Bidirectional Perfect Absorber Using Free Substrate Plasmonic Metasurfaces

Much effort has been made to achieve metasurface absorbers with polarization‐insensitive, wide‐angle, multiband, or broadband performance from gigahertz to terahertz to optical ranges. However, there are well‐known challenges in optimizing the performance of metasurface absorbers, due to the existence of a theoretical limit. It is urgent to extend and improve the optical properties of metasurface absorbers to satisfy the requirements for practical applications. Here, a novel free substrate metasurface absorber is demonstrated that has high spatial frequencies on the surface and can prevent light transmission and reflection by transforming free‐space incident waves into evanescent waves. Experimental and simulation results show that the proposed metasurface absorber can realize bidirectional perfect absorption and that the absorption of the light incident on one side of the metasurface is independent of the light incident on the other. Hence, the bidirectional perfect absorption does not depend on coherence of the beams. The proposed metasurface provides new degrees of freedom to realize a perfect absorber with twice the absorption capacity. It could have a wide range of applications and profound implications in photodetection, solar light harvesting, and sensing.