Absorption of light by a TiN nanocube


I’m PhD student and is currently using FDTD to solve for a light absorption problem. The geometry of simulation looks like this: a gauss source (633nm) falls on a TiN cube. I used the analysis group Pabs_adv to determine the power absorbed by cube. The cube lies on a silicon substrate. Area of source is 8 um* 8 um. The beam is focused on the edge of the cube for better excitation of plasmons. I want to use focused lens (N.A. 0.7) and for that I have to use “thin lens”. Here you write that for this you need to enable the function “fill lens”: Boundary conditions for focusing source
And I have a question: Why, when I enable “fill lens” , the absorption becomes more than without it? But with “fill lens” when this is observed diffractionIs. It possible to do without “fill lens”? And can I use “scalar approximation”, but chose waist radius which matches NA 0.7 ?

I have attached the FDTD file with my simulation.

Thanks in advance!

TiN 100nm NA0,7 2.fsp (290.3 KB)

Hello @ElAChernyh,

Sorry for the late reply.

The beams with and without the fill lens option are very different. Here is the field with “fill lens”:


And here is the beam without it:


The beam without the fill lens option has a much larger radius, which is why the results are different. Without the “fill lens” option, most of the light will not be near the cube, so the normalized absorption will be lower.

If you turn this option off you can set a beam radius that is less than the lens radius, so the input beam will not be clipped. Whether or not you use this option depends on what type of beam you want to create, or what type of experimental setup you are trying to duplicate. There is more information about these settings on this page:

You can use the scalar approximation if the beam waist is much larger than the wavelength. If the waist radius is on the order of the wavelength, then the “use thin lens” option is best. In this case, because the beam waist appears to be fairly small, I would recommend you use the thin lens option.

I hope this helps. Let me know if you have any questions.