Phonon focusing and mode-conversion effects in silicon at ultrasonic frequencies

Using a scanned point-source, point-receiver technique based on laser generation and piezoelectric sensing, we have observed pronounced anisotropy in the amplitudes of ultrasonic waves propagated in silicon single crystals. The source of this anisotropy can be attributed to focusing of acoustic-ray vectors brought about by the elastic anisotropy of the medium. We present our results in the form of scan images that contain a wealth of structures corresponding to reflection and mode-conversion processes. Head waves are also clearly identified for the first time in a single-crystal specimen. The transduction process also has an important influence on the radiation pattern, and is responsible for the almost complete extinction of certain wave modes. Our results are well accounted for with Monte Carlo ray simulations, and are consistent with the known phonon-focusing pattern of silicon.