Lagrangian-Eulerian micromotion and wave heating in nonlinear self-excited dust-acoustic waves

We investigate particle-wave microdynamics in the large amplitude self-excited dust acoustic wave at the discrete level through direct visualization. The wave field induces dust oscillations which in turn sustain wave propagation. In the regular wave with increasing wave amplitude, dust-wave interaction with uncertain temporary crest trapping and dust-dust interaction lead to the transition from cyclic to disordered dust motion associated with the liquid to the gas transition, and anisotropic non-Gaussian heating. In the irregular wave, particle trough-trapping is also observed, and the heating is nearly Gaussian and less anisotropic.     

Localisation and damping in resonators with complex geometry

Based on numerical studies, we show that localisation is a common phenomenon in resonators exhibiting some kind of geometrical complexity. In two-dimensional (2d) shallow cavities of irregular shape, localisation effects are due to spatial decoherence in a major fraction of the volume. In 2d shallow cavities of regular geometry with embedded absorbing material of irregular shape, one observes the appearance of eigenmodes localised in both, the absorbing and the non-absorbing media. Those modes are thought to be responsible for increased dissipation. These results may be a hint to understand why natural or practical systems absorbing wave energy are found, or built, with complex geometry.     

Generation of linear waves in Bose-Einstein condensate flow past an obstacle

A theory of linear wave patterns developing in Bose-Einstein condensate flow past an obstacle is developed. The results obtained characterize the wave crestline geometry and the far-field dependence of the wave amplitude on coordinates. The theoretical predictions agree with the results of previous numerical simulations and provide a qualitative explanation of experiments on the flow of a Bose-Einstein condensate released from a trap past an obstacle.     

Solitary autowaves in magnetic flux tubes

The weakly nonlinear dynamics of surface magnetoacoustic waves in magnetic flux tubes in the presence of the effects of dissipation and active non-adiabaticity is considered. The wave is described either by the extended Benjamin-Ono equation (slab geometry) or by the extended Leibovich-Roberts equation (cylindrical geometry). For the slab geometry, it is shown that solitary waves with arbitrary initial parameters evolve to solitons with specific amplitude, length and speed defined by the physical parameters of the magnetic slab.     

各向同性介质中任意偏振光的简并四波混频

在各向同性介质的简并四波混频中,就共线(探测光波矢平行于泵浦光的波矢)和非共线装置,线偏振和圆偏振等情况,推导出反射波的复振幅(包括偏振)与探测光和泵浦光的振幅、偏振、介质的三阶非线性张量元之间的关系式。在非共线装置中,泵浦光为平行的线偏振光时,可利用探测波与反射波的偏振方向间的关系求出x₁₁₁₁与x₁₂₂₁的比值;泵浦光为同旋性的圆偏振光时,可获得理想的复共轭波。在共线装置中,在反射波中总含有一定的非共扼的成分,振幅反射系数与样品厚度的关系可能是正切、双曲正切或线性函数关系;泵浦光为平行的线偏振光时,若|x₁₂₂₁|>|x₁₁₂₂|,可产生自振荡现象;在泵浦光为旋性相反的圆偏振光时,对圆偏振的探测光可产生理想的平面镜反射的效果,等等。结果可用来解释一些已知的实验结果,并对新的实验设计具有一定的指导作用。 关键词：     

Effect of substrate dimensions on dynamics of ultrasonic consolidation

An FEM model is developed for a fundamental study of the time-dependent mechanical behavior of the substrate and its dimensions on ultrasonic consolidation. The simulation shows that for a given vibration condition, the amplitude of contact friction stress and displacement stabilizes to a saturated state after certain number of ultrasonic cycles. With the increased substrate height, the amplitude of contact frictional stress decreases, while that of contact interface displacement increases. The reason for the decrease in the frictional stress at the contact interface for certain substrate heights is the complicated wave interference occurring in the substrate. An analytical wave model has been built to validate the FEM model. A specific substrate geometry (height:width = 1.0) generates a minimum frictional strain state at the interface as a result of wave superposition. Such minimum strain state is believed to have produced the “lack of bonding” defect for the geometry. The energy density and transfer coefficient at the contact interface with different substrate heights is used as an indicator to correlate with the bond formation in ultrasonic bonding.     

