Reflection from 2D Photonic Crystal is Greater than 1

I am trying to simulate the reflection from a photonic crystal at normal incidence (incident medium is air, as seen in the file). I set up a plane wave source and a monitor behind the source. The results from the Reflection exceed 1 at several frequencies. Changing the size of mesh step size didn’t affect this result; I also tried increasing the number of layers in the PML, and that also did not appear to have an effect. I think the y boundaries are far enough from the source that there shouldn’t be spurious interactions there. I’ve attached the file. If you could tell me why I am measuring a reflection over 1 and how I can fix it, I would appreciate it.

PC_reflection.fsp (303.1 KB)
Graph of reflectance.


It seems the main issue is the simulation time. The default value (1000fs) is too short for this structure as the light stays trapped in the PC and it creates these ripples.

If you set the simulation with a longer time (in the FDTD object properties), you can get rid of these ripples: