# discripencies between analytical simulations and lumerical

#1

Hello!
I’m writing to you to know the origin of the discrepancies I’m having between matlab simulation using Green’s function and FDTD lumerical solution. I’m trying to compare the absorption cross section. I have using Lumerical (sigma_abs=3.4e4 nm2) while matlab (sigma_abs=1.5e4 nm2). Here is my FTDT simulation and then I get sigma_abs using:

sigma_abs=Pabs_tot*sp/out;
where Pabs_tot is the total power absorbed from a Pabs_adv monitor, sp=sourcepower, out=sourceintensity.

Is there anyway to improve the accuracy? test.fsp (276.9 KB)
Thank you in advance

#2

Dear @abeer.al-mohtar

Absorption cross section (sigma) is defined as:
sigma = transmission(f) * sourcepower(f) / sourceintensity(f);

In your simulation file, Pabs-tot is the total absorbed power within the analysis box and is normalized to source power. Thus, this quantity can not be used as transmission.

To calculate transmission you can use script command transmission("monitor_name") (and then sum over all monitors). Alternatively you can use absorption cross section analysis group as is used in this direct.

Thanks

#3

Thanks for your prompt answer. There are a few things that I didn’t understand:

1. We can’t calculate the absorption cross section from the power absorbed?
2. Can we use the analysis group given in the example you provided for an inhomogeneous medium (particle over a substrate)? The unit of the sigma it calculates is in m2 right?
Thanks again,
Abeer

#4
1. Absorbed power calculated from electric field intensity and imaginary part of permittivity should be equal to net transmitted power through the analysis box. The only concern is that this value is already normalized in your simulation file and then is multiplied by input power. With some care on how we defined Pabstot you should be able to use it as well.

2. I am not seeing a limitations of using it on inhomegenous material as in your case. It calculates the net transmission on each monitor and sums over all the monitors.

Yes, Transmission is unitless as is normalized to source power, sourcepower has units of Watts if CW norm is selected, and sourceintensity is Watts/m^2 in CW norm. So, the unit is m^2.

Please let me know if you are still getting different results from what you expect.
Thanks