Time Reversal Focusing beyond the Diffraction Limit Using Near-Field Auxiliary Sources

In this letter, a novel approach for obtaining focal spots smaller than one-half wavelength of time-reversed waves is presented. By placing the extra designed transmitters in the near field of the original source in the first step of time reversal, the time-reversed waves, emitted by time-reversal mirrors, focus on the source location beyond the diffraction limit. The auxiliary sources do not exist in the reconstruction step, and the subwavelength focus is caused by superoscillation. The theoretical analysis of the phenomenon and numerical experiments in the finite-difference time-domain method are provided to demonstrate the proposed approach. The designed controllable sources can be incorporated into the subwavelength structures to further improve the super-resolution.