Cross-spectral density matrix of a random electromagnetic beam propagating through an apertured axially nonsymmetrical optical system

Using the fact that a hard-edged rectangular aperture function can be approximated by a two-dimensional multi-Gaussian series, analytic formulas for the elements of the cross-spectral density matrix of a random electromagnetic beam truncated by a rectangular aperture and passing through an axially nonsymmetrical optical system is derived. The analysis is illustrated by numerical examples relating to changes in the spectral degree of polarization of electromagnetic Gaussian Schell-model beams propagating through free space, focal system and dual-focus system. In particular, the effect of the size of the hard aperture on the spectral degree of polarization of the beam along its optical axis is discussed.     

交叉传播激光脉冲与等离子体相互作用产生的等离子体密度光栅

理论上研究了两束交叉传播的激光束与等离子体相互作用产生的电子和离子密度调制. 用一维粒子模拟程序(particle-in-cell,PIC)研究了两束激光脉冲产生的干涉场激发的等离子体布拉格光栅. 研究表明等离子体初始密度、脉冲强度和宽度共同影响等离子体布拉格光栅的演化. 光栅的密度峰值可以达到初始等离子体密度的8倍以上,并且可以维持几皮秒的时间. 等离子体布拉格光栅可以囚禁由受激拉曼散射形成的电磁孤子,从而形成准稳态的孤子结构,很大程度上降低了形成电磁孤子所要求的激光脉冲强度. 关键词： 等离子体布拉格光栅 电磁孤子 交叉传播激光束 粒子模拟     

Ku波段球体目标系的电磁复合散射研究

研究了球形目标之间的电磁相互作用,得到了目标二次散射场的表达式,给出了目标的复合散射场以及双站复合散射截面,并在Ku波段进行了数值仿真.结果表明：目标的尺寸对前向散射和后向散射均有较大的影响,周围目标的影响呈现出一定的周期性,二次散射随着目标间距的增大而减小,周围目标离前向散射方向越近,对前向散射的影响也越大.目标的二次散射场与一次散射场强度之比在10^-4数量级以上.在研究多粒子的相互影响时,周围的粒子可近似看作尺寸一定的粒子.利用所得结果以及坐标变换可以研究三维体系内粒子间的电磁相 关键词： 散射截面 二次电磁散射 电磁相互作用     

电磁波在负折射材料填充的3层平板波导中的传播特性

从电磁场理论出发对负折射材料填充的3层平板波导的传播特性进行研究,并进行了数值计算,分析TE和TM波在平板中传播的性质,得到了电磁波的模式方程,与电磁波在右手材料填充的平板波导中的传播特性做了对比,考察了TE模式的能流密度,进行了归一化计算。结果分析表明：基模、一阶模不存在,且任何模式都存在一个截止厚度,随着厚度的增大,模式数量也增多;在一定的入射频率下,平板薄膜厚度趋向一定数值时,可以同时传播多种模式的波,并且入射波频率越高,波导同时存在多种模式的可能性越大;与右手材料相比,左手材料填充相同尺寸的3层平板波导可以传播更高能量的电磁波,导波效果更好。     

Microwave surface resistance of pristine and neutron-irradiated MgB<Subscript>2</Subscript> samples in magnetic field

We report on the microwave surface resistance of two polycrystalline Mg¹¹ B₂ samples; one consists of pristine material, the other has been irradiated at very high neutron fluence. It has already been reported that in the strongly irradiated sample the two gaps merge into a single value. The mw surface resistance has been measured in the linear regime as a function of the temperature and the DC magnetic field, at increasing and decreasing fields. The results obtained in the strongly irradiated sample are quite well justified in the framework of a generalized Coffey and Clem model, in which we take into account the field distribution inside the sample due to the critical state. The results obtained in the pristine sample show several anomalies, especially at low temperatures, which cannot be justified in the framework of standard models for the fluxon dynamics. Only at temperatures near T_c and for magnetic fields greater than 0.5H_c2(T) the experimental data can quantitatively be accounted for by the Coffey and Clem model, provided that the upper-critical-field anisotropy is taken into due account.     

Amplitude-squared squeezing in superposed coherent states

We study amplitude-squared squeezing of the Hermitian operator Z_θ=Z₁ cosθ+Z₂ sin θ, in the most general superposition state , of two coherent states and . Here operators Z_1,2 are defined by , a is annihilation operator, θ is angle, and complex numbers C_1,2 , α, β are arbitrary and only restriction on these is the normalization condition of the state . We define the condition for a state to be amplitude-squared squeezed for the operator Z_θ if squeezing parameter , where N=a⁺a and . We find maximum amplitude-squared squeezing of Z_θ in the superposed coherent state with minimum value 0.3268 of the parameter S for an infinite combinations with α- β= 2.16 exp [±i(π/4) + iθ/2], and with arbitrary values of (α+β) and θ. For this minimum value of squeezing parameter S, the expectation value of photon number can vary from the minimum value 1.0481 to infinity. Variations of the parameter S with different variables at maximum amplitude-squared squeezing are also discussed.     

Chaos in the unbalance response of journal bearings

The behaviour of non-linear systems often yield unexpected phenomena which are extremely sensitive to initial conditions. The hydrodynamic journal bearing is a common machine element which is strongly nonlinear for large excursions within the clearance space. A simple model of a rigid journal, supported hydrodynamically using a short bearing theory is shown to behave chaotically when the rotating unbalance force exceeds the gravitational load. At these values of the force ratio the time history of the response is very sensitive to initial conditions and a spectral analysis demonstrates a significant broadening from the expected peak at the rotational frequency. A once per revolution sampling of the time history (Poincaré plot) revealed an apparent aperiodic pattern. An estimate of the fractal dimension using the Grasberger-Procaccia algorithm resulted in a lower bound of 2.15, a typical result for low dimensional systems with significant dissipative action. The required levels of unbalance are only an order of magnitude greater than acceptable levels for rotating machinery and thus could be achieved with in-service erosion or minor damage. The subsequent non-synchronous response could result in fatigue and potential shaft failure.     

第一周期元素的电磁辐射与“冷聚变”的可能机制

根据一些室温核聚变的实验事实，提出了第一周期元素（包括类氢碱金属）的束缚态模型新概念，并用“束缚态”放能概念解释“室温核聚变”放能机制。对（ｐ，ｅ，ｐ）（Ｄ￣＋，ｅ，Ｄ￣＋）多体问题须考虑核与电子同时运动以代替玻恩－奥本海默近似。用量子力学粗估了束缚态能量。所谓室温核聚变放出的超热主要来自束缚态的Ｘ射线辐射能，而核反应几率很小，约低１０￣（－４）～１０￣（－６）量级，能量约低１０￣（－２）～１０￣（－４）量级。指出与此相应的物质存在是一种新的物质结构形态。并将所得理论结果与一些实验进行了对比验证。     

Signal enhancement in a r.f. SQUID using stochastic resonance

Summary We describe an experiment on a simple superconducting loop with a Josephson junction subject to a time-sinusoidal magnetic flux embedded in a noise background. The response is shown to display thestochastic-resonance effect, wherein the output signal-to-noise-ratio passes through a maximum at a critical value of the noise strength. Escape times probability density functions are also obtained experimentally Paper presented at the International Workshop ?Fluctuations in Physics and Biology: Stochastic Resonance, Signal Processing and Related Phenomana?, Elba, 5–10 June 1994.