We use ultrasonic experiments to study the acoustic analogue of photonic crystals. As an illustrative example, we present recent pulsed transmission measurements on hexagonal close packed (hcp) arrays of 0.80-mm-diameter stainless steel beads immersed in water. High quality crystals are prepared, starting with extremely monodisperse steel beads, by using a manual assembly technique. We directly measure the ultrasonic wave field transmitted through slab-shaped samples of different thicknesses, allowing us to determine both the dispersion curve and amplitude transmission coefficient. We compare our experimental results with theoretical calculations based on multiple scattering theory (MST) for elastic waves, which is ideally suited to the spherical scatterer geometry of the present experiments. We find that the low-frequency band structure and transmission coefficient calculated using this approach are in very good agreement with our experiments.
Work performed in collaboration with A.L. Goertzen, Zhengyou Liu, C.T. Chan and Ping Sheng