ULTRASONIC-IMAGING OF THE GROUP-VELOCITY SURFACE ABOUT THE CUBIC AXISIN SILICON

This article reports measurements of the group velocity surface of sil icon in the region of the [100] axis. Pulsed ultrasonic beams generate d by a piezoelectric longitudinal mode transducer are transmitted thro ugh water and focused onto a small spot on the surface of a (001) orie nted silicon single crystal disk. This gives rise to transient elastic waves which propagate in a wide range of directions through the speci men and they are detected by a small piezoelectric longitudinal mode t ransducer fixed at the center of the opposite face. The imaging of the group velocity surface about the (100) axis is accomplished by scanni ng the focused beam in various directions along lines that intersect a t epicenter. The scanned images reveal complex foldings of the slow tr ansverse sheet of the ray surface about the [100] axis, in general agr eement with the predictions of ray acoustics. However, the observed im age manifests pronounced wave motions well beyond the cuspidal edges p redicted by ray acoustics. This phantomlike feature or eidolon can be explained on the basis of diffraction resulting from the finite wavele ngth of the ultrasonic waves. (C) 1996 American Institute of Physics.