Acousto-optical effect induced by ultrasound pulses in a nematic liquid-crystal film.

The reorientation of the director of a homeotropically aligned nematic liquid-crystal film subjected to ultrasound pulses is discussed. The relationship is investigated for the first time, to our knowledge, between the transmitted optical power and the high-frequency (approximately megahertz) pulsed ultrasound intensity. It is evident that there exists a power law governing the ultrasound-intensity-dependent optical transmission. However, the data indicate no existence of a characteristic threshold for the acousto-optical effect. Our results suggest that acoustic streaming, known to be responsible for acousto-optical effects in cw ultrasound experiments, should also play a similarly important role in the case involving a pulsed ultrasound field.