A low-threshold and high-power oxide-confined 850-nm AlInGaAs strained quantum-well vertical-cavity surface-emitting laser

A low-threshold and high-power oxide-confined 850-nm AlInGaAs strained quantum-well (QW) vertical-cavity surface-emitting laser (VCSEL) based on an intra-cavity contacted structure is fabricated. A threshold current of 1.5 mA for a 22 μm oxide aperture device is achieved, which corresponds to a threshold current density of 0.395 kA/cm². The peak output optical power reaches 17.5 mW at an injection current of 30 mA at room temperature under pulsed operation. While under continuous-wave (CW) operation, the maximum power attains 10.5 mW. Such a device demonstrates a high characteristic temperature of 327 K within a temperature range from -12℃ to 96℃ and good reliability under a lifetime test. There is almost no decrease of the optical power when the device operates at a current of 5 mA at room temperature under the CW injection current.     

Optical Absorption in Periodic Graphene Superlattices: Perpendicular Applied Magnetic Field and Temperature Effects

A detailed study of the magneto‐optical absorption is presented for graphene superlattices (SLs) subjected to a perpendicular magnetic field. For a given temperature, this quantity exhibits a resonant peak structure whose characteristics depend on the magnetic field regime, circular polarization of light and SL barrier height. For the intermediate field regime, we demonstrated that the resonant peak structure of is directly correlated to the partial joint density of states. Specifically, the latter exhibits van Hove‐like singularities and peaks at energies where takes its maximum values. We also investigated the magnetoabsorption in the weak field regime for SLs exhibiting one and extra Dirac points in the absence of the field. It was found that for SLs with only one Dirac point, the absorption spectra consist of resonant peaks satisfying the same circular polarization dependent selection rule as that for pristine graphene, except for one of them. For SLs with extra Dirac points, the resonant peaks arise from transitions between singlet subbands or between doublet subbands and satisfy a circular polarization and peak intensity dependent selection rule. It was also found that the resonant structure of can be observed experimentally at room temperature in clean SLs.     

无铅四方相钙钛矿短周期超晶格压电效应机理研究

超晶格压电行为与内部正离子之间的内在联系尚缺乏相关的研究.本文基于密度泛函理论的第一性原理方法,研究了三种无铅四方相钙钛矿铁电超晶格(BaTiO_3/SrTiO_3,KNbO_3/KTaO_3和BaTiO_3/KNbO_3)中A,B位正离子对整体的极化和压电贡献.通过计算超晶格不同轴向应变条件下原子结构和Born有效电荷,获得了超晶格和各个正离子的极化值和压电系数.结果表明,在轴向压缩应变条件下(-0.15—0 A),无铅超晶格中的正离子位移D(A)和D(B)受到抑制,在拉应变时位移才显著增大,因此极化和压电行为不明显.在轴向拉伸应变作用下(0—0.15 A),无铅超晶格中各原子的极化贡献显著增大,特别是B位原子Ti,Nb和Ta的极化贡献使得总的极化强度也显著提高,并当拉应变达到一定值,超晶格才会出现明显的压电行为.无铅超晶格的极化和压电行为主要由B位正离子贡献.     

Effect of exchange interaction in ferromagnetic superlattices:A Monte Carlo study

The Monte Carlo simulation is used to investigate the magnetic properties of ferromagnetic superlattices through the Ising model. The reduced critical temperatures of the ferromagnetic superlattices are studied each as a function of layer thickness for different values of exchange interaction. The exchange interaction in each layer within the interface and the crystal field in the unit cell are studied. The magnetic coercive fields and magnetization remnants are obtained for different values of exchange interaction, different values of temperature and crystal field with fixed values of physical parameters.     

不同脉宽激光和频产生蓝色超短激光的模拟研究

为了扩展现有飞秒激光的波段,采用非周期的准位相匹配光学超晶格进行了两束激光的和频研究.光学超晶格的结构由遗传算法根据要求模拟计算得到,在此结构晶体上进行平顶长脉宽激光与飞秒激光和频,可以有效地抑制和频光的脉宽展宽,生成457nm的蓝色飞秒激光.结果表明,该技术提供了一种通过两束不同激光和频产生新波长超短脉冲激光的全新技术方案.     

