Interaction of electromagnetic and acoustic waves in a stochastic atmosphere

In the Stanford radio acoustic sounding system (RASS) an electromagnetic signal is made to scatter from a moving acoustic pulse train. Under a Bragg-scatter condition maximum electromagnetic scattering occurs. The scattered radio signal contains temperature and wind information as a function of the acoustic-pulse position. In this investigation RASS performance is assessed in an atmosphere characterized by the presence of turbulence and mean atmospheric parameters. The only assumption made is that the electromagnetic wave is not affected by stochastic perturbations in the atmosphere. It is concluded that the received radio signal depends strongly on the intensity of turbulence for altitudes of the acoustic pulse greater than the coherence length of propagation. The effect of mean vertical wind and mean temperature on the strength of the received signal is also demonstrated to be insignificant. Mean horizontal winds, however, shift the focus of the reflected electromagnetic energy from its origin, resulting in a decrease in received signal level when a monostatic radio-frequency (RF) system is used. For a bistatic radar configuration with space-diversified receiving antennas, the shifting of the acoustic pulse makes possible the remote measurement of the horizontal wind component.     

Shear surface acoustic waves at the periodically nonuniform boundary between two solids

It is shown that along the boundary of two isotropic solids in mechanical contact there may propagate pure shear surface acoustic waves if this boundary is periodically nonuniform.     

Transmission-line analogue of carrier waves in solids

A general transmission-line analogue of carrier waves in solids has been developed. Collision and thermal-diffusion effects have been taken into account. This general result will be reduced to the well known transmission-line equivalent for an electron beam in a vacuum if both the collision and thermal effects are neglected and reduced to that of the electroacoustic waves if the carriers in the solid are stationary.     

Nonlinear interaction of acoustic surface waves with carriers

We calculate the 2nd-order signal generation due to the space-charge nonlinearity induced by two interacting surface waves in a piezosemiconducting material. Using the Bleustein?Gulyaev mode as a typical example, it is shown that this 2nd-order term is not zero for two oppositely travelling surface waves of the same frequency ? in the absence of electron drift. This calculation clearly shows that this nonzero signal arises from the decrease in the ultrasonic electric field toward the interior of the sample. The general result can be extended easily to other types of elastic surface waves. Complementary experiments are also reported on the interaction of surface waves propagating in the same direction, and it is shown that there is a satisfactory agreement between the experimental data and the results of the analysis.     

Continuous deflection of light with longitudinal acoustic waves

With a quartz-crystal acoustic light modulator, 600 spots have been obtained by only varying the frequency of the longitudinal acoustic waves. The center frequency of the acoustic waves was 780 MHz.     

Interaction of surface acoustic waves with a two-dimensional electron gas in the presence of spin splitting of the Landau bands

The absorption and variation of the velocity of a surface acoustic wave of frequency f=30 MHz interacting with two-dimensional electrons are investigated in GaAs/AlGaAs heterostructures with an electron density n=(1.3–2.8)×10¹¹cm² at T=1.5–4.2K in magnetic fields up to 7 T. Characteristic features associated with spin splitting of the Landau level are observed. The effective g factor and the width of the spin-split Landau bands are determined: g*≅5 and A=0.6 meV. The greater width of the orbital-split Landau bands (2 meV) relative to the spin-split bands is attributed to different shielding of the random fluctuation potential of charged impurities by 2D electrons. The mechanisms of the nonlinearities manifested in the dependence of the absorption and the velocity increment of the SAW on the SAW power in the presence of spin splitting of the Landau levels are investigated. Fiz. Tekh. Poluprovodn. 33, 979–985 (August 1999)     

Convenient method of studying helicon waves in solids

A convenient method of studying helicon-wave propagation in solid-state plasmas is described. The method is particularly useful with ordinary laboratory magnets and for low-temperature studies. Some typical experimental results are described.     

