Omnidirectional retroreflectors based on flexural wave metagratings

Omnidirectional retroreflectors that reflect the obliquely incident wave along its incident direction in the omnidirectional angle range are highly desired for the control and manipulation of flexural waves. With an enhanced signal-to-noise ratio (SNR) and suppressed signal losses, high-efficiency omnidirectional retroreflectors can be widely used for flexural wave guiding, detection, imaging, and cloaking. In this article, we propose a different but competent design paradigm to realize omnidirectional retroreflection on the basis of metagratings. By applying the multi-objective optimization algorithm to manipulate as many as nine different diffraction channels, we successfully demonstrate a high-efficiency retroreflector over a broad and continuous range of incident angles spanning from −78.2° to 78.2°. Two interesting applications are suggested for the metagrating-based retroreflectors. In the first one, the metagrating structure is utilized to suppress the diffuse reflection induced by irregular or curved boundaries of a thin plate, restoring the spectral reflection effect and enhancing SNR. In the second one, the metagrating unit cells are used to decorate the flat boundaries of a straight waveguide to stop the wave propagation along the waveguide, resulting in a localization of a wave signal. Considering the relatively simple configuration, the proposed design approach of the omnidirectional retroreflector is expected to find applications in various fields such as nondestructive evaluation and structural health monitoring.