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Designing high efficiency asymmetric polarization converter for blue light: a deep reinforcement learning approach.

Conventional polarization converters selectively preserve the required polarization state by absorbing, reflecting or refracting light with unwanted polarization state, leading to a theoretical transmittance limit of 0.5 for linearly polarized light with unpolarized light incidence. In the meanwhile, due to the high-dimensional structure parameters and time-consuming numerical simulations, designing a converter with satisfactory performance is extremely difficult and closely relies on human experts' experiences and manual intervention. To address these open issues, in this paper, we first propose an asymmetric polarization converter which shows both high transmittance for one linearly polarized light and high transmittance for the orthogonal linearly polarized light with 90° rotation in blue wavelength region. To maximize the performance of the proposed structure, a deep reinforcement learning approach is further proposed to search for the optimal set of structure parameters. To avoid overly long training time by using the numerical simulations as environment, a deep neural network is proposed to serve as the surrogate model, where a prediction accuracy of 96.6% and 95.5% in two orthogonal polarization directions is achieved with micro-second grade simulation time respectively. With the optimized structure, the average transmittance is larger than 0.5 for the wavelength range from 444 to 466 nm with a maximum of 0.605 at 455 nm, which is 21% higher than the theoretical limit of 0.5 of conventional polarization converters.