also observed in GaAs/AlGaAs heterostructures near cyclotron resonance filed at which new Shubnikov–de Haas-like oscillation was observed by Zudov. The oscillatory structure has a period not in inverse magnetic field but magnetic field. The origin of the oscillation observed is caused by Rayleigh interference of two-dimensional electron gas.     

Experimental observation of edge magnetoplasma excitations in a two-dimensional electron system in the quantum-hall-effect regime

New low-frequency modes corresponding to acoustic edge magnetoplasma excitations have been observed in the resonance microwave absorption spectra of a two-dimensional electron system in a transverse magnetic field. The additional excitation modes have been shown to appear only in the quantum-Hall-effect regime (in narrow magnetic-field regions near the integer values of the filling factor), when the resonance microwave absorption lines exhibit sharp narrowing. The absolute values of the resonance absorption frequencies and their dependence on the parameters of the electron system coincide (without any fitting parameters) with the respective theoretical predictions of the formula describing the properties of the acoustic modes of edge magnetoplasma excitations.     

Influence of an electric field on the ferromagnetic resonance in a plane-layered magnetic system

The influence of an electric field on the ferromagnetic resonance (FMR) in a multilayer magnetic system consisting of two magnetic layers separated by a thin nonmagnetic interlayer has been investigated. It has been shown that, upon the excitation of magnetization oscillations by a microwave magnetic field, the eigenfrequencies of the ferromagnetic resonance depend on the stationary electric field applied in the plane of the layers. It has also been demonstrated that, in this system, high-frequency magnetization oscillations can be excited by an electric microwave field. The results of the investigation of the polarization properties of the excitation mechanism indicate that this effect can be observed experimentally.     

微波场对电磁诱导透明谱的局域调制

当用两个微波场作用到Λ型三能级原子的耦合跃迁的基态与另两个超精细能态构成的跃迁时,由微波场诱导产生的两个高对比度共振吸收线将单重电磁诱导透明谱分裂成三个透明窗口。通过调节微波场的强度或失谐量,可实现对原子透明谱进行局域调制。采用缀饰态理论对展示的物理现象进行了解释。这些研究对多通道光通信和原子的可控调制等方面有一定的作用。     

Principal mode of the nonlinear spin-wave resonance in perpendicular magnetized ferrite films

The paper reports a theoretical and experimental study of the nonlinear spin-wave resonance (SWR) modes in normally magnetized ferrite films. Particular attention is focused on the principal, lowest frequency, SWR mode. It is shown theoretically that, as the precession amplitude increases, the profile of the principal mode changes to make the excitation distribution across the film thickness more uniform. The nonlinear shift of the resonance field depends on the surface-spin pinning parameters. An experimental study has been made of YIG films with a strong uniaxial anisotropy field gradient over the film thickness, as well as of YIG films of submicron thickness. As the microwave power was increased, the principal-mode resonance field was observed undergoing a sublinear shift accompanied by a superlinear growth of absorbed power. This behavior is attributed to a change in the profile of the spatial distribution of ac magnetization.     

多窗口可调谐电磁诱导透明研究

当两束激光以Λ-构型作用于三能级原子系统并满足双光子共振条件时，探测激光场吸收谱呈现电磁诱导透明(EIT)特征.若再加一个微波控制场作用于该三能级系统的两个低能级跃迁之间，会导致探测吸收特性明显变化，EIT窗口将发生劈裂.通过求解相应的密度矩阵方程，揭示了外加微波场作用下EIT窗口的变化规律，并给出了相应的缀饰态解释.研究结果表明，在适当的条件下, 电磁诱导透明呈现三重结构，而EIT窗口的频率位置取决于微波控制场的拉比频率及频率失谐量.因此通过改变微波控制场的参数可以实现多EIT窗口的频率调谐. 关键词： 电磁诱导透明 量子相干 频率调谐 多窗口EIT     

Coexistence of collective and single-particle effects in the photoresponse of a 2D electron gas to microwave radiation

The photoresponse of magnetoresistance of a high-density two-dimensional electron system to microwave electromagnetic radiation is studied. The damping of the Shubnikov-de Haas oscillation by radiation with a non-monotonic dependence of this effect on the magnetic field and the radiation-induced oscillations of magnetoresistance are observed. The damping is most pronounced within isolated narrow magnetic field intervals that closely correspond to the expected positions of magnetoplasma resonances in the sample under study and also near the cyclotron resonance position. A “window” is observed in the photoresponse near the field value predicted on the basis of a single-particle electron spectrum consisting of broadened Landau levels. The radiation-induced oscillations, the window in the photoresponse, and the damping of the Shubnikov-de Haas oscillations near the cyclotron resonance are described in terms of the theory based on the concept of the nonequilibrium filling of single-electron states. Thus, it is demonstrated that the photoresponse pattern observed in the experiment is formed by both single-particle and collective (magnetoplasma) effects.     

高功率微波输出窗内侧击穿动力学的PIC/MCC模拟研究

高功率微波在受控热核聚变加热、微波高梯度加速器、高功率雷达、定向能武器、超级干扰机及冲击雷达等方面有着重要的应用.本文针对高功率微波输出窗内侧氩气放电击穿过程,建立了二次电子倍增和气体电离的一维空间分布、三维速度分布（1D3V）模型,并开发了相应的PIC/MC程序代码.研究了气压、微波频率、微波振幅对放电击穿的影响.结果表明：在真空情况下,介质窗放电击穿只存在二次电子倍增过程;在低气压和稍高气压时,二次电子倍增和气体电离共存;在极高气压时,气体电离占主导.给出了不同气压下电子、离子的密度和静电场的空间分布.此外还观察到,在500 mTorr时,随着微波振幅或微波频率的变化,气体电离出现的时刻和电离产生的等离子体峰值位置有较大差异,尤其是当微波频率（GHz）在数值上是微波振幅（MV/m）的2倍时,气体电离出现的较早.     

