Learning Forum

[Seyed Mohammadhossein Enjaviarsanjan]

Hello there,

I am working on the 1D photonic crystal which I expect to show a quality factor of about 1 million. I used the high q analyzer embedded in Lumerical FDTD. 
I recently found out that the quality factor changes using changing frequency points in the global properties of the "monitors" section in the software.

So I simulate my cavity considering random dipole in the region to excite the resonant modes, I am using this method to find the intrinsic quality factor of my structure. 

As far as I know, the time monitor which is used in this method to calculate the decay of energy from the cavity is not dependent on the number of frequency points, right?

Would you please guide me, so I can make sure that my simulation is reliable?

[Guilin Sun]

The number of frequency points is to output how many resonant wavelengths. 

Does the change happen to the largest Q? did you check the resonant wavelength? 

For such devices if the periodicity does not change but only hole diameters change, you will make sure each period has exactly the same mesh, eg, you will need an override region with mesh size delta=period/N where N is an integer. However when both the diameter and peioidicity change it is challenge to have integer number of meshes. In this case you may need to use finer mesh along the direction of local periodicity. 

[Seyed Mohammadhossein Enjaviarsanjan]

So I use auto-nonuniform mesh 3. For the same structure(all of the parameters are exactly the same except for a number of frequency points), the highest quality factor changes dramatically.

But the wavelength of resonance doesn't change that much(less than 1 nm -- resonance wavelength is 775 nm) 

This is the highest Q which is my desired one.

I can not realize how this number can change the quality factor since the time monitor does not have any input related to that.

Is it related to the post-processing of calculating the quality factor?

Also, I created a defect using different distances between holes in the cavity region and I kept the diameter fixed.

just for your kind information even with mesh 3, the quality factor is about 1M, and finer mesh size doesn't change the resonance wavelength.
It just increases the Q.

[Muhammad Danial Haziq Azizan]

Hye,

Can I get your files cause I am stuck on doing that? My Q factor changes if I change the source. I do not really understand about this. Really need help. Here is my email. (md.haziq@graduate.utm.my)

[Guilin Sun]

It meeds careful meshing and selecting the time widow for post processing. 

The uniform mesh is to guarantee the periodicity. Without proper periodicity the result might deviate the theory. Please note that the discrete mesh matters for the simulation.

As far as 2008 Harvard team reported over 1M Q factor for Photonic crystal cavity so high Q is not an issue for Lumerical FDTD.

Once the meshing is proper, both the dipole sources and the time monitors can be modified to be placed where the field intensity is strongest for around the desired wavelength. You can place one or more profile monitors to record the fields with much longer simulation time than Q simulation, with apodization setting:

https://optics.ansys.com/hc/en-us/articles/360034902473-Understanding-time-apodization-in-frequency-domain-monitors

Thus it will be iterative: get rough resonant wavelength and Q, modify the locatios of the dipoles and time monitors, simulate Q again,... when the mesh is proper. Please note in the two cases the simulation time can be very different.

 

[Muhammad Danial Haziq Azizan]

Can you share it file? my result for FHWN is around 1.2. It is to large and make qfactor value is low.

[Guilin Sun]

Please visit online examples as we are not allowed to share any files.

https://optics.ansys.com/hc/en-us/articles/360041567754-Photonic-crystal-cavity

https://optics.ansys.com/hc/en-us/articles/1500006350801-Tips-for-simulating-high-Q-Photonic-Crystal-PhC-Cavity 

https://optics.ansys.com/hc/en-us/articles/360042088453-Bragg-microcavity 

You can also invite peers in the feed for a specific topic.

[Muhammad Danial Haziq Azizan]

I dont have member to discuss in my area. I am doing alone my research because in my department, only me use this software.