Dynamic Visualization of Surface Phonons in Phononic Crystals

Using ultrashort pulsed optical excitation and interferometric detection, we dynamically image ultrahigh-frequency surface acoustic phonon wavepackets on two-dimensional (2D) phononic crystals. The samples consist of a square lattice of air-filled holes etched in a silicon substrate. Good agreement is found when compared with time-domain finite element numerical simulations. By Fourier transformation, the dispersion relation is derived and stop bands are revealed. The spatial distributions of the acoustic eigenmodes at specific frequencies are also presented.