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!