IMAGING OF SURFACE ACOUSTIC-WAVES

A new experimental method has been devised that directly determines th e group velocities of surface acoustic waves. A point source and a poi nt detector are employed to measure the ultrasonic transmission across a solid surface as a continuous function of the propagation direction . Results for single pulses give the times-of-flight for both Rayleigh surface waves (RSW's) and pseudo-surface-waves (PSW's). Calculations and measurements of the group velocities of the surface waves on silic on show some unanticipated behavior: fluid loading qualitiatively chan ges the group velocity curves for both RSW and PSW. In particular, the RSW branch gains an additional component which we denote here as an i nduced Rayleigh wave (IRW). If a wave train is employed in the experim ent, the analog of phonon focusing is observed for the ultrasonic wave s, modified by ''internal-diffraction'' effects. Systematic measuremen ts of the wave intensities on silicon as a function of propagation dis tance are consistent with expected acoustic losses into the surroundin g water: the attenuation length of a wave depends on the mode and freq uency. A survey of surface-wave images on other crystals is included i n this study.