Low-index second-order metagratings for large-angle anomalous reflection.

A large-angle anomalous reflector based on a low-index polylactic acid metagrating is designed around 140 GHz. By breaking the limit of fixed bar-to-bar distance and directing the beam into the second diffraction order through the optimization of the 4π phase supercell, the efficiencies for 70° and 80° reflection under normal excitation both reach 0.82 with a wide bandwidth. In contrast, the efficiency is less than 0.2 in conventional designs. The success of the second-order low-index metagrating for large-angle deflection originates from the appropriate number of Bloch modes with well-engineered mode interactions and couplings. The design can be potentially fabricated by three-dimensional printing, with promising applications in designing flat lenses of high numerical aperture and extending the material system of metadevices.