Laser-generated surface waves provide new tools for studying material properties, from linear elastic behavior to fracture.

Surface Acoustic Waves in Materials Science

Determination of elastic modulus and thickness of surface layers by ultrasonic surface waves

The generation of ultrasound in coating-substrate systems subjected to laser beam illumination has been studied quantitatively by using the finite element method. Taking into account the temperature dependence of material properties, the transient temperature and temperature gradient field can be obtained in different coating-substrate systems. According to the thermoelastic theory, these temperature gradient fields are taken as bulk sources to generate ultrasound in coating-substrate systems. The typical surface acoustic waves (SAWs) in systems, a slow coating on a fast substrate and a fast coating on a slow substrate, are obtained. The influence of the propagation distance and the coating thickness on the SAWs is analyzed.

Finite element model of laser-generated surface acoustic waves in coating-substrate system

Scanned irradiation of a Si (111) crystal with a focused cw mode‐locked argon or neodymium:yttrium aluminum garnet laser to its melting threshold has generated a type of surface morphology, ripples, with a periodicity which is dependent on the laser pulse width We interpret these ripples as being a thermoelastically generated surface acoustic wave frozen out on the crystal surface

Laser-Pulse Width Dependent Surface Ripples on Silicon

Elastic properties of nanoscopic materials, structures and thin films are important parameters controlling their growth, as well as their optical and electronic properties. Acoustic microscopy is a well-established method for elastic imaging. In order to overcome its micrometer-scale diffraction-limited lateral resolution, scanning probe microscopy-based acoustic near-field techniques have been developed. Among the acoustic modes used for microscopy, surface acoustic waves (SAWs) are especially suited for probing very small and thin objects due to their localization in the vicinity of the surface. Moreover, the study of SAWs is crucial for the design of frequency filter devices as well as for fundamental physical studies, for instance, the probing of composite fermions in two-dimensional electron systems.This review discusses the capabilities and limitations of SAW-based scanning probe microscopy techniques. Particular emphasis is laid on the review of surface acoustic waves and their interaction with elastic inhomogeneities. Scattering, diffraction and wave localization phenomena will be discussed in detail. Finally, the possibilities for quantitative acoustic microscopy of objects on the nanoscale, as well as practical applications, are presented.

http://iopscience.iop.org/article/10.1088/0034-4885/73/1/016102/pdf

Surface acoustic wave-assisted scanning probe microscopy?a summary

Short laser pulses were used for generating surface acoustic waves (SAW’s) on optically opaque solids via the thermoelastic effect. The spatial and temporal shapes of the laser pulses were either a spot or a line of variable total width δ=2a with a Gaussian power distribution. Maximal efficiency is obtained when a=vRτ(vR: Rayleigh velocity, 2τ: total laser‐pulse length). This condition is caused by interference effects between the individual frequency components radiating from the area of the thermoelastic source. If the intensity of the laser beam were 100% modulated at a given frequency, maximal efficiency would occur if kR=(2)1/2/a (kR: wave vector of the SAW’s).

Efficient generation of surface acoustic waves by thermoelasticity

We have experimentally verified the conditions for maximal generation of surface-acoustic via the thermoelastic effect using a pulsed laser.

https://www.jstor.org/stable/pdfplus/37919.pdf

B. Betz

Papers

Efficient generation of surface acoustic waves by thermoelasticity

Surface-acoustic-wave generation by thermoelasticity