High efficiency gradient index GaN metasurfaces

Metasurfaces are promising optical components for accurate phase, amplitude and polarization control of the transmitted light. Their fabrication simplicity and their extreme compactness with respect to more traditional... [ view full abstract ]

Metasurfaces are promising optical components for accurate phase, amplitude and polarization control of the transmitted light. Their fabrication simplicity and their extreme compactness with respect to more traditional volume-based diffraction components make them attractive for lightweight and integrated applications. In this contribution, we designed efficient metasurface beam deflectors, able to address a wide range of deflection angles from zero to almost 90 degrees. The devices are designed to work in transmission. They are optimized in order to maximize constructive interference in the first diffractive order while maintaining destructive interference for its symmetric counterpart, leading to maximal transmission into the design direction. In this study, we designed various GaN grating couplers for various deflection angles and obtained very high efficiencies up to 95 % for deflection angles of less than 63° at a wavelength of 600 nm. The performance decreases down to 70 % for a deflection angle of 81° (see Figure 1).

During the design phase, we relied on a goal-oriented optimization procedure based on Rigorous Coupled Wave Analysis (RCWA) simulations. These results have been benchmarked with the Discontinuous Galerkin Time Domain (DGTD) method. Moreover, we have studied the broadband behavior with the DGTD simulations (see Figure 2).  In order to ensure the robustness of our design, we have performed Uncertainty Quantification regarding the metasurface's geometry, taking into account fabrication tolerances of the e-beam lithography. Fabrication processes of GaN-based metasurfaces and the experimentally measured performances of these structures will be reported during this presentation. The utilization of GaN material offers interesting perspectives in particular due to its negligible absorption in the entire visible range.