根据激光单脉冲能量空间分布测量激光横模结构的方法

针对脉冲激光器,提出了一种根据激光单脉冲能量空间分布测量激光横模结构的方法,构造了相应算法,并通过理论和实验验证了该方法.通过对调Q二极管泵浦固体激光器的调Q过程进行数值模拟,计算出单脉冲能量空间分布曲线和各阶横模比例,同时根据单脉冲能量空间分布曲线上一些离散点计算出各阶横模比例,且与调Q过程数值模拟的直接计算结果相等.同时在实验中根据测量出的单脉冲能量相对值,计算出各阶横模比例,拟合出腔外不同距离处的单脉冲能量空间分布曲线.对腔外不同距离处单脉冲能量相对值和相应拟合曲线进行比较,发现测量值和拟合曲线吻合.     

Effective absorption enhancement in small molecule organic solar cells using trapezoid gratings

We demonstrate that the optical absorption is enhanced in small molecule organic solar cells by using a trapezoid grating structure. The enhanced absorption is mainly attributed to both waveguide modes and surface plasmon modes, which is simulated by using finite-difference time-domain method. The simulated results show that the surface plasmon along the semitransparent metallic Ag anode is excited by introducing the periodical trapezoid gratings, which induce the increase of high intensity field in the donor layer. Meanwhile, the waveguide modes result in a high intensity field in acceptor layer. The increase of field improves the absorption of organic solar cells significantly, which is demonstrated by simulating the electrical properties. The simulated results also show that the short-circuit current is increased by 31% in an optimized device, which is supported by the experimental measurement. Experimental result shows that the power conversion efficiency of the grating sample is increased by 7.7%.     

Directed spontaneous emission from an extended ensemble of N atoms: timing is everything

A collection of static atoms is fixed in a crystal at a low temperature and prepared by a pulse of incident radiation of wave vector . The atoms are well described by an entangled Dicke-like state, in which each atom carries a characteristic phase factor exp(ik0.r(j)), where is the atomic position in the crystal. It is shown that a single photon absorbed by the N atoms will be followed by spontaneous emission in the same direction. Furthermore, phase matched emission is found when one photon is absorbed by N atoms followed by two-photon down-conversion.     

Supernova explosions and constraints on mass and lifetime of heavy neutrinos

Constraints on mass and lifetime of heavy neutrinos imposed by supernova explosions are investigated. It is found that in the mass range of 10–70 MeV the constraint imposed by supernovae is more stringent than those given by cosmology. Any lifetime in this mass range is almost ruled out by the present constraint imposed by supernovae together with those imposed by high-energy experiments. It is suggested that if heavy neutrinos have mass and lifetime not ruled out by these constraints, the energy released by neutrino decay can induce supernova explosions even if the standard bounce-shock mechanism fails in explosions.     

声表面波在线性调频声栅上作Bragg衍射后的聚焦声场

本文应用准周期概念给出声表面波在线性调频声栅上Bragg衍射的透射函数。将一级衍射波的分布作为有限孔径内的已知分布。将求解透射区的一级声束的场分布问题作为求上述有限孔的衍射问题来处理。分析结果表明,对偏转声束来说,在Fresnel区存在一个Fraunhofer平面。线性调频声栅同时起两个作用:它既相当于一个等周期的声束偏转器,同时又是一个Fresnel透镜,在焦平面上的声场分布,是声栅出射平面上偏转声束幅度分布的Fourier变换。分析结果还表明:聚焦声轴随入射声波频率而平行移动。这一现象属首次发现,实验结果证实了上述理论分析预计。 关键词：     

Ferromagnetic spin coupling of 2p impurities in band insulators stabilized by an intersite Coulomb interaction: nitrogen-doped MgO

For a nitrogen dimer in insulating MgO, a ferromagnetic coupling between spin-polarized 2p holes is revealed by calculations based on the density functional theory amended by an on-site Coulomb interaction and corroborated by the Hubbard model. It is shown that the ferromagnetic coupling is facilitated by a T-shaped orbital arrangement of the 2p holes, which is in its turn controlled by an intersite Coulomb interaction due to the directionality of the p orbitals. We thus conjecture that this interaction is an important ingredient of ferromagnetism in band insulators with 2p dopants.     

三维非均匀不稳定渗流方程的自适应网格粗化算法

将渗透率自适应网格技术应用于三维非均匀不稳定渗流方程的网格粗化算法中,在渗透率或孔隙度变化异常区域自动采用精细网格,用直接解法求解渗透率或孔隙度变化异常区域的压强分布,在其它区域采用不均匀网格粗化的方法计算,即在流体流速大的区域采用精细网格.用该方法计算了三维非均匀不稳定渗流场的压降解,结果表明三维非均匀不稳定渗流方程的三维非均匀自适应网格粗化算法的解在渗透率或孔隙度异常区的压强分布规律与采用精细网格的解非常逼近,在其它区域压强分布规律与粗化算法的解非常逼近,计算速度比采用精细网格提高100多倍.     

