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.