How to understand the negative two-photon transition strength

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Discussion Overview

The discussion centers on the calculation of negative two-photon transition strengths (DeltaTPA) using the CCSD response approach, specifically addressing the implications of negative TPA absorption coefficients (beta) and their potential relation to saturable absorption processes.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Lan reports calculating DeltaTPA for ten excited states, finding mostly positive values but some negative ones, raising questions about the implications of negative beta.
  • Lan inquires whether negative beta can be related to saturable absorption processes and seeks references or comments from others.
  • One participant suggests that negative absorption coefficients may indicate a gain situation, typically associated with stimulated emission and inverted populations, but does not confirm or deny Lan's specific findings.

Areas of Agreement / Disagreement

Participants have not reached a consensus, as there are differing views on the interpretation of negative absorption coefficients and their implications.

Contextual Notes

The discussion lacks detailed definitions of terms such as "saturable absorption" and "inverted population," and the assumptions underlying the calculations of DeltaTPA are not fully explored.

sandf
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Dear all,
I have calculated the rotationally averaged two-photon transition
strength (DeltaTPA) using the CCSD response approach. Degenerate TPAs of ten excited states have been calculated. For most of the excited states considered, I got the positive DeltaTPAs, while for some excited states, I got the negative DeltaTPAs. The negative DeltaTPA will lead to the negative TPA cross section, then to the negative TPA absorption coefficience (beta). My question is:
whether the negative beta can be related to the saturable absorption process.
Any references and comments are appreciated.
Best regards.
Lan
 
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I'm sorry you are not generating any responses at the moment. Is there any additional information you can share with us? Any new findings?
 
Greg Bernhardt said:
I'm sorry you are not generating any responses at the moment. Is there any additional information you can share with us? Any new findings?

Dear Bernhardt,
Thanks for your attention. I also ask for help on the forum of DALTON program
which implements the CCSD response calculation for TPA. Unfortunately, I haven't gotten the response.

Bests
Lan
 
I'm not familiar with with the specific technique you have used but I can give a more general comment.

Generally, when one is talking about negative absorption coefficients, what is meant is that you have a gain situation, i.e. stimulated emission, most often coming from some form of inverted population.
 
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