Frequency Acoustic Waves Research Articles

Electronic absorption of surface acoustic waves by quantum rings in a magnetic field

Absorption of a surface acoustic wave by a quantum-ring array occupied by one or two electrons is considered. Dependences of the absorption coefficient on the magnetic flux through the quantum ring and the acoustic-wave frequency with allowance made for the Coulomb interaction between electrons are calculated. It is shown that the absorption coefficient is an oscillatory function of the magnetic flux with a period equal to the magnetic-flux quantum.

Modeling mid-to-high frequency acoustic signal fluctuations induced by fetch limited sea surface roughness in shallow water

Surface waves are among several environmental parameters that significantly influence mid-to-high frequency (1–18 kHz) acoustic wave propagation. In coastal regions, surface waves are fetch limited with reduced spectral level at lower frequencies. In order to assess the detail of an acoustic signal interaction with the sea surface in such regions, a combined approach based on experimental observation and modeling of both surface waves and acoustic waves has been adopted. Data from two broadband shallow water acoustic experiments are presented. These data include simultaneous wind speed and acoustic propagation measurements. The experimental design allowed an examination of received signals corresponding to single surface bounced ray paths. Measured data analysis shows a high correlation between time–angle–intensity fluctuations of received signals and varying sea surface conditions. An empirical fetch-limited ocean wave spectrum has been combined with an acoustic ray-based model to study acoustic wave propagation. Rough sea surface realizations are generated and used as sea surface boundaries with the acoustic model. This combined sea surface/acoustic model predicts the variability of acoustic signal fluctuations as a function of varying sea surface. Modeled time–angle–intensity signal fluctuations compare excellently with field data at lower wind speeds.

Unsteady beam-plasma discharge: I. Mathematical model and linear theory

A mathematical model is constructed that describes the development of the beam-plasma instability in a traveling-wave tube amplifier in the presence of a neutral gas. Steady solutions are derived for conditions of microwave discharges in a magnetized plasma-filled traveling-wave tube amplifier, and their stability is investigated. It is shown that the steady-state amplification regime may become unstable and change to the self-modulation regime. The relationships between the amplifier parameters at the instability threshold are obtained, and the frequencies of the excited ion acoustic waves are determined. The results of numerical modeling are found to agree well with the analytical results.

The investigation of preferred orientation growth of ZnO films on the PbTiO 3-based ceramics and its application for SAW devices

Poly-crystal ZnO films with c-axis (0 0 2) orientation have been successfully grown on the lead-based ceramic substrates by RF magnetron sputtering technique. The deposited films were characterized as a function of deposition time, argon–oxygen gas flow ratio, and RF power. Crystalline structures of the films were investigated by X-ray diffraction, scanning electron microscopy and atomic force microscopy. Highly oriented films with c-axis normal to the substrates can be obtained by depositing under a total pressure of 10 mTorr containing 50% argon and 50% oxygen and RF power of 70 W for 3 h. The phase velocity, electromechanical coupling coefficient and temperature coefficient of frequency of surface acoustic wave (SAW) device with ZnO/IDT/PT-ceramic structure were investigated. It shows that the preferred oriented ZnO film is beneficial for improving the electromechanical coupling coefficient of SAW device.

Anisotropy of the phonon-phason dynamics and the pinning effect in icosahedral AlPdMn quasicrystals

A minimal model of the phonon-phason dynamics in icosahedral quasicrystals with inclusion of the pinning effect is suggested. Resonant attenuation of low-frequency acoustic waves in the temperature range corresponding to thermal activation of phasons is considered. In the long-wave length limit, the velocity of acoustic phonons is isotropic; however, the phonon-phason coupling causes anisotropy of the velocity and of the attenuation of acoustic waves with small wave vectors. These effects manifest themselves most strongly at an acoustic wave frequency close to the inverse relaxation time of phasons with the same wave vector. The pinning effect can cause a significant decrease in the anisotropy of the velocity and of attenuation of acoustic waves.

An Improved 2.5 GHz Electron Pump: Single-Electron Transport through Shallow-Etched Point Contacts Driven by Surface Acoustic Waves

We present an experimental study of a 2.5 GHz electron pump based on the quantized acoustoelectric current driven by surface acoustic waves through a shallow-etched point contact in a GaAs/AlGaAs heterostructure. At low temperatures and with an additional counter-propagating surface acoustic waves beam, up to n = 20 current plateaus at I = nef could be resolved, where n is an integer, e the electron charge, and f the surface acoustic waves frequency. In the best case the accuracy of the first plateau at 0.40 nA was estimated to be ΔI/I = ′25 ppm over 0.25 mV in gate voltage, which is better than previous results.

