Gain-dispersion coupling induced by transient light shifts in an atomic medium

A weak resonant femtosecond pulse propagates through an optically thick atomic medium whose properties have been modified by a strong nonresonant ultrashort pulse. We study the energy redistribution between the spectral components of the weak resonant pulse using both experimental and theoretical methods. The transmitted spectrum exhibits an asymmetric feature that reveals an important gain-dispersion coupling. For light shifts that perturb slightly the propagation of the weak resonant pulse, this asymmetry maps out the asymptotic part of the linear susceptibility.

Figure 1

(a) Excitation scheme of the two-level system (transition $5s^2{S}_{1∕2}\to 5p^2{P}_{1∕2}$ in the rubidium atom) by the bichromatic field (issue experimentally from a single femtosecond pulse). The strong nonresonant ultrashort pulse modifies the optical response of the system that at the same time probes the weak resonant ultrashort pulse. (b) Adiabatic representation of the two-level system. In our case, the propagating pulse preferentially connects levels (1) and (4). (c) Experimental setup.

Figure 2

Spectrum intensity of the two-pulse sequence. (a) Experiment. (b) Numerical simulations. In black is the spectrum at the entrance of the medium and in gray is the transmitted spectrum. See text for parameters values.

Figure 3

Differential spectrum $S_p(1,W)-S_p(0,W)$ between the exit and the entrance of the sample in the region of the propagating pulse. (a) Experiment. (b) Numerical simulations. A dispersionlike behavior is observed.