Optical generation of long-lived surface vibrations in a periodic microstructure

We use the laser-induced transient grating technique for the excitation and detection of surface vibrational modes of a periodic microstructure on a silicon substrate forming a one-dimensional phononic crystal. Two standing wave eigenmodes with zero-group velocity corresponding to the top and bottom of the bandgap in the dispersion of the zone-folded Rayleigh waves are produced by setting the spatial period of the excitation pattern to twice the structure period. These modes do not radiate acoustic energy into the substrate, yielding an enhanced lifetime. We show that the relative amplitude of the two modes is controlled by the spatial phase of the excitation pattern, and discuss the dependence of the confinement time of the acoustic oscillations within the excitation area on the curvature of the dispersion surface.