Learning Forum

[naarinfatehi_1995]

hello, I proposed a photonic crystal solar cell ; then i simulated that in FDTD module. but the absorption in some special wavelengths was more than 1 !!

what's the problem?

[Guilin Sun]

Most of the case this happens is due to not long enough simulation time, or too large the autoshutoff min level. Please set sufficient long simulation time, then using check point feature to reduce autoshutoff min and see if the result improves. When the device has strong resonance, it needs long simulation time to get accurate result.
Of course, due to simulation error, small amount of larger than 1 is acceptable.
Check point information:
https://support.lumerical.com/hc/en-us/articles/360034382534-Simulation%20-%20FDTD
https://support.lumerical.com/hc/en-us/articles/360036896474-resume-Script-command

[naarinfatehi_1995]

thanks a lot,
i increased the simulation time as you said(from 1000 to 1500) but instead of reducing the autoshutoff min, i incresed it (from 1e-05 to 0.001) . and finally i got the satisfactory result! you think does it have any problem???( i mean maybe it happened by chance!)

[Guilin Sun]

Sorry, it is by chance! The autoshutoff min should not be increased except you have a strong reason. It is used to evaluate the residual energy left inside the simulation volume. When it is small, we can in general say that the frequency-domain result is accurate, since we get the cw result by Fourier transforming the signal.
If the absorption is significantly larger than 1, say 1.2, then most likely it is due to shorter simulation time. Please follow the previous suggestions, in particular the simulation time can be significantly increased and use the autoshutoff min (and checkpoint) to terminate the result.
Please refer
Simulation time and frequency domain monitors
Transmission efficiency greater than 1?