Characterization of isotropic solids with nonlinear surface acoustic wave pulses

The nonlinear propagation of high-amplitude surface acoustic wave ~SAW! pulses in two isotropic materials, polycrystalline aluminum and synthetic fused silica, was studied and exhibited qualitatively different types of nonlinear behavior. A single SAW pulse excited by a nanosecond laser pulse through a strongly absorbing layer was detected at two probe spots along the propagation path with a dual-probe-beam deflection setup. In this way the nonlinear changes of the SAW pulse shape were observed. For aluminum, the compression of the SAW pulse and formation of one negative ~inward the solid! narrow peak in the registered normal surface velocity and a shock front in the in-plane velocity were detected. This nonlinear behavior corresponds to a positive value of the nonlinear acoustic constant responsible for the local nonlinearity. For fused silica, the temporal extension of the SAW pulse and formation of two positive sharp peaks in the normal velocity related to two shock fronts in the in-plane velocity were registered. In this case the acoustic constant of the local nonlinearity is negative. The nonlinear acoustic constants for each material were evaluated by fitting the theoretical model based on the nonlinear evolution equation to the registered SAW pulses. The values obtained were found to be consistent with those calculated from nonlinear elastic moduli of the third order, measured previously with different techniques for similar materials. The characterization of solids by their nonlinear acoustic and elastic constants promises to be complimentary and more specific than the characterization based on the linear properties.