Calibration of Curie temperatures for LiTaO/sub 3/ single crystals by the LFB ultrasonic material characterization system

The line-focus-beam ultrasonic material characterization (LFB-UMC) system is applied to a standardized comparison and evaluation of the Curie temperatures, T/sub C/, exclusively used in evaluating the chemical compositions of commercial LiTaO/sub 3/ crystals by measuring the velocities of Rayleigh-type leaky surface acoustic waves (LSAWs), V/sub LSAW/. We measured V/sub LSAW/ and T/sub C/ (standardized) under the same T/sub C/ measurement conditions for 36/spl deg/Y X-LiTaO/sub 3/ single-crystal wafers produced by four manufacturers and related the results to the T/sub C/ (individual) measured by the individual manufacturers. The relationships between V/sub LSAW/ and T/sub C/ (individual) varied from one company to another, and a single straight line of the proportional relationship between V/sub LSAW/ and T/sub C/ (standardized) was obtained for all wafers regardless of the manufacturer. These experimental results clarify that the problem associated with T/sub C/ measurements lies in the measurement conditions and the absolute accuracy of the measurement instruments. Measurements of the center frequencies of SH-type surface acoustic wave (SAW) filter devices are compared with V/sub LSAW/ measurements. A method of calibrating T/sub C/ using this ultrasonic system is proposed to establish standardized specifications of SAW-device crystal wafers.

Computational high frequency wave propagation

Numerical simulation of high frequency acoustic, elastic or electro-magnetic wave propagation is important in many applications. Recently the traditional techniques of ray tracing based on geometrical optics have been augmented by numerical procedures based on partial differential equations. Direct simulations of solutions to the eikonal equation have been used in seismology, and lately approximations of the Liouville or Vlasov equation formulations of geometrical optics have generated impressive results. There are basically two techniques that follow from this latter approach: one is wave front methods and the other moment methods. We shall develop these methods in some detail after a brief review of more traditional algorithms for simulating high frequency wave propagation.

Theory of the acoustic instability and behavior of the phase velocity of acoustic waves in a weakly ionized plasma

The amplification of acoustic waves due to the transfer of thermal energy from electrons to the neutral component of a glow discharge plasma is studied theoretically. It is shown that, in order for acoustic instability (sound amplification) to occur, the amount of energy transferred should exceed the threshold energy, which depends on the plasma parameters and the acoustic wave frequency. The energy balance equation for an electron gas in the positive column of a glow discharge is analyzed for conditions typical of experiments in which acoustic wave amplification has been observed. Based on this analysis, one can affirm that, first, the energy transferred to neutral gas in elastic electron-atom collisions is substantially lower than the threshold energy for acoustic wave amplification and, second, that the energy transferred from electrons to neutral gas in inelastic collisions is much higher than that transferred in elastic collisions and thus may exceed the threshold energy. It is also shown that, for amplification to occur, there should exist some heat dissipation mechanism more efficient than gas heat conduction. It is suggested that this may be convective radial mixing within a positive column due to acoustic streaming in the field of an acoustic wave. The features of the phase velocity of sound waves in the presence of acoustic instability are investigated.

Local coupled modes and volume scattering in heterogeneous anisotropic shallow water environments

Seafloor bottom/subbottom interactions are significant for low frequency acoustic wave propagation in shallow water environments. We investigate the significance of deterministic and stochastic volume scattering for generally anisotropic seafloor bottoms. The following assumptions are made: (i) the model is laterally heterogeneous with an elastic bottom/subbottom, (ii) anisotropy is hexagonally symmetric, (iii) the bottom/subbottom structure is dominated by deterministic elastic moduli with additional small-scale stochastic variations. The local coupled mode rough interface scattering theory of Park and Odom [Geophys. J. Int. 136, 123 (1999)] is extended by including volume scattering terms. New volume scattering terms are derived by applying perturbation theory to the elastic equations of motion. The acoustic wavefield is expressed as a sum of a mean deterministic wavefield and a small-scale scattered wavefield. Perturbations in the elastic moduli contribute to energy loss and the loss of signal coherence. The local mode formalism brings insight into the efficiency of volume scattering by indicating directly how much energy of coherent mode q is redistributed into scattered mode r. Numerical calculations indicating the effects of volume scattering are presented. [Work supported by ONR.]

