Pure spin currents generation in magnetic tunnel junctions by means of adiabatic quantum pumping

We study the spin polarized currents generation in a magnetic (ferromagnetic/ferromagnetic) tunnel junction by means of adiabatic quantum pumping. Using a scattering matrix approach, it is shown that a pure spin current can be pumped from one ferromagnetic lead into the adjacent one by adiabatic modulation of the magnetization and the height of the barrier at the interface in absence of external bias voltage. We numerically study the characteristic features of the pure spin current and discuss its behavior for realistic values of the parameters. We show that the generated pure spin current is robust with respect to the variation of the magnetization strength, a very important feature for a realistic device, and that the proposed device can operate close to the optimal pumping regime. An experimental realization of a pure spin current injector is also discussed.     

基于电子倍增电荷耦合器件的哈特曼-夏克波前传感器质心探测误差分析

哈特曼-夏克(Hartmann-Shack,H-S)波前传感器的探测误差是自适应光学系统中的一个主要误差源.本文分析了电子倍增电荷耦合器件(electron multiplying charge-coupled devices,EMCCD)的各种噪声源对H-S波前传感器质心探测误差的影响,推导了基于EMCCD的H-S波前传感器的质心探测误差的数学表达式,并进行了数值仿真和实验研究.结果表明,基于EMCCD的H-S波前传感器的质心探测误差与增益直接相关,在增益小于P_max/(Q·N_mean)时质心探测误差随着增益的增大而减小,在增益大于P_max/(Q·N_mean)时质心探测误差随着增益的增大而增大,其中P_max是EMCCD的峰值电荷,Q是量子效率,N_mean是平均入射光强. 关键词： 自适应光学 电子倍增电荷耦合器件 波前传感器 质心探测误差     

基于铌酸锂晶体的电控λ／2波片

利用有限元法分析了双面电板下铌酸锂晶体内部的电场分布。在规定匀强电场区域条件下,得到匀强电场区域大小随晶体宽度与电极宽度比值增大而增大的规律。在此基础上利用铌酸锂晶体的双横向普克尔效应设计了一种适用于任意光波长的电控λ／2波片。制作并测试了晶体宽度与电极宽度之比分别为2：1和3：1的电控铌酸锂λ／2波片,结果表明：这种波片可将一束线偏振的输入光转换成任意方向偏振的输出光,也可将任意偏振方向的输入光转换成固定方向偏振的输出光,并且晶体宽度与电极宽度比值越大,实验值与理论值符合得越好。     

Flexible resistance memory devices based on Cu/ZnO:Mg/ITO structure

Resistance memory devices based on a Cu/Mg‐doped ZnO/indium‐tin‐oxide structure on a PET (polyethylene terephthalate) flexible substrate were fabricated. The devices showed stable bipolar resistance switching property and good flexibility. The high to low resistance ratio was larger than 30 times, the endurance was more than 10² cycles, and the resistance retention was longer than 10⁴ s. The resistance values of both high and low resistance states were not significantly changed by bending in a radius (≥20 mm) for more than 10³ times. This resistance switching phenomenon of our devices can be explained by creation/rupture of metal conductive channels induced by electrochemical migration of Cu ions. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Defect assisted coupled resonator optical waveguide: Weak perturbations

Coupled resonator optical waveguides (CROW) comprised of only a few resonators including one defective element are investigated analytically and numerically. The defect is introduced by perturbing weakly the resonator at the center of the chain, creating a ‘cavity’ in what is an analogue of a Fabry-Pérot resonator or a defect-assisted photonic crystal structure. The device is found to have a very narrow resonance within the original stopband of the unperturbed CROW, and an accompanying extremely large group velocity excursion, that extends over both the subluminal and the superluminal ranges. The properties of the resonance and the magnitude of the group velocity dispersion are found to be conveniently controllable by coupling strength and by the number of resonators used. The perturbation can be caused thermo-optically or electro-optically. The complex effects of waveguide loss are extensively discussed.     