Interaction of electromagnetic waves with general bianisotropicslabs

An efficient, elegant, and systematic formulation technique which, combining Fourier transform with matrix analysis methods, is suitable for problems related to radiation by dipole or other sources in the presence of an arbitrarily general stratified anisotropic medium has been recently developed. This technique is adapted further extended to allow the presence of general bianisotropic media described by four tensors with no limitations on their elements. Two specific applications pertaining to some canonical problems of fundamental importance are included to exemplify the method and demonstrate its usefulness: radiation by an arbitrarily oriented elementary electric dipole source located in the vicinity of a general bianisotropic slab, either grounded or ungrounded, leading to the expressions of the dyadic Green's function of the structure, and reflection and transmission of an arbitrarily polarized plane wave incident upon such a slab, leading to closed-form concise expressions for the reflection and transmission coefficient matrices     

Visualisation of surface acoustic waves

A simple method for acoustic-surface-wave visualisation is described which is applicable to both piezoelectric and non-piezoelectric surfaces. Briefly, a uniform layer of small particles, comparable to the wavelength, is distributed over the acoustic surface. The acoustic-beam pattern of an incident surface wave is then traced out by scattering the particles out of the propagation path. Experiments at 100 MHz on lithium-niobate surfaces are described.     

Energy-trapped shallow bulk acoustic waves

A low-loss high-sidelobe-rejection shallow bulk acoustic filter has been constructed. The delay line consisted of apodised and finger-withdrawal-weighted transducers as well as a metallic energy-trapping grating. Experimental results are given and discussed.     

Amplification of acoustic waves in semiconductors with dominant ionized impurity scattering

Using the Boltzmann equation with modified collision term we obtain an expression for the attenuation/amplification coefficient of an acoustic ewave in a nondegenerate semiconductor in the presence of a d.c. electric field. The ionized impurity scattering has been taken as the sole mechanism of the scattering. A comparison of our results with those obtained by using well known Cohen, Harrison and Harrison model is also made.     

State of the art in wireless sensing with surface acoustic waves

Surface acoustic wave (SAW) passive devices can be used in novel applications such as wireless identification and sensing. For identification purposes, a SAW transponder picks up an electromagnetic request signal and stores it until all echoes caused by multipath propagation have died away. Then, a characteristic response is beamed back to the receiver. In radio-link sensors, a physical or chemical quantity influences the propagation properties of the SAW and consequently changes the response pattern of the device. This paper surveys the operating principle of such sensors and their state-of-the-art performance. Examples include temperature sensors and sensors for mechatronic applications     

Optical probing of surface acoustic waves

Light diffraction by surface acoustic waves is used in studying the propagation characteristics of Rayleigh waves on y-cut z-oriented LiNbO3. In reflection, only the surface deformation contributes to the diffracted light. In transmission, the photoelastic effect, which is incident beam polarization dependent, must also be considered. Such optical probing has been used to study the Fresnel diffraction radiation pattern of an interdigital transducer, to measure beam steering effects related to transducer misalignment on an anisotropic substrate, and to measure reflection and transmission coefficients of an electrically loaded transducer. Attenuation measurements have been made over frequencies as high as 3.5 GHz. The power dependence of attenuation and harmonic generation is also examined.     

Amplification of acoustic surface waves with two sets of carriers

It has been found that the present form of acoustic-surface wave amplifiers consisting of a semiconductor adjacent to an insulating piezeolectric substrate can be improved if the piezoelectric substrate is also semiconducting. When the conductivity of the substrate is properly chosen, the point of maximum gain occurs when the drift velocity of the carriers in the semiconductor is near the velocity of sound.     

Wavefront visualisation of acoustic surface waves

Acoustic surface waves (a.s.w.) propagating on yz-cut lithium niobate have been visualised by two imaging techniques. The wavefronts of straight-crested and convergent a.s.w. have been rendered visible on the screen of a storage oscilloscope.     

Frequency mixing of acoustic waves in piezoelectric semiconductors

A phenomenological theory has been developed to study the frequency mixing of the acoustic waves in piezoelectric semiconductors in the presence of an oscillatory electric field (OEF) as well as a d.c. electric field. The numerical results have been presented for the typical case of Cadmium Sulphide. It is found that the results are considerably altered by the application of an OEF.     

Comments on `Transmission-line analogue of carrier waves in solids?

The letter points out the differences between the transmission-line analogue of carrier waves in solids recently developed by Bong Ho et al. and the one previously developed by K. Fujisawa. Both have difficulties in constructing clear physical pictures. Here, another alternative of transmission-line analogue is given, which has simple physical pictures.     

Resonant phenomena in laser excitation of surface waves on solids

Excitation of surface waves by a moving laser beam has been studied both theoretically and experimentally. Using periodic surface structures as an example, it is demonstrated that the type of excited wave and the rate of its growth depend resonantly on the beam velocity. Experimental data show that resonant excitation of different periodic surface structures can change the spatial distribution of the absorbed laser energy