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[Clovis Darrigan <clovis.darrigan@univ-pau.fr>]

Dear Wu,

Perhaps can I give you a little indication?

> The nonlinear optics calculation has been implemented in the abinit code. For semiconductors the second harmonic generation coefficient varies greatly around their respective energy band gaps. But for abinit, it gives out a single value, I wonder to know at what frequency range, the second harmonic generation coefficient, or chi(2) is, or all most, is a constant?

I have not seen the formula of Sum Over States method in Abinit, but normally, when you use the SOS, it's possible to plot all (non)linear properties versus the frequencies of perturbing fields.

For linear effect, there is a great variation arround transitions energies, of course when transitions momentum are not null. Generally, the first possible transition is a direct gap transition (but not necessary!) so a great variation can occurs at energy band gap.

For nonlinear effects, where two or more photons are diffused, you can have great variations (i.e. resonant terms) for omega, omega/2, omega/3, etc, where omega is a possible transition frequency (or energy).
SHG effect have a resonant term at omega and omega/2, so great variations can be observed, not only at energy band gap, but also before this energy. So consider that SHG effect can be constant between zero frequency and just before omega/2 frequency.

I hope this will help you.

Clovis Darrigan

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