High-efficiency, polarization-insensitive 1400-lines/mm retroreflective metagrating with cascaded nano-optical modes

A retroreflective grating with a high line density is one of the most desirable optical elements, as it is capable of channeling the reflected light toward the source with high angular resolution. Metagratings can steer the incident light into the desired diffraction order with ultrahigh deflection angle and unitary efficiency. However, in practice, the diffraction efficiency of a metagrating with a small period is very sensitive to the unit cells, thus leading to significant challenges in the fabrication process. In this Letter, a kind of high-efficiency, polarization-insensitive retroreflective metagrating with cascaded nano-optical modes on high-reflection films is proposed. It is found that the conception may increase the fabrication tolerance by a factor of more than two. A 1400-lines/mm metagrating is experimentally realized for the first time, to the best of our knowledge, which shows an excellent broadband diffraction spectrum with 96% efficiency over 50 nm in TE and TM polarization. This proposed novel metagrating will benefit miniature optical systems.