Double-resonant extremely asymmetrical scattering of electromagnetic waves in non-uniform periodic arrays

A new type of Bragg scattering – double-resonant extremely asymmetrical scattering (DEAS) of optical waves in oblique, non-uniform, periodic Bragg arrays is analysed theoretically and numerically. Steady-state DEAS is demonstrated to occur in the extremely asymmetrical geometry where the scattered wave propagates parallel to the front array boundary. The non-uniform array is represented by two joint uniform, strip-like, periodic arrays with different phases (and amplitudes) of the grating. DEAS is characterised by a unique combination of two simultaneous resonances with respect to frequency and phase variation at the interface between the joint arrays. As a result, a strong resonant increase in the scattered wave amplitude compared with the amplitude of the incident wave is predicted and investigated theoretically. The amplitude of the incident wave inside the array is also shown to increase resonantly in the middle of the array where the step-like variation in the phase of the grating takes place. The effect of different widths of the joint arrays, and magnitudes of the grating amplitudes on DEAS is analysed. Physical explanations of this type of scattering, based on the diffractional divergence of the scattered wave from one of the joint arrays into another, are presented.     

Analysis of photorefractive volume holographic lenses with different recording and read-out wavelengths

The photorefractive volume holographic lenses for converting a plane wave to a lateral-spread spherical wave with different recording and reconstructing wavelength has been studied. Using the coupled-wave theory, the half-analytical solutions for the amplitudes of the space harmonics of the field inside the transmission geometry are presented. The values of the off-Bragg parameter at the reconstructed process and the diffracted beam's amplitude distribution are analyzed. In addition, the dependences of diffraction properties on the focal length of recording spherical wave and on the geometrical dimensions of the grating are discussed. Furthermore, the focusing properties of this photorefractive holographic lens are analyzed.     

Theory of light diffraction in thick anisotropic reflection holographic gratings

Diffraction of light in thick anisotropic holographic media is considered. Solutions for the wave amplitude, diffraction efficiency, and angular mismatch are given in the reflection geometry for the case of refractive index modulation. A comparison with Kogelnik’s isotropic theory is made.     

Nonlinear skin effect in semimetals

The influence of nonlinearity on propagation of radio waves into semimetals in the geometry where the constant magnetic field H is directed along the trigonal axis of the crystal has been investigated. Strong magnetic Landau damping occurs in the linear mode in this geometry. It is shown that the electron capture by the magnetic field of a radio frequency wave of large amplitude decreases this absorption. As a result, the depth of the skin layer in the semimetal becomes the function of the amplitude of the exciting radio frequency field and can increase many times.     

Perception of amplitude and frequency modulated signals (mixed modulation)

This article discusses the detection of mixed modulation, i.e., simultaneous amplitude and frequency modulation (MM). The investigations have incorporated both a sine wave modulating signal and an irregular modulating signal, a very narrow noise band, of a specified center frequency. The results revealed that for a sinusoidal low-frequency modulating signal, amplitude and frequency changes that were separately subthreshold could be detected by listeners in mixed modulation. This indicates summation of sensations caused by simultaneous AM and FM modulation. This effect was not observed in the case of the irregular modulating signal. A hypothesis is advanced that the perception of modulated signals is governed by two mechanisms, viz., temporal and spectral. The operation of the two mechanisms depends mainly on the modulating frequency. The type of modulation does not play any significant role in this case.     

Modified impulse method for the measurement of the frequency response of acoustic filters to weakly nonlinear transient excitations

In this paper, a modified impulse method is proposed which allows the determination of the influence of the excitation characteristics on acoustic filter performance. Issues related to nonlinear propagation, namely wave steepening and wave interactions, have been addressed in an approximate way, validated against one-dimensional unsteady nonlinear flow calculations. The results obtained for expansion chambers and extended duct resonators indicate that the amplitude threshold for the onset of nonlinear phenomena is related to the geometry considered.     

