Transmission of a structure over 1

Hi there,
I am trying to simulate a simple structure to see the transmission of it. currently I have a plane source within a material of 1.5 refractive index. The source has a wavelength of 0.65um. When I do not have any material present, increasing the mesh accuracy gives me transmission near 1. When I include my material, it shoots over 1! I have been playing with mesh size, auto shut off time, boundary conditions etc. but yet to find any solution. Currently my source angle theta is zero.

Eventually I will be studying the effect of source angle, so have been using bloch BCs but as I do not have a broadband range opted not to use BFAST for my simulation.

I have tried to give an overview of what I am trying to achieve here and also attached my simulation file herewith. Kindly have a look and let me know what’s wrong in my simulation.


Sample.fsp (256.7 KB)

Dear @qxu1

Thank you very much for providing simulation file and having a very descriptive explanation of the problem.

When I ran your simulations, I noticed that transmission is slightly over 1 (~1%). While this error is small (in the range of simulation/numerical errors), I tried to modify the precision of the simulations.

Since you are using periodic BCs, I decreased the x and y FDTD span (you can decrease it to the point that you have only one mesh cell, for example see the multilayer stack example) to speed up the simulation time. Please note that since you are using plane wave and your geometry is symmetric, you can run 2D simulations to even speed up the simulations instead of 3D ones. I increased the z-span to leave a bit more space between the edge of PML and rectangle object. Using stablized PML with 48 layers, transmission dropped to below 1 (0.993!):

and here is the simulation file for your review:
Sample_modified.fsp (257.5 KB)

Please note that minimal errors (in the order of few percent) are expected to be observed in numerical simulations.

I hope I could answer your question.