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Ripples on crystalsPhysical Review Letters 88, 185504 (18 April 2002) Recently, surface acoustic waves (SAWs) have aroused interest for their possible use in areas such as high-speed signal processing. But the minute size (about 0.2 atomic widths) and rapid speed (over 3,000 m s-1) of the atomic vibrations involved mean that attempts to capture the propagation of SAWs have so far met with little success. Now, writing in Physical Review Letters, Yoshihiro Sugawara et al. present the first real-time images of SAWs in motion.
The authors used high-speed laser interferometry to capture minute variations in the motion of atoms on the surface of glass and various crystals. The samples were coated with a thin film of gold, and a short laser pulse (about 1-picosecond long) generated a surface wavepacket of coherent phonons Ethe atomic vibrations that make up SAWs Eat a point on the substrate. The effect of these phonons on the motion of surface atoms was then determined by measuring the rapid variations in interference between two successive 'probe' pulses. For atomically isotropic materials such as glass, the results confirmed Lord Rayleigh's prediction that the sound waves would spread out from a point source in concentric circles. But the images obtained also unveiled the beautifully intricate patterns that emerge in strongly anisotropic crystals such as tellurium oxide (see figure). This work opens up possibilities for a whole new field of surface phonon 'optics', allowing the study of SAW propagation through devices such as phononic crystals Eartificial structures with periodic surface features at the nanometre-scale Eand phononic waveguides, as well as providing new insights into the mechanical properties of crystals. More about surface wave propagationWatching Ripples on Crystals |
