ON EPICENTRAL WAVE-FORMS FOR LASER-GENERATED ULTRASOUND

A laser pulse incident on a metal plate produces, by the thermoelastic process, an inward displacement at the epicentre, except for an initial outward-displacing spike. This initial displacement is not predicted by a point source approximation for this source of elastic waves. It is shown that the finite size of the source resulting from thermal diffusion into the bulk of the plate is a major cause of this initial spike in the epicentral waveform. As the rise time of the incident pulse is increased, the duration of the spike increases, but its amplitude decreases.