Discussion Forum

Yevgeniy Davletshin posted a mie scattering model under "model exchange" and you can have a look. I'm tring to figure out that model. Maybe you can ask him but I don't know when he will show up here.

Scattering cross-section in 3D model can be calculated by Boundary integration (over all domain before PML) of scattered power per incident power.

Thank you both for the comments. I think before I can start calculating my cross section I need to sure up my model. I am not getting the results that I should be, so I have been searching for the solution.

What I am trying to do first in 2-D (cylinder/circle) and will eventually move to 3-D like Yevgeniy's model. This should be easy I would think in 2-D, but I am missing something.

Here is a breakdown of what I am doing maybe someone on here will see something obvious I am missing. I have actually attached a COMSOL report with the details as well. I have a 30nm Ag cylinder (circle in 2-D) that I am using a Drude model to model the material. I want to have a TM wave incident on the cylinder and measure the far field scattering cross section at a wavelength range from 300-700nm. I know there should be a peak in the Scr at ~340nm and know what the surface plot should look like. I am using RF Module/TM-waves /Harmonic Propagation right now. I tried using Scattered Harmonic Propagation with the same results.

I have a PML, an intermediate ring of just vacuum where I will measure Scr and the Ag cylinder. Just looking at one wave length say 340nm (where I expect my resonance), I get a very strange result. When I plot my surface of the incident Ey wave (Ey_rfweh-scEy_rfweh) my incident field looks very distorted. I think this be my first problem. I attached a screen capture.

Any input would be greatly appreciated. I know I probably haven’t provided enough information as there is a lot that goes into each model. Hopefully some has had similar issues when they first started and can suggest things to check from experience.

Yevgeniy, have you ever done this in 2-D?

Thanks,
Jacob

Upload the model.

Here it is.

Maybe I am doing this all wrong? Once again, all I am trying to do is to measure the scattering cross section of a 30nm (radius) Ag cylinder. The model I uploaded is only evaluating at 340nm where I expect a resonance, which should be visible in a surface plot of Ey and also Hz. Once the model is correct I will sweep frequencies between 300 and 700nm. I hope to generate a Cscatter plot which looks like my attachment (done with analytical approach in Maple)

Any help would be greatly appreciated.

I never used 2D, so I'm not sure what is that you need to get from the simulation.

But for sure decrease subdomain size, make mesh more fine (34nm for outersubdomains and even less for cylinder). And try other modes. And try to get good results with complex index of refraction for Ag, and then continue to Drude model.

Yevgeniy, thank you so much for your help. I ended up making adjustments based on your suggestions and it worked beautifully.

I guess one last question is on the maximum mesh size. Is there a rule of thumb mesh based on geometry and wavelength used for excitation? I have ordered a FEM modeling book, but I am still waiting for it to arrive.

I am going to have a 100nm cylinder with a 10nm "jacket" around it of a different material (metal coated nanowire). I worry in the 10nm shell that my mesh maximum mesh size will need to be <1nm. I don't even know if that sounds feasible or not.

I guess we will see what happens! Thanks again.

Jacob

Rule of thumb,
If you make your mesh finer and your solution changes dramatically then you probably need to further decrease mesh size, if however solution didn't change much, then your mesh probably OK.

Also always look at your solution if you see that it has artifacts then decrease size of the mesh. If it not converging then again decrease size of mesh.

Hi

I have got the message to ensure at least 5 nodes per wavelegth period, as a minimum. But it's as usual, ask yourself how fast is your solution modulating, what kind of gradient do you have, you must adapt the mesh to be able to catch correctly the gradients slope, or the slope of your changes, otherwise you might get aliasing and strange results.

In structural, and probably it's rather general, doubling the node numer and expect less than 10% change then you are generally in a good situation.

You have also the mesh refinement option in the solver, you might need to adapt the L2 optimisation function for your case.

Good luck
Ivar

Great advice guys. Thanks so much for the help!

So I was able to calculate a scattering cross section, but am having trouble producing an extinction cross section or an absorption cross section.

In the end I want a extinction, and if I can calculate an absorption cross section then I can get there.

Cext = Cabs + Csca

Anyone every calculated the absorption or extinction?

Check the following paper:
Nanoshells to nanoeggs to nanocups: optical properties of reduced symmetry core/shell nanoparticles beyound the quasy-static limit, by Mark W. Knight and Naomi Halas, in New Journal of Physics...
You will see there that absorption cross section is proportional to integral of resisting heating over your metallic nanoparticles...
Cheers
Srdjan

Thank you for the info Srdjan. The paper proved to be very useful!

Rule of thumb,
If you make your mesh finer and your solution changes dramatically then you probably need to further decrease mesh size, if however solution didn't change much, then your mesh probably OK.

Also always look at your solution if you see that it has artifacts then decrease size of the mesh. If it not converging then again decrease size of mesh.

Higher in the thread, someone indicated that you posted a 3D mie scattering model.

I don't find it in the Model Exchange - is it still available?

Thanks,

Lloyd

It's moved to the Modell Galerie:
www.uk.comsol.com/showroom/gallery/3459/

Has anyone done this model with 4.0a?

nopes.
i had done for silver particle.

Hey can u share ur model of silver particle if you have done with 4.0a?

i wanted to simulate "mie theory" model with comsol 4, but i think, variables are not same in two version, and i couldnt adjust incident field same as comsole 3.5, any body did it, can help me?

how to calculate this molde scattering cross-section and Abs cross-section .

very thankful.

Do you know how to convert this in comsol 4.2?

Im currently trying to simulate the same (version 3.5a) and I managed to get the proper shape of the scattering and abs efficiencies with the help of the formulas in the paper "nanoshells to nanoeggs to nanocups" that Srdjan recommended to you.

However, the abs efficiency is on the order of 10^10 and the scattering efficiency on the order of 10^23. So i guess there is someting wrong with the constants in front of the integrals.

Did you applied these formulas straight away? Or did you make any change due to the fact that we are dealing with 2D simulations instead of 3D? (I guess tat the cross sections should have dimensions of m in 2D, instead of m^2 in 3D)

Any help is more than welcome!
Jorge

Did you solve this ?

Hello All,

This thread seems to touch on some very old versions of the software. To address the general question of computing the cross section of an object illuminated by a plane wave we would suggest that you first start with this example:
www.comsol.com/model/computing-the-radar-cross-section-of-a-perfectly-conducting-sphere-10332
This example computes the radar cross section of a perfectly conductive sphere, and compares the computational result to analytic solutions and shows agreement. Most of the older questions in this thread are moot or addressed by this model.

but this model with mph is unfortunately not available. can u plz send mph file