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Physical Review Letters

Coherent Control of Light Shifts in an Atomic System: Modulation of the Medium Gain

Jean-Christophe Delagnes, M. A. Bouchène

Published Jan 30, 2007• 20 citations
Publisher

Abstract

A sequence of two femtosecond coherent pulses--a strong pi-polarized pulse and a weak sigma-polarized pulse--excite the S1/2-P1/2 transition of atomic rubidium in an optically dense vapor. The sigma pulse induces transitions between the adiabatic states with a coupling strength that is different for identically and oppositely light-shifted coupled states, and that can be modified by tuning the relative phase between the pulses. An efficient control of the medium gain for the sigma pulse is experimentally demonstrated. It is shown to be the result of interference between the absorption and the stimulated emission paths for sigma photons.

DOI
10.1103/physrevlett.98.053602

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A sequence of two femtosecond coherent pulses--a strong pi-polarized pulse and a weak sigma-polarized pulse--excite the S1/2-P1/2 transition of atomic rubidium in an optically dense vapor. The sigma pulse induces transitions between the adiabatic states with a coupling strength that is different for identically and oppositely light-shifted coupled states, and that can be modified by tuning the relative phase between the pulses. An efficient control of the medium gain for the sigma pulse is experimentally demonstrated. It is shown to be the result of interference between the absorption and the stimulated emission paths for sigma photons.
We consider a duplicated two-level system where each two-level subsystem is driven by a strong resonant femtosecond pulse and a weak resonant femtosecond pulse connects cross transitions. Strong interference effects are induced in such a configuration leading to a modulation of the medium gain as a function of the relative phase between the pulses []. We study here the influence of the temporal overlap between the pulses on the dynamics of the system and the medium gain. We show that the dynamics is dominated by two competing phenomena: Light shifts (LS) induced by the driving pulse that prevails for small delays and free induction decay (FID) that prevails for large delays. LS enhance the medium gain for and reduce it for , whereas FID always leads to a decrease of the medium gain.
A sequence of two femtosecond pulses---a strong driving $\ensuremath{\pi}$-polarized pulse and a weak propagating $\ensuremath{\sigma}$-polarized pulse---excites resonantly the ${S}_{1∕2}\ensuremath{\rightarrow}{P}_{1∕2}$ transition of an atomic system. Strong interference effects take place in the system between absorption and emission paths leading to a substantial amplification of the $\ensuremath{\sigma}$ pulse. We study the influence of the fine structure on the medium gain when the contribution of the off-resonant P3∕2{P}_{3∕2}P3∕2​ level is taken into account. A drastic reduction of the medium gain is obtained. This effect is explained within the bright-state--dark-state formalism where the strong driving pulse creates a wave packet that can be trapped in a state---the bright state---leading to a significant reduction of the gain for the $\ensuremath{\sigma}$ pulse. Finally, we also show that periodical gain dependence with the driving pulse energy exhibits a significant change in its period value (compared with expected Rabi oscillations).
The interaction of strong laser fields with matter causes light-shifts that play an important role in many fields as quantum optics, molecular physics and chemistry. Here the control results from the interference between two excitation quantum paths, corresponding to the absorption and to the emission by the dressed atomic system of one photon from/on the weak field mode respectively. This effect paves the way to many applications and we will describe a new method to slow the light in an atomic medium.
Coherent excitation of an atomic excited state occurs during the propagation of near-resonant light pulses and is responsible for the induced polarization. Simultaneously, incoherent excitation occurs due to the relaxation processes described by the absorption coefficient. Here, the theory for the coherent and incoherent excitation is initially presented in terms of the traditional vector model. While a complete understanding of the two-level system is provided by the vector model, it is shown to be incomplete when the problem of directly monitoring the coherent and incoherent excitation is considered. This is because this latter problem involves more than two levels. For this more complicated multilevel problem, adiabatic states are introduced to gain further understanding. The adiabatic states are the stationary states of the atom in the presence of the near-resonant laser field; they help to explain the intimate connection between the coherent excitation and the two-photon resonance. Experimental measurements of the coherent and incoherent excitation associated with near-resonant pulse propagation in Rb vapor are presented. The double-resonance technique used a relatively strong pulsed dye laser tuned near the $5{S}_{\frac{1}{2}}\ensuremath{\leftrightarrow}5{P}_{\frac{1}{2}}$ transition (7948 \AA{}A) of Rb to produce the coherent and incoherent excitation, and a weak, tunable cw dye laser tuned in the region of the $5{P}_{\frac{1}{2}}\ensuremath{\leftrightarrow}6{D}_{\frac{3}{2}}$ transition (6206 \AA{}A) to monitor this excitation. In agreement with theory, the experimental results demonstrate that coherent excitation is responsible for two-photon absorption, while the incoherent excitation corresponds to one-photon absorption to the 5P125{P}_{\frac{1}{2}}5P21​​ state.
We consider a double two-level system in which each single two-level system is driven by a strong pulse. The light shifts induced by the strong field are probed by a weak pulse that connects the two sub-system in a crossed manner connecting the ground state of one system with the excited state of the other and vice versa. We are interested in coherent control of medium gain for the weak pulse and we demonstrate the conditions on laser and atomic parameters that render this control possible. Both ultra-short and long pulse regimes are discussed