The performance of a Q-switched Nd:YAG laser as an ultrasonic source in a single-crystal silicon sample has been studied using a wide-bandwidth modified Michelson interferometer as an absolute displacement sensor. At low power densities the source is an extended buried thermoelastic source; however, the thermal expansion is less than the contraction due to the negative pressure coefficient of the energy gap and the illuminated 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.