- #26

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I am a comsol beginner. Can you advice me how to start with comsol simulation of gold nanoparticles,,,can u send me some documents and/or sample examples using comsol v4..

Your help is highly appreciated

- Thread starter Srbasket
- Start date

- #26

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I am a comsol beginner. Can you advice me how to start with comsol simulation of gold nanoparticles,,,can u send me some documents and/or sample examples using comsol v4..

Your help is highly appreciated

- #27

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http://www.comsol.com/community/forums/

I am a comsol beginner. Can you advice me how to start with comsol simulation of gold nanoparticles,,,can u send me some documents and/or sample examples using comsol v4..

Your help is highly appreciated

- #28

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Thanks tatarin

- #29

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Thanks to my good friend V, I am able to update u all about COMSOL issues related to nanoparticles.

There is a mistake that I and many more were doing. If you take a look at Stratton-Chu formulae that is implemented directly in comsol, you will notice that it works only for scatterer embeded in uniform medium, more precisely vacuum. If it is embeded in water, lets say, spectra will keep the same shape, so no worries about that.

If you think that it can be implemented and modify easily for substrate case, you are wrong.

To keep it as simple as possible: Scattered light by particle is composed of many k-vectors, and if you have substrate, everty k-vectror will behave different when passing through the interface, so for each k-vector one needs to incorporate guess what, Fresnel coefficients, so some components can feel TIR, some not, and that is tricky to implement into Stratton-Chu. For more detailes type in in google, "Forbiden light dipole" and you would get some papers by famous Lukas Novotny.

So, what was accounted to be COMSOLs advantage over other programs (substrate calculations), it is not.

Thanks V for these remarks

Question is what to do:

1) You can disregard this post and continue as you do. If you get something fairly similar to experiments, you are fine (Of couse, someone can say that it is not correct)

2) We can forget about SC formulae, and combine approach where calculating Scattered field in one point for each wavelength, where this point should be as far from the antenna as possible (in far-field zone, meaning one wavelength away). Same as always, incorporate Fresnel coeff. into excitation, and if you choose good point for evaluation, you might get something useful.

3) There are also methods (see on COMSOL website, about nanoantennas) where you put particle on 200nm thick glass substrate, and below is air. You can get some results, but again it is not the optimum solution. (I have tried once, and got some numerical or bad meshing related peaks, but there was also main plasmonic peak. Probably can be optimized...See the document). There you solve for Electric field. You can use also semy infinite substrate, but the excitation can only come from air side. If it comes from glass side, there is some inclear problem about defining excitation: I will probably try to solve this if I get some spare time...

4) I thank once again to V, and if I or anyone else get some great idea about solving this problems, we will update this thread

I will try to convince V to join the thread, since he is the theoretician who knows what he is doing, acontrary to us experimentalists

Cheers, folks

Enjoj WC

Yours (to some extent dissapointed in COMSOL) Srbasket

- #30

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We have been checking if SC gives similar result to Green dyiadic method for substrate case, and the found resonance by both methods coincide pretty much. However, it looks like for anything else, like radiation pattern calculations, or real physical values of cross sections, one should not rely on SC in case of substrates.

So, hope that COMSOL guys will find some solutions to this problem.

Cheers

- #31

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thanks

- #32

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Hi,

thanks

go for RF module. You should be able to download example of Mie scattering calculations from comsol website. That is very good starting point. Also there are many more models that might be usefull. You should also read carefully all the posts. Check also Discussion on comsol web site, and there is one group on yahoo.

For magnetics you will need to use tensors, but shouldn´t be that difficult.

Good luck

Also, if u have subscription for comsol supprt, that is cool, since they reply quite fast

- #33

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This might help, it is under construction, but....

srdjancomsol.weebly.com

srdjancomsol.weebly.com

- #34

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Hi Srbasket,

I found your thread and website are very useful.

Thanks man!!

I found your thread and website are very useful.

Thanks man!!

- #35

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Hi Srbasket,

your information is very usefull for me. I tried to do tests of PML as you described using Fresnel coefficients. But I still have big reflections.

