Discussion Forum

1. lambda/2 could work fine. If you think it might be too close, try putting the boundary farther away and trying again.
2. That said, may I assume that you are using the Comsol Far-Field computation tool, and not instead simply (and unwisely) using the field values locally on or near the finite-element boundaries, all while naively hoping those points are far enough away? You wouldn't make that mistake, right?
:-)

Regardless, I also recommend you search for "far field" in the Comsol help files.

Thanks Robert ..

Actually i want to calculate the electric field components in far field region and for this i used far field domain option in comsol.

I selected a domain in far field domain and applied proper boundary condition of PEC and PMC according to my geometry. Then for evaluating the far field expression of electric filed i performed point evaluation and selected the point and put the expression Efarz, Efarx, Efary for calculating it.

Did i perform the right operation??

Sounds good. So, did you get different or the same results when trying larger volumes? If the results are not consistent, perhaps you should post your model so that others may examine the details.

Dear Robert,

I simulate the dipole emission near dielectric substrate. Unfortunately, Comsol Far-Field computation tool gives different results for different surrounding volumes. These results barely correspond to theory.
I will be grateful, if you suggest me something.

P.S. model is attached.

Aleksandr,

First thing to check is mesh of PML domain. As is, you mesh of PML domain is not acceptable. Use Swept Mesh for PML and make sure to have 5-6 elements through the thickness of this domain (Right-click Swept node and select Distribution. In the Distribution sub-node define the Number of elements accordingly).

Regards,
Sergei

Dear Sergei,

Thank you for your reply.

I have already tried to use swept mesh according to the sample applications (for example www.comsol.com/model/dipole-antenna-8715).
I checked it once again today (plus I kept mesh size below lambda/5). Unfortunately, it didn't solve the problem.

Kind regards,
Aleksandr

Dear all:

My name is You Zhe Ho. I am a Ph.D. student in National Taiwan University.

I have read the paper"Enhanced spontaneous emission inside hyperbolic metamaterials". It is very interesting and excellent.

Now I want to reproduce the same simulation in COMSOL.

But I do not know how to calculate the Purcell factor in COMSOL 5.1.

The attached file is version COMSOL 5.1.

If you are available, please tell me how to setting the calculation in COMSOL.

Thanks for your kind reply.

Thanks for your help.

You Zhe

Hello Robert,

I found your suggestions extremely informative and helpful. But I have another question concerning about this topic. I am working on the scattering of a particle in a heterogeneous environment (a nanosphere on a glass slide in water). I want to study the influence of the surrounding environment on the scattering pattern of the sphere. Should I still just use the Efarx, Efary, Efarz to evaluate the field? Because my initial attempt was to integrate the poynting vector of the relative field on the outer boundaries of the entire geometry (inside PML of course). But I realized that the size of the entire geometry can actually influence my integration a lot. So I don't know what will be the best thing to do. Any advice will be greatly appreciated! Thank you very much in advance.

Hi,
It's correct to made:
Efar=sqrt( abs(Efarx)^2 + abs(Efary)^2 + abs(Efarz)^2 )
to calculate electric far field ?

Efarx, Efay and Efarz :
Efarx(sin(theta)*cos(phi),sin(theta)*sin(phi),cos(theta))
Efary(sin(theta)*cos(phi),sin(theta)*sin(phi),cos(theta))
Efarz(sin(theta)*cos(phi),sin(theta)*sin(phi),cos(theta))
where the variables theta and phi are defined to represent the angular direction in radians.
Thanks

Maybe I can shed some light on the problem. I recently contacted support about a problem I was having with a changing FF result, and after about 2 months they finally issued an answer.

My setup is a air/Si interface with a metallic inclusion, and I was wishing to capture the FF of the cross-polarized scattered field component. However, I found that changing the port length changed the FF phase/amplitude, even while keeping the mesh minimally unchanged (kept boundary mesh density constant) and adjusting for phase addition of the incremental port length. At first, I thought it was due to my cross-pol scattered field being extremely low in amplitude w/r/t the incident field, and that slight changes in the incident field would induce large changes in the cross-pol scattered field.

However, the COMSOL support response is that the FF Domain node is ONLY appropriate for application to the boundary of a homogeneous domain that completely surrounds the scatterer. In my case, the scatterer being placed at the air/Si interface cannot meet this criterion, and thus the FF results are invalid. They state they are looking to address this in future releases. There is a work-around; though I do not have the directions at this computer. I can post if there is interest, and I hope to be generating a discussion on the matter at the conference!

-Bryan

Dear Bryan,

I am stuck and the very same point for a problem that seems to be the same as yours (scatterer at interface) and it would be great if you'd post the workaround you mentioned. I'd be happy to have a look at it.

Cheers,
Alex