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Thermoelastic Coupling to Lamb Waves

We consider the thermoelastic generation of Lamb waves produced by incidence of a light beam on a solid plate. In addition to the variables relevant for thermoelastic SAW generation -- frequency and the optical, thermal, and acoustical properties of the materials used -- one has the plate thickness and mode type (symmetric, anti-symmetric). The characteristics of the problem depend on the ratios of plate thickness to thermal wavenumber and acoustic wavelength, and on the symmetry of optical beam incidence. Initial results indicate that the thermoelastic coupling efficiency for plate modes can be appreciably higher than for SAWs. Furthermore, if the plate is a suitable semiconductor structure with an applied electric field, one can increase the local heating over that produced by optical absorption alone, as shown by surface displacement measurements made in the photo-thermal-displacement microscope. These features may be useful in novel sensors that employ Lamb waves.