Microwave-Controlled Light-Pulse Propagation in a Driven Λ-Type Atomic System with Two-Folded Levels

The temporal and spatial dynamics of one weak probe laser pulse, propagating through a Λ-type atomic medium with two-folded levels under the resonant excitation of one microwave driving field and one strong control field, is investigated in this paper. By numerically solving coupled Bloch-Maxwell equations, it isfound that, in the absence of the microwave driving field, the atomic medium is transparent to the probe pulse at line center, which propagates over sufficiently long distances. By contrast, when the microwave driving field is applied, the probe pulse at line center can be rapidly absorbed on propagation. This substantial reduction of probe transmittance caused by the microwave driving field may lead to potential applications in designing a new kind of optical switching.     

Electron cyclotron heating in the WT-1 tokamak

When a high power microwave is injected into a tokamak plasma, the intensities of microwave radiations emitted from the plasma and those of impurity lines are increased. Such increases are observed only when the electron cyclotron resonance field is present in the torus. The upper hybrid resonance heating is also observed near the plasma surface.     

A low-cost implementation of EPR detection in a dissolution DNP setup

The implementation of electron paramagnetic resonance (EPR) detection in a low-temperature dissolution dynamic nuclear polarization (DNP) setup is presented. Using a coil oriented parallel to the static magnetic field, the change of the longitudinal magnetization of free radicals is measured upon resonant irradiation of an amplitude or frequency modulated microwave (mw) field. The absorption EPR spectrum is measured if the amplitude of the mw field is modulated, whilst the first derivative of the spectrum is obtained with frequency modulation. Using a burst of pulses, it is also possible to perform pump-probe experiments such as saturation-recovery or electron-electron double resonance experiments. Furthermore, the magnetization could be monitored in a time-resolved manner during amplitude modulation, thus making it possible to record its transient as it is approaching an equilibrium value. Experimental examples are shown with frozen solutions of trityl radical and TEMPO, two commonly used radicals for dissolution DNP experiments.     

Electron spin resonance on a two-dimensional electron gas in a single AlAs quantum well

Direct electron spin resonance (ESR) on a high mobility two-dimensional electron gas in a single AlAs quantum well reveals an electronic g factor of 1.991 at 9.35 GHz and 1.989 at 34 GHz with a minimum linewidth of 7 G. The ESR amplitude and its temperature dependence suggest that the signal originates from the effective magnetic field caused by the spin-orbit interaction and a modulation of the electron wave vector caused by the microwave electric field. This contrasts markedly with conventional ESR that detects through the microwave magnetic field.     

Portable X-band system for solution state dynamic nuclear polarization

This paper concerns instrumental approaches to obtain large dynamic nuclear polarization (DNP) enhancements in a completely portable system. We show that at fields of 0.35 T under ambient conditions and at X-band frequencies, 1H enhancements of >100-fold can be achieved using nitroxide radical systems, which is near the theoretical maximum for 1H polarization using the Overhauser effect at this field. These large enhancements were obtained using a custom built microwave transmitter and a commercial TE102 X-band resonant cavity. The custom built microwave transmitter is compact, so when combined with a permanent magnet it is readily transportable. Our commercial X-band resonator was modified to be tunable over a range of approximately 9.5-10 GHz, giving added versatility to our fixed field portable DNP system. In addition, a field adjustable Halbach permanent magnet has also been employed as another means for matching the electron spin resonance condition. Both portable setups provide large signal enhancements and with improvements in design and engineering, greater than 100-fold 1H enhancements are feasible.     

微波脉冲与不同形状带矩形孔缝腔体耦合的数值模拟

 利用时域有限差分法对微波脉冲与带矩形孔缝的矩形和圆柱形腔体两种系统的线性耦合过程进行了研究。首先用数值方法分析了耦合过程中的场增强现象、脉宽展开现象和腔体调制现象,并发现了耦合过程中微波脉冲存在频谱分离现象。当微波脉冲的电场与孔缝窄边平行时,借助耦合函数对两个系统内部耦合场的分布特性进行了研究,结果表明在与孔缝窄边垂直的平面内,越靠近腔体壁,耦合场越弱。此外,两种腔体内部的耦合场在腔体截面内均呈现准周期振荡分布,矩形腔体内部耦合场振荡的幅值较均匀,而圆柱形腔体内部耦合场幅值在其截面中心附近区域最大;除了孔缝附近区域外,圆柱腔体轴线两端的耦合场远大于矩形腔体相应的耦合场。最后,研究了孔缝耦合共振频率与孔缝尺寸的关系,结果表明系统耦合共振频率不只与孔缝尺寸有关,而是由孔缝尺寸和腔体形状及其对微波脉冲的反射特性共同决定。     

Nonlinear regimes of resonance precession of the magnetization in a (111) iron-garnet film

The special features of precessional motion under ferromagnetic resonance conditions with perpen-dicular magnetization of the film are investigated on the basis of a numerical solution of the equations of motion of magnetization in a (111) type iron garnet film. Several nonlinear magnetization precession regimes exist for fixed values of the crystallographic and induced anisotropies. Depending on the value of the magnetizing field and the amplitude of the microwave field, precession occurs around the normal to the film with the third harmonic of the fundamental frequency making a small or large contribution to the nutation motion and with a small or large amplitude of the precession angle. Precession regimes around one of three symmetric directions different from the direction of the normal are possible. Narrow ranges of the static field, where dynamical bistability and regimes with a period which is a multiple of the period of the microwave field are realized, exist.