Learning Forum

[sagar]

In my simulation, I am using the BCC for the oblique incident of plane wave source to measure reflection and transmission, and I am always getting reflection more than 1, which might be because the FDTD status is showing 1 even with a longer simulation time. Please suggest the reason for it and a solution. Even with RCWA solver also I getting the same result. 

[Guilin Sun]

Please note that Bloch BC is not good for braodband simulation: 

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

https://optics.ansys.com/hc/en-us/articles/1500006417822-Bloch-BCs-in-broadband-sweeps-over-angle-of-incidence

you can use BFAST  https://optics.ansys.com/hc/en-us/articles/360034902273-Broadband-Fixed-Angle-Source-Technique-BFAST-

If the incident angle is not very large, say over 70deg.

The larger than 1 result usually is due to autoshutoff min is too large. Please reduce it and do tests. Of course the simulation time should be long enough to ensure the simulation terminates before reaching 100%.

 

 

[sagar]

Dear Guilin,

 I have tried with the BFAST source and observed the auto-shutoff has started increasing. I  have increased the simulation time up to 50 ps, but nothing has improved. 

Another question, What is the significance of Grid in FDTD? 

this is the design I am using for my simulation. The broadband plane wave source is injected at a particular angle (BFAST), PBC in the four sides and at the top and bottom, I have used PML, and the pillars are arranged in a hexagonal array. 

 

 

 

[sagar]

Every time I get this, look at the auto shutoff value. Please help to solve this issue, 

[Guilin Sun]

I believe there are higher diffraction orders and the angle is large, say larger than 85 deg. Then you will need to use Steep Angle PML with 32 or 64 PML layers.

Please roughly check the diffraction angle even it does not converge, by using visualizer with 10 by 10 periods, for an esitmation.

If at some wavelengths the dirraction angle is close to 90deg, then it can be very challenge to converge, the PML reflection is strong in physics.

[sagar]

In this case, you can see the angle I put is 5 degrees. The violet arrow is the k vector in the image. Even in my case, I need the simulation for a maximum angle of 20 degrees. 

 

[Guilin Sun]

Please try normal incidence and find the diffraction orders and angles with periodic BCs. If the diffraction orders change from one wavelength to another, there will be wavelengths that have 90 deg diffraction.

[sagar]

How do I find the diffraction orders?