Learning Forum

[hamid.h.javadi]

Is there an example of band structure calculation in DGTD similar to the one in FDTD: https://optics.ansys.com/hc/en-us/articles/360042038933-Bloch-mode-profile-Photonic-crystal

[Guilin Sun]

Not yet. Please refer to the FDTD examples to get script, groups and run the simulation script file to get results, and compare if they are similar.

Thank you!

[hamid.h.javadi]

 

I want to run bandstructure calculation of a 1-D array of plasmonic gold nanowires in DGTD.

I modified DGTD example “dgtd_plasmonic_dolmen” to include only one gold wire. I added “periodic” to the “boundary condition” and choose “simulation region” confined to “source_rectangle” (-0.06um to 0.06 um), I get

               period=getperiodicity(“DGTD”);

               ?period;

                            0.06 um as xmin of “simulation region” is forced to zero.

I chose "x: set based on source angle" and changed source angle “theta” from 0 to 10 to 45 degrees. No difference in resonance frequencies (PEC or PMC symmetry) was observed (between different theta runs).

Is this the right approach?

 

[Guilin Sun]

Unfortunately band structure simulation requires to use Bloch boundary conditions, and DGTD has not implemented yet:

https://optics.ansys.com/hc/en-us/articles/360034918593-Boundary-conditions-in-DGTD-Simulation-Object

https://optics.ansys.com/hc/en-us/articles/360034382714-Bloch-boundary-conditions-in-FDTD-and-MODE

https://optics.ansys.com/hc/en-us/articles/360034382734-Periodic-boundary-conditions-in-FDTD-and-MODE

So even though you set angles it will be like normal incidence.

Please file a feature request for this: 

New Feature vote:   Vote new features, and file your feature request

 

[hamid.h.javadi]

Concentrating again on Ansys FDTD bandstructure calculation:

I would like to introduce a perturbation in a 1D array of periodic plasmonic nanowire array (analyzed by a unit cell with Bloch boundary condition in 1D).

How can I add perturbion/defect in one unit at the end of the array? Can I calculate the E- and H-fields on the edge surface of the array and introduce it as a source in the perturbed unit? How do I achieve cyclic response between the array and the defect on one end?

Should I calculate the bandstructure to detect the change in mode frequencies? Or reflection minimum as a function of kx?

[Guilin Sun]

Since bandstructure simulaiton needs Bloch boundary condition and DGTD has not implemented, please try to use FDTD instead.

Not sure what do you mean "perturbation" here: anything that breaks the periodicity will not be considered as "periodic">

For band structure edge, you can specify the correct K value, for example at the edge of symmatric point Km. You still need to simulate the band structure. YOu may also use the transmission band to find the bandgap.eg, T vs frequency.

 

[hamid.h.javadi]

Thanks for your response. Here I am concentrating in FDTD:

to make my objective clear, let's assume that I want to introduce a defect in an infinite array of plasmonic nanowires.

i can assume that the defective unit has an infinite array on the left and an infinite array on the right hand side. It is very memory-intensive to analyze the whole array with a defect in the middle. Can this be done similar to the Lumerical Bragg example by considering coupling between:

1- the fields at the interface between the infinite array on the left and the defective unit

2- the fields at the interface between the infinite array on the right and the defective unit

and calculate transmission through the whole system at resonance frequencies of the eigenmodes (of the infinite array)?

 

Thanks.

 

 

 

[Guilin Sun]

Just give you a quick reply that, this is more physics than simulation. Simulation obeys strictkly the math. Periodic BCs ensure the infinitely duplicated unit cells. When there is a defect, you will need to use larger perioridicity, and compare different periodicity and see if the result changes. If no change, it means the pertubaltion is local and you can use the smaller periodicity. I hope this helps you to consider it.

[hamid.h.javadi]

Thanks for your insight.

Are you suggesting that I simulate bandstructure of a unit cell (N times the original one) containing the defect? What should be the x-axis boundary condition (Bloch or PML)? What is the range of kx; [0 - pi/(N*a)] or [0 - pi/a]? 

And then change N till the bandstrcture no longer changes.

Thanks,

[Guilin Sun]

In the periodic direction you should use Bloch BC.

"a" is the period;

and the absolute value of "k" is  [0 - pi/a].

when you change N, I am not sure if the bandstructure can keep the same since their absolute K is not the same. What I mean is their transmission changes a little, when you do a regular transmission simulation, like a grating. This should be done before you do the band structure simulation.