Learning Forum

[Debasish Biswasray]

Hi there!!

I want to calculate the electric field profile for two metallic nanostructures separated by a range of gaps. First, I have calculated optical extinction (absorption + scattering) cross-section spectra for all the separations. For each separation, one peak in visible regime was found, among which the peak corresponsing to the lowest separation ocurred at a maximum wavelength of 730 nm (largest redshift).

But when I calculated the electric field profile with a frequency-domain power field monitor, I found that the electric field enhancement is highest for a slightly higher separation. However, I was expecting that the electric field enhancement should also be maximum at the same separation where the extinction peak had shown maximum redshift. I wonder why this happened, even though the setup in both the simulations was the same, including mesh size.

Can you let me know what I can do to solve this problem?

Thank you in advance!!

[Amrita Pati]

Hi Debasish,

Are you looking at the extinction cross-section and the electric field intensity from the same run (of each separation)? I would expect the field intensity at the smallest separation to be maximum. When you say that a slightly higher separation has the highest intensity, does it mean that you see a lower intensity for the smaller gap?

Regards,
Amrita

[Debasish Biswasray]

Hi Amrita,

Many thanks for you reply.

Yes, I am simulating both the extinction cross-section and the electric field intensity of the system in a single run for each separation.

I too expected the maximum intensity to be at the smallest separation, but the it is obtained at slightly higher separation, while the smaller separations showed decreased intensities. I wonder why that happened. Is there any technical aspects related to the software that might have affected the electric field simulation?

I should also mention here that the mesh size is large enough to separate both the nanostructures in the system.

I hope, I have answered your questions. Kindly help understand the issue.

[Amrita Pati]

Hi Debasish,

Although the electric field intensities can reduce in sub-nanometer gaps due to quantum effects, FDTD doesn't simulate that behavior. I think we should perform convergence testing to make sure that the simulation results are accurate. This is especially important in plasmonic applications where the fields change very rapidly. The mesh size in particular can have significant effects due to this reason. You can consider the smallest gap and run the simulation with increasingly finer mesh until the field intenstites converge. 

Typically, we require finer mesh for smaller features, so it is quite possible that with the current mesh size, we are not capturing enough information in the smaller gap. So, we can not be sure until we do convergence testing. 

Regards,
Amrita

[Debasish Biswasray]

Thank you for your sugesstion. I will try and let you know the result.