ULTRASOUND GENERATION IN SINGLE-CRYSTAL SILICON USING A PULSED ND-YAGLASER

The performance of a Q-switched Nd:YAG laser as an ultrasonic source i n a single-crystal silicon sample has been studied using a wide-bandwi dth modified Michelson interferometer as an absolute displacement sens or. At low power densities the source is an extended buried thermoelas tic source; however, the thermal expansion is less than the contractio n due to the negative pressure coefficient of the energy gap and the i lluminated region contracts rather than expands. Above a certain power density, ablation occurs, giving rise to a localized normal force at the surface of the sample.