Discussion Forum

1. Are you (and carefully!) using Floquet-type periodic boundary conditions? You probably should be. Or... are you instead using some other type of periodic condition(s)? If so, that's likely at least part of your problem.

2. A computationally brute-force alternative (avoiding the challenges of correctly setting up the Floquet conditions) is to model a much larger, but still finite structure (many cells) and hit it with a ~Gaussian (or other tapered) beam illuminating a bunch of the cells, then observe the resulting transmission and/or reflection (of the integrated power or whatever else is of interest to you). This may or may not be practical, depending on the nature of your problem and the capabilities of your computer.

1) As for the physics I have used the wave equation (EM frequency domain) with electric displacement field model: refractive index. I have used 2 periodic ports one for input from above and one for transmission below. I have used a Floquet periodicity condition on both sides of the geometry and the k-vector from periodic port. Is there anything I might need to check in the periodicity condition?

2) I will try the simulation with multiple channels instead of a single unit and see if it solves the problem.

3) In order to simulate over a broad band of wavelenghts, I have created a parametric sweep for lambdas and plotted them against abs(emw.S11)^2 and abs(emw.S21)^2 where 1 and 2 are the input and output ports respectively to get the reflectivity and transmittivity plots. Is this the right approach?

Thank you for your inputs.

Edit 1: I am getting accurate results for reflectivity of plain silicon i.e. without creating any features but when I add features (to obtain periodic surface structures) the results fluctuate and some of the reflectivity values at certain wavelenghts are even higher than that of the untextured surface.

Edit 2: I have tried using multiple channels and I am still getting highly oscillatory results. Do you have any more suggestions? Thank you.

I am still not able to resolve the issue with the simulation. I have attached an image of the simulation results.
Also when I use a flat surface without any features, I am not getting the right reflactance values for higher angles of incidence but for normal incidence the values are correct. I am starting to wonder if that is the reason the simulation with features is incorrect as the values of reflectance for higher angles of incidence are incorrect.
Any comments on this would be appreciated. Thank you.

I am still not able to resolve the issue with the simulation. I have attached an image of the simulation results.
Also when I use a flat surface without any features, I am not getting the right reflactance values for higher angles of incidence but for normal incidence the values are correct. I am starting to wonder if that is the reason the simulation with features is incorrect as the values of reflectance for higher angles of incidence are incorrect.
Any comments on this would be appreciated. Thank you.

Could you share your model file.Then I can get an idea about what might be the problem.

I have the same problem too. My transmission spectrum seems correct, because I checked it with Lumerical, however, it is oscillatory, whereas I expect a clear peak.