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Towards the performance limit of catenary meta-optics via field-driven optimization
Catenary optics enables metasurfaces with higher efficiency and wider bandwidth, and is highly anticipated in the imaging system, super-resolution lithography, and broadband absorbers. However, the periodic boundary approximation without considering aperiodic electromagnetic crosstalk poses challenges for catenary optical devices to reach their performance limits. Here, perfect control of both local geometric and propagation phases is realized through field-driven optimization, in which the field distribution is calculated under real boundary conditions. Different from other optimization methods requiring a mass of iterations, the proposed design method requires less than ten iterations to get the efficiency close to the optimal value. Based on the library of shape-optimized catenary structures, centimeter-scale devices can be designed in ten seconds, with the performance improved by ~15%. Furthermore, this method has the ability to extend catenary-like continuous structures to arbitrary polarization, including both linear and elliptical polarizations, which is difficult to achieve with traditional design methods. It provides a way for the development of catenary optics and serves as a potent tool for constructing high-performance optical devices.
To achieve the performance limit of catenary optical devices, precise phase modulation is employed. Geometric phases are strictly calculated based on the optical Jones matrix, while the propagation phases are corrected using the FDO method. The catenary structures can be optimized to achieve perfect wavefront modulation, with a resulting diffraction efficiency near 100% in 10 iterations. Besides, our work expands the catenary optical devices to incidence with arbitrary polarization. Although our approach currently only demonstrates optimization for 1D metalenses, 2D metalenses can be designed and optimized by modifying the integral path of the catenary-like structures and adjusting the phase distribution of the metalenses, while the entire optimization progress remains the same. To enable the rapid design of large-scale high-performance optical devices, we establish a comprehensive library of parametrizable shape-optimized structures, which greatly facilitates the rapid design of high-performance optical devices over a large scale. Simulation and experimental results show that devices designed by the library of only nine curved structures significantly improve performance by ~15% compared to devices with equal-width structures. The addition of more structures to the library would increase the available options and enable finer adjustments, resulting in more precise interpolations and better-performing devices. In addition, although the library is currently limited to design devices for CP light only, the method has the potential to establish a library for arbitrarily polarized incidence. In the future, it may be possible to achieve rapid design of devices with arbitrary polarized light incidence by establishing a library that contains the information about the incident polarization state. Besides, combining this method with deep learning may further improve the efficiencies of the devices.
(From:https://www.oejournal.org/article/)
