I did not find the method you used to calculate the absorption. It can be one of the following:
1: use A=1-R-T where R and T are the reflection and transmission coefficient
2: using a spatial 3D monitor to calculate the absorbed power density and integrate it, which is used in solar and opto-electric devices.
I would strongly suggest to use the 1st method and do convergence testing. Please check the material fit also. You can get the fit data by getfdtdindex - Script command.
In addition, please use stackrt to verify the result with experiment: stackrt - Script command
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