Zn0.8Cd0.2Se-ZnSe应变层超晶格的激子发光和受激发射

本文用MOCVD技术在GaAs衬底上成功地制备了具有波导结构的Zn0.8Cd0.2Se-ZnSe应变层超晶格样品,在77K温度的光致发光光谱中观测到n=1的重空穴和轻空穴激子的辐射复合。在光泵浦下,在波导结构的F-P腔中观测到具有多模结构的受激发射,受激发射谱中的不同模具有不同的阈值功率密度;时间延迟衰减曲线的半宽度越窄,阈值光强越大.     

Developments in nano-oscillators based upon spin-transfer point-contact devices

We review the current status of research on microwave nano-oscillators that utilize spin transfer devices with point-contact geometry, with an emphasis on the open questions that still prevent our full understanding of device properties. In particular, we examine those aspects that might affect irreproducibility of device performance. While there is a clear picture of the general principles that underlie the properties of the spin torque nano-oscillator, there are a number of details complicating the picture. We suggest that these details are potentially responsible for adversely affecting uniformity of performance from device to device. These details include (1) nonlinearities, (2) the Oersted field, (3) thermal and deterministic noise sources, and (4) non-uniformity of the spin accumulation. We suggest what role that these details might have in determining spin torque dynamics, and suggest particular avenues of investigation that might clarify whether or not these details are indeed responsible for device variability. This article is one of a series devoted to the subject of spin torque in this issue of the Journal of Magnetism and Magnetic Materials.     

理想浅海波导中声场奇异点与声源深度的关系

为了有效利用声场奇异点蕴含的声源参数信息,研究了在理想浅海波导中,远场不同邻阶模态组的声场奇异点与声源深度之间的关系。推导计算了典型浅海声源声场的邻阶模态组奇异点位置,并通过仿真对奇异点的分布进行分析,结果显示邻阶模态组的阶数和阶差越大,奇异点分布越复杂。进一步研究发现,邻阶模态组第一组奇异点的深度和声源深度之间存在联系,并且基于奇异点与声压场的对应关系,在获得准确模态分布的前提下,可以通过两个邻阶模态组的第一组奇异点深度逆运算获得对应声源深度信息,也可以通过第一组奇异点深度反演获得声源深度信息。该文为获取浅海声源深度提供了思路。     

Self oscillations of a bistable element in two coupled hybrid ring resonators

A CdS crystal showing thermally induced optical bistability is incorporated into two coupled hybrid ring resonators with different delay times. Both delay times are much longer than the relaxation time of the nonlinearity. The resulting self-oscillations are investigated both experimentally and theoretically. We find two different types of oscillation modes. If the crystal is on the lower branch of the bistability during the longer delay time, step like oscillations similar to the case of a single resonator occur. If the crystal is in the lower state onlh for the shorter delay time (also the shorter of the two delay times is much longer than the relaxation time) we find more complicated modes with plateaus and stairs because the long resonator acts as a memory for the system state before the switching process. We find complex mode locking structures exhibiting Farey-tree like transitions between different oscillation modes as well as mode coexistence. Based on an adiabatic theory we compute the regions of extstence of the different oscillation modes and compare them with experimental results.     

Logarithmic terms in entanglement entropies of 2D quantum critical points and Shannon entropies of spin chains

Universal logarithmic terms in the entanglement entropy appear at quantum critical points (QCPs) in one dimension (1D) and have been predicted in 2D at QCPs described by 2D conformal field theories. The entanglement entropy in a strip geometry at such QCPs can be obtained via the "Shannon entropy" of a 1D spin chain with open boundary conditions. The Shannon entropy of the XXZ chain is found to have a logarithmic term that implies, for the QCP of the square-lattice quantum dimer model, a logarithm with universal coefficient ±0.25. However, the logarithm in the Shannon entropy of the transverse-field Ising model, which corresponds to entanglement in the 2D Ising conformal QCP, is found to have a singular dependence on the replica or Rényi index resulting from flows to different boundary conditions at the entanglement cut.     

Nondegenerate mirrorless oscillation in silicon waveguide

We propose nondegenerate four-wave mixing mirrorless oscillation in a multimode silicon nonlinear waveguide. Thanks to the large modal dispersion between two spatial modes caused by the high-index-contrast waveguide structure, two counterpropagating pumps of one spatial mode can generate two new optical waves of the other spatial mode at different frequencies. The phase-matching condition can be satisfied with the higher-order modes involved; therefore, frequencies of the newly generated light can be tuned by simply changing the pump frequency. The threshold power and conversion efficiency of the proposed mirrorless oscillation are investigated under different waveguide parameters.     

