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The Journal of Atomic and Molecular Sciences

Phase Control of the Temporal Envelope of an Ultrashort Pulse Propagating in a Strongly Driven Atomic Medium

M. A. Bouchène, Jean-Christophe Delagnes

Published Feb 1, 2011• 0 citations
Publisher

Abstract

We consider a double two-level system in which each single two-level system is driven by a strong linearly polarized pulse. The system is probed by a weak pulse orthogonally polarized. We study the distortion experienced by the probe temporal envelope when the relative phase between the pump and the probe is varied. We show that pulse behavior oscillates between two regimes that correspond to transparency and amplification respectively. Interpretation of this effect is given within adiabatic representation.

DOI
10.4208/jams.071611.073011a

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Physical Review A•Nov 30, 2006•7 citations
We consider a double two-level system in which each single two-level system is driven by a strong linearly polarized pulse. The system is probed by a weak pulse orthogonally polarized. We study the distortion experienced by the probe temporal envelope when the relative phase between the pump and the probe is varied. We show that pulse behavior oscillates between two regimes that correspond to transparency and amplification respectively. Interpretation of this effect is given within adiabatic representation.
This thesis deals with the study of propagation effects experienced by weak light pulses as they propagate in atomic media driven by strong pulses. We explore both the ultra-short and long pulse regime and investigate the phenomena that arise at these different time scales. In the short pulse regime, a strongly driven two level atomic system presents transient light shifts, and non-adiabatic transitions occur between these adiabatic levels. We have studied a method to probe these light shifts in real time by propagating a weak probe through the medium. The light shifts enrich the spectrum of the probe and the probe gets shaped as a result. In this way a strongly driven two-level system can act as an active pulse shaper, and can introduce oscillations in the temporal profile of an ultra-short pulse at a time scale shorter than the pulse duration. We also show that by driving the system with two time delayed non-resonant strong fields, the non-adiabatic effects can be rendered phase dependent. This gives very sensitive phase control of the excited state population and can be used to formulate new techniques in interferometry. In the long pulse regime we present a new method of slowing light that can be realized in a double two-level system interacting with two orthogonally polarized light pulses that propagate along different axis. Spatio-temporal dependence of the total polarization induces a grating in the ground Zeeman coherence. The stronger of the two fields (the control field) is diffracted from this grating into the direction of the weak probe field compensating for the absorption of this latter field. A transparency window is thus created in the absorption spectrum of the probe leading to the slowing down of light. The transparency window exhibits characteristics similar to the one obtained by EIT (electromagnetic induced transparency) method. However, the important difference between our method and the traditional EIT method, is that ours doesn't rely on realizing dark state in the system. This may open the possibility of slowing down light in more complex atomic media. Moreover, when the linear absorptive response of the medium is cancelled in this manner, the nonlinear response becomes more important. In the situation where fields propagate in the same direction and have same frequency, two regimes have been investigated. For small optical depths, the effective susceptibility behaves as chi_lin exp[2 i phi] (with phi the phase difference between two fields). This renders phase control of the medium response, and the medium can be changed from an absorber to an amplifier, with normal or anomalous dispersion, by adjusting the relative phase phi.  In the regime of large optical thickness, phase saturation takes place and the effective susceptibility turns into chi_lin*, changing an absorber into an amplifier without effecting the dispersive response (chi_lin is linear susceptibility).
In this paper, I numerically investigate the propagation of ultrashort pulses through an extended collection of multilevel systems under the condition of electromagnetically induced transparency. The transparency of a weak probe pulse is induced by a much stronger coupling pulse. In the limit of a weak probe excitation and a large number of excited states, I show that the free-induction decay signal that would naturally follow the excitation pulse is strongly suppressed, and the envelope of the probe pulse suffers a distortion in its temporal profile as the pulse propagates into the medium. This distortion is characterized by a stronger absorption of the leading edge of the pulse than that of its trailing edge. The temporal phase of the pulse, however, remains constant throughout propagation.
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
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.
We present the theory of a method to slow a linearly polarized probe pulse as it propagates through a duplicated two-level system driven by an orthogonally polarized control field. The method makes use of Zeeman coherence oscillations that arise in the atomic system because of the spatial and temporal modulation of the total polarization. This method exhibits properties similar to those that rely on electromagnetically induced transparency but without the existence of any trapping dark state. We demonstrate also the propagation of a polariton in the medium.
An ultrashort strong nonresonant pulse induces transient phenomena in an optically thick medium consisting of an ensemble of nondegenerate two-level atoms (nonadiabatic transitions, transient light-shifts,\dots{}). A weak resonant pulse propagates in the system and we study theoretically and numerically the temporal and spectral modifications that are induced. Moreover, we show that the atomic media, modified by a strong pulse can act as a ``pulse-shaper'' device that can modify, in a controlled way, the shape of the weak pulse. It maps out the transient light shift induced by the strong pulse and reduces efficiently the long time dispersion tail of the weak pulse. We study and discuss the physical phenomena that allow such shaping effects and the necessary conditions to satisfy in order to observe them.