Abstract
Metasurfaces are ultrathin optical elements that are highly promising for constructing lightweight and compact optical systems. For their practical implementation, it is imperative to maximize the metasurface efficiency. Topology optimization provides a pathway for pushing the limits of metasurface efficiency; however, topology optimization methods have been limited to the design of microscale devices due to the extensive computational resources that are required. We introduce a new strategy for optimizing large-area metasurfaces in a computationally efficient manner. By stitching together individually optimized sections of the metasurface, we can reduce the computational complexity of the optimization from high-polynomial to linear. As a proof of concept, we design and experimentally demonstrate large-area, high-numerical-aperture silicon metasurface lenses with focusing efficiencies exceeding 90%. These concepts can be generalized to the design of multifunctional, broadband diffractive optical devices and will enable the implementation of large-area, high-performance metasurfaces in practical optical systems.
Highlights
Metasurfaces are optical devices that utilize subwavelength-scale structuring to shape and manipulate electromagnetic waves[1]
Assessment of computational efficiency To benchmark the improvements in computational efficiency that are afforded by our approach, we perform adjoint-based topology optimization on metagratings that are made of silicon ridges
A more detailed discussion of the adjoint optimization method that is applied here is provided in the Aperiodic Fourier modal method To apply these concepts to the design of isolated, finitesized device elements, we have developed an aperiodic Fourier modal method (AFMM), which is a hybrid method that combines a solver for periodic systems with perfectly matched layers (PMLs)
Summary
Metasurfaces are optical devices that utilize subwavelength-scale structuring to shape and manipulate electromagnetic waves[1]. The most widely used methods, which we will term “conventional methods,” sample the desired phase profile using discrete phase-shifter elements to form a nanoscale phased array (Fig. 1a) These methods utilize a library of simple, physically intuitive building blocks, including anisotropic waveguides[6], Mie resonators[7], plasmonic resonators[8], and dielectric transmit arrays[9], and can quickly produce macroscale device designs. These approaches lack the necessary degrees of freedom for realizing high-efficiency in devices that are designed for large-angle deflections, multiple functions, and broadband responses[10], thereby preventing metasurfaces from being practically applied in many contexts
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