When I simulated propagation of planar wave in air, scattered field was was in order of magnitude 1e-2 which is insufficient. For air/glass interface was scattered fiedl about 2e-1 magnitude, so very poor performance.

I computed my result in 3.5a+fix and also 4.2 versions.

http://physics.fme.vutbr.cz/files/zlamal/air3_5a.mph" [Broken]

http://physics.fme.vutbr.cz/files/zlamal/glass3_5a.mph" [Broken]

http://physics.fme.vutbr.cz/files/zlamal/3Dair4_2.mph" [Broken]

Can you have a look at my projects and tell me please what I am doing wrong?

Can you describe how to tune up PML if it is cause of my problems (I tried also much denser meshes but no advance)?

your information is very usefull for me. I tried to do tests of PML as you described using Fresnel coefficients. But I still have big reflections.

When I simulated propagation of planar wave in air, scattered field was was in order of magnitude 1e-2 which is insufficient. For air/glass interface was scattered fiedl about 2e-1 magnitude, so very poor performance.

I computed my result in 3.5a+fix and also 4.2 versions.

http://physics.fme.vutbr.cz/files/zlamal/air3_5a.mph" [Broken]

http://physics.fme.vutbr.cz/files/zlamal/glass3_5a.mph" [Broken]

http://physics.fme.vutbr.cz/files/zlamal/3Dair4_2.mph" [Broken]

Can you have a look at my projects and tell me please what I am doing wrong?

Can you describe how to tune up PML if it is cause of my problems (I tried also much denser meshes but no advance)?

Last edited by a moderator:

- #36

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Dear team, minds :)

The thing is that I have micrcro membrane with optical gratings and I need to have it with optical signal registration: i.e. some pressure to the membrane is applied and using laser light interference of diffraction we need to model it To callibrate diffraction minimums and maximums to pressure units.

P.S. this should be done in COMSOL

Sincerely

PHD . Karolis

- #37

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Problem - In the far-field analysis : When I tried to calculate the scattering cross section using the near field the results are incorrect but for the same program the absorption cross section calculated using the near field match perfectly.

I am trying to simulate the scattering in the same way as it is explained in mie scattering 3.5 example.

some info :-

radius of : scatterer -100nm,auxiliary boundary=200nm,inner pml=250nm,outer pml=300nm. Its the aux boundary where Efar is being defined.

any help will be appreciated...i wonder if someone has encountered same problem cuz in 3.5 it works well but the same formulation is giving incorrect result as explained above but absorption cross section is fine.

thanks.

- #38

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Problem - In the far-field analysis : When I tried to calculate the scattering cross section using the near field the results are incorrect but for the same program the absorption cross section calculated using the near field match perfectly.

I am trying to simulate the scattering in the same way as it is explained in mie scattering 3.5 example.

some info :-

radius of : scatterer -100nm,auxiliary boundary=200nm,inner pml=250nm,outer pml=300nm. Its the aux boundary where Efar is being defined.

any help will be appreciated...i wonder if someone has encountered same problem cuz in 3.5 it works well but the same formulation is giving incorrect result as explained above but absorption cross section is fine.

thanks.

- #39

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Thank you so much for sharing such useful information about how to solve a scattering problem by Comsol! The website http://srdjancomsol.weebly.com is also very helpful.

Actually, I followed the 2-step procedure as you provided to solve my case which is light scattering from a waveguide. The waveguide made of polymer (refractive index 1.58) has a square cross-section (1 micrometer times 1 micrometer) and it is supported by a layer of the same polymer with the thickness of 0.3 micrometer. Beneath the polymer layer, there is SiO2 substrate (refractive index 1.45). In this case, The effect of the 2-layer substrate on the background excitation field should be taken into account. However, it is not straightforward to get the analytical expression of the (Fresnel formulation) background field in the presence of the polymer layer and the SiO2 substrate. On the other hand, it is either incorrect to simply put a plane wave onto the whole structure including the scatter and the supporting layers since the supporting layers are then limited in lateral size and it also causes scattering.

