Inverse design of dielectric metasurface by spatial coupled mode theory

Modeling metasurfaces with high accuracy and efficiency is challenging because they have features smaller than the wavelength but sizes much larger than the wavelength. Full wave simulation is accurate but very slow. Popular design paradigms like locally periodic approximation (LPA) reduce the computational cost by neglecting, partially or fully, near-field interactions between meta-units and treating them in an isolated manner. The coupling between meta units has been fully considered by applying temporal coupled mode theory to model the metasurface. However, this method only works for resonance-based metasurfaces. To model the broadly studied dielectric metasurfaces based on the propagation of guided modes, we propose to model the whole system using spatial coupled mode theory (SCMT) where the dielectric metasurface can be viewed as an array of truncated waveguides. An inverse design routine based on this model is then devised and applied to gain improvements over LPA in several scenarios, such as high numerical aperture lens, multi-wavelength focusing, and suppression of coma aberrations. With its accuracy and efficiency, the proposed framework can be a powerful tool to improve the performance of dielectric metasurface on various tasks.