Spatial statistics of a partially coherent wave in a randomly inhomogeneous cubic medium

The spatial statistics of a partially coherent light wave in a randomly inhomogeneous medium is considered on the basis of perturbation theory. The equations for the spatial coherence function of the light wave in a nonlinear irregular medium were solved. The variation in the fluctuation field dispersion, in its amplitude and phase, as well as the behavior of the radius of spatial correlation for various perturbations of the incident radiation, were investigated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 27, No. 1, pp. 56–64, January, 1984.     

Interaction of terahertz electromagnetic radiation with a probe-object system in a terahertz apertureless near-field microscope

The mechanism of the interaction of coherent terahertz radiation with a probe-nanoobject system has been experimentally investigated in a terahertz apertureless near-field microscope. It has been found that the type of the material of a sample under the probe, as well as the geometry of the probe, determines the form of the dependence of the differential signal of the terahertz field on the distance between the probe and sample. The amplitude of the modulation of the probe height significantly affects the spectral composition of the differential terahertz signal, which is directly related to the amplitude and phase of a terahertz wave scattered by the probe-nanoobject system.     

Experimental study on polarization holographic storage in genetic mutant BR-D96N film

The information storage is developing toward high speed access and massive stor-age capacity. Optical holographic storage has not only high speed parallel data access and high addressing speed without mechanical motion but also high storage density with multiplexed volume holographic storage[1]. In recent years, optical holographic storage is studied intensely in the field of information storage. It has been shown that the polari-zation of light could also be recorded with polarization hologra…     

Terahertz wave generation via pre-ionized air plasma

We report the terahertz(THz) wave generation from a single-color scheme modulated by pre-ionized air plasma via an orthogonal pumping geometry. It is found that the amplitude of the THz signal generated by the pump beam tends to decrease gradually with the increase of the modulation power. We believe that the ponderomotive force plays an important role in the process of the interaction between the pump beam and the pre-ionization beam. The hydrostatic state of the electrostatic separation field caused by the modulation beam will directly affect the generation efficiency of the THz wave. Our results contribute to further understanding of the theoretical mechanism and expanding of the practical applications of THz wave generation and modulation.     

Guided wave arrays for high resolution inspection

The paper describes a general approach for processing data from a guided wave transducer array on a plate-like structure. The raw data set from such an array contains time-domain signals from each transmitter-receiver combination. The technique is based on linear superposition of signals in the frequency domain with some amplitude and phase factors and can be applied to any array geometry and any types of array elements. The problem of finding optimal coefficients, which allow the best resolution to be achieved with the minimum number of array elements, is investigated. It is shown that improvements in resolution are obtained at the expense of sensitivity to noise. A method of quantifying this sensitivity is presented. Results are shown that illustrate the application of the technique to a linear array and an array of circular geometry (containing a single ring of elements). Experimental data obtained from a guided wave array containing electromagnetic acoustic transducer elements for exciting and detecting the S0 Lamb wave mode in a 5-mm-thick aluminum plate are processed with different algorithms and the results are discussed. Generalization of the technique for the case of multimode media is suggested.     

Paritätsreinheit der Kernzustände in Rb85

Nuclear γ-radiation must be circularly polarized if parity conservation does not hold strictly in nuclear states. From a polarisation measurement of the 513 keV γ-line in Rb⁸⁵ an upper limit for the relative amplitude of the irregular wave function F≦5,5·10^?6 was obtained.

     

不规则区域上并行求解Poisson方程

提出一种并行求解不规则区域上的Poisson方程方法,将不规则区域转化为带约束的三维结构网格表示,在该区域采用红黑排序并行求解Poisson方程.数值实验表明,方法可较好地解决不规则区域上的Poisson并行求解问题.同时评估了不规则区域对并行性能带来的影响.