Broadband flattened underwater acoustic Luneburg lens

Conventional Luneburg lenses are symmetric circular gradient-index lens with no aberration, but they are restricted by their circular focal surface. Here, we show the design, realization and measurement of an underwater acoustic Luneburg lens with flattened focal surface. The flattened lens is designed by using the quasi-conformal transformation technique and constructed by metamaterials based on air-filled photosensitive resin. Through numerical simulations and experimental studies, the flattened lens is demonstrated to have excellent focusing performance for the incident plane waves propagating at desired angles over a broad frequency band from 20 kHz to 35 kHz. The proposed flattened Luneburg lens can be potentially applied in the applications such as biomedical imaging, underwater acoustic sensing and communication.