The optical design and analysis of micro-structured optical elements with high index contrasts and large numerical apertures is still challenging. Due to severe numerical efforts, rigorous algorithms like Fourier Model Method are always time consuming and consequently only valid for periodic structures with small enough period. While approximated algorithms like Thin Element Approximation is simple and fast but only valid for to the description of very thin structures with a thickness of only a few wavelengths and paraxial conditions for the incoming light. Therefore, simulation methods that provide accurate results at low computational cost are preferred.
In this work, "Split-step" method that is suitable for simulating micro-structured surfaces are introduced. Concept of "Split-step" contains two aspect. One is that propagation area is split into thin layers; The other is that propagation in each layer is split into two parts, homogeneous propagation and inhomogeneous phase correction.

Pic-Split step method
An Ex-polarized plane wave is used as input field, with the wavelength of 632.8nm. The binary grating used in this simulation is a 1 to 6 beam-splitter.
The period of the grating is 10