Large-Angle, Broadband, and Multifunctional Directive Waveguide Scatterer Gratings

Abstract

Conventional surface-relief gratings are inefficient at deflecting normally incident light by large angles. This limits the overall efficiency of grating-based optical instruments and also restricts their use in many applications. Here, we demonstrate a simple approach for the design of diffraction gratings that can be highly efficient for large deflection angles, while also offering additional functionality. Such gratings are composed of a unit cell comprising a vertically oriented asymmetric slot waveguide. The unit cell shows oscillating unidirectional scattering behavior that can be precisely tuned as a function of the waveguide length. This occurs due to interference between multiple modes excited in the structure by the incident light. In contrast to metasurface-based gratings with multiple resonant subelements, a periodic arrangement of such nonresonant diffracting elements allows for broadband operation and a strong tolerance for variations in angle of incidence. Full-wave simulations show that our grating designs can exhibit diffraction efficiencies ranging from 94% for a deflection angle of 47° to 80% for a deflection angle of 80°. To show the multifunctionality of our approach, we also demonstrate a flat polarization beamsplitter and a polarization insensitive grating.