Nonlinear coupling of surface and quasitransverse bulk modes in cubic crystals

In crystals the speed of the surface acoustic wave mode may approach that of the lowest-speed quasitransverse bulk mode in some directions of propagation. Under these circumstances, it is possible for energy to be transferred from the surface mode to the bulk mode by nonlinear coupling. In the present paper we investigate the possibilities for mode coupling in the (001), (110), and (111) planes of cubic crystals. A condition is given for determining the range of propagation directions with significant coupling, and numerical results are provided for eight different crystals with a range of anisotropy ratios. It is shown that even for significant excitation amplitudes the coupling is negligible for most propagation directions in the aforementioned surface cuts.