Acoustic nonlinearity parameter measurements in solids using the collinear mixing of elastic waves

An alternative method for measuring the nonlinearity parameter β of longitudinal acoustic waves propagating in solids is presented. The method is based on the detection of the phase modulation resulting from the parametric interaction between a high frequency acoustic wave and a lower frequency acoustic pulse. Results are reported for various materials: fused quartz, duraluminum, titanium, and polystyrene.

Effects of Thickness Extension Mode Resonance Oscillation of Acoustic Waves on Catalytic and Surface Properties. IV. Activation of a Ag Catalyst for Ethanol Decomposition by Overtone Resonance Frequencies

The effects of resonance frequencies of acoustic waves on catalytic and surface properties were studied. The overtone resonance frequencies of 3.5, 10.8, and 17.9 MHz were applied to a 100 nm thick Ag catalyst deposited on a ferroelectric z-cut LiNbO3 crystal which generated thickness extension mode resonance oscillation (TERO). For ethanol decomposition, the TERO enhanced ethylene production without significant changes in acetaldehyde production for all the frequencies. The extent of catalyst activation strongly depended on the resonance frequency. In a low power region ( 1.0 W), it increased in the order 3.5 > 10.8 > 17.9 MHz. The activation energy for ethylene production decreased remarkably in the presence of TERO, the extent of which strongly depended on the frequency. Laser Doppler measurements showed that with increasing resonance frequency, the number of standing waves increased markedly, whereas the...

Apparatus and method for vasodilation

A method and apparatus for dilating blood vessels in vasospasm through the use of high frequency wave, e.g., hydraulic or acoustic waves. The invention is suitable to treat any vasospasm, including any vasospasm intractable to medication, including cerebral vasospasm, which currently is not susceptible to any mechanical or chemical treatments. The apparatus for dilating blood vessels includes a catheter having a fluid-filled lumen, a wave generator arranged within the catheter lumen for generating a wave front that propagates through the fluid in the lumen and is transmitted from the distal end of the catheter to propagate through the fluid in the blood vessel, and an energy source connected to the wave generator to provide energy to produce the wave front.

Nonlinear Interaction Saw with Periodic Domain Structures in LiNbO 3 Crystals

The propagation of acoustic waves trough a nonlinear media is interesting for both the fundamental physics and for many practical applications. In such materials the second acoustic harmonic generation and other acoustic nonlinear effects were investigated. The application on ferroelectric crystals such as LiNbO 3, LiTaO 3 and others with periodic domain structures (PDS) gives a possibility to develop a new class of acoustoelectronic devices: bulk acoustic waves (BAW) and surface acoustic waves (SAW) transducers, filters, delay lines, frequency standards and analogue signal processors. In this paper theoretical calculations and experimental results of surface and bulk acoustic generation at the first and second harmonics using a PDS in single crystal of lithium niobate are reported.

Electromagnetic excitation of high frequency acoustic waves and detection in the liquid phase

Excitation of acoustic waves in a quartz disk has been instigated by exposing the piezoelectric substrate to the electromagnetic field of a planar spiral coil placed in close proximity to the disk. It is argued that reciprocally induced magnetic and electric fields lead to secondary electric fields which couple with the piezoelectric tensor. A comparison of acoustic resonance envelopes recorded in air and in electrolyte demonstrates that spurious acoustic modes are damped in the liquid medium, and that the dielectric properties at the device–solution interface contribute to the exciting electric field. In agreement with previous acoustic device work, increased viscosity of the surrounding fluid leads to a rise in damping which manifests itself in changes in sensor frequency, amplitude and quality factor. With respect to work at high frequencies it is possible to operate the device at as high a harmonic as the 75th, although at this level of frequency resonance begins to break down because of a significant reduction in acoustic Q value. Finally, the use of the system in the flow-injection mode has also been demonstrated through the on-line detection of the adsorption of the protein, neutravidin, to the device surface. The glycosylated parent molecule is very widely employed as a linker for the immobilization of biological macromolecules in bioanalytical chemistry.

