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mode analysis question

Posted Mar 6, 2015, 12:03 a.m. EST RF & Microwave Engineering Version 4.4 3 Replies

Runyu Liu
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Hi All,

Recently I am trying to setup a model that can find the waveguide mode that we expected to exist in a special geometry (which has been illustrated in the ppt file).

basically, the upper space are infinite air, with an infinite slab of dielectric, and the bottom is a thick layer of Au, a narrow slot has been made to create a channel in the Au.

I am looking for a waveguide mode in the slot. right now I am using the PML boundaries. but it looks like the pml didn't work very well, it has some reflections. and I was told that within the same pml layer, the materials should be identical. I just don't know if the model that i set is reasonable or not.

I attached both the mph and ppt file together.

really appreciate the help.

Thanks,
Runyu
3 Replies Last Post Mar 9, 2015, 3:49 p.m. EDT
Sergei Yushanov Certified Consultant
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Posted: 9 years ago Mar 9, 2015, 8:42 a.m. EDT
Runyu,

A couple of thoughts regarding your model:

1. PMLs are not designed for guided waves, so their use in your model is inappropriate.
2. Plasmonic propagation mode should be localized at the metal/dielectric interface and decay exponentially away from interface. It means that top and bottom boundary condition in your geometry are not essential.
3. I would impose Periodic BC at the left and right boundaries.
4. Most difficult part in you model is to provide educated guess to solver for initial value of the effective mode index. Experimental data would be very helpful.
5. If experimental data are not available, then I would start with classical DMD plasmonic waveguide configuration by setting the refractive index of the top dielectric layer to 1. There are analytical solutions for such configuration which would help you to get confidence that numerical model is correct and behaves as expected. Then, gradually increase the refractive index of the top dielectric layer by providing good guess to the solver for each iteration.

Regards,
Sergei
Runyu, A couple of thoughts regarding your model: 1. PMLs are not designed for guided waves, so their use in your model is inappropriate. 2. Plasmonic propagation mode should be localized at the metal/dielectric interface and decay exponentially away from interface. It means that top and bottom boundary condition in your geometry are not essential. 3. I would impose Periodic BC at the left and right boundaries. 4. Most difficult part in you model is to provide educated guess to solver for initial value of the effective mode index. Experimental data would be very helpful. 5. If experimental data are not available, then I would start with classical DMD plasmonic waveguide configuration by setting the refractive index of the top dielectric layer to 1. There are analytical solutions for such configuration which would help you to get confidence that numerical model is correct and behaves as expected. Then, gradually increase the refractive index of the top dielectric layer by providing good guess to the solver for each iteration. Regards, Sergei
Runyu Liu
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Posted: 9 years ago Mar 9, 2015, 11:22 a.m. EDT
Hi Sergei,

Thank you very much for you time and effort, it is helpful!

For the periodic BC, I suppose to use the "continuity" type right? But I looked after the previous post about periodic BC, it said that the continuity is somehow similar with PEC, why is that? and I know for my case I shouldn't use the floquet periodic because i can't define the kx vector.

I tried the periodic BC, and find the mode that I wanted, however, the mode kinda of spread out in the entire simulation area. I have attached the png file, you can see that there are some field distribution outside my waveguide. I am wondering how these parts will affect the effective index? Will they increase the imaginary part of the effective index?

Thanks,
Runyu
Hi Sergei, Thank you very much for you time and effort, it is helpful! For the periodic BC, I suppose to use the "continuity" type right? But I looked after the previous post about periodic BC, it said that the continuity is somehow similar with PEC, why is that? and I know for my case I shouldn't use the floquet periodic because i can't define the kx vector. I tried the periodic BC, and find the mode that I wanted, however, the mode kinda of spread out in the entire simulation area. I have attached the png file, you can see that there are some field distribution outside my waveguide. I am wondering how these parts will affect the effective index? Will they increase the imaginary part of the effective index? Thanks, Runyu
Runyu Liu
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Posted: 9 years ago Mar 9, 2015, 3:49 p.m. EDT
and also, does COMSOL has the function that allows me to do some post-calculation/manipulations?

like truncate a specific area and re-calculate the neff?

Thanks,
Runyu
and also, does COMSOL has the function that allows me to do some post-calculation/manipulations? like truncate a specific area and re-calculate the neff? Thanks, Runyu

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