Discussion Forum

Posted: 1 decade ago 26 juil. 2011, 22:54 UTC−4

[QUOTE] Hi all, I'm trying to simulate a periodic array of gold nanorods under normal-incident light using the RF module (3D, scattered harmonic propagation). I believe the proper boundary conditions for the sides of the unit cell are PEC and PMC, orthogonal to the electric and magnetic fields respectively. At the moment I'm using a scattering BC on the top and bottom boundaries (have also played around with PMLs). The problem is that I want to vary the polarization of the incoming beam as part of a parametric sweep. It's easy to introduce a polarization angle by modifying the x- and y-components of the electric field, but with a rectangular unit cell there's no good way to rotate the orientation of the PEC/PMC BCs to match (except for 0 and 90 deg, obviously). Does anyone know of a way to handle this issue? Should I be using some type of periodic BC? [/QUOTE] Hello, You probably need to use Floquet conditions to achieve your goal. Please refer to http://www.comsol.com/community/forums/general/thread/17516/ for details. All the bests David

Posted: 1 decade ago 13 sept. 2011, 18:40 UTC−4

David, thanks for your reply. I've been experimenting with Floquet periodicity but it doesn't seem to be working. Based on the "plasmonic wire grating" tutorial you linked to and a couple of others, it looks like the Floquet k-vector should be the zero vector for normal incident light (regardless of polarization?). The "periodic boundary condition" (http://www.comsol.com/showroom/gallery/8610/) model documentation, by contrast, seems to show a Floquet k-vector that is the same as the k-vector of the incoming wave. In the 3D case therefore, I've tried k=(0,0,0) and k=(0,0,k0_rfw) for the Floquet boundary conditions. Neither one gives the same results as a PEC/PMC BC combination even for the same polarization. Instead I get strange spots of high electric field at the edges of the unit cell. I haven't even added a scattering subdomain at this point; I'm just trying to get the wave to propagate through an empty unit cell and it keeps failing. I feel like I'm missing something obvious here... any ideas what it might be?

Posted: 1 decade ago 29 sept. 2011, 10:56 UTC−4

It might have to do with your meshing conditions. Basically you have to make sure that all boundaries with periodicity have the same meshing conditions, because otherwise you will get indeed really numerical values at these boundaries. You should use the "copy boundary mesh" for all of the boundaries involved. I struggled with this for a long time as well, but this solved all the issues I had before..... Good Luck!