what is the optical gap?

[waynebeibei]

Hi all

I am confused about the definition of optical gaps after calculating the absorption spectrum using BSE: below is three possible statements:

1) the position of the first excitonic peak( although it may be a dark exciton)
2) the position of the first bright excitonic peak
3) similar to the experimental definition, it relates to the fundamental absorption edge. We need to plot the relation between the absorption coefficient and photon energy following Urbach rule, i.e. ahv =(Eg-hv)~2. By extrapolating the linear portion of the curve to zero absorbance, we can get Eg, which is considered as the optical gap.

Which one is correct?

[Daniele Varsano]

Dear Waynebeibei,
that's question of definition as usual. I would define it as the onset of the absorption spectrum. In presence of bright bound excitons, I would mean the energy of the first bright excitons, that turns to be lower than the electronic gap (measured by photoemission). Note that if in you calculation you do not consider temperature effects, vibrionic effects (phonons), the results of your solution are discrete peaks, broadened by an artificial damping, so the the extrapolation you mention in point 3 would be fictitious.

Best,
Daniele