Visible Light, Wide-Angle Graded Metasurface for Back Reflection

Metasurfaces, or phase-engineered quasi-2D interfaces, enable a large degree of control over the reflection, refraction, and transmission of light. Here we demonstrate the design and realization of a visible light gradient metasurface tailored for highly efficient back reflection based on the Huygens-Fresnel principle. The metasurface emulates the functionality of a Littrow grating, capable of efficiently channeling light into the first negative Floquet order over a broad angular range and bandwidth at visible frequencies. Our theoretical results predict unitary efficiency for extremely low profiles and an optical response that is robust against discretization and design modifications. The experimentally realized metasurface is comprised of high-index TiO_x nanowires over a protected Ag mirror, enabling back reflection with efficiency above 85% in the visible range, close to the reflectivity of the bare silver mirror. The presented analytical design methodology and the resulting low-profile device are advantageous compared to conventional gratings, while offering broadband efficiencies over a range of incidence angles.