Inverse design of dielectric metasurface by spatial coupled mode theory

TL;DR

This paper introduces a spatial coupled mode theory-based inverse design method for dielectric metasurfaces, offering improved accuracy and efficiency over traditional local periodic approximation, enabling advanced optical functionalities.

Contribution

The paper develops a novel spatial coupled mode theory model for dielectric metasurfaces and applies it to inverse design, surpassing LPA in accuracy and enabling complex optical functionalities.

Findings

Outperforms LPA in high-NA lens design

Enables multi-wavelength focusing

Suppresses coma aberrations effectively

Abstract

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.