Influence of coherent adiabatic excitation on femtosecond transient signals

TL;DR

This paper investigates how coherent adiabatic excitation influences femtosecond transient signals in pump-probe experiments, providing a model that explains the transition between coherent and incoherent descriptions based on state distributions.

Contribution

It introduces a model treating the system as independent two-level subsystems to explain the coherence effects in femtosecond signals and challenges the conventional focus on state density.

Findings

Coherent and incoherent treatments converge under certain conditions.

State distribution outside the laser bandwidth determines the validity of incoherent models.

Analytical and numerical explanations of the transition between coherence regimes.

Abstract

The transient signals derived from femtosecond pump-probe experiments are analyzed in terms of the coherent evolution of the energy levels perturbed by the excitation pulse. The model system is treated as the sum of independent two-level subsystems that evolve adiabatically or are permanently excited, depending on the detuning from the central wavelength of the excitation laser. This approach will allow us to explain numerically and analytically the convergence between the coherent and incoherent (rate equations) treatments for complex multi-level systems. It will be also shown that the parameter that determines the validity of the incoherent treatment is the distribution of states outside and inside the laser bandwidth, rather than the density of states as it is commonly accepted.