COMSOL band structure of photonic crystal waveguide

Click For Summary

Discussion Overview

The discussion focuses on the challenges of calculating the band structure for a planar photonic crystal waveguide using COMSOL, specifically addressing a quasi-3D problem with finite thickness. Participants explore methods for implementing boundary conditions and normalizing electric fields, while sharing experiences and troubleshooting issues related to computational time and solver errors.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant describes their setup for calculating the band structure, detailing the use of Floquet periodic boundary conditions and the adaptation of a tutorial for a purely 2D photonic crystal.
  • Another participant mentions encountering an error with the parametric solver not recognizing the electric field components Ex, Ey, and Ez when normalizing the entire electric field.
  • A different participant shares their experience of long computation times, suggesting that it may be due to a fine mesh in a 3D problem, and emphasizes the need for a full vectorial treatment in 3D as opposed to decoupled modes in 2D.
  • One participant requests access to the COMSOL document referenced by the original poster.
  • Another participant asks for code or scripts to learn from, while one clarifies that they used the GUI and followed instructions for the 2D case, highlighting the importance of identical meshes on periodic boundary condition pairs for convergence.

Areas of Agreement / Disagreement

Participants express various challenges and approaches without reaching a consensus on solutions. There are differing experiences regarding errors and computation times, indicating that multiple views and methods are present in the discussion.

Contextual Notes

Participants mention specific dependencies on the RF module and the necessity of proper meshing techniques for periodic boundaries, which may not be automatically handled by the software.

BrillouinPie
Messages
2
Reaction score
0
Setup:
I'm trying to make the band structure for a planar photonic crystal with finite thickness, i.e., a quasi-3D problem.

I only want the x-direction band structure. So, I'm using variable floquet periodic BCs for the x-direction boundaries, and 0 degree floquet periodic BCs for the y-direction. The top and bottom parts of the cell have either PEC or PMC BCs.

I'm trying to adapt the method used here: http://www.comsol.com/showroom/documentation/model/798/

This tutorial shows how to a make band diagram for a purely 2D photonic crystal. The main idea in the tutorial is following a particular band while ramping k by force feeding the previous frequency into the current parametric solver. Their method also uses an integration coupling variable with an ODE on the frequency, by normalizing the z-comp of the electric field (they're using 2D > RF Module > In-Plane Wave > TE waves > eigenfrequency).

Problem:
I try to adapt the problem by normalizing the entire electric field using: Ex*conj(Ex) + Ey*conj(Ey) + Ez*conj(Ez). For subdomain ICs I use Ex(to) = Ex, etc., for all parts of the E-field.

When I try to run the parametric solver and ramp k, the solver immediately halts, saying that it doesn't know what Ex, Ey, and Ez are.

Any ideas?
 
Physics news on Phys.org
Nothing specific to this problem, but an FYI: COMSOL has started a discussion forum (as of only a week ago) where you might be able to get help from active users.

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

You should be able to use the forum if you are a registered user with an account. Be sure to read and follow the Guidelines posted at the bottom of the welcome thread.
 
Oh great. Thanks!
 
Dear Sir,

I am a started using COMSOL. And I would like to do the band structure for a planar photonic crystal as well. Can you be so kind to share the document from COMSOL web you got to me?

Thanks in advance!
David
 
I am also trying to do this. I haven't got the arrors you have but the computation time seems to be never ending, but perhaps that's because the mesh is very fine and it's a 3D problem.

Have you made sure you are using a 3D RF module where the dependent variables are Ex, Ey and Ez? Because in the 2D problems the two mode polarisations decouple and there is only a single variable Ez (or Hz). Such decoupling does not occur in 3D and you need a full vectorial treatment (although from your post it seems as though you already knew that).

I'll keep you posted and if I get the thing working I can send you my scripts if you wish.
 
Can you please load your code so that I may learn how you were able to accomplish this? Thank you.
 
I didn't actually write any code. I just used the GUI and followed the instructions for the 2D case, but changed the geometry I drew. Unfortunately the file, with the mesh and results is too large to upload here.

The most important thing to realize when doing things in 3D is the the mesh on the periodic boundary condition boundary pairs must be identical. This is not done automatically and you must thus mesh one boundary then manually copy the mesh from it to another when there is a periodic pair. Once you have done all the periodic pairs then mesh what's remaining using "Mesh remaining free". If you do not do this your computation will not converge.
 

Similar threads

Graduate Photonic Band Structure Calculation
  • · Replies 0 ·
Replies
0
Views
7K
Graduate Avalanche photon detectors
  • · Replies 0 ·
Replies
0
Views
449
Graduate Photonic Crystal Fibres and COMSOL
  • · Replies 3 ·
Replies
3
Views
8K