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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Propagation of frequency-chirped laser pulses in a medium of A-atoms

G. Demeter, David Dzsotjan, Gagik P. Djotyan

Published Jun 1, 2007• 0 citations
Publisher

Abstract

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.

DOI
10.1109/cleoe-iqec.2007.4386857

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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.
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.
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.
The spatial and temporal dynamics of two short pulses propagating in an optically dense medium of resonant three-level Λ-atoms is investigated numerically and analytically. The maximum coherence for the Raman transition due to coherent population trapping. It is shown that, at the initial stage of propagation, the waveforms of such pulses only slightly change along the length of the medium, which may considerably exceed the length of linear absorption for a single weak pulse. As the length of the absorbing medium increases, the energy of the probe (first) pulse is completely transferred into the second (control) pulse.
Adiabatic passage induced by chirped laser pulses in three-level $\ensuremath{\Lambda}$ systems is investigated. Two different strategies for efficient population transfer (equal and opposite sign of the chirp) are analyzed. The role of detuning, crucial for the attempt to reduce the population of the intermediate state, is demonstrated. The scheme of adiabatic passage by laser pulses with chirps of different sign is not symmetric with respect to the sign of the one-photon detuning. Blue detuning results in a smaller transient population of the intermediate state than does red detuning. The results are explained on the basis of dressed states. Numerical results for a fully off-resonant adiabatic passage scheme, based on two pulses with the same sign of the chirps, are also considered. The robustness of the schemes in terms of the Rabi frequency, chirp rate, detuning, and time delay of the pulses is explored.
The spatial and temporal dynamics of two short laser pulses propagating in absorbing three-level medium under conditions of induced transparency is investigated in adiabatic approximation. We analyze two cases of electro-magnetically induced transparency creating--coherent population trapping and adiabatic population transfer. It is shown that in both cases the probe pulse can penetrate into a medium at a distance considerably exceeding the length of linear absorption of a single weak probe pulse in absence of a coupling pulse at adjacent transition. The difference of spatial and temporal evolution of level population in processes of coherent population trapping and adiabatic population transfer is although demonstrated.