Negative Transmission problem


I have no idea why the transmission from monitor is negative.(see the second picture below)
The direction of propagation is forward(y direction in 2D simulation).
Did I have a wrong FDTD setup? (the below FDTD file is my setup)
Thank you.

2D_ethanol_PML12_goldinboundary_20170413_gapgoldthickness2_1.fsp (1.7 MB)


Hi @r04223194
The direction of propagation determines the the sign of the transmission. Let’s consider that, the power is flowing through the monitor in the negative direction. As a result, the transmission gets negative.


Thank you for the reply.
I did a series of simulation by changing the gold nanostructure length.
but only this dimension gets negative transmission, all the others are positive.
I have no idea why only this one gets negative result.


I checked the simulation file and I noticed that the simulation did not reach the auto shutoff threshold, so I increased the simulation time setting of the FDTD solver region object. If the simulation time is too short and the simulation is ended before the fields have fully decayed it can cause errors in the frequency domain monitor results. Increasing the simulation time resulted in a positive value of transmission.

The following post discusses how you can tell if the auto shutoff was reached:


Thank you for your reply.
I increased the simulation time and made sure the simulation isn’t ended before the fields have fully decayed it.
Because I set the x span of gold nanostructure big enough, so I just changed the x span of FDTD box to simulate a series of length of the gold nanostructure.
But in some simulation, there are still negative values.
2D_water_PML12_goldinboundary_20170418_gapgoldthickness0_6.fsp (3.8 MB)
Because the source is under nanostructure and direction is forward, and the monitors of transmission are on the top of the nanostructure.
I think the transmission value should be all positive.
I also increased the PML layer of y boundaries, but the results are also strange.

I need to know the exact transmission value to design my nanostructure.


There is some reflection from the PML which could be minimized by increasing the number of PML layers. However, even with 64 PML layers I was still seeing a very small negative value of reflection on the order of 1e-5.

By checking the far field projection, it seems that for this particular setup there is light diffracted to 90 degrees from normal which can be challenging to simulate since fields can continue to propagate sideways for a long time without decaying, and the results can be sensitive to the auto shutoff theshold.

I think that this could be the cause for the remaining error in the result. It may be possible to get a better result with a lower the auto shutoff min setting and running the simulation for a longer time. Otherwise, you may choose to disregard the results for the particular simulation settings where the structure diffracts light at 90 degrees. You can typically tell when this is the case by plotting the number of grating orders supported by the device like in the following example, and at the point when the number of grating orders changes, there will be light diffracted at 90 degrees: