Time-Resolved Cuspidal Structure in the Wave Front of Surface Acoustic Pulses on (111) Gallium Arsenide

The first experimental observation of the cuspidal structure in the surface acoustic wave (SAW) front propagating from a point source in a crystal is reported. Nanosecond SAW pulses are generated on the (111) surface of GaAs by focused nanosecond laser pulses and detected with a cw probe laser beam. Multiple SAW arrivals are observed in the cuspidal region near the $〈11\overline{2}〉$ direction. A peak of the SAW amplitude due to “phonon focusing” is observed in the cusp direction. The measured SAW wave forms are well described by a model calculating the surface response to a localized pulsed force.