Surface resonant states and superlensing in acoustic metamaterials

We report that the negative material responses of acoustic metamaterials can lead to a plethora of surface resonant states. We determine that negative effective-mass density is the necessary condition for the existence of surface states on acoustic metamaterials. We offer the microscopic picture of these unique surface states; in addition, we find that these surface excitations enhance the transmission of evanescent pressure fields across the metamaterial. The evanescent pressure fields scattered from an object can be resonantly coupled and enhanced at the surface of the acoustic metamaterial, resulting in an image with resolution below the diffraction limit. This concept of acoustic superlens opens exciting opportunities to design acoustic metamaterials for ultrasonic imaging.