Radio-frequency Subwavelength Time-reversal Imaging and Focusing: A Review of Theory, Methods, and Applications

Time reversal is an established wave imaging and focusing method that has proved to be robust and compatible with super-resolution imaging and focusing, i.e., to provide images and foci with subwavelength features beyond the diffraction limit. The method has been applied to numerous wave systems. We propose a systematic review of super-resolution time reversal applied to electromagnetic waves in the radio-frequency regime. We examine the theoretical foundation, the methods and the applications of radio-frequency super-resolution time-reversal. We explain a seeming contradiction between a widely used model of resolution due to the time-reversal cavity, and a common approach of Fourier optics highlighting the significance of evanescent waves for super-resolution. We also present an application of one of the algorithms of time-reversal imaging (known as TR-MUSIC) to measurements in a highly reflective environment, such as a resonant cavity. Finally, we outline open questions and applications.