We first show experimentally and theoretically that super-resolution can be achieved while imaging with a flat lens consisting of a phononic crystal exhibiting negative refraction. The phononic crystal is composed of a triangular lattice of steel rods in a methanol matrix. The theoretical study uses the Finite Difference Time Domain (FDTD) method. Band structure calculations show that the phenomenon of super-resolution is related to the coupling between the incident evanescent waves and a bound slab mode of the phononic crystal lens. This coupling leads to amplification of evanescent waves by the slab mode. Super-resolution is only observed when the source is located very near to the lens, and is very sensitive to the location of the source parallel to the lens surface as well as to site disorder in the phononic crystal lattice. Other effects on super-resolution such as lens length, thickness, and frequency are reported. The extension of this work to solid/solid phononic crystals exhibiting negative refraction of longitudinal waves will also be discussed.
We will also report on the properties of a phononic crystal consisting of a square array of cylindrical Polyvinylchloride inclusions in air. This phononic crystal exhibits positive, negative, or zero refraction depending on the angle of the incident sound beam. These properties are analyzed theoretically using the FDTD method and are demonstrated experimentally. For all three cases of refraction, the transmitted beam undergoes splitting upon exiting the crystal. Band structures and equifrequency surfaces (EFSs) calculated with the Plane Wave Expansion method show that the observed properties result from the unique geometry of the phononic crystal’s EFS as compared to that of the incident media.
Finally, we will present perspectives and challenges for future research in the field of acoustic metamaterials and their applications. The concept of band structure design and its impact on phononic crystal properties will be discussed in the context of developing novel acoustic functionalities and devices.