Issue with angled injection

I am designing a plasmonic nanolaser where I tune the emission spectra by varying the injection angle of a broadband pump pulse (It is a TFSF source). But merely changing the theta angle does not have the desired effect due to the intrinsic limitation of Lumerical FDTD which keeps the in-plane wavevector constant. I cannot use BFAST type source because I have a nonlinear plugin material (gain medium) in my design. And the method described here does not yield any significant result:

what should I do now? Basically, I need a way to effectively sweep both angle and frequency and then add the effect of single frequency simulations to get the broadband result. TIA

Hi @Zaheen.azad,

Thank you for the question. Can you give more information on why the technique in the link you posted does not yield any results? I believe this technique should work with the TFSF source. Otherwise, I think you will simply have to sweep both angle and frequency.

Let me check this procedure again and then get back to you. In the meantime, I have one question:If I sweep both angle and frequency and then add the effects, will I be able to get the desired result, considering the fact that there is a nonlinear plugin material in my design? TIA

That’s a good point. We cannot combine the results of different simulations in this way when a nonlinear material is involved, so this technique cannot be used. There are a number of difficulties introduced when using a nonlinear gain material, which are discussed in our nonlinear and gain simulation examples.

When a nonlinear material is involved we cannot use a broadband input pulse to obtain results for a large frequency range as we could with a linear system. Because the simulation is nonlinear, the results are only applicable to that specific input pulse, so the strategy of running several single frequency simulations to generate a broadband result would not work. The source spectrum in the simulation has to be the spectrum you are trying to simulate.

You could try simplifying your gain material to a linear gain model. However, this would not include effects like gain saturation.

It would be helpful for me to see exactly what you are trying to simulate, could you attach your simulation file?

Check it out: there’s an analysis toolbox named “Analysis_SPR” which is at the interface between Metal and Top (Gain medium). I have designed the toolbox in such a way that it would integrate the normal component of the Poynting vector and give me the power spectrum, which is dubbed as “Spectra” in the results category of the toolbox. I need to sweep the injection angle across 4 values: 0, 15, 20 and 25 degrees and then record the change in the spectrum. As per previous literature, the mode should blueshift. Your help is very much appreciated. Please, ask me if you need to know anything else.

P.S.: You can also implement the method described in the following link. I did not get any significant result. Maybe you would. Hoping for the best here :vulcan_salute: :v: :crossed_fingers:

Hey there, I had a breakthrough on my end. Despite the warning that BFAST would not work with a plugin material, I ran the simulations and surprisingly enough, received the results I was looking for:


As dictated by the literature, this is exactly what is supposed to happen: blueshift and reduction in magnitude (of the modes centered around 875nm). Now all I need is the exact behavior from the simulation file I sent you, the one including a TFSF source. I have an idea: is there any way you can help me build an analysis toolbox that mimics the behavior of a TFSF source? What I mean by that is I need to create a scattering region just like a TFSF source does and then I can place my monitor there to record the effect of the plasmons. Would that be possible?

I am glad to hear you are making progress. Can you send the file you used to obtain these results? I am curious how you set this up. I know that the complex fields don’t work with, for example, the Kerr nonlinear materials (squaring a complex field is different than squaring a real field). I suppose it is possible that the complex fields work with the 4 level 2 electron material, I will have to look into the exact implementation.

I had a couple questions about the file you sent me. First, I am wondering why you want to use periodic BCs. Unless you want to simulate a periodic array of holes, it seems to me like the structure you sent is not periodic. I think normal PML boundaries would be more appropriate for this structure.

Second, I am not sure if the TFSF source is the right source for this, considering this is not a scattering simulation. Why are you using the TFSF source? Are you trying to remove the pump pulse?

As for the analysis group that mimics the TFSF source, I am not sure I understand what you are trying to do. What results are you trying to obtain exactly?

  1. I am using periodic BC in the x and y direction because it is a periodic nanohole array.
  2. As you might have seen in the file, the Analysis_SPR is in the metal-gain medium interface where the plasmonic mode would emerge. I am using the TFSF source so that I can record the spectrum in the scattering region to get the effect of plasmons. The picture I gave you is the mode shift on the transmission side of the laser. Now I need to record the effect of plasmons in the metal-dielectric interface. So yes, I am trying to remove the pump pulse.
  3. If I have an analysis toolbox that can simulate a scattering region, then I can just place that box in the interface and then get the spectrum. That’s why I require such a toolbox.I already have the plasmonic mode for normal incidence. If I have the scattering region toolbox that I was talking about, then I do not require TFSF source. I can just use a BFAST type plane wave source, use that toolbox and record the the plasmonic mode.
    I hope I have clarified the issues.

