Learning Forum

[Mike Lee]

Hello! I am trying to design a device using inverse design, But it seems necessary to use this command (define the n on each coordinate points) to define the structure：

But our commonly used structural definition is based on coordinate intervals, as shown in the following picture, which specifies the start and stop point of the X coordinate:

so the question here is, how to define the refractive index n when the coordinate point is on the interface between two materials?

I tried a simple example here:

as you can see, I set four 1*1μm rectangles leaning together under 0.1μm mesh (Red-Si, White-SiO2). I set a index monitor to observe the data recorded by it. The data is a 21*21 matrix, so I think the refractive index is not recorded within each grid, but at each grid endpoint.

So here you can see in the 11th column (the interface between Si and SiO2) of the matrix, refractive index is 1.46. Does that means when we define the n on the interface between two kinds of materials, we use the smaller value?

Looking forward to your reply!

[Guilin Sun]

This post involves several different concepts.

1: inverse design uses binary permittivity/refractive index. However it does not need users to import binary material. 

2:importnk2 imports refractive index varying with space. As for what part is used will depend on FDTD region.

3: Refractive index monitor: 

in the preview before simulation, it is not recalculated at the material interace. It simply uses staircase depending on which material takes more than 50% of a Yee cell.

After simulation, it will show what exactly is used i simulation, from the "Mesh refinement" method: https://optics.ansys.com/hc/en-us/articles/360034382614-Selecting-the-best-mesh-refinement-option-in-the-FDTD-simulation-object

Therefore, it is normal to have a gap in index preview. If you use finer mesh it will reduce the gap. No worry about the gap as this is a preview. Please note that for this discrete method, everyting must be discretized, and the geometry/material is no exception. of couse the finer the mesh and the better the accuracy.