Watching Ripples on Crystals

Abstract

PHYSICS Throwing pebbles in a pond, or the patterns of sand that develop on a drum skin, are familiar examples of surface excitations that are ubiquitous in nature. Imaging the development of such surface waves that arise from local disturbances is fairly straightforward, because the surface tension of the material is low and so the waves produced are large and propagate relatively slowly. In a solid material, a crystal for example, the surface tension is relatively high and imaging the subsequent fast-moving surface excitations can present a challenge, especially when trying to assess their dynamical motion. Using ultrashort laser pulses, Sugawara et al. have generated and dynamically imaged tiny ripples of sound—acoustic phonons—on the surface of a tellurium oxide crystal. An intense laser pulse created the local disturbance of the crystalline lattice, and a train of weaker probe pulses was used to image the development of the waves as they propagated across the surface. In addition to providing beautiful patterns, the technique should prove useful in studying effects such as phonon focusing or phonon waveguiding.— ISO Phys. Rev. Lett. 88 , 185504 (2002).