Acoustic properties of a dielectric with cavities in a polarizing electric field

The effect of an external polarizing electric field on the shear wave propagation in a centrosymmetric crystal with electrostriction, whose body is penetrated with parallel cylindrical cavities (pores), is considered. The cavities are distributed throughout the crystal at random and with a low density. The waves are assumed to be polarized along the cavity generatrices, and the wave propagation occurs in the elastic isotropy plane, which is orthogonal to the cavity axes. The external field is assumed to be axial. Possibility of controlling the propagation of shear waves by the polarizing field is demonstrated for the case of metallized cavity surfaces.     

Scattering of obliquely incident shear waves from a cylindrical cavity

Prior work has proposed the use of ultrasonic angle-beam shear wave techniques to detect cracks of varying angular location around fastener sites by generating and detecting creeping waves. To better understand the nature of the scattering problem and quantify the role of creeping waves in fastener site inspections, a 3D analytical model was developed for the propagation and scattering of an obliquely incident plane shear wave from a cylindrical cavity with arbitrary shear wave polarization. The generation and decay of the spiral creeping waves was found to be dependent on both the angle of incidence and polarization of the plane shear wave. A difference between the angle of displacement in 3D and the direction of propagation for the spiral creeping wave was observed and attributed to differences in the curvature of the cavity surface for the tangential and vertical (z) directions. Using the model, practical insight was presented on measuring the displacement response in the far-field from the hole. Both analytical and experimental results highlighted the value of the diffracted and leaky spiral creeping wave signals for nondestructive evaluation of a crack located on the cavity. Last, array and signal processing methods are discussed to improve the resolution of the weaker creeping wave signals in the presence of noise.     

Diffraction of Elastic Waves from a Cylindrical Infinite Cavity with Circular Cross Section Filled by a Liquid

Based on the equations of elasticity theory and hydroacoustics, we calculated the field structures arising when a plane wave is incident on an infinite cylindrical cavity with circular cross section filled by a liquid. The variable pressure component on the cavity axis for longitudinal and shear incident waves is determined.     

Geometrical freedom for constructing variable size photonic bandgap structures

In order to study the design flexibility of photonic bandgap structures, we investigate different examples of 1D traditional Bragg layers and 2D photonic crystals. We have also considered a simple case of 3D woodpile structures. It turns out that in systems with large gaps, the evanescent waves penetrate into the bulk only distances comparable to one lattice constant. Therefore confinement of light can also be achieved without long range order, which leads to the introduction of novel photonic bandgap designs. Adhering to some constraints, the changes in the photonic bandgap in disordered structures are negligible. The important quantity to characterize the presence or absence of modes is the local photonic density of states, however bandgap phenomena in size and position disordered arrangements can also be verified with plane wave supercell calculations as well as finite difference time domain techniques.     

Anomalous photoacoustic behavior of semiconductors: Evidence for thermally generated surface deformations

The photoacoustic signal measured for a solid sample is generally thought to originate from plane thermal waves, created by exposing the sample to modulated irradiation. For illumination of energy near or below the bandgap energy of a semiconductor, photoacoustic response was observed which could not be explained with conventional, thermal wave theory. This behavior can be explained if it is assumed that the signal is dominated by thermal deformations, generated at the semiconductor surface. This paper will show how this and other types of sample vibration can affect photoacoustic measurements of semiconducting materials.     

Theoretical study on photonic devices based on a commensurate two-pattern photonic crystal

In two-dimensional optical chip applications, the optical wave is classified to two modes according to its polarization: TE and TM. It is desirable to integrate multiple optical wave control devices for different polarizations onto the same plane at the scale of the wavelength. Here we demonstrate a periodic two-pattern photonic crystal with a large, complete photonic bandgap (PBG). It comes from the superposition of two substructures: one contributes the TM PBG and the other contributes the TE PBG. By purposely introducing defects into the substructures, photonic devices for different polarizations can be integrated to bend, split, and resonate TM/TE waves simultaneously on the same plane.     

Parametric scattering of high-frequency elastic waves by a small spherical cavity oscillating under the action of a Rayleigh wave

Scattering of high-frequency transverse and longitudinal plane waves incident on a spherical cavity located at a small depth under the surface of a half-space is considered. The cavity oscillates as a whole in the field of a low-frequency Rayleigh surface wave, the oscillation vectors of the longitudinal, transverse, and surface waves being coplanar. The cavity radius is assumed to be small compared to the wavelengths of the sounding wave and the pumping surface wave. The scattered compression and shear waves at the combination frequencies ω±Ω are calculated in the dipole approximation. Expressions obtained describe the qualitative behavior of the combination-frequency signal levels produced at the outputs of horizontally and vertically oriented geophones moving over the free surface of the elastic half-space.     