Full-wave analysis of photonic-crystal structures with multi quantum well active region

In this paper, a semi-classical formulation for modeling photonic-crystal (PC) structures with a four-layer multi quantum well (MQW) active region is presented. The formulation treats the electromagnetic fields as a classical entity, while employing the quantum-mechanically derived rate equations to analyze the active medium. Using the Poynting theorem, we interrelate the two sets of equations, namely the MQW rate equations and Maxwell's equations. The formulation involves the stimulately emitted photon density in the former and a virtual volume current density representing the generated photons in the latter. With the help of this formulation, we then model a PC optical amplifier as well as a PC hexagonal-defect cavity laser and solve the obtained equations after replacing the involved spatial and temporal derivatives by pseudo-spectral and Runge-Kutta's formulations, respectively. The formulation also takes the effect related to gain saturation and frequency broadening into account.     

All-optical steering of the interactions between multiple spatial solitons in isotropic polymers

All-optical steering of the nonlinear interactions between multiple spatial solitons can be performed in an isotropic photoisomerization polymer, by propagating an external control beam in perpendicular direction. Fusing, giving birth to another new soliton, and transferring energy can take place in the interactions of signal beams, which can be achieved by changing the incident position of the control beam, the initial relative phase and the power ratio between the signal beams and the control beam. These phenomena are physically explained, and they have significantly potential applications in optical signal readdressing, logic gating, and all-optical switching, etc.     

Effect of initial stress on Love waves in a piezoelectric structure carrying a functionally graded material layer

The effect of initial stress on the propagation behavior of Love waves in a piezoelectric half-space of polarized ceramics carrying a functionally graded material (FGM) layer is analytically investigated in this paper from the three-dimensional equations of linear piezoelectricity. The analytical solutions are obtained for the dispersion relations of Love wave propagating in this kind of structure with initial stress for both electrical open case and electrical short case, respectively. One numerical example is given to graphically illustrate the effect of initial stress on dispersive curve, phase velocity and electromechanical coupling factor of the Love wave propagation. The results reported here are meaningful for the design of surface acoustic wave (SAW) devices with high performance.     

Conductivity of Poly(pyrrole)‐Poly(styrene sulfonate) Core–Shell Nanoparticles

The dielectric properties of poly(styrene) nanoparticles decorated at their surfaces with poly(styrene sulfonate) [PSS] brushes and subsequently loaded with polypyrrole (PPy) were studied. These film‐forming materials which may serve as hole‐injection layers in organic light‐emitting diodes, exhibit a core–shell‐type morphology with a core of electrically insulating poly(styrene) and a shell consisting of a corona of PSS chains which form the matrix in which the electrically conducting complex of PPy and PSS is embedded. This conducting complex exists in form of domains of nanoscale dimensions. Thin compressed pellets of these nanoparticles were studied using mainly impedance spectroscopy. Measurements were carried out in the temperature range between 123 and 453 K and frequency range from 10^?1 to 10⁶ Hz. While earlier studies were centered around the effect of polypyrrole volume fraction on the conductivity films and pellets composed of these nanoparticles, the present study reveals in which way the conductivity can be modified by exchange of the mobile inorganic counter ions of PSS. Besides the free‐acid form (H⁺), the Li⁺‐, Na⁺‐ and Cs⁺‐salts of PSS were investigated. The PPy volume fraction was the same for all PPy/PSS core–shell nanoparticles. The distance for phonon‐assisted hopping between next‐neighbor polypyrrolium chains is influenced by the presence of these inorganic cations. For all samples containing PPy, a transition from insulating to conducting behavior in the range of 300‐350 K was found. Using the fluctuation‐induced tunneling model, the average tunneling distance, as well as the potential energy barrier separating neighboring conducting grains was estimated. Finally, a detailed analysis of the dielectric spectra suggests the localization length of the charge carriers to be about 0.33 nm.     

Towards graphene nanoribbon-based electronics

The successful fabrication of single layer graphene has greatly stimulated the progress of the research on graphene. In this article, focusing on the basic electronic and transport properties of graphene nanoribbons (GNRs), we review the recent progress of experimental fabrication of GNRs, the theoretical and experimental investigations of physical properties, and device applications of GNRs. We also briefly discuss the research efforts on the spin polarization of GNRs in relation to the edge states.