Surface acoustic waves with negative group velocity in a thin film structure on silicon

A surface acoustic wave dispersion curve of a thin film structure on silicon measured with the laser-induced transient grating technique displays a local minimum in frequency at nonzero wavenumber. For a range of wavenumbers to one side of this minimum the dispersion curve is of negative slope, implying the existence of surface waves with negative group velocity. Unlike in the well studied case of “backward traveling” Lamb waves in plates, the anomalous behavior here is exhibited by the lowest acoustic mode of the structure. The existence of the local minimum in the dispersion curve is corroborated by numerical simulations.