Surface Acoustic Wave Amplitude Research Articles

Impulsive fracture of silicon by elastic surface pulses with shocks.

During nonlinear evolution of surface acoustic waves (SAWs) stress increases with propagation, and may cause fracture of brittle materials. This effect was used to evaluate the strength of crystalline silicon with respect to impulsive load in the nanosecond time scale without using seed cracks. Short SAW pulses propagating in the [11(macro)2] direction on the Si(111) plane induce fracture at significantly lower SAW amplitudes than the mirror symmetric wave propagating in the [112(macro)] direction. This effect is explained by the differences in elastic nonlinearity of the two propagation directions.

Excitation of elastic surface pulses by fiber optics and near-field optical devices

Elastic surface pulses or surface acoustic wave (SAW) pulses were launched at the surface by confining pulsed laser radiation with single-mode fibers and tapered fiber tips, produced by the focused ion beam technique, in near-field configuration. The propagation of the elastic surface pulses was studied in a point-source point-probe experiment. The transient surface displacements of the propagating SAW pulses were detected by an actively stabilized Michelson interferometer with sub-angstrom resolution. For tip aperture sizes in the micrometer range confinement effects could be observed, extending the frequency spectrum of the launched SAW pulses to approximately 800 MHz. The actual frequency spectrum was limited by the applied detection method. An inverse square root dependence of the SAW amplitude with propagation distance was confirmed experimentally, as expected for the excitation geometry employed. Results are presented for layered systems with dispersion of the ultrasonic surface pulses. The relevance of the frequency spectrum of the SAW pulse for the determination of elastic and mechanical film properties in dispersive media is discussed.

Non-contact acoustic microscopy

We demonstrate a fast all-optical surface acoustic wave (SAW)microscope. This acoustic microscope may be thought of as a non-contact (hencenon-perturbing) surface acoustic microscope. The key to producinga sufficiently high SAW amplitude for imaging without producing surfacedamage is to tailor the generating optical distribution. This can be used tospread the optical power on the sample surface (preventing damage) and tofocus the acoustic waves (increasing the amplitude). This paper discusses thegeneral and specific design features of our microscope and importantnew developments in the general design of such instruments. Our microscopebased on this technology is capable of producing high quality, quantitativeimages of SAW amplitude and phase (velocity) on many materials; this and new,unique, forms of acoustic contrast are demonstrated and discussed.

X-ray imaging and diffraction from surface phonons on GaAs

Surface acoustic waves (SAWs) are excited on the GaAs (001) surface by using interdigital transducers, designed for frequencies of up to 900 MHz. The emitted phonons with wavelengths down to 3.5 μm are visualized and characterized by combined x-ray diffraction techniques. Using stroboscopic topography, the SAW emission of a parallel and a focusing transducer geometry are imaged. High-resolution x-ray diffraction profiles show up to 12 phonon-induced satellite reflections besides the GaAs (004) reflection, with a width of 9 arcsec each. The diffraction pattern is simulated numerically, applying the kinematical scattering theory to a model crystal. From fits to measured diffraction profiles at different excitation voltages, the SAW amplitudes were calculated and found to be in the sub-nm range.

Spatially resolved photoluminescence in GaAs surface acoustic wave structures

The interaction between a surface acoustic wave (SAW) and the excitonic photoluminescence (PL) in GaAs SAW structures is investigated. The dependence of the PL on the SAW amplitude and illumination intensity is explained by a simple model based on the field-induced ionization of the excitons and on the screening of the SAW electric field by photogenerated carriers. Microscopic PL constitutes a powerful technique for spatially resolving electric-field distributions in SAW structures.

