Acoustic flat lensing using an indefinite medium

Abstract

Acoustic flat lensing is experimentally confirmed, using optical feedback interferometry, for a phononic array of rigid spheres placed in air. The array behaves as an indefinite medium characterized by tensors whose principal elements have different signs, allowing for effects such as negative refraction and high impedance surface reflection. Here, this behavior is achieved in a narrow frequency region around 10.6 kHz, where the acoustic band displays a hidden singularity unveiled by an unusual path in the irreducible Brillouin zone.