Waveguide modulated non-local optical interaction of semiconductor microdisks

The non-local optical interaction of two semiconductor microdisks with a waveguide bridged at radial direction is proposed and studied by three dimensional finite-difference time-domain （FDTD） electromagnetic simulations. The strong and weak optical interactions between two microdisks are observed and ascribed to the internal coupled modes with different coupling ratios. The vertical radiation losses and the related mode quality factors are modulated by waveguide length and present oscillation characteristics for the resonant modes. In addition, the optical leakage of coupling system is affected by the etching depth of disks due to tile emission of minor components of electric field.     

Singly resonant optical parametric oscillation in periodically poled lithium niobate waveguides

We report quasi-phase-matched singly resonant optical parametric oscillation in electric-field-poled lithium niobate waveguides. Parametric gains as high as 250%/W, an oscillation threshold of 1.6 W (peak), idler output powers of 220 mW, and a tuning range of 1180-2080 nm for pump wavelengths of 756-772 nm have been observed. Pump depletion is limited to 40% because of the multiple launched transverse modes at the pump wavelength. We predict that fully optimized waveguide singly resonant oscillators can have thresholds of ~100 mW, accessible to cw diode pumping.     

Mathematical simulation of self-oscillations in a fiber optic laser with a microresonator reflector

In this paper we obtain approximate equations that describe the dynamics of a fiber optic laser with an external reflector, constructed on the basis of microresonance structures under inhomogeneous pumping conditions. The results demonstrate the possibility of the existence of selfmodulation modes with frequencies equal to the frequency of natural vibrations of the microresonator structure. The parameter ranges in which self-oscillations are possible are calculated.     

Rabi oscillations and generalizations

A unified, quantum mechanical account of the behavior of a model atom with few relavant energy eigenstates and subject to intense monochromatic incoming radiation is given. The modifications of the standard application of quantum electrodynamics needed to include incoming fields of finite intensity are described. A projected Green's function method for evaluating either the time evolution or spectral properties of systems is developed and applied to a variety of systems, including: (1) An atom with two to four levels interacting with a radiation field of one or two modes, resulting in various forms of Rabi oscillation; (2)_An atom interacting with all modes of the radiation field in its vacuum state, resulting in the possibility of spontaneous radiation; (3) An atom interacting with all modes of the radiation field with one highly excited, resulting in damped Rabi oscillation, resonance fluorescence, optical pumping, and in a continuing sequence of photon emissions which are studied in some detail.     

Charge,spin and valley pumping in silicene junction

The feasibility of generating polarized and unpolarized current in silicene by means of quantum pumping is discussed within the framework of Floquet scattering matrix. Charge pumping current is induced at zero magnetization splitting whereas spin and valley pumping current emerge when the symmetry between Dirac points K and K′ is broken. The intensity and direction of pumped current are shown to be dependent on pumping amplitude, phase between barriers, exchange energy and electric field. By careful control of external parameters, it is demonstrated that the ferromagnetic-silicene junction could be operated as a pump device that generates pure spin and valley pumping current.     

Experimental realization of a quantum spin pump

We demonstrate the operation of a quantum spin pump based on cyclic radio-frequency excitation of a GaAs quantum dot, including the ability to pump pure spin without pumping charge. The device takes advantage of bidirectional mesoscopic fluctuations of pumped current, made spin dependent by the application of an in-plane Zeeman field. Spin currents are measured by placing the pump in a focusing geometry with a spin-selective collector.     

Dipole-exchange spin wave spectrum in an anisotropic ferromagnetic waveguide with a rectangular cross section

A theory has been constructed that strictly describes the spectrum of dipole-exchange spin waves in an arbitrarily magnetized anisotropic ferrite waveguide with a rectangular cross section. The theory takes into account the spatial inhomogeneity of the internal magnetic field in the waveguide cross section. The influence of parameters of the ferrite waveguide on the distribution of the internal magnetic field in the waveguide cross section is analyzed. The dispersion characteristics of two waveguide types most widely used in practice are investigated. The dipole-exchange spin wave spectra calculated for a transversely magnetized waveguide are presented and the distributions of the dynamic magnetization in the waveguide cross section for several types of volume and localized spin-wave modes are constructed.     