The effects of surface acoustic waves on the optical properties of AlGaAs-GaAs quantum-well structures

The effects of a propagating surface acoustic wave (SAW) on the transitions in single- and multiple-quantum-well structures and the change in their absorption and refractive index are presented here. The fundamental difference between the effects of an applied electric field and a propagating SAW are those due to a linear and a nonlinear induced field, respectively. The electron and hole energy eigenvalues and envelope functions of the unperturbed structure are determined. The effect of the SAW-induced strain and electric fields on the confining potential of the quantum-well (QW) structure is then calculated and the energy eigenvalues and envelope functions for this perturbed case are determined. The complex refractive index of the structure is then determined for both the perturbed and unperturbed cases, to give the change in refractive index and the absorption coefficient as a function of SAW wavelength, amplitude, barrier composition, well width, and number of wells. The wavelength (/spl lambda//sub ac/) and power of the SAW were found to be important in setting the induced changes in the refractive index.

Scanning Acoustic Force Microscope Investigations of Surface Acoustic Waves

We report on the investigation of surface acoustic wave (SAW) fields by scanning acoustic force microscopy (SAFM), reaching submicron lateral resolution. The SAFM is based on a standard atomic force microscope and utilizes the non-linear force curve in the sense of a mechanical diode. The surface oscillation therefore leads to a shift of the cantilever's rest position. With SAFM we investigated SAW transducers operating at frequencies above 600 MHz. We measured the dynamic behaviour of the wave pattern within the transducers when sweeping the frequency and found a local influence of mass loading on the standing SAW amplitude. Furthermore, the first images of SAW diffraction and scattering are shown.© 1997 John Wiley & Sons, Ltd.

Surface acoustic wave propagation in Langmuir-Blodgett layers

The amplitude of surface acoustic wave (SAW) was monitored during the Langmuir-Blodgett deposition. This in-situ measurement enabled to detect the changes caused by superimposing individual monomolecular layers on LiNbO 3 crystal. The experimental dependence of the SAW-echo on the film thickness was fitted by the theoretical curve derived from the model of SAW propagation at the thin layer-substrate interface.

Improved dew point measurements based on a SAW sensor

Optical dew point hygrometers have several drawbacks including high cost, frequent mirror contamination, instability under continuous use, and inability to detect the frost-point transition. In this work a novel hybrid sensor, and a surface acoustic wave (SAW) oscillator were used to compare optical and SAW dew point measurement techniques in the areas of condensation sensitivity, surface contamination effects, frost point transition behavior and dew point measurement resolution. The SAW technique was found to better control condensation density than the optical technique, allowing for a resolution of 0.03°C versus 0.2°C for a commercial optical dew point hygrometer. The SAW technique was also found to be much less susceptible than the optical technique to surface contamination. The frost point transition was found to cause significant measurement instability when controlling condensation density with SAW velocity or optical reflection voltage while SAW amplitude provided stable measurements. A Teflon Al coating provided accurate SAW dew point measurements at extremely low condensation densities, which may allow for improved measurements at very low relative humidities.

Surface acoustic wave studies of quantum nanostructures

Surface acoustic wave (SAW) scattering has been used as a contactless probe of the electronic properties of quantum nanostructures. Interdigital transducers, operating at 70 MHz, were deposited on the edges of samples containing large-area arrays of quantum wires and dots. Due to the piezoelectric coupling between the SAWs and the electronic systems, very small changes in the attenuation and velocity of a SAW pulse are measured with magnetic field at low temperatures. With the SAW propagation direction perpendicular to the wires, oscillations were observed due to the magnetic depopulation of 1D sub-bands. Using a simple model, characteristic confinement potentials were calculated at different electron densities. With the SAW propagation direction parallel to the wires, a monotonic decrease in amplitude is seen with increasing electron density. In quantum dots, the transmitted SAW amplitude shows a broad asymmetric minimum as a function of magnetic field, which reflects the density of single-electron energy level crossings per unit magnetic field interval. A simple model, assuming parabolic confinement, predicts a peak in the level crossing density at low magnetic fields, and we observe a corresponding decrease in SAW amplitude.