High efficiency and enhanced ESD properties of UV LEDs by inserting p-GaN/p-AlGaN superlattice

Significantly improved electrostatic discharge(ESD)properties of InGaN/GaN-based UV light-emitting diode(LED)with inserting p-GaN/p-AlGaN superlattice(p-SLs)layers(instead of p-AlGaN single layer)between multiple quantum wells and Mg-doped GaN layer are reported.The pass yield of the LEDs increased from 73.53%to 93.81%under negative 2000 V ESD pulses.In addition,the light output power(LOP)and efficiency droop at high injection current were also improved.The mechanism of the enhanced ESD properties was then investigated.After excluding the effect of capacitance modulation,high-resolution X-ray diffraction(XRD)and atomic force microscope(AFM)measurements demonstrated that the dominant mechanism of the enhanced ESD properties is the material quality improved by p-SLs,which indicated less leakage paths,rather than the current spreading improved by p-SLs.     

Enlargement of omnidirectional electronic gap in the graphene superlattice heterostructures with Gaussian profile potential voltages

A valid method is used to extend the omnidirectional electronic gap (OEG) of Gaussian gapped graphene superlattices (GSLs) heterostructure. The heterostructure consists of two superlattices with different width ratios of potentials. Each superlattice comprises a periodic repetition of a unit cell consisting of 21 layers with the potential voltages varying according to a Gaussian function and another layer with a fixed potential voltage. The potential width ratios of constituent Gaussian gapped GSL are established utilizing the lower and upper energy edges of omnidirectional electronic gap depending on the width ratio of potentials. Moreover, it is shown that the width of OEG of the heterostructure is sensitive to lattice constant, which can be applicable to the development of graphene-based electronics.     

Stress‐dependent correlations for resonant Raman bands in graphite with defects

We have studied the Raman features characteristics of defects generated in graphite under high stress conditions. Defects are generated in pristine highly oriented pyrolytic graphite by squeezing the samples in a high‐pressure anvil cell and monitored in situ by Raman spectroscopy. On the basis of our Raman measurements and existing literature correlations, we conclude that vacancies and grain boundaries are generated during compression–decompression cycles, being the defects mostly generated during the decompression stage. Our results demonstrate that the relative intensities of the D, D′, and (D + D′) bands are strongly correlated. Which is important for practical application of Raman spectroscopy in the characterization of carbon materials is that such correlations are essentially constant over the whole stress range covered in the experiments (~7 GPa). An additional interesting result concerns the relative intensities of the denoted 2D₁ and 2D₂ contributions, which are correlated with the intensity of the G band; the intensity ratio between both features is modified by stress indicating that the stress affects the stacking order of pristine graphite. Finally, we find that the decrease in intensity of the 2D₂ band with decreasing crystallite size found in existing studies on unstrained graphite remains under stress conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Electrical field analysis of metal‐surface plasmon resonance using a biaxially strained Si substrate

Electrical field components of metal‐surface plasmon resonance were analyzed in detail. Both longitudinal optical (LO) and transverse optical (TO) phonon modes of a biaxially strained Si layer can be excited by surface‐enhanced Raman spectroscopy (SERS). The z to y polarization ratio in SERS measurements was calculated to be 0.78 using the intensity ratio of TO to LO phonon modes. The electrical field components of SERS were also calculated by the finite‐difference time‐domain method. Copyright © 2014 John Wiley & Sons, Ltd.     

类W-势阱与应变超晶格量子阱的带间跃迁

假设量子阱是类W-势阱,应变效应表现为势阱底部出现了类抛物线鼓包。在量子力学框架下,讨论了应变效应对输出波长的影响。结果表明,在应变作用下,量子阱出现了能级分裂,正是这种分裂为高性能量子阱光学器件的研制提供了更大的设计空间,为量子阱激光器件输出波长的调节提供了理论基础。