Surface acoustic waves can be used for the characterization of mechanical properties of materials, as well as to investigate the near-surface region of a solid for cracks and other flaws by probing for the presence of scattering sources. In the non-destructive characterization of solids, laser generation of ultrasound as well as interferometric detection of the surface waves are particularly attractive in view of the non-contacting nature of such systems. In recent studies, accurate detection of surface wave speed and attenuation have been shown to be possible by the use of dual-probe laser interferometers[1,2]. A number of authors have also shown the feasibility of generating ultrasound through the use of high-power lasers that generate acoustic waves either by thermoelastically heating the solid, or by ablating the material. For nondestructive applications, however, it is undesirable to ablate the material, and hence one must confine laser generation to the thermoelastic regime. However, in view of the relatively low sensitivity of typical optical detection systems, the generated surface waves have to be sufficiently strong and can be obtained only in the ablation regime if a single laser source is used.
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