Plasmonic Ring

[Noha Anous]

Hello,

I was trying to simulate a plasmonic ring resonator (MIM and IMI),
I use 2D simulation, and Lorentz-Drude model for metals,
but I faced 2 problems:

1 – Calculating the effective index, ‘mode not found’ appears a lot to me, and inconsistently, I mean, I’m not using cut-off wavelengths or guide width, but I get strange results like having symmetric and antisymmteric modes at lambda = 633 nm, then at lambda= 900, only antisymmetric, then at lambda = 1000 nm, symmetric and antisymmetric modes appear again!

2 – When I operate at a wavelength at which a mode exists, no matter how I increase the simulation time or decrease the mesh size or time step, I don’t get the resonance effect of the ring,
actually I’m not using my ring design, yet, I’m using already published designs so as yo make sure I’m operating OptiFDTD correctly, however, these previously done designs don’t seem to work with me in OptiFDTD.

So please,
Does OptiFDTD have any limitations when simulating plasmonics?
And how can I overcome the difficulties I’ve stated above?

Thanks a lot,
Noha

[Damian Marek]

Hello Noha,

Would it be possible to attach your project file along with the citation for a reference you are trying to match? I’ll see what I can find!

Regards

[Noha Anous]

Thanks a lot Damian!

The file is attached and the reference I’m trying to imitate is:

http://iopscience.iop.org/2040-8986/14/5/055001

I’m trying to simulate the design in Fig 2(a)(the linear ring not the one with the non-linear material … at least as a beginning :))
which is a dielectric ring with n = 1.12 on a Silver wafer using Drude model.

The dimensions are as follows:
Rcore = 100 nm
Waveguide width = 50 nm
gap between ring and waveguide is 10 nm.

Actually, using the previous design, I can’t even determine the effective index of the modes that’ll propagate in the structure … I get no modes!

and please if an example for a plasmoni ring (the ring itself is metal) is added to the manual or OptiFDTD site, that will be very useful …

Thanks a lot for your help,

[Damian Marek]

I used a different reference for the Drude model of silver and found modes. Where did you reference the material properties of the silver from?

[Noha Anous]

Dear Damian,

I was using the built in Ag model in Optiwave, but chose the option of Drude-only model …

Did it work properly when you used another Drude model?

Also I tried using Lorentz-Drude from Rakic’s paper, but still couldn’t get effective indices except at some wavelengths … and I have no idea what eh problem is.

Thanks a lot

[Damian Marek]

Ah ok, I think you should try to find the exact parameters the paper used to simulate their design. They reference Johnson and Christy:

http://www.its.caltech.edu/~seheon/FDTD_related/Drude_param.pdf

I used Set 1 for my simulation and it worked.

[Noha Anous]

Dear Damian, thanks a lot for your time, I’ll try this.

But actually I also have another problem,
in the attached file, I’m trying to find the effective index of the symmetric and antisymm. modes at different wavelengths, but I face the following:
1- At 0.2 um, and 0.3 um strange modes appear, at 0.4 um, 4 modes appear, then as we increase wavelength, they disappear again.
2- symmetric mode ONLY exists starting 0.4 um, disappears at 0.67 um,
3- at 0.67, 0.68, 0.69 um no modes at all exist ….
4- starting 0.7 um the antisymmetric mode starts to appear till 1.3 um
5- Beyond 1.3 um, no more modes exist…

Also, when I change the W.G. width to 2 um, I almost get no modes at most wavelengths …

I don’t know if I can trust these results, are they right or wrong … they seem strange.

I used the silver Lorentz-drude model of:
Rakic et al., Optical properties of metallic films for vertical-cavity optoelectronic devices, Applied Optics (1998)

Thanks in advance

[Damian Marek]

Hi Noha,

I’m away from the office for the next two weeks, so hopefully my responses will be fast enough. I have not run the simulation yet, but to me it seems like the mesh may not be fine enough. For some widths the mesh maybe not small enough to resolve the geometry. Also for plasmon polariton modes they become HIGHLY confined to the interface for thicker widths, which might be much smaller than the mesh defined.

If you are still having trouble let me know, and I’ll see what I can do to help!

Regards

[Noha Anous]

Dear Damian:

Thanks for your reply.
The no. of points in the mesh is something I considered, I minimized the mesh size to 1e-6 um , yet I’m facing the same mode problems, modes not found …

[Damian Marek]

When I simulate modes for much thinner waveguides as in the paper you referenced which are approximately 50 nm or 0.05 um I get modes that to me seem correct plasmon polaritons. The Ez field has the exponential decay you would expect for a plasmon polariton. Is there a reason why you are using much thicker waveguides? The results do seem strange, but the paper you referenced has much thinner waveguides.

Regards

[Noha Anous]

Dear Damian:

I really apologize for bothering you with so many questions,

first, about my first file (in which I referred to an IOP paper), I tried Set 3 in Drude model following the link you sent, and other sets as well, I only get a mode that it a decaying sinusoidal … it doesn’t seem like a plasmonic mode, this is at w.g. width of 50 nm.

second, as for my other file, I increased the w.g. width because I couldn’t get modes at 50 nm or smaller width + I need to study the effect of the width, so I need the file to operate at diff. wavelengths and different widths.

Do you have any idea what the problem maybe?
And please if you have any working file, please send it to me.

Thanks a lot ,
Really thanks.

[Damian Marek]

Not at all Noha! I did my thesis on the same type of surface plasmon polaritons, except in my case it was thin width metal strips surrounded by dielectric. Like I mentioned elsewhere though I am out of office so I cannot give all my attention to this problem at the moment.

I know that you should be able to solve these modes analytically which results in a dispersion relation. Here is a text on the subject:

http://cdn.intechopen.com/pdfs-wm/40367.pdf

I am not sure myself what the modes should look like, but if you can solve those equations using Matlab or a graphical approach you could make sure for yourself.

Regards

[Noha Anous]

Thanks a lot Damian,
I really appreciate your help

I’ll study the document you sent and see what I can reach …

but mainly my question was if the OptiFDTD is reliable to extract all types of plasmonic modes,
as I tried OptiMode, and the modes I get there are more than what I get in OptiFDTD and somehow different !

Thanks again!

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