Surface acoustic wave generation with customized optical beam distributions

Laser generated ultrasound has many attractive properties. This paper discusses two methods of controlling the distribution of optical energy on the sample by the use of an acousto-optic cell and the use of computer generated holograms. These methods may be used to enhance the generation of harmonics of the fundamental frequency of the ultrasound. It is possible with this technique to reduce the peak optical intensity on the sample without decreasing the amplitude of the ultrasound. The acousto-optic cell technique splits a single generating source into two separated by a controllable distance. By varying the separation of the sources different harmonics can be enhanced or suppressed. Computer generated holograms may be used to generate complex optical distributions which can be used to focus ultrasound and enhance high frequency harmonics. Furthermore holograms may be designed to focus the light onto a curved surface allowing examination of samples with difficult geometries. Experimental results are presented which demonstrate these principles.