# Obtain absorption in percentage

#1

Hi,
I’m trying to get absorption in this structure. As you know, Lumerical FDTD calculate absorption and scattering curves in Cross-section vs wavelength, but I want to obtain absorption in percentage.
In other words I want to know What percentage of incoming light in 675 nm (the peak of absorption), is absorbed by my structure (pentagram with graphene) and what percentage is scattered?

101.fsp (291.5 KB)

Best regards.

#2

Hi,

It will totally depend on the illumination area of the beam. Cross-section has a unit of m2. When you divide the cross-section by the illumination area you will get the fractions or percentages you want.

Best Wishes,

Vivek

#3

You can also use the “Power Absorbed” analysis groups from the “Object Library”. They will give you the power absorbed per unit volume but also the total power absorbed, normalized to the source power (that is what you are looking for).

The methodology is described at: https://kb.lumerical.com/en/index.html?layout_analysis_pabs.html.

#4

Hi,
Dear @vivek and @gbaethge, thank you for reply.
I read and practice examples of https://kb.lumerical.com/en/index.html?layout_analysis_pabs.html, but I was confused. unfortunately the description was unclear.
I change my simulation like that, but I don’t know how can I obtain absorption in percentage unit yet!
I send my modified file, please guide me how can I solve my problem?
102.fsp (292.4 KB)

best regards.

#5

Hi @nahal, there’s actually different ways to calculate the absorption, depending on what you are interested in. The Pabs analysis groups will give you the absorption in the considered volume. The downside is the amount of data generated, as we store the field in a 3D monitor. So it will be better to reduce the size of the group to the part of the structure you want to get the absorption.

A simpler way, if you would like to get the absorption in the Si, for instance, would be to use 2D frequency monitors, place them at the start and at the end of the layer and calculate the power transmitted through them. Then the absorption between the 2 is just the difference of the transmissions. This will work well for a periodic structure (with a plane wave as a source), or if the monitors are large enough to capture all the fields.

Finally, in your case, I’m not too sure how to deal with the graphene layer, as it is a 2D element. Maybe other users will have some ideas!

#6

Thanks for your reply. As you suggest, I reduce the size of Pabs group to the part of the pentamer.This will be reduce the memory requirement.
As I said earlier, I want to calculate the absorption of pentamer and graphene (Pabs region) into source in percentage amount.
I utilized both of your suggestion ways to obtain absorption. because I want to know Which method gives the right result.

I replace graphene 3D instead of 2D type. I hope this change help us to obtain result.
So please rectify it to solve my problem. because I don’t know my result is true or not!

.103.fsp (286.1 KB)