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Journal of the Optical Society of America B

Population transfer in three-level Λ atoms with Doppler-broadened transition lines by a single frequency-chirped short laser pulse

Gagik P. Djotyan, Jozsef S. Bakos, G. Demeter, Zs. Sörlei

Published Jan 1, 2000• 24 citations
Publisher

Abstract

We examine interaction of a single frequency-chirped laser pulse with three-level atoms that have a Λ configuration of levels. We show that it is possible to produce complete fast and robust population transfer of all atoms of the ensemble with Doppler-broadened transition lines from one ground state into the other ground state with negligibly small temporary population of the excited state by controlling the intensity of the laser pulse and the direction and speed of the frequency chirp.

DOI
10.1364/josab.17.000107

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Chinese Physics•Jul 1, 2006•2 citations
We examine interaction of a single frequency-chirped laser pulse with three-level atoms that have a Λ configuration of levels. We show that it is possible to produce complete fast and robust population transfer of all atoms of the ensemble with Doppler-broadened transition lines from one ground state into the other ground state with negligibly small temporary population of the excited state by controlling the intensity of the laser pulse and the direction and speed of the frequency chirp.
We investigate the behavior of 85Rb{}^{85}\mathrm{Rb}85Rb atoms in the field of a sequence of frequency-chirped short laser pulses. The analysis is based on a numerical solution of equations for the probability amplitudes of the hyperfine levels of the ${5S}_{1/2}{\ensuremath{-}5P}_{3/2}$ transition in the 85Rb{}^{85}\mathrm{Rb}85Rb atom and the dressed-states analysis. We analyze different regimes of interaction, including relatively short laser pulses (when the width of the pulse envelope spectrum is of the order of or exceeds the frequency interval between the hyperfine levels resulting in effective mixing of them) and relatively long ones (when the ground hyperfine levels are resolved but the excited ones are not resolved). In the latter case dependence of the population transfer efficiency on the initial coherence of the ground states is analyzed. The case of long laser pulses when all working hyperfine levels are resolved is also discussed using numerical simulations and a dressed-states analysis. We show that in all regimes considered, the interaction of a frequency-chirped laser pulse with the multilevel 85Rb{}^{85}\mathrm{Rb}85Rb system is similar to the interaction with an effective two-level atom at sufficiently large peak intensities of the pulses. It allows us to perform efficient excitation of the multilevel atom by transferring populations of two hyperfine ground states to the excited ones and back to the ground states using a pair of frequency-chirped laser pulses. We propose to utilize this scheme of population transfer for the coherent manipulation of a beam of 85Rb{}^{85}\mathrm{Rb}85Rb atoms using sequences of counterpropagating frequency-chirped short laser pulses.
We present a scheme of population transfer between two metastable (ground) states of the $\ensuremath{\Lambda}$ atom without considerable excitation of the atom using single frequency-chirped laser pulses. The physics of the process is generation of the ``trapped'' superposition of the ground states by the laser pulse at sufficiently high laser peak intensity. The main conditions for realization of this regime are the following: The width of the transform-limited laser pulse envelope frequency spectrum (without chirp) must be smaller and the peak Rabi frequency of the pulse must be larger than the frequency interval between the two ground states of the $\ensuremath{\Lambda}$ atom. During the frequency chirp, the laser pulse must first come into resonance with the transition from the initially occupied ground state to the excited state and after that with the transition between the excited and second initially empty ground states. In the case when the envelope frequency spectrum width (without chirp) of the pulse exceeds the frequency interval between the two ground states, we show a possibility of controllable generation of superposition of the ground states with a controllable excitation of the $\ensuremath{\Lambda}$ atom.
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.
Aim The Stark-chirped rapid adiabatic passage(SCRAP) is studied in a Λ-three-level states system.Methods Level-crossing technique and dressed states theory are used.Results SCRAP uses chiefly an intense far off resonance Stark laser pulse,which modifies the transition frequency and resonates with the Bohr transition frequency for two level crossings.Conclusion A population-transfer maximal efficiency approaching unity is provided from the ground state to the excited state,when the delay of the Stark-shifting pulse is strong at one of these crossings and is weak at second one.
The stark-chirped rapid adiabatic passage (SCRAP) is analyzed, which can give a complete transfer of atomic or molecular population between two bound states in the two level states and Λ(-three)-level state, by means of level-crossing technique and dressed states theory. SCRAP uses two laser pulses: a nearly-resonant pump laser pulse drives the population between a ground state and an excited state, and an intense far-off-resonance Stark laser pulse modifies the transition frequency and resonates with the Bohr transition frequency for two level crossings. A population-transfer maximal efficiency approaching unity is provided from the ground state to the excited state, when the delay of the Stark-shifting pulse with respect to the pulse of the coupling-laser frequency is appropriate and guarantees the pump pulse is strong at one of these crossings and is weak at second one.
By using the time-dependent multilevel approach, we have calculated the coherent population transfer among the quantum states of potassium atom by a single frequency-chirped laser pulse. The results show that the population can be efficiently transferred to a target state and be trapped there by using an `intuitive' or a `counter-intuitive' frequency sweep laser pulse in the case of `narrowband' frequency-chirped laser pulse. It is also found that a pair of sequential `broadband' frequency-chirped laser pulses can efficiently transfer population from one ground state of the Λ atom to the other one.