Intensity and linear response

[wei wang]

Dear developers,
I've recently learned GW-BSE simulations and have some questions while studying the Real-time & Nonlinear Response tutorial as a beginner in real-time methodologies:

I noticed that the Field1_Int parameter corresponds to very large values (1000 kW/cm² or even 10,000 kW/cm²) in all calculations. What does this parameter represent (field strength or power density)? Isn't this value too large for a linear response calculation? Or perhaps I misunderstood, as the tutorial states:
*"The standard input of Lumen is designed for non-linear response, so we need to modify parameters to calculate linear response. Set the field direction along y-axis, field type to DELTA, simulation duration to 55 fs, number of bands from 3 to 6, dephasing to zero, and energy steps to one (highlighted in red above)."*

I don’t fully grasp why adjusting these parameters converts a non-linear response simulation into a linear one. Could you clarify the reasoning behind these specific settings? And,where can I learn about the meanings of these parameters in rt-simulations?
Thanks!

Best Regards,

wwang

[myrta gruning]

I suppose you are looking at the tutorial:
https://wiki.yambo-code.eu/wiki/index.p ... erry_Phase

If you need to do just linear response, it is more efficient to use the the linear response or BSE parts of Yambo. This real-time part is mostly intended for nonlinear response. This tutorial has more an educational purpose.

Regarding the strengths and units.

Field1_Int is the intensity (you can see this in the generated input)

Field1_Int= 1000.00 kWLm2 # [RT Field1] Intensity

It may seem large but optical nonlinear effect are very small and even with such intensity the response is linear.
There is no conversion from nonlinear to linear. The real-time simulates an experiment: You shine a laser, you measure the output polarization. This is the total polarization of the system carrying the linear and nonlinear components. The linear part is 6-7 order of magnitude larger than the largest nonlinear component, 12-13 than the second-largest, so it is the dominant part. In fact, extracting the nonlinear response is a nontrivial task.

Hope this helps

[wei wang]

Dear myrta,
Thanks again for your help earlier!

I saw the "Non-equilibrium absorption in bulk silicon (under construction)" tutorial and want to use its feature to calculate non-equilibrium absorption. But since it’s labeled "under construction," is it already working? If yes, I have one quick question:

In the tutorial, the electron-hole chemical potential settings — do they directly represent the pump excitation energy from the referenced paper?("Non equilibrium optical properties in semiconductors from first–principles: a combined theoretical and experimental study of bulk silicon", Phis. Rev. B 93, 195205 (2016)) Basically, can I simulate different pump conditions (like changing pump energy) by adjusting this parameter?
% RTmuEh
-0.223 | 0.10 | eV # [RT] Chemical potentials hole | electron
%