Perfect Diffraction with Multiresonant Bianisotropic Metagratings

Abstract

One of the most desirable functions of a diffraction grating is its ability to scatter incident light into a specific diffraction order with near-perfect efficiency. While such asymmetry can be achieved in a variety of ways, for example, by using a sawtooth (blazed) geometry, a recently emerged approach is to use planar metagratings comprised of designer multiresonant periodic units (metamolecules). Here we demonstrate that a bianisotropic metamolecule supporting four resonances of the appropriate symmetry can be used as a building block for achieving perfect diffraction. Coupled-mode analysis shows that these resonant modes provide a small number of orthogonal electromagnetic radiation patterns that are needed to suppress transmission/reflection into all but one diffraction order. Bianisotropy caused by breaking the metamolecule’s mirror symmetry enables a normally incident wave to excite two otherwise “dark” resonant modes through near-field couplings. We design and experimentally realize such bianisotropic metamolecules, which are subwavelength in all three dimensions, achieving efficient and directional diffraction in the mid-infrared spectral range. We show that optical beams tightly focused onto just a few unit cells of the metagrating can also be asymmetrically deflected with high efficiency, paving the way for compact broadband optical devices.