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Physical Review A

Propagation of frequency-chirped laser pulses in a medium of atoms with a Λ -level scheme

G. Demeter, David Dzsotjan, Gagik P. Djotyan

Published Aug 30, 2007• 8 citations
Publisher

Abstract

We study the propagation of frequency-chirped laser pulses in optically thick media. We consider a medium of atoms with a $\ensuremath{\Lambda}$ level-scheme (Lambda atoms) and also, for comparison, a medium of two-level atoms. Frequency-chirped laser pulses that induce adiabatic population transfer between the atomic levels are considered. They induce transitions between the two lower (metastable) levels of the $\ensuremath{\Lambda}$-atoms and between the ground and excited states of the two-level atoms. We show that associated with this adiabatic population transfer in $\ensuremath{\Lambda}$-atoms, there is a regime of enhanced transparency of the medium---the pulses are distorted much less than in the medium of two-level atoms and retain their ability to transfer the atomic population much longer during propagation.

DOI
10.1103/physreva.76.023827

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European Quantum Electronics Conference (EQEC)•Jan 18, 2006•0 citations
We study the propagation of frequency-chirped laser pulses in optically thick media. We consider a medium of atoms with a \ensuremathΛ\ensuremath{\Lambda}\ensuremathΛ level-scheme (Lambda atoms) and also, for comparison, a medium of two-level atoms. Frequency-chirped laser pulses that induce adiabatic population transfer between the atomic levels are considered. They induce transitions between the two lower (metastable) levels of the \ensuremathΛ\ensuremath{\Lambda}\ensuremathΛ-atoms and between the ground and excited states of the two-level atoms. We show that associated with this adiabatic population transfer in \ensuremathΛ\ensuremath{\Lambda}\ensuremathΛ-atoms, there is a regime of enhanced transparency of the medium---the pulses are distorted much less than in the medium of two-level atoms and retain their ability to transfer the atomic population much longer during propagation.
In the present paper, we examine the propagation of frequency-chirped laser pulses through a medium of Lambda atoms, pulses that are capable of producing adiabatic population transfer between the stable states. We compare the propagation of these pulses to their propagation in a medium of two-level atoms, and show that the distortion of the pulses is greatly reduced in the three-level case. In effect, there exists a transparency phenomenon for frequency-chirped laser pulses propagating in the three-level medium that is somewhat similar to electromagnetically induced transparency. It is concluded that the population transfer induced by chirped pulses does populate the excited state during the process to a slight extent, even if the transfer is perfectly adiabatic. Thus the transparency of the medium is also imperfect, the pulse is eventually distorted and its capability to produce the population transfer is lost, even in the perfectly adiabatic limit.
A quasi-transparent propagation phenomenon is presented for a pair of frequency-modulated laser pulses in an optically thick medium of cold atomic gas. The noninteracting, identical lambda-structured atoms are driven by two classical laser fields, which are frequency-chirped in the same way around the corresponding atomic transition frequencies, maintaining two-photon (Raman) resonance. It is shown by numerical analysis that after propagating over a relatively short distance (determined by the absorption length), the frequency-chirped pulse pair is affected by the atoms in such a way that instead of exciting the atoms (as it would happen in an optically dilute medium), they create a certain coherent superposition of the ground states (which can be varied by the parameters of the incoming fields), and they propagate in the remainder of the medium without significant further losses. This quasi-lossless propagation effect of the Raman-resonant frequency-chirped pulses, described above, is not only interesting in the point of view of the laser fields, but the on-demand creation of coherent superpositions among atomic states along the optically thick medium may also find applications in quantum optical experiments and quantum informatics.
We investigate the behavior of a three-level atomic system with \ensuremath{\Lambda} structure of levels in the field of a sequence of short frequency-chirped bichromatic laser pulses (BLP's) in the adiabatic passage regime of interaction. We show that the efficiency of the population transfer in this atomic system is sensitive to the relative phase of the pulses forming the BLP when the atom is prepared in a coherent superposition of its two ground states initially. We propose to use this sensitivity for coherent fast and robust writing and reading of optical phase information. The initial preparation of the atom in the ``dark'' superposition state is suggested through the action of a sequence of a few frequency-chirped BLP's with duration longer than the atomic relaxation time.
Spatio-temporal dynamics of two short laser pulses propagating in an absorbing medium, which consists of one- and two-photon resonant (Lambda) -atoms, is investigated in the adiabatic approximation. We give an analytical solution and compare it with numerical simulations. It is shown that pulses may propagate in a medium, whose thickness is considerably greater than the length of the linear absorption of a single weak probe wave. Finally, full transfer of the probe pulse energy into the coupling pulse takes place. A vector model of the adiabatic interaction of two pulses with three-level atom is proposed.
This paper shows that a single laser pulse with a chirped frequency can generate trapped superposition of the ground states in the /spl Lambda/-atom when the width of the transform-limited laser pulse envelope frequency spectrum (without chirp) is smaller but the peak Rabi frequency of the pulse is larger than frequency interval between the two ground states of the /spl Lambda/-atom. This study also analyzes the propagation of the frequency-chirped laser pulse in a medium consisting of /spl Lambda/-atoms analytically and by exact numerical solution of coupled Bloch-Maxwell equations. The results show that there is nearly absorptionless propagation of the single frequency-chirped laser pulse on distances more than 40 times the pulse duration, when the formulated above conditions of negligible excitation of the /spl Lambda/-atom are fulfilled. The described effect of absorptionless propagation of the single frequency-chirped pulse is referred to as electromagnetically self-induced transparency.