# Curvilinear coordinate Generalized Source Method for gratings with sharp edges

## TL;DR

This paper introduces a new curvilinear coordinate Generalized Source Method for gratings with sharp edges, enhancing computational accuracy and efficiency in diffractive and metasurface optics applications.

## Contribution

It extends the Generalized Source Method to handle sharp edges in gratings by treating corners as effective media interfaces, improving accuracy and computational efficiency.

## Key findings

- The method achieves higher accuracy for gratings with sharp edges.
- It maintains $O(N \, log N)$ complexity, ensuring efficiency.
- Corners are effectively modeled as interfaces, simplifying calculations.

## Abstract

High-efficient direct numerical methods are currently in demand for optimization procedures in the fields of both conventional diffractive and metasurface optics. With a view of extending the scope of application of the previously proposed Generalized Source Method in the curvilinear coordinates, which has theoretical $O\left(N\log N\right)$ asymptotic numerical complexity, a new method formulation is developed for gratings with sharp edges. It is shown that corrugation corners can be treated as effective medium interfaces within the rationale of the method. Moreover, the given formulation is demonstrated to allow for application of the same derivation as one used in classical electrodynamics to derive the interface conditions. This yields continuous combinations of the fields and metric tensor components, which can be directly Fourier factorized. Together with an efficient algorithm the new formulation is demonstrated to substantially increase the computation accuracy for given computer resources.