Laser-generated nonlinear surface wave pulses in silicon crystals

The absorption-layer method for inducing pressure shocks is employed to generate finite-amplitude, broadband surface wave pulses in crystalline silicon. Spectral evolution equations are used to compute the wave form distortion from the first to the second measurement location, and the results are shown to be in quantitative agreement with the measured data. The measurements also confirm that a nonlinearity matrix which describes the coupling of harmonics provides a useful tool for characterizing wave form distortion. In the (001) plane, the measurements show that the longitudinal velocity wave forms develop rarefaction shocks along [100] and compression shocks along 26degrees from [100]. In the (110) plane, compression shocks are observed in the longitudinal velocity wave forms in the direction 37degrees from [100], whereas rarefaction shocks are seen along [1(1) over bar 0]. The results in the (001) and (110) planes are consistent with sign changes in the nonlinearity matrix elements. In the (111) plane, the measured wave form distortion is consistent with the phase changes associated with the computed complex-valued matrix elements. In particular, the characteristics of propagation in the [11(2) over bar] and [(11) over bar 2] directions are shown to differ. This specific case is proved to follow from a more general result based on the symmetry properties of surface acoustic waves in this plane. In all the planes, it is demonstrated that, unlike bulk waves, the peak acoustic amplitude of surface waves can increase as they propagate, thereby allowing large stresses to be generated at surfaces. Finally, the power flux and total power of the pulses are shown to be substantially higher than in previous reports.