  We propose and demonstrate a compact acoustic retroreflector that reroutes
probing signals back towards the source with minimal scattering. Gradient
refractive index (GRIN) 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 relatively broad 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 (UAV) sonar systems, robotic ranging detectors, ultrasonic imaging
systems.