Directional properties of an array of sound receivers positioned in an impedance screen recess

Expressions for calculating the directional characteristics of an array of sound receivers positioned in a waveguide with impedance walls are obtained from the solution to the problem on the diffraction of a plane sound wave by the waveguide open end with impedance flanges. The waveguide can be of a finite length, and, in this case, it can be considered as an open cavity in an impedance screen. The solution of the integral equation for the sound pressure distribution over the opening area is reduced to the solution of an infinite system of algebraic equations for the coefficients of the field expansion in normal waveguide waves. Examples of calculated directional characteristics are presented for arrays with receivers positioned at different distances from the opening and for different values of the impedances of the waveguide walls and flanges.     

Creation of tunable absolute bandgaps in a two-dimensional anisotropic photonic crystal modulated by a nematic liquid crystal

Photonic crystals (PCs) have many potential applications because of their ability to control light-wave propagation. We have investigated the tunable absolute bandgap in a two-dimensional anisotropic photonic crystal structures modulated by a nematic liquid crystal. The PC structure composed of an anisotropic-dielectric cylinder in the liquid crystal medium is studied by solving Maxwell's equations using the plane wave expansion method. The photonic band structures are found to exhibit absolute bandgaps for the square and triangular lattices. Numerical simulations show that the absolute bandgaps can be continuously tuned in the square and triangular lattices consisting of anisotropic-dielectric cylinders by infiltrating nematic liquid crystals. Such a mechanism of bandgap adjustment should open up a new application for designing components in photonic integrated circuits.     

Measurement of surface figure of plane optical surfaces with polarization phase-shifting Fizeau interferometer

A Fizeau interferometer based set up for measurement of surface forms of plane optical surfaces has been discussed. Phase shifting interferometry has been applied using polarization phase shifter. A linearly polarized (632.8 nm) He–Ne laser has been used as the source. Light reflected from the object and the reference/master surfaces are made circularly polarized in opposite senses by means of two properly oriented quarter wave retardation plates placed at appropriate positions, one inside and other outside the interference cavity of the interferometer, and phase shifts are introduced between the object and the reference/master waves by varying angular orientation of a polarizer/analyzer. Final result is made free from any residual wave-front aberrations introduced by the (intra-cavity) wave plate by subtracting phase values obtained by PSI technique between a high optical quality master surface and the reference surface from that obtained for the test object surface with respect to the same reference surface for each point of the interference field. Results are shown for a plane surface.Advantages of the technique presented are linearity and high accuracy in phase stepping, no perturbation of the interference cavity during the phase shifting and possibility of real time or dynamic interferometry.     

Investigation of a point-like and plane-wave excitation in 2D photonic bandgap microcavities using Green’s function method

We report on a new model for simulating wave propagation in two-dimensional photonic crystals (PhCs) by means of Green’s functions rearranged to fit on the geometrical and physical properties of the structure under investigation. The model can take into account physical effects occurring when the PhC is excited by either a point-like source, for the analysis of extended crystals with line and point defects, or a plane wave coming from infinite, to investigate mirrors and microlenses. The model has been used for studying a Fabry–Peròt cavity to evaluate its response in presence of a Hankel source or a plane wave excitation. A parametric investigation of the filter response as a function of the cavity length has been carried out and the best conditions to obtain an increase of quality factor of each resonant cavity mode have been determined.     

Effect of a dielectric film on the characteristics of an open Fabry-Perot resonator cavity

It is demonstrated that placing a thin dielectric film inside the cavity of an open Fabry-Perot resonator, at an angle to its axis, will remove the polarizational degeneracy of the principal TEM_q00 oscillation modes. Upon excitation of such a resonator cavity by a plane wave, the reflected electromagnetic field is in general elliptically polarized. The shape of the resonance curve depends on the thickness of that film, on its orientation inside the cavity, on the conditions under which the cavity has been excited, and on the conditions under which the reflected signal appears in the receiver. Changing the polarization plane of the incident plane wave does not result in a smooth variation of the resonator Q factor.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 21, No. 11, pp. 1648–1652, November, 1978.     

球面电磁波斜入射建筑物内场的数值分析

为了分析窄带电磁脉冲源近、远场辐照大型建筑物内部空间电磁场分布特点,采用时域有限差分方法,对比分析平面波、球面波场源正面斜入射多层多单元建筑物的计算结果,并较为全面地分析了各房间各水平面中心电场强度幅值、各层各水平面电场强度最大值分布情况。建筑物各层相对应高度面上场强分布近似,在电磁波传播方向上窗户房间内部区域场强较强,两种波场源入射结果基本一致;在其余区域场强较弱,特别是在较大空间内的区域,球面波场源入射下场强相对更弱。其中通过与待模拟建筑物外形相紧凑性匹配的球面波场源构建,模拟仿真了窄带球面电磁波在真空空间的辐射传播,计算结果验证所构建球面波近场源准确可行。     

