How to identify modes (FDTD)

Dear All,

I’m using FDTD to simulate mode distribution in hexagonal nanowire (NW) (axial GaAs quantum well between axial GaAsP barriers) and use Mode source alingned with NW to model QW emission.

If I check properties (Edit object) of the Source and then choose “User Selected” and then “Calculate modes” around max n (refractive index), it shows me 20 modes - but doesn’t identify them.

Of coursem I can look at field distribution and guess, but I’d prefer FDTD to indentify modes for me (e.g., mode 1 = TE01, mode 2= TM01, mode = HE31 etc.).

WIll the Mode expansion monitor do the job? If it will - where to place it? If not - how else this can be done to identify modes?

Thank you

Anton

P.S. Apologies if my question is too naive, I’m a newbie to FDTD.

Hello @a.velychko,

FDTD will not provide the names of the modes in question. These mode labels are somewhat arbitrary labels for solutions to Maxwell’s Equations. You will have to compare the found solutions with eg solutions as the underlying physics does not care about labels. With practice it does become very easy to identify common modes in regular sorts of geometry, but due to the diversity of possible wave guiding structures this technique is not generally applicable.

Mode expansion will calculate the overlap in each specified mode through the DFT monitor used for expansion.

To help identify the modes you are interested in you can sort modes by polarization, mode area, effective index etc.

Regards,

Hello trobertson,

Thank you for your reply.

You will have to compare the found solutions with eg solutions as the underlying physics does not care about labels.

I didn’t get that - what do you mean by “to compare the found solutions with eg solutions?” Is there a word missing?

To help identify the modes you are interested in you can sort modes by polarization, mode area, effective index etc.

How do I do that, please? Let’s start simple with effective index, e.g. - how do I distinguish say fundamental mode TE01 for hexagonal waveguide (nanowire) from other modes based on effective index only? What values etc. should I expect for TE01 to have?

Also, is Mode Solutions better for identifying modes or it it similar to FDTD in this sense?

Thank you

Hello @a.velychko,

This means that you should look at the TE/TM example modes for comparison.

Click on column headers to sort by that category, and change from ascending to descending.

The fundamental TE mode will have the highest effective index and will be mostly TE polarized. The n_eff will be less than the material index. You should refer to the literature for reference on these modes.

MODE operates in a similar fashion to FDTD’s mode source, but provides more calculation options. Furthermore the integration with other solvers makes it more efficient for many waveguiding applications

Regards,