Compact acoustic retroreflector based on a mirrored Luneburg lens

We propose and demonstrate a compact acoustic retroreflector that reroutes incident signals back toward the source with minimal scattering. Gradient refractive index acoustic metamaterials, based on Archimedean spiral structures, are designed to fulfill the required refractive index profile. The experiments show that the compact acoustic retroreflector, whose radius is only approximately one wavelength, works in an incident angular range up to 120 degrees over a bandwidth of about 27% of the central frequency. Such compact acoustic retroreflectors can be potentially applied in pulse-echo-based acoustic detection and communication, such as unmanned aerial vehicle sonar systems, robotic ranging detectors, and ultrasonic imaging systems.