ELASTIC WAVE SCATTERING AND DYNAMIC STRESS CONCENTRATIONS IN CYLINDRICAL SHELLS WITH A CIRCULAR CUTOUT

In this paper, based on the theory of elastic wave motion for open cylindrical shell, wave scattering and dynamic stress concentrations in open cylindrical shells with a hole are studied by making use of small parameter perturbation methods and boundary-integral equation techniques. The boundary-integral equations and iterative imminent series of scattered waves around the cavity of the cylindrical shell are derived. By employing this method, the approximately analytical solutions of scattered waves on the edge of cutout are gained. The computational formula for getting the dynamic stress concentration factors on the contour of cavity is developed. As an example, the numerical results of these dynamic stress concentration factors are graphically presented and discussed. The analytical methods put forward in the present work have practical significances for solving the problem of elastic wave scattering and dynamic stress concentrations in cylindrical shells with a circular cutout.     

The measurement of sound transmission at structural junctions by an impulse method

With short duration impulsive signals used to provide the excitation the vibration level difference across a T-junction of Perspex plates has been measured both as a function of frequency and angle of incidence. Problems associated with high velocity waves and relatively short path differences have been reduced or eliminated by suitable choice of material and geometry, and by use of spatial as well as temporal averaging. Problems associated with the dispersive nature of bending waves have been reduced by use of low pass filtering. Results are presented for point source excitation and comparison is made with plane wave excitation theory. In addition, plane wave excitation has been simulated by means of line averages of the plate response to a point source and these results are also presented and compared with the theoretical ones.     

The penetration of highly directional acoustic beams into sediments

A planar array of seven hydrophones buried in a sand sediment was used to study the propagation of 20 kHz sound waves through a water/sediment interface. Two different sources were used: one a parametric source with a beamwidth near 2°, and the other a linear source with a beamwidth near 10°. Measurements were made with each source at four grazing angles between 14° and 78°. It was found that at low grazing angles the behavior of the wave produced by the parametric source departed significantly from the prediction of plane wave theory for a lossy sediment. When the parametric source was used, the wave fronts penetrated more steeply into the sediment and the attenuation with depth was less than predicted by plane wave theory. It was observed experimentally that near the critical grazing angle the narrow acoustic beam is displaced along the interface before entering the sediment.     

Wave optical analysis of integral method for three-dimensional images

We analyze by wave optics an integral method that produces three-dimensional (3D) images. The point light source is given at the pickup stage, and the light wave passing through each elemental lens is obtained at the display stage. The amplitude distributions of the waves from each elemental image are the same around a specific point where a 3D image is formed. Since the light waves approaching the image plane from different elemental lenses are incoherent, the synthesized value is the sum of the squared amplitudes of the waves. Therefore the modulation transfer function of this integral method is given by that of a single elemental lens.     

Diffraction of a sound wave by the open end of a flanged waveguide with impedance walls

Diffraction of a plane sound wave by the open end of an impedance-wall waveguide connected to an opening in an impedance screen is considered. The plane wave is incident on the waveguide from a free half-space. Two versions of the problem are considered: for a semi-infinite waveguide and for a finite-length waveguide with a specified bottom impedance; the impedances of the walls, screen, and waveguide bottom can be different. The finite-length waveguide can be treated as an open cavity in the impedance screen. For the cavity of zero length, the problem is reduced to the diffraction by an impedance insert in the impedance screen. The solution in the external region determines the scattered field; the solution in the internal region allows one to determine the directional pattern of an array of receivers located in the cavity. The problem is solved using the integral Helmholtz equation with a specially selected Green’s function that provides the fulfillment of the boundary conditions. Formally, the problem is reduced to an infinite system of algebraic equations. The computational results obtained for bistatic and monostatic scattering patterns are presented.     

准光激励毫米波圆波导旋转TE6 2模式产生器

 介绍了采用准光方法激励圆波导产生旋转TE_{6 2}模式的设计原理、测试方法和实验结果。该模式产生器由毫米波光学系统和开放同轴波导谐振腔系统组成:毫米波光学系统由角锥喇叭天线、双曲面反射镜、抛物面反射镜、修正抛物面反射镜等部件组成;开放同轴波导谐振腔系统由开放同轴波导谐振腔、圆波导、测试辐射喇叭天线组成。通过网络分析仪和毫米波近场自动测试系统测试表明:该模式产生器在频率为96.4 GHz附近产生的圆波导旋转TE_{6 2}模式的纯度达到97%。     

一维超声光子晶体禁带结构分析

超声波在介质中传播时可以引起介质的折射率发生周期变化,当光波垂直于超声波的传播方向时,这种介质可以被用来作为光栅使用,称为超声光栅。当光波沿超声波的传播方向通过这种介质时,它还可以用作一维光子晶体,并称其为一维超声光子晶体(1D-USPC)。利用平面波法证明了一维超声光子晶体具有一般一维周期层叠结构光子晶体的禁带特征。同时这种光子禁带是可以通过超声波的波长和振幅来改变的,这就为控制光的行为方面提供另一种新的方法。