# negative reflectance???

#1

Dear all,
Im simulating a thin layer of 2 nm of a material (see index of refraction in picture 1; I cannot show the material name, sorry ) with the configuration of transmission (down) and reflection (up) monitors depicted in pic 2. the source is TFSF and the boundaries are PML. Substrate is glass (n=1.52). The results shows a negative transmission, but also a negative reflectance (pic 3). I need to calculate the extinction as Ext=1—(R+T), so my question is how to interpret this results and if this is the best way to calculate R and T. I know that T must be represented as (-Re), but R must be positive as it is represented in pic 2, isn’t it? I cannot use stackRT because I will add scatterer objects next in my project.
Cheers,

#2

Hi,

Looks like your reflectance monitor is in front of the source. Hence, the monitor will be measuring the light initially coming from the source and then the light that is reflected from the surface. Ideally place the reflectance monitor behind the source.

Also, check the direction of the source, if remember that backward direction sources lead to a negative value in transmission. You can verify that.

Best Wishes,

Vivek

#3

Dear vivek,
thanks a lot for your answer. This pic corresponds to R and T from monitor outside the source (note I plot Re and -Re for each case). It corresponds to noise, so there is no actual light coming out of the box. I know the R is cancelled at the source face of the box, this is why I have to put inside the box (after the source), in order to get the reflectance signal, probably mixed with the input light. I realize that add +1 to the signal that the monitor gave me I get the proper Reflectance.

Another example is a Au disc (diameter=100; h=30nm) (see picture). I put monitors inside and outside the TFSF source box. Here are the results for T and R.

(Note the values less than -1, inside the red circle).
In this case, the monitors outside gave me the correct results. So my question is why it doesn’t happen the same with a thin layer, where I have to put the monitors inside the box? It is because the TFSF source only allows me to measure the scattering, that is bigger for the disk than from the film? What I really want to measure is T and R of this combined system (disc+layer), so which is the place to put the monitors, inside or outside the source box? Maybe I have to use another source?
I hope it is clear, so please do not hesitate to ask me any questions.
Thank you very much.

#4

Dear @pedrueza

Since I haven’t seen your simulation file, it will be hard to tell you what is causing the problem, maybe related to source intensity:
https://kb.lumerical.com/en/ref_sim_obj_tfsf_cross_section_and_normalization.html

https://kb.lumerical.com/en/ref_sim_obj_tfsf_power_normalization.html