Hi there,
I need to make a rectangular toolbox that mimics scattering region characteristics, that is, plane wave and waves reflected from a flat surface would be subtracted at the wall boundaries and the rest would be let through inside the box, exactly like the scattering region of a TFSF source. In need of dire help here. Can you provide me such a toolbox? My paper sort of hinges on it. Thanks in advance

Hi @Zaheen.azad,

I have combined your topics because they are regarding similar issues. Sorry for the slow reply, this is a difficult case and I needed to discuss it with my colleagues.

Regarding the analysis group that mimics a TFSF source, I am not convinced that this is necessary or that it would work for what you intend. If you want to measure the transmission into the surface plasmons, a square of transmission monitors around the hole should work, provided they are not placed too close to avoid measuring non-plasmon transmission.

You should note that we do not support the use of BFAST boundary conditions and nonlinear materials. Even if using them together gives the results you expect, there is no guarantee the results are reliable.

I would recommend you consider whether or not the nonlinear gain material is necessary for modelling the effect you are trying to observe. It is possible that the shift in the mode excitation is not dependent on the nonlinear gain, so you can replace the four level two electron material with an equivalent linear material with the same refractive index. You could then use an angled source and BFAST boundary conditions. You should try running the simulations with the nonlinear gain material replaced with linear material and see if you observe the expected effects.

If this does not work, you will have to perform this simulation without periodic boundary conditions in the x direction. You will have to include multiple periods and use a diffracting plane wave or Gaussian source with PML x min/x max boundary conditions. You should still be able to use standard periodic y min/y max boundaries because your source is not angled in the y direction. This approach will require a long simulation time, however. I would recommend you try the linear approach I described above first.

Hi @kjohnson thanks for getting back to me. Actually, it is my fault that I could not properly explain the situation to you. Plasmonic Nanolaser is an area that I have been working on for the past six months. It is hard to explain such a vast field within a few sentences. The thing is that I would require the nonlinear gain material. The gain molecules transfer excitonic energy to the plasmons that act as nanoresonators. This is the crux of a plasmonic nanolaser. So I cannot replace the gain material with a linear material. I mean I can and I have. But then a very small resonance occurs around the pump pulse wavelength which is not part of lasing action. Again, sorry for such a brief description of what is actually happening. To reiterate the main point: nonlinear gain material is at the center of the action at 875nm.
If you can provide the toolbox I have asked for, then it is great. Otherwise I would have to look for other avenues. If you cannot, no problem at all. Thanks for trying :v: :vulcan_salute: :slightly_smiling_face:

Thank you for the explanation. If the nonlinear material is required, I would recommend you use the approach I outlined above, where you simulate many periods of the device in the direction you are angling the source with PML BCs in that direction. Although this would require a long simulation time, I believe this would be the best approach to obtain reliable results. You could then use the TFSF source if you would like.

Unfortunately providing custom analysis groups is beyond the scope of what we support, but if you would like you can build it yourself, and ask any specific questions about issues you come across here. However, I would still recommend you use the approach above, and simply use the TFSF source instead of trying to make your own.

Let me know if you have any questions.

Okay, so let’s say I want to angle my source with the x-direction. I will take 5-6 periods in that direction and use PML BC for x-direction. Then I will record the spectrum. I can do the same for TFSF source, right? Basically, using PML BC in the direction with which I am angling would change the in-plane wavevector and not keep it constant, as outlined here:

Okay, so let’s say I want to angle my source with the x-direction. I will take 5-6 periods in that direction and use PML BC for x-direction. Then I will record the spectrum. I can do the same for TFSF source, right?

Yes, I believe a diffracting plane wave or Gaussian source would work better for this simulation, but you could use a TFSF source if you would like.

Basically, using PML BC in the direction with which I am angling would change the in-plane wavevector and not keep it constant, as outlined here:

You are correct. with this setup the injection angle would not be constant as a function of frequency. Unfortunately this is just a limit of the software, I don’t think there is a way around this if you are using a nonlinear material, which cannot be used with BFAST.

The change of inplane wavevector is what I am actually looking for. So you’re saying the inplane wave-vector would remain constant even if I make the necessary changes in my simulation (multiple periods included in PML BC with diffracting plane wave source). Or are you saying the opposite? Sorry for the pedantic behavior. A little confused here :stuck_out_tongue: :stuck_out_tongue:

I should have been more clear, if you are not using a BFAST source the in-plane wave vector will be constant for all frequencies of a plane wave source. This means you can set the angle of the plane wave/Gaussian/TFSF source, but this angle will only apply to the central frequency. The angles of plane waves at other frequencies will be different, as described on the page you linked. I hope this clarifies the issue, let me know if you have any questions.

See this is the issue. Without the change in in-plane wavevector, I would not get the effect that I am looking for. Anyways, I am looking for other means now. Thanks a lot for the all the effort you put in.

I regret that our software could not accomplish what you are trying to do. Feel free to contact us for any other questions you have. Best of luck with your paper!

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