Learning Forum

[takuya.sobue]

Now I try to investigate scattering phenomena of a particle on a substrate.
Simulation conditions:
Substrate:SiO2
Particle:Au
Source:TFSF
Wavelength:355nm
Mesh:uniform, maximum mesh step 8.8nm (defined by sphere override mesh)
Geometry information is below screenshots.

And then, I arranged multiple DFT monitors above the source at different height as below screenshot.

After simulation, I checked farfield E^2 profile and I realized that the profiles were different among monitors. I think the simulation region was large enough and the mesh size was fine enough. However, the higher the position of the monitor, the more discretized the profile was generated. How should I interpret this result? And how should I determine the position of DFT monitor properly? Is there any problem on setting the monitor very close to the scattering body?

・0.5um above substrate

・1um

・1.5um

[Kirill]

Dear Subscriber,

Ideally, the monitor should be positioned close enough to capture the entire scattered field. In your example, since there is high-angle scattering, I would trust the closest monitor more. You can place the monitor as close as 2–3 cells outside the TFSF source. Additionally, you can use the movie monitor in your simulation to visually inspect the scattering process.

You may find the following references helpful:

1. Defect scattering and detection – Ansys Optics
2. Mie scattering (FDTD) – Ansys Optics
3. Tips and best practices when using the FDTD TFSF source – Ansys Optics
4. Scattering - list of examples – Ansys Optics

Best regards,
Kirill

[takuya.sobue]

Thank you.
Should the position of the monitor be closer to the scatterer when using other sources (plane waves or Gaussian beams) as well?
For example, When I want to put a monitor under the source, should I set the monitor 2-3 cells away from the scatterer?

[Kirill]

First of all, if you use sources other than TFSF, you will capture a combination of incident and scattered waves, similar to what happens inside the Total-Field region of a TFSF source.
Second, the monitor should be positioned close enough to capture the full scattered field but far enough to avoid capturing evanescent (non-propagating) fields.

Best regards,
Kirill

[takuya.sobue]

 

 

 

 

 

 

 

 

 

Thank you. By the way, if I set the monitor behind the TFSF source, and if I set the monitor in front of the plane wave source and extract only the reflected components, I should theoretically get the same result except for very low numerical aperture region.
But in the latter case, when I tested with only sphere structure(no substrate), far field was like a cross mark, even with or without scatterers. What causes it?

p.s. I used bloch boundary condition in x and y boundaries. I tried expanding simulation region gradually(~10um), The above phenomena became smaller. Even though, the signal of cross mark much larger than defect signal. So in my opinion, it’s impossible to conduct appropriate simulation by using plane wave. However, some templates (ex. Optical defect metrology (s-matrix)) adopts relatively small region (2.16um), why is it not problem?

 

 

 

 

 

 

 

 

 

[Kirill]

Could you please add pictures or sketches to clarify your questions? Specifically:
1. What do you mean by "behind the TFSF source" and "in front of the plane wave source"?
2. How does the "cross mark" appear?

Visuals would help to better understand the issue and provide more accurate assistance.

Best regards,
Kirill

[takuya.sobue]

OK! I used SiO2 sphere which diameter was 400nm. And wavelength was 355nm.

"behind the TFSF source" 



 "in front of the plane wave source"


After filtering low NA region

After filtering "cross mark", scattering signal is observed slightly. 

I wanna know what causes "cross mark".

Best regards

[Kirill]

Dear Subscriber,

In general, to obtain the scattered field alone and reproduce results similar to those from a TFSF source, you need to ensure that all other parameters in the simulation region remain the same. This includes the source size and position, mesh settings, simulation region size, monitor size and position, and boundary conditions.

In your case, I believe the ‘cross’ you observe is diffraction from your structure. By setting Bloch BC, you effectively simulate a 2D lattice of scatterers. Try replicating the same simulation conditions as for the TFSF source, starting with PML boundary conditions, and observe the results.

Best regards,
Kirill