Time‐reversal Inverse‐designed Metasurfaces for On‐demand Resonance Tailoring and Dispersion Engineering

Abstract Metasurfaces, endowed with sophisticated spectral tuning capabilities such as broadband optical resonance tailoring and dispersion engineering, play an indispensable role in a range of applications, including optical sensing, filtering, pulse shaping, and integrated optics. However, due to issues such as spectral non‐uniform discrete sampling and multi‐objective optimization, it remains a significant challenge to design freeform metasurfaces with sophisticated optical spectral responses using traditional frequency‐domain topology optimization methods. Here, a straightforward but effective time‐domain topology optimization method based on time‐reversal for inverse design of broadband resonance and dispersion metasurfaces is proposed. By incorporating time‐reversal symmetry and Green's function symmetry, the time‐causal gradient of the figure of merit from the metasurfaces' forward and adjoint time‐domain pulse responses is extracted. This strategy enables the entire response spectrum of metasurfaces to be captured simultaneously in a single simulation, instead of calculating that of metasurfaces at each frequency individually, thus circumventing the problem of multi‐wavelength multi‐objective optimization. As a proof‐of‐concept demonstration, two freeform broadband EIT resonant metasurfaces with different Q factors (2971 and 131) over a bandwidth of 100 nm are demonstrated, as well as a freeform broadband dispersion metasurface exhibiting anomalous group delay dispersion of −12fs 2 ${\rm fs}^2$ over a 128 nm bandwidth. This research paves the way for arbitrary design of optical resonance and dispersion in metasurfaces and may find exciting applications in metaoptics, integrated optics, and nanophotonics.