Resolution limit of a phononic crystal superlens

We report on the subwavelength imaging capabilities of a phononic crystal (PC) flat lens consisting of a triangular array of steel cylinders in methanol, all surrounded by water. The image resolution of the PC flat lens beats the Rayleigh diffraction limit because bound modes in the lens can be excited by evanescent waves emitted by the source. These are modes that only propagate in the direction parallel to the water-lens interface. These modes resonantly amplify evanescent waves that contribute to the reconstruction of an image. By employing the finite difference time domain method and ultrasonic experiments, we also explore the effect on the image resolution and focal point on various structural and operational parameters, such as source frequency, geometry of the lens, source position, and time. The mechanisms by which these factors affect resolution are discussed in terms of the competition between the contribution of propagative modes to focusing and the ability of the source to excite bound modes of the PC lens.