Q Factor

[Lucas MoggParticipant]

Dear Optiwave Team,

I am a student that has been on placement at University and this software has been great to try designs of photonic crystals, thanks so much for making it available to try, and the 32 Bit version. My question is related to use of the 32 Bit version. Please excuse any naivety in this post, I am new to the world of FDTD software…

I have modeled a photonic crystal cavity (a H1 cavity comprising of array of GaAs pillars). Using the 2D band gap simulator I have the required band gap in the photonic crystal lattice for the modeled point source. I would like to know if I am calculating the Q factor correctly using the software, this is my method:

1. Positioning a point source in the center of the cavity
2. Placing an observation point in the cavity.
3. Running the simulation on the auto settings (I have also tried decreasing the mesh size and time steps to the memory limit of my computer). I run the simulation for 30000 steps which seems to allow the standing wave in the cavity to ‘settle’.
4. Then use the OptiwaveAnalyser software to perform observation point analysis. I use the FFT and DFT (with seemingly similiar outcomes) to perform the following: frequency/(FWHM) at the observation point.

I am getting very low Q factors of nothing higher than 480 at best…

I would like to make sure I am using a valid method to calculate the Q factor? Is this possible in 32 Bit version of Optiwave FDTD?

I hope you can help.

Regards,

Lucas

[Damian MarekParticipant]

Hi Lucas,

Have you tried the time domain approach? Excite the cavity with a plane source or line source with a short pulse. Then using the observation point look at the decay of the time signal, measuring the decay at a time after the input has “turned off”.

You only need enough time steps for the signal to drop to a insignificant level. Try using a couple observation points in the cavity at different points to see how long it takes for the power to drop.

This should be possible in the 32 bit version, its limitation only being amount of memory that it can use.

Let me know if you are still having trouble!

+1

[Lucas MoggParticipant]

Thanks Damian,

I have since run the simulation for longer and it seems to yield better Q factor results. I also tried your method and I get good Q factor results that way too. Much appreciation for your help.

Regards,

Lucas

[Dr. Subhashish TiwariParticipant]

Hi Lucas,

I am doing Phd and working on the similar topic. The problem I am facing is that I am not getting bandgap in my simulation. A following paper is attached, on which I am working. Could you please help.

Thanks

[Saurav ChandaParticipant]

check out the PWE band solving parameters for PBG.

Regards,
Sourav

[Lucas MoggParticipant]

Hi Subhashish

I cant see an attached paper anywhere. Are you working to a pre-existing crystal structure or designing from scratch?

Lucas

[Dr. Subhashish TiwariParticipant]

I got the bandgap. Thanks Lucas