Negative refraction and focusing of acoustic waves using a foam-like metallic structure

A phononic crystal (PC) slab made of a single metallic phase is shown, theoretically and experimentally, to display perfect negative index matching and focusing capability in water. The PC slab is a centimeter scale air-filled foam-like metal structure arranged in a regular hexagonal lattice in which the acoustic energy is mediated via the metal lattice. The structure has the density and longitudinal velocity of water at low frequency. The negative index property arises from an isolated higher frequency branch of the dispersion curves corresponding to a mode that couples to incident acoustic waves in water. This band intercepts the sonic line at a frequency in the ultrasonic range, which leads to perfect index matching. The metallic structure is consequently a candidate for the negative refraction of incident longitudinal waves, and the realization of flat superlenses which can focus acoustic waves with a resolution lower than the diffraction limit.