Dispersion characteristics of two-dimensional unmagnetized dielectric plasma photonic crystal

This paper studies dispersion characteristics of the transverse magnetic (TM) mode for two-dimensional unmagnetized dielectric plasma photonic crystal by a modified plane wave method. First, the cutoff behaviour is made clear by using the Maxwell--Garnett effective medium theory, and the influences of dielectric filling factor and dielectric constant on effective plasma frequency are analysed. Moreover, the occurence of large gaps in dielectric plasma photonic crystal is demonstrated by comparing the skin depth with the lattice constant, and the influence of plasma frequency on the first three gaps is also studied. Finally, by using the particle-in-cell simulation method, a transmission curve in the \Gamma -X direction is obtained in dielectric plasma photonic crystal, which is in accordance with the dispersion curves calculated by the modified plane wave method, and the large gap between the transmission points of 27~GHz and 47~GHz is explained by comparing the electric field patterns in particle-in-cell simulation.     

电子散斑干涉载频调制形貌测量方法的分析

根据电子散斑干涉载频调制测量物体形貌的基本原理,物体表面的微小偏转可引入包含物体高度信息的载波干涉条纹。将物体的偏转视为变形,对物体变形与载波干涉条纹之间的关系进行了讨论,得出了离面位移引入载波和面内位移引入含有物体高度信息相位的结论。发现利用典型电子散斑干涉系统测量物体形貌效果最好,并通过实验得到了验证。     

碳化硅反射镜坯体光学加工的残余应力测量与分析

测量并分析了碳化硅反射镜坯体光学加工的残余应力。采用X射线衍射法测定了磨削成形、研磨以及抛光过程引入的表面残余应力的性质和大小;采用逐层抛光法测定了在用120#粒度金刚石砂轮磨削时引入的残余应力层厚度。研究结果表明:在用120#金刚石砂轮磨削加工时沿磨削方向和垂直于磨削方向分别引入了残余拉应力和残余压应力,其大小分别为40MPa和70MPa,应力层深度约为60μm,大于裂纹层深度;在用W7金刚石微粉研磨时引入了残余压应力,在其作用范围内残余应力平均值为60～80MPa;在抛光时理论上会引入残余压应力。在此基础上提出了在碳化硅反射镜坯体的光学加工过程中,可以通过研磨消除磨削引入的裂纹层和残余应力层。     

Electric field modulated Raman scattering in CdS

We have observed electric field modulated Raman scattering by A₁ LO phonons in CdS. The field induced scattering is observed with a geometry in which Raman scattering by A₁ LO phonons is normally allowed. The interference of the field induced and allowed terms in the transition susceptibility leads to a modulated Raman scattering intensity proportional to the applied field. This is contrasted with data previously reported on field induced Raman scattering by E₁ LO phonons in a configuration in which the Raman scattering is normally forbidden and in which there is no interference between linear wavevector dependent and field induced terms in the transition susceptibility. Electric field effects on Raman scattering by TO phonons and by 2 LO phonons is also discussed.     

Nonlinear dispersion in the process of quasistationary excitation of plasma waves

A quasistationary problem of Lengmuir wave excitation by external sources in uniform plasma is considered. It is established that energy is transferred from external sources to the wave if during its excitation the wave phase velocity changes in addition to an increase in the wave amplitude. A nonlinear dispersion equation for the plasma wave of finite amplitude excited by the external sources is derived. The nonlinear contribution of this dispersion equation is caused not only by an increase in the wave amplitude but also by the wave frequency shift.     

The role of propagating and evanescent waves in scanning near-field optical microscopy

Theoretical study on the image formation in scanning near-field optical microscopy is carried out in the framework of the direct moment method. Information brought, respectively, by the propagating and evanescent components in the optical near field that is collected by a scanning fiber tip with a sub-wavelength aperture is numerically and systematically analyzed in the light of the resolution achieved by the microscope. The analyses reveal that resolutions beyond the diffraction limit can be achieved even in the absence of the evanescent waves. That is, it is incorrect or at least incomplete to believe that a microscope that collects only the propagating waves is limited by the diffraction. Our studies show that a scanning near-field optical microscope can achieve resolutions beyond the diffraction limit by collecting only the propagating waves.     