Oscillations at a difference frequency in the middle and far infrareds in GaP semiconductor waveguides

The feasibility of oscillation at a difference frequency in the middle and far infrareds is considered under the condition of phase matching between a nonlinear polarization wave and a difference mode produced by two fundamental modes with a wavelength near 1 μm excited in a GaP dielectric waveguide. With 10-W short-wave modes propagating in a 100-μm-wide planar waveguide, the power of the difference mode can be as high as 300 μW at 1–8 THz at room temperature. When the GaP waveguide leans upon a silicon substrate, the power of the difference mode may reach 5 mW at 10–14 THz at the same temperatures.     

Analysis of magnetization instability patterns in spin-transfer nano-oscillators

The stability of large precessional magnetization motions induced by spin-polarized currents in spin-transfer nano-oscillators is discussed. Quantitative analytical predictions are obtained for the critical values of spin-polarized injected current and external magnetic field at which the oscillator magnetization precession becomes unstable. It is shown that the mechanism leading to instability is parametric resonance of well-defined pairs of magnetostatically coupled perturbation modes. The amplitude of these modes grows to large non-thermal values when the oscillator frequency matches the mean of the natural frequencies of the two coupled modes. Analytical predictions are obtained for the space-time structure and symmetry of the magnetization patterns that are formed at the instability. Analytical results are compared with numerical simulations of spin-transfer-driven magnetization dynamics.     

Coupling efficiency for phase locking of a spin transfer nano-oscillator to a microwave current

The phase locking behavior of spin transfer nano-oscillators (STNOs) to an external microwave signal is experimentally studied as a function of the STNO intrinsic parameters. We extract the coupling strength from our data using the derived phase dynamics of a forced STNO. The predicted trends on the coupling strength for phase locking as a function of intrinsic features of the oscillators, i.e., power, linewidth, agility in current, are central to optimize the emitted power in arrays of mutually coupled STNOs.     

10m高分布式负载有界波电磁脉冲模拟器的模式分析

利用时域奇异值分解(SVD)技术对并行时域有限差分(FDTD)方法计算得到的分布式负载平行板有界波电磁脉冲(EMP)模拟器内的场进行模式分析,给出有、无效应物时模拟器内场的3个主要模式(即TEM,TM1和TM2模)奇异值的变化及该奇异值随效应物的位置及高度的变化规律。研究结果表明:电磁波在模拟器中传播时,其主模TEM模的高频分量在衰减;效应物越靠近源时测试点场的3个主要模式所对应的奇异值变化越大;且当效应物的高度与所在位置处两极板距离之比小于等于60%时,效应物附近位置处场的TEM模对应的奇异值会出现两个极大值点,两点的水平间距与效应物的水平尺寸相当。     

Nearly noninvasive readout and manipulation of spin in double quantum dot using spin bias

Dynamics of two quantum dots coupled to electrodes with spin bias is investigated theoretically by means of the master equations. The two dots are coupled via exchange interaction. When the exchange interaction is much smaller than the lead-dot 2 coupling and dot 2 is under a symmetric spin bias, an initially fully polarized electron spin in dot 1 undergoes an oscillation with ignorable attenuation. Meanwhile, the direction of charge current flowing through dot 2 oscillates in the same period as that of the spin in dot 1. This allows to reverse or nearly noninvasively read out the spin in dot 1, by switching on and off the exchange interaction for a duration of half-integer or integer periods of the oscillation, respectively.     

三维声场边界元法几乎奇异积分问题分析

以三维声场问题为例,提出一种准确计算高阶单元几乎奇异积分的半解析算法.首先分析高阶单元几何特征,构造近似几何量,然后应用扣除法,将奇异积分核函数分解为规则核函数与近似几何量表达的奇异核函数.规则核函数积分采用常规Gauss数值积分计算,奇异核函数积分采用半解析算法计算.给出三维声场内问题和外问题经典算例,计算了近边界点的声压,结果证明本文半解析算法的有效性和准确性.     

Anomalous fluctuations in phases with a broken continuous symmetry

It is shown that the Goldstone modes associated with a broken continuous symmetry lead to anomalously large fluctuations of the zero field order parameter at any temperature below T(c). In dimensions 2相似文献