Fortunately, I found your message online and tried to follow your directions. First, I calculated the background field with the supporting layers (by setting periodic boundary conditions to ensure the infinite lateral size) but without the scatter. Second, I stored the solution (Solve Manager/Store Solution) of the first step and then used it as an initial value (Solve Manager/Initial Values/Stored Solution) for the second-step calculation. However, I found some problem because the initial value is not regarded as the excitation field by Comsol so that this 2-step calculation just gives the same result as simply putting a plane excitation wave, exp(-j*k0_rfweh*y), onto the scatter and the supporting layers which also serve as scatters due to its limited lateral size.

I also tried total field calculation, but the limited lateral size of the substrate is always the problem. Vice verse, if I eliminate the problem of the limited lateral size of the substrate by setting periodic boundary conditions, the solution is according to an array of scatters instead of a single one.

Would you please give me some more suggestions?

Thank you!

Jing Zhou

jizhou@umich.edu

One clarification:

In 3) second time you are solving press RESTART, not the SOLVE

Hi, all,

folks keep contacting me about different things regarding this subject. I have advanced a bit, but still some issues remaining.

1) In excitation field definition you can forget about using complicated formulae with different expressions for z>0 and z<0. It looks like that if u use k_rfw, COMSOL will automatically consider refractive index of material into k vector in different media.

2) THERE is a big difference if u are solving for SCATTERD or TOTAL field. You should read Jiaming Jin's book to find out more. In short, if u are solving for SCATTERD field your excitation should be defined in the most precize analytical way, using Fresnel coefficients, and in that case you need complicated formulae with z>0 and z<0, since in SCATTERING MODULE you have to define field present everywhere except PML. In total field calculation, you have to insert incident wave on one boundary (port or scattering boundary condition with excitation), and propagation should be calculated... There are some advantages/disadvantages in both cases, depending what you whant to calculatet, how much memory you have, and some FEM issues that you can read in Jim's book...

3) About 2 step procedure: When u create new model name your dependents variables to scEx1,etc, and Application mode name to rfw1. Then you can make your geometry. After, you go to Multyphysics/Module navigator and ADD new scEx2,...,rfw2, and now you have two models on the same geometry. In both models you define separately Subdomain setting, Boundary settings, etc. Meshing is the same, since you have one geometry, and 2 models defioned on the same geometry. So, first you solve situation when your particle is made of air let's say, and later use that solution as excitation in your second model. Thus, in Physics/Scalar Variables you set Eoix... with Fresnel coefficients (or not if u don't want), and for E0ix2 you set Ex, E0iy2 set Ey, etc.... First step is to SOLVER MANAGER/Solve for you highlight rfw1, and for the output rfw1 (not necessary), and when it is done, you go to SOLVER MANAGER/Initial value, there you first Store solution, and check STORED SOLUTION option as initial values, SOLVE FOR you highlight rfw2, and the same for OUTPUT... That should work. Theoretical benefits of this procedure are not so clear to me. IF SOMEONE COMPARES IT; PLEASE PUT YOUR RESULTS AND CONCLUSION HERE....

4) Someone asked me "How did you separate the scattered field from the substrate and from the nanoparticle?" Maybe this 2 step procedure is the answer to that. Maybe you can normalized what you get for particle air and partice made of metal case, I don't know exactely.

5)Scattering cross-section: Read RF module reference guide. There is a Stratton-Chu formula implemented ion COMSOL for calculating far-fileld components. Onece you have them, you need spherical surface (it doesn't have to be the same as one for Stratton-Chu, can be smaller to save you integratin time) and one should do boundary integration of normEfar*normEfar, and that is proportional to scattering cross section. If you want to be more precise calculate this expresion: (normEfar*normEfar)/(Surface of particle*Ein*Ein*R*R) where R is the radius of your integrating spherical surface.

6) IF ANYONE HAVE GOOD RECIPE FOR PML THAT WORKS FINE, please POST IT!

Cheers

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