Dependence of surface acoustic wave nonlinearity on
propagation direction in crystalline silicon
R. E. Kumon, M. F. Hamilton (Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712-1063), P. Hess (Institute of Physical Chemistry, University of Heidelberg, 69120 Heidelberg, Germany), A. M. Lomonosov, and V. G. Mikhalevich (General Physics Institute, Russian Academy of Sciences, 117942 Moscow, Russia).
Nonlinear distortion of a surface acoustic wave in a crystal depends not only on the second and third order elastic constants, but also on the plane and direction of propagation. A theory developed for nonlinear surface waves in arbitrary anisotropic solids [Hamilton et al., J. Acoust. Soc. Am. 105, 639-651 (1999)] reveals a strong dependence on propagation direction in the (001) plane of crystalline silicon. Three distinct regions may be identified in terms of the angle q with respect to the á100ñ direction. In region I, defined by 0° £ q < 21°, the nonlinearity is ``negative" in the sense that positive segments of the in-plane particle velocity waveform steepen backward in space, and negative segments steepen forward (i.e., opposite what a sound wave does in air or water). In region II (21° < q < 32°) the nonlinearity is ``positive," with waveform distortion the reverse of that in region I. Measurements of laser-generated, broadband, surface wave pulses in regions I and II support these predictions. In region III (32° < q £ 45°) the nonlinearity is again positive, although the surface wave behaves increasingly like a shear wave for q®45°, and the nonlinearity is very weak. Propagation is expected to be nearly linear at q @ 21° and q @ 32°.