Bianisotropic all-dielectric metasurfaces for efficient diffraction of mid infrared electromagnetic waves

A blazed grating maximizes the diffraction efficiency in a specific diffraction order. We design and experimentally demonstrate a bianisotropic all-silicon metasurface that shows a near unity diffraction efficiency by implementing a 4-mode interference in the far field. A coupled mode analysis shows that these modes provide a small number of orthogonal radiative electromagnetic moments that are needed for a perfect blazed grating. Bianisotropy due to asymmetric meta-atoms allows a normally incident plane wave to access through near field couplings two other modes which would have vanishing dipole moments without the symmetry breaking. Resonant and non-resonant properties of these modes and respective interaction can be mediated by sculpting meta-atoms of the metasurface. Because of these low order electromagnetic moments being utilized for the blazed grating, these high index meta-atoms can be subwavelength in all dimensions using simple geometries. At the same time, optical properties of these modes are relatively less sensitive to local fabrication imperfections. Further numerical simulations show an array consisting of a small number of these meta-atoms can efficiently diffract tightly focused Gaussian beams.