Influence of viscosity on aerodynamic sound emission in free space

The theory of aerodynamic sound is reformulated with account taken of the influence of viscosity in the source flow on the sound emission in free space. This is based on the Ffowcs Williams form of the Lighthill equation. The source flow is assumed to be determined by a localized vorticity field and characterized by low Mach numbers and high Reynolds numbers. It is found that the acoustic pressure emitted by the viscous vortex motion is composed of a quadrupole and two kinds of monopole. The Reynolds stress in the source flow is decomposed into isotropic and non-isotropic parts. The non-isotropic part leads to the quadrupole wave derived by Möhring, while the isotropic part is related to one of the monopole-like waves radiated when the total kinetic energy changes. The other monopole wave is associated with entropy production by viscous dissipation of the kinetic energy. All three components are influenced by viscosity.     

Boson Mean Field Theory of Effective Holon-Holon Interaction in the RVB Model

The RVB model is investigated by a mean field theory, in which the holon is expressed by a fermion operator, and the spinon by a boson operator. The effective holon-holon interaction by exchanging the spinons is derived for T≠0. It is found that the interaction is attractive.     

Surface decoration: Localization of crystallites along the steps

The interest of decorating surfaces by metallic particles is to point out defects and in particular, steps. However, the precise localization of these particles is not known. It is shown here, by statistical measurements, that gold and palladium nuclei settle in the upper part of diatomic and monoatomic steps of (100) KCl. This unexpected localization is interpreted by the consideration of the elastic strain field, induced in the substrate by an in-contact crystallite. A crystallite placed near to the lower part of a step finds itself in a repulsive situation. When localized in the upper part, an attractive force counterbalanced by the line tension of the step exists. It is understood that decorating crystallites moving along the steps do not leave them, as it has been revealed by measurements and observations.     

Three dimensional stagnation flow of Casson nanofluid through Darcy-Forchheimer space: A reduction to Blasius/Sakiadis flow

Heat and mass transport properties in the neighbourhood of stagnation region are analyzed in this paper where the region is created by the motion of Casson nanofluid inside a porous space. Porous space bounded by a sheet from one side is modeled by Darcy-Forchheimer rule, and is under the influence of heat source/sink and normal magnetic field. The developed mathematical model is solved for numerical solutions by the application of Runge-Kutta scheme due to Fehlberg along with Newton Raphson Shooting approach. This model is further reduced to Blasius and Sakiadis type flows to correlate them. For the validation of code and results, a comparison graph is prepared. Solutions depicted by figures and tables, reveal that velocity is declined in Sakiadis flow with Casson parameter however opposite nature is noted for Blasius flow. In Blasius flow, it is observed that temperature is greater and concentration is lower if compared with Sakiadis flow.     

Observation of trapped and passing runaway electrons by infrared camera in the EAST tokamak

In EAST, synchrotron radiation is emitted by runaway electrons in the infrared band, which can be observed by infrared cameras. This synchrotron radiation is mainly emitted by passing runaway electrons with tens of Me V energy. A common feature of radiation dominated by passing runaway electrons is that it is strongest on the high field side. However,the deeply trapped runaway electrons cannot reach the high field side in principle. Therefore, in this case, the high field side radiation is expected to be weak. This paper reports for the first time that the synchrotron radiation from trapped runaway electrons dominates that from passing runaway electrons and is identifiable in an image. Although the synchrotron radiation dominated by trapped runaway electrons can be observed in experiment, the proportion of trapped runaway electrons is very low.     

Electron capture by charged impurities in semiconductors under conditions of spatial diffusion

The capture of electrons by charged impurities in semiconductors due to spatial diffusion is investigated theoretically. In a semiconductor, an electron either can be captured by the field of a charged impurity if this electron loses energy by emitting phonons or can be ionized from the trapping state if it acquires energy by absorbing phonons. The electron trapping is governed by a change in the distribution function of electrons in both coordinate and momentum space. The trapping coefficient is calculated under the condition where it is determined by the diffusion redistribution of the electron density in the field of a charged impurity.     

基于倍频波长转换中走离效应产生多个脉冲的研究

利用倍频光与泵浦光脉冲之间的走离效应,通过在周期性极化的LiNbO3波导中间增加两段未被极化的波导,1.55 μm波段的泵浦脉冲在极化的波导部分产生0.77 μm波段的倍频光,在未被极化的部分不产生倍频光,形成了三个被间隔开来的倍频子脉冲.通过差频将三个倍频子脉冲还原成1.55 μm波段的三个差频脉冲,实现了用一个泵浦脉冲产生三个转换脉冲.用数值模拟计算展示脉冲的产生及其演化过程,并讨论了用LiNbO3波导参量控制脉冲特征参量的方法.