Hybrid topology optimization combining simulated annealing for designing unpolarized high-efficiency freeform metasurface in-coupler for augmented reality waveguide.

High-efficiency in-couplers with unpolarized responses are crucial for the performance of waveguide augmented reality displays. Freeform quasi-3D metasurfaces (FQ3DM), which integrate freeform metasurfaces with multilayer films, is one possible solution to achieve this. However, the performance of FQ3DM is limited by the lack of inverse design algorithms capable of optimizing its overall structure. In this work, we proposed a hybrid topology optimization combining simulated annealing (HTO-SA) algorithm that alternates between topology optimization and simulated annealing to find the global optimum for both the shape and thickness of FQ3DM. With the HTO-SA algorithm, we designed an unpolarized high-efficiency in-coupler that achieves an average efficiency of 90% across a 20° field-of-view for both transverse electric and transverse magnetic polarization. We envision that our proposed approach can be generalized to the design of high-performance diffractive optical devices.