Near-perfect retroreflection of flexural waves via optimized elastic metagratings

The potential of elastic wave retroreflection is substantial for various applications such as structural identification and detection. However, realizing accurate retroreflection presents significant challenges owing to the need for high-efficiency anomalous reflection from boundaries. This study delves into the relatively less-explored field of flexural wave retroreflection in thin plates and proposes the use of metagratings for its realization. The present metagratings, which are built from unit cells created by possibly-overlapping circular holes, offer a simple fabrication process owing to their straightforward geometry. They hold an advantage over metasurfaces, which often struggle with wave steering because of their unmanageable higher-order diffraction modes. However, no systematic approach has been proposed so far to design metagratings for flexural wave retroreflection. To address this gap, we have established an optimization problem that identifies the optimal geometries and locations of the circular holes built into the proposed retroreflecting metagratings using a gradient-based optimizer. The design process involves diffraction grating theory and spatial fast Fourier transform. Both single and multiple retroreflections at various angles were considered and the numerical findings were substantiated by experimental results. The findings of this study imply that the investigated metagratings and design approaches have promising potential for more sophisticated wave manipulations.