# Problem with calculated transmittance values

Hi all,

Background
I am studying the optical properties of a material, namely trying to obtain the values for absorbance, transmittance and reflectance for a model which is meant to be a leaf. I have a plate with dimensions of 10 000 x 10 000 x 357 microns, and I intend to study the optical properties in the Z dimension for a 1 x 1 x 357 microns piece.

Problem
I have placed the 2 DFT power monitors behind the source and on the other side of the structure. However, I seem to get a value for 0 for the monitor meant to give the value for Transmittance (on the other side of the structure). To investigate, I placed DFT power monitors at intervals from the Plane wave source and I realised that the values for transmittance drops with increasing distance from the source. This is so even for monitors which have been placed in between the source and the structure, which is supposed to be a region of air with background index 1 as seen in the screenshot below.

This decrease in Transmittance values is also observed within the leaf. May I know why this occurs? I have tried increasing the amplitude of the plane wave, but it does not seen to have helped.

Attached is the simulation file for your reference.
Leaf model 1.fsp (354.5 KB)

If anybody could suggest how I should change the any of the simulation settings or rectify the above problem, I would be very grateful. Thank You!

You should use periodic boundary conditions just like:

or use anti-symmetric and symmetric boundary conditions like:

But, in my opinion, the problem that you want to study is not suitable for FDTD simulation.

Hey @151293e

In addition to what @Junyu_Li said; the material fit that you are using is creating loss. Check the Material Explorer next to the run button. I can see that you are using some experimental data for the real part of the index, but have forced the imaginary part to zero.

It is obviously not physical since a leaf is by its nature designed to absorb light. The fit that the FDTD solver uses is in blue, so as you can see the material is not being modeled correctly.

To capture the features in this material I would recommend using a couple smaller bandwidth simulations with less weight on the imaginary component (which is clearly incorrect).
See below.

A better approach would be to treat this as a 1D problem using stackrt.

> #Define the wave space that you are interested in.
> start_wave = 0.4e-6;
> stop_wave = 2.5e-6;
> wave_res = 1000;
>
> lambda = linspace(start_wave,stop_wave,wave_res);
>
> #Create index matrix for each layer and wavelength
> n = matrix(3,wave_res);
> n(1,:)=1; n(3,:)=1; #First and last layer vacuum
>
> #Loop over wavelength and get sampled material data
> for (p=1:wave_res){
> n(2,p) = getindex("Leaf", c/lambda(k));
> }
>
> #define thickness vector
> d = zeros(3,1);
> d(2)=357e-6;
>
>
> RT = stackrt(n,d,c/lambda);


You would still have the problem that the material is incorrect which you could solve by improving the material fit as I describe above and using the script command getFDTDindex

-Regards

1 Like

Thank you very much for your help! However, my mentor has suggested that I study this as a 3D problem in FDTD without the use of stackrt.

After correcting the issue caused by the anti-symmetric and symmetric boundary, I have been able to obtain a more consistent result, yet there seems to be the ripple effect present as seen in the image below, where blue represents the values for Transmittance and Green the values for Reflectance.

Ive looked through some of the past answers regarding how to resolve the ripple effect and this includes increasing the simulation time and decreasing the auto shutoff min. I have tried both of these, wherein the result obtained above was obtained with a simulation time of 100 000 fs and an auto shutoff min of 1e-08 (simulation did not reach this). However, the issue still persists. I’ve tried replacing the material with the dielectric material but the issue still persists.
Could you please look through my simulation file to see if i should make any changes?
Thank you very much!

The simulation file is attached below:
Leaf model 1-actual.fsp (4.5 MB)

You have energy stored in the leaf that you have modeled as a thick film. The round trip transit time is ~ 6ps, so simulating 17 round trips is obviously not enough time for the energy to decay sufficiently.

Hi, I have rerun the simulation, increasing the simulation time by 30 times such that it is now 3 000 000 fs. However, the ripples still remain in the values for transmittance and reflectance. Here are the results for Transmittance and Reflectance in blue and red respectively:

The auto shutoff level was 1e-08, and the simulation time was 3000 ps, which equates to 500 round trips. The Auto Shutoff reached 3.07664e-006 at the end of the simulation, hence I would have been led to believe that the energy has indeed decayed sufficiently.

Below is the simulation file:
Leaf model 1- long time.fsp (4.5 MB)