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Hi,

I have a COMSOL Multiphysics model in which I simulate an optical cavity with a high Q-factor mode around 194 THz. My goal is to first solve for the eigenmodes of the optical cavity using the 'Eigenvalues' study step. From this, I obtain the extact eigenfrequency of my desired mode. In a next step I would like to excite this mode using a 'Frequency Domain' study step. To do this I place a magnetic dipole in the cavity to excite this mode.

I first performed this sequence of steps on a model which uses a PEC (XZ plane) and PMC (YZ plane) to exploit the symmetry of my mode. Everything works well and in my simulation with the 'Frequency Domain' study step, I can see that I have excited the correct mode.

When I move the PMC boundary condition to investigate a design that is not symmetric in the x-direction, I can solve the 'Eigen Frequency' problem. However, when I now plug the eigenfrequency of my mode in the 'Frequency Domain' step, the simulation gives the following error:

The following feature has encountered a problem: - Feature: Stationary Solver 1 (sol1/s1) *Undefined value found. - Detail: NaN or Inf found when solving linear system using BiCGStab. *

I have tried the same for a model without any symmetry boundary conditions and got the same error. I have verified that the correct BC apply when I remove the PMC. Strange enough I get this error after the following lines in the output file:

Iter SolEst Damping Stepsize #Res #Jac #Sol LinIt LinErr LinRes ------- Current Progress: 71 % - Solving linear system Memory: 47703/48062 59739/60208 1 - 1.0000000 - 1 1 1 1 1.2e+51 1 ---------- Current Progress: 100 % - Solving linear system Memory: 5231/49874 9221/61944 Solution time: 262 s. (4 minutes, 22 seconds) Physical memory: 49.87 GB Virtual memory: 61.94 GB Ended at Jun 13, 2024, 10:07:23 AM.

Which seems to suggest that my model did actually work and solved the Frequency Domain study step. Can anyone help me understanding what is going wrong here?