Inverse Design Of Metasurfaces Based On Coupled-Mode Theory And Adjoint Optimization

Metasurfaces typically have sizes much larger than the wavelength yet contain a large number of subwavelength features. Thus, it is difficult to design entire metasurfaces using full-wave simulations. However, without full-wave simulations, most existing design approaches cannot accurately model the interactions between the individual elements comprising the metasurface. Here, we demonstrate an approach for the design of resonant metasurfaces based on coupled-mode theory. Our approach fully describes wave dynamics and coupling in metasurfaces and is much more computationally efficient than full-wave simulations. As an example, we show that the combination of coupled-mode theory and adjoint optimization can be used for the inverse design of high-numerical-aperture (0.9) metalenses with sizes as large as 10000 wavelengths. The computation efficiency of our approach is orders of magnitude faster than full-wave simulations. Complex functionalities such as angle-multiplexed metasurface holograms can also be realized. With its accuracy and efficiency, the proposed framework can be a powerful design tool for large-scale resonant flat-optics devices.