Influence of fetch limited surface roughness on mid-to-high frequency acoustic propagation in shallow water

Surface waves are among several environmental parameters that can influence broadband mid-to-high frequency (1–18 kHz) acoustic wave propagation. Understanding the interaction of sound waves at a rough surface requires a detailed description of the ocean wave spectrum. In shallow water regions, due to proximity to land, surface waves are usually fetch and duration limited with reduced spectral level and higher frequency components. A model that approximates the wave spectra for a fetch limited sea has been combined with acoustic ray-based method for analysis of forward scattered acoustic signals in such a coastal environment. Numerical modeling is employed to investigate time-frequency-angle characteristics of ray paths reflected from a rough sea surface. Temporal variability of acoustic signal fluctuations has been examined as a function of varying sea surface. To validate this model, results are compared against a unique set of experimental data collected in a fetch limited region. The experimental design allowed an examination of time evolution of a single surface bounced ray path. Simultaneous wind speed and acoustic propagation measurements allowed correlation between the individual ray paths and the sea surface at varying sea state conditions. For low wind speed conditions, model results predict the temporal fluctuations of the measured acoustic signal propagation.

Generation of cumulative sum frequency acoustic waves of shear horizontal modes in a solid plate

The physical process of generation of the cumulative sum frequency acoustic waves (SFAWs) of shear horizontal (SH) modes propagating in a solid plate is described in this paper. Due to the bulk elastic nonlinearity of material of the solid plate, there will be the SFAWs and difference frequency acoustic waves (DFAWs) once an excitation source of SH modes include two or more frequencies. A technique based on second-order perturbation is given for analyzing generation of the SFAWs and DFAWs, which arise from the nonlinear cross interaction of the partial shear waves of SH modes with the different frequencies. The following conclusions have been acquired in this paper. The DFAWs must not have a cumulative growth effect, the SFAWs may have a cumulative growth effect once some conditions are satisfied, and the SFAW fields with cumulative growth effect may be symmetrical or antisymmetrical. This paper also derives the analytical solution of the cumulative SFAWs, which satisfies the boundary and initial conditions of excitation. Numerical results clearly reveal the distortion and growth effects of the cumulative SFAW field patterns along the propagation direction, as well as the dependence of the cumulative SFAW field patterns on the position of the excitation source. The process of analysis shown in this paper yields clear physical insight into the cumulative SFAW generation of SH modes with different frequencies not previously available.

Brillouin light scattering on the two quasi-transverse waves in rotated sample of the LiNbO 3 crystal

The paper presents results for the Brillouin light scattering experiment, using a LiNbO 3 crystal, and identifies Brillouin scattering switch from one quasi-transverse acoustic branch to the second one during sample rotation. Measured hypersonic frequencies of quasi-transverse acoustic waves observed in different crystallographic directions are examined. A selection rule for the switching phenomenon is described.

Influence of environmental parameters on mid-to-high frequency acoustic signal propagation in coastal regiona)

Broadband mid-to-high frequency (0.6–18 kHz) acoustic wave propagation in shallow coastal waters (20 m) is influenced by a variety of oceanographic conditions. Physical parameters such as temperature and salinity as well as hydrodynamic parameters such as surface waves, tide, and current can influence amplitude and travel time of acoustic transmissions. While some of these physical parameters can be extracted from the acoustic wave propagation, understanding the forward model is vital to a fundamental understanding of the inverted parameters in shallow coastal waters. A unique set of simultaneous ocean and acoustic observations that reveal interesting temporal behavior of the acoustic signal and its correlation with environmental variability are presented. Some forward modeling using full-wave and ray-based methods is employed to investigate the time-frequency-angle characteristics of the measured acoustic field. Results of acoustic inversion for current, salinity, and sea-surface roughness, as well as the limitations of such inversions when using broadband signals in very shallow water regions, are also discussed. [Work supported by ONR and Sea Grant.] a)For Underwater Acoustics Best Student Paper Award.

Nonlinear transformation of seismic waves into ultra-low frequency atmospheric acoustic waves

A nonlinear mechanism of the transformation of low-frequency seismic waves into ultra-low frequency atmospheric acoustic waves is discussed. The spectrum of the seismic waves caused by the earthquake is wide but only ultra-low frequency acoustic waves reach the ionosphere. This process can be amplified due to the nonlinear interaction of low frequency waves and their transformation into ultra-low frequency acoustic wave at the difference frequency. It is shown that thus the efficiency of this transformation is high.