# the optical current difference between PEC contact and the aluminum contact

Hi all
recently I do a FDTD simulation and want to extract the optical current from the FDTD.
I use two contact metal for comparision.the picture below is the generation rate of Si.It can be easy to find that there is no optical generation in the region.However ,the optical current extracted from the generation script is 0.000125174 A.Where does the optical current come from?? I also use the PEC in the FDTD simulation and the optical current I got is 0 A. What cause this big difference?
Hope to get your feedback.Thanks a lot.Si Al.fsp (663.3 KB)
PEC Si.fsp (656.7 KB)

Hi @jbwei,

I looked into this and the generation rate analysis group first calculates the power absorption over space, then relates that to the number of generated electron-hole pairs.

Power absorbed is calculated using the equation Pabs=-0.5w|E|^2*imag(eps) where w is the angular frequency and eps is the permittivity of the material. The equation is from the “theory” section on this page:
https://kb.lumerical.com/en/index.html?layout_analysis_pabs_simple.html

In the Al material, there can be some absorption since both the electric field intensity and the imaginary part of the permittivity of Al is non-zero. However, in the PEC material, since it is a perfect electrical conductor the electric field is forced to be 0 inside the material which means that the absorption is 0.

I suspect that the small amount of current that you get is due to interpolation error of the monitor data at the interface between Al and Si and if you decrease the z span of the analysis group by 1 mesh cell you would get 0 current again. Please let me know if this makes sense.

A post was merged into an existing topic: optical current in the VPD in your official website