Ripples in the total transmission graph

When I ran the following FDTD simulation,I noticed that the total transmission is rippling at certain wavelength regions.
I have tried the following

1.Changing all the PML layers to metal but the transmission peaked to as high as 50 near 1.4 microns.
2.My simulation time is 20k fs and it takes around 40 mins in real time.I am afraid I can’t increase the simulation time any greater than this as I need to run as many as 100 simulations in a loop.
3.I’m currently using 64 PML Layer but the results we’re not much different with 128,256 PML layers.

Please find the relevant files below.

I request you to kindly respond at the earliest possible with your suggestions.LSC_HCG_Randmozed.fsp (696.0 KB)

Dear @me12b099

I ran your simulation and I noticed that your simulation does not reach auto shutoff level. Increasing the simulation time will be one thing to try.

Also, it looks like your device is periodic? If this is the case, we can design one period and use periodic BCs. There was some concerns regarding meshing, but I will hold on it until we decided on the final geometry design.


Hi BK,

I tried increasing the simulation time to 40k fs but it still does not reach the auto shutoff level.So I am afraid it’s not ideal to increase it even more as mentioned earlier.[quote=“bkhanaliloo, post:2, topic:5372”]
Also, it looks like your device is periodic? If this is the case, we can design one period and use periodic BCs. There was some concerns regarding meshing, but I will hold on it until we decided on the final geometry design

Can you please elaborate more on this?
I did use periodic conditions at one point but the ripples are even more higher at certain wavelengths.So I avoided it.
What kind of changes do you recommend in mesh settings?

I would appreciate if you can make the possible changes to the simulation file send it to me explaining what changes did you make and why.

Thanking you!
Kiran Vaddi

Dear @me12b099

Since light gets trapped in gratings, it takes a long time for light to escape the geometry. If you don’t give enough simulation time, ripples will be unavoidable.

Can you please tell me what you are trying to simulate? Depending on the application, we can come up with different strategies:

  1. If it is OLED, please take a look at this example:
    Since you used a few dipoles in your simulations, I am not quite sure if this is what you wanted to do. Please note that there are technique on how to simulate a quantum dot behavior with dipoles with only a few simulations and using symmetries.

  2. If you have multiple quantum dots with random position in a periodic array, you can try something like in this KB page:

in which you design only one period with a single dipole under a hole and use periodic BCs. Then you run another simulation with dipole located between holes, and finally average the results.

Please clarify what you want to do and we can come up with a good technique to solve the problem.


Hi BK,
Thanks for your inputs,
So here is my design.
I have to model a Luminsicent solar concentrator which has embedded Quantum dots in a waveguide/substarte.We are looking at adding a grating layer on the top to increase the efficiency.
So the procedure we adapted for this is.
1)Have a wave guide of dimensions 100x10
2) to one side have a randomly distributed in phase and location dipoles(as many as we can)
3)Add thin layer(to increase angular sensitivity) and grating layer(to increase guiding efficiency) on the other side
4)and collect the transmission in the monitors placed at the edges (which is also guiding efficiency)

So, if you look in the simulation file,I have kept two transmission monitors at the edges covering wave guide thickness.
Since for us the geometric gain factor(100/2*10 = 5) is also important I think i would need to look at a finite structure such as 100x10 But I am not sure about this.Any insights on this would be grateful.

I hope this clearly explains what I want to do.
Please do make the necessary changes needed in the simulation file and send it back to me if possible.

Thanking you!
Kiran Vaddi.

Dear @me12b099

Sorry for my delayed reply.

I spend some time with my colleague to assess the best approach for your simulation. Since your device has specific dimension (i.e. not infinite), use of periodic BC will not be a good idea.

Also, as you mentioned, dipoles have different phases. Does this mean that you are looking for the coherent sum of the results? If this is the case, you will need to run at least 50-100 simulations, each with dipole ensemble, and then add the results coherently. Of course this approach will be time consuming.

If the results are incoherent (for example, dipoles emit at different frequencies) you can simulate only one dipole at a time and then add them incoherently. A good link that discusses these approaches is:

If the incoherent sum is what you want, I think we can simplify simulations a lot. Lets assume that grating structure is large enough such that dipole transmission to the left and right monitors is not sensitive to the dipole position along the x-direction (you can check this by putting dipole at different location along the x-axis). Also, from your dipole cloud, it looks like they are located up to only 1.5um below the substrate. So, I think we need only 4 simulation with four dipole position (shown by blue star in the screenshot below):

You can learn more about this approach by looking at OLED examples in our KB:

Please let me know how you want to treat your simulations and I am glad to be of a help.