Learning Forum

[Sayan Bandyopadhyay]

Hi,

I have a 2D photonic crystal cavity that I am simulating using Lumerical, I am trying to calculcate the high-Q value. I have a periodic cavity structure with an L3 defect. Before I was setting the index of refraction of the FDTD region to be 1.0 and the holes material to be of etch type as well. Now I want to change the index of refraction of the FDTD region to 1.44 and index of refraction of etch type I have already created with 1.44. However, I am now getting negative Q values, and very high dQ. What changes do I need to make to the mesh region and the other conditions so as to simulate a pulse propagation with 1550nm dipole at the center?

Thanks,
Sayan

[Guilin Sun]

negative Q values, and very high dQ definitely mean something is wrong. Depending on which method (findresonance or fft) please check some settings, like simulation time, time sginals. If it is fft method, the time section might not be proper to collect the resonance. Please check the analysis script in the group, and plot some intermediate results for diagnosing the issues.

Since FDTD is a time domain method, most of the time we use broadband source. However you can limit the frequency-domain monitor to record specified wavelength or wavelength range. Time monitor picks up signals traveling back and forth, which can be used to extract resonance information.

You can inject a pulse centered at the desired wavelength in source settings:

Please submit future posts in the Photonics Forum.

 

[Sayan Bandyopadhyay]

I believe the dipole settings are fine. I think there is something wrong in the way the mesh settings are working. Since the different refractive index, of the holes need a different grading of the mesh. Is this automatically handled by CMT2? Even the E-profile looks accurate and should be how the mode is propagating. This is using the dipole monitor. Only the high Q analysis script is getting a negative value.

[Guilin Sun]

Did you check what method you use now?

One suggestion for better result is to use integer number of meshes in one period along the periodic direction, instead of the equivalent index. Otherwise  it will not keep the periodicity after discretization.

[Sayan Bandyopadhyay]

Have switched it over to a custom non-uniform. Is there any setting that needs to be adjusted here. The dy is adjusted with a multiplication of (sqrt(3)/2) since the mesh is at an angle.

[Guilin Sun]

As suggested previously, please use uniform mesh, and the number of meshes should be an integer in one period, in order to maintain periodicity of the discretized periodic section. This is important for the reflector sections.