薄膜初期成長過程に及ぼす金属シード層の効果

Ag and Cu films were deposited by vacuum evaporation on a LiNbO3 single-crystal substrate on which surface acoustic waves (SAW) at 45 MHz were propagating. Prior to the deposition, ultra-thin Ti, Cr, Cu, and Ag films were fabricated as seed layers. The coalescence processes in an earlier stage of the film growth were investigated by measuring the SAW amplitudes during deposition. The critical thickness, at which the film structure changes from islandlike to continuous, was found to decrease with increasing seed thickness, except in the case of Cu films deposited on Ag seed layers. Ag films deposited on ultra-thin seed layers had topologically smooth surfaces in comparison with films directly deposited on the substrate. These results suggest that islands of the seed layer capture the adatoms diffused over the substrate surface and become the growth centers for the subsequent film deposition.

Time-resolved cuspidal structure in the wave front of surface acoustic pulses on (111) gallium arsenide.

The first experimental observation of the cuspidal structure in the surface acoustic wave (SAW) front propagating from a point source in a crystal is reported. Nanosecond SAW pulses are generated on the (111) surface of GaAs by focused nanosecond laser pulses and detected with a cw probe laser beam. Multiple SAW arrivals are observed in the cuspidal region near the $〈11\overline{2}〉$ direction. A peak of the SAW amplitude due to ``phonon focusing'' is observed in the cusp direction. The measured SAW wave forms are well described by a model calculating the surface response to a localized pulsed force.

SAW velocity measurement of crystals and thin films by the phase velocity scanning of interference fringes

We present principle and application of a novel noncontact velocity measurement of surface acoustic waves (SAW) on crystals and thin films using laser interference fringes scanned at the phase velocity of SAW. The scanning interference fringes (SIF) are produced by intersecting two laser beams with a frequency difference. The SAW velocity within the laser beam spot is measured as the ratio of observed SAW frequency and predetermined wave number of the SIF. The frequency measurement can be quite precise because of a large number of generated SAW carriers and amplitude enhancement effect. The SAW velocity measurement is free from the water loading effect accompanying the leaky SAW measurements. This principle was successfully applied to evaluate Si/sub 3/N/sub 4/ and SiO/sub 2/ films deposited on Si [001] surface.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Recent new experiments in magnetic thin film fabrication: Plasma nitriding and SAW exciting substrate

The physical properties of plasma and their effects on film structure such as phase and morphology are discussed based on the results of plasma diagnosis for plasma evaporation and sputtering, respectively. α″−Fe 16N 2 films, as an example, are systematically synthesized by using the above two independent methods and the degree of nitriding process of Fe atoms is discussed in connection with the results of plasma diagnosis. A new thin film fabrication technique which uses a substrate excited by surface acoustic wave (SAW) is proposed. The structural and magnetic behaviours of ultrathin metallic films are discussed in connection with the intrinsic film growth mode evaluated from the propagation properties of SAW during the film deposition. Metallic multilayers, Co/Cu, are formed as a function of oscillation amplitude of SAW and the GMR effect is discussed in connection with the change of interface conditions.

Optical Probe Detection of High-Frequency Surface Acoustic Waves Generated by Phase Velocity Scanning of Laser Interference Fringes

Laser-generated high-frequency surface acoustic waves (SAWs) and their optical detection for noncontact and nondestructive materials evaluation were presented. Scanning interference fringes (SIF) produced by intersecting two coherent pulsed laser beams on a tested object with different frequencies were used as a thermoelastic source to generate a 110 MHz SAW. The SAW was detected by the optical knife edge method using another continuous wave (CW) laser as a probe beam. Estimation of the SAW amplitude was carried out and the amplitude enhancement effect of the phase velocity scanning (PVS) method was verified. As a result, the SAW amplitude of 1.4 nm was obtained on an aluminum specimen with 0.71 MW/cm2 power density of second-harmonic waves (532 nm) of a Q-switched Nd:YAG laser. An application to anisotropic SAW velocity measurement on a silicon (100) surface was carried out.

