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Hi Guilermo,

Problems that are large compared to the wavelength are heavy to solve. As a starting point, you should consider whether diffraction effects are important in your model. If not, the Ray Optics Module, is a better choice for you.

As a general advice, you should try to reduce the model domain as much as you can using symmetry considerations (mirror symmetry? periodic symmetry?) and by truncating the calculation domain using scattering boundary conditions or PMLs to absorb the outgoing waves.

You can try using a coarser mesh in parts of your structure where you don't have much power density. However, consider first if you can remove those parts completely from the calculation. If you make a coarser mesh in some parts, it is still good to make a mesh convergence study, where you refine the mesh in steps, to see that you have a result that seems to be converged to a physically sound solution.

The Beam Envelopes interface is good if the solution consists of mainly one or two propagation directions/modes that you know of in advance. If you have a scattering proplem, where the scattered wave propagates in many different directions, the Beam Envelopes interface will not help you.

If you consider coupling the regular Frequency Domain interface with the Beam Envelopes interface, you can find some information about this procedure in this blog post: https://www.comsol.com/blogs/2-methods-for-simulating-radiated-fields-in-comsol-multiphysics/

Good luck with your modeling project.

Best regards,

Ulf

Hello Ulf,

Thanks for your reply. Of course diffraction effects are important in my model, take into account that in the end what I want my structure for is to diffuse and polarize light which is going from one medium to air.

What do you mean by trying simmetry considerations? My model is periodic since it is an array but I don't understand how the "model domain" can be reduced? What will the advantages be and how to apply this reduction? I used PML for the "edges" of the air domain and Perfect Electric Conductor for the bottom of my geometry to achieve ideal reflexion. In general, scattering should not be a problem since I'm not considering roughness in any interface and the material i'm using is homogeneous.

Best regards,

Guille

I'll add two suggestions to the above:

1. If you have a large periodic structure (i.e., an array of many elements, equally spaced) and if you can convince yourself that its local physics behavior can be reasonably represented by an infinite array of such elements, then you may wish to employ Floquet boundary conditions. This approach will let you model just one unit cell rather than a large array, but as immersed in an (equivalent) infinite array. You have to set the Floquet conditions carefully, but it can be done. I've done it in the RF module. (I don't know if the Wave Optics module includes the same feature, but I'm guessing it does.)

2. As another potentially useful technique, you may find it possible to extend your computations to greater distances via a near-field aperture-integration approximation: See https://www.comsol.com/community/exchange/672/ .

Good luck.