Optical generation of coherent surface acoustics: an optically based probe of surface structure and dynamics

Above-band-gap picosecond pulses are used to generate holographically high-frequency, coherent, surface acoustic modes on semiconductor surfaces. Optical diffraction from the surface acoustics is in superposition to a free-carrier phase grating that acts as an amplifying cross term in the diffraction process. The detection limits are of the order of 10(-4) nm for the surface displacement. Frequencies up to 2 GHz have been realized, with frequencies >20 GHz possible. A quantitative theory for the photothermal coupling has been developed. In addition, propagation of the optically generated surface modes has revealed a solid-liquid phase transition of the water layer at TiO(2)-H(2)O interfaces.