Nonlinear regime of laser-induced piezoexcitation of the Rayleigh acoustic wave in CdS1−xSex crystal

The Rayleigh-type surface acoustic wave piezoexcitation induced by the interband absorption of laser pulses in CdS1−xSex crystal placed in a dc electric field was investigated. We detected the surface acoustic wave (SAW) pulses with a profile of the particles’ vibrational velocity consisting of three consequent phases and we observed nonlinear (logarithmic) increase of the SAW amplitude with the increase of laser intensity. Our experiments directly demonstrated that the efficiency of SAW laser-induced piezoexcitation is more than 4 orders of magnitude higher than the efficiency of SAW laser-induced thermoelastic generation.

弾性表面波で励振された基板上への金属薄膜の作成とその構造制御

Pure Ag, Cu, Ni, and Co films were prepared by vacuum evaporation on a LiNbO3 single-crystal substrate excited by surface acoustic waves (SAW) at 45 MHz under various amplitude conditions. The surface morphology of the films and the SAW propagation properties during deposition were investigated in order to determine the influence of SAW excitation on the structural changes in the films. In the Ag and Cu films, the surface roughness Ra and the thickness dp at which the SAW attenuation reached a maximum changed drastically as a function of the SAW amplitude, in contrast to the case for the Ni and Co films. Taking into account that dp corresponds to the thickness at which the film structure changes from island to continuous, it was considered that SAW influences the initial stage of film growth and that a large SAW amplitude enhances the agglomeration of islands. By in situ monitoring of the propagation properties of SAW, microscopic structural changes were also found in the Ag and Cu films during the initial growth stage.

The Influence of Surface and Volume Effects on Light Diffraction by Surface Acoustic Waves in Lithium Iodate Crystals

The influence of surface ripplings and periodical fluctuations of the refractive index in the layer closest to the surface on the phenomenon of light diffraction by surface acoustic waves (SAW) is investigated. The analysis includes a study of parallel interactions in α-LiIO3 crystals. The cases of external reflection, internal reflection, and transmission of light from or by the surface on which the SAW is propagating are considered. A theoretical analysis, based on numerical calculations, makes it possible to determine which interaction mechanisms take part in this phenomenon and which components of the SAW amplitude contribute to the intensity of the diffracted light. For this reason it is of interest to study, at the same time, all the basic geometries of interactions.

High Frequency Conductivity in the Fractional Quantum Hall Regime

Surface acoustic wave (SAW) propagation on high quality AlGaAs/GaAs heterostructures is examined in the fractional quantum Hall regime By measuring the transmitted SAW amplitude and frequency the 2D electron system (2DES) conductivity at the SAW frequency can be deduced and compared to the measured d.c. conductivity. Up to about 500MHz the SAW derived conductivity does not differ substantially from the measured d.c. conductivity. At higher frequencies some deviations are observed at FQHE and IQHE states However, at Landau level filling ν=1/2 SAW propagation is markedly different from that observed in the neighboring filling factor range; the response to the SAW is the opposite of that suggested by the d.c. conductivity In addition, anomalous sound propagation is also observed in the small filling factor range ν≲1/5, where electron solid formation is suspected. The properties of these distinct SAW anomalies are described and their possible origins discussed.

Rayleigh-type surface acoustic wave piezoexcitation by optical generation of the charge carriers in semiconductors

A linearised description of the process of surface acoustic wave (SAW) piezoexcitation by electric fields distributed in the volume of the crystal is proposed. These non-stationary inhomogeneous electric fields may be initiated by the spatial separation of optically generated electrons and holes. A simple nonlinear model is developed which explains the effects of SAW amplitude saturation with increasing laser intensity or external electric field observed in recent experiments. The estimates show that the conversion efficiency of optical power into SAW signal voltage may be three to seven orders of magnitude higher than obtained previously by thermoelastic mechanism of optoacoustic transformation.