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AIP conference proceedings

Effects of interference among two-photon coherences on the population dynamics of a Four-level Λ system

A. S. Choe, Yongjoo Rhee, Jongmin Lee

Published Jan 1, 1997• 0 citations
Publisher

Abstract

We show that optimal detuning method describing population transfer passage in a three‐level system has an essential difference from the adiabatic following passage by counterintuititive sequence of two coherent laser pulses, by examining the dressed eigenstates and the Bloch vector precessing around a torque vector. In a four‐level Λ system, population dynamics is studied for the case where, on the optimal detuning condition, two‐photon coherences causing adiabtic transfer interfere each other. For a scheme of three degenerate levels, we show a possibility of complete population inversion by the optimal detuning method.

DOI
10.1063/1.52224

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We show that optimal detuning method describing population transfer passage in a three‐level system has an essential difference from the adiabatic following passage by counterintuititive sequence of two coherent laser pulses, by examining the dressed eigenstates and the Bloch vector precessing around a torque vector. In a four‐level Λ system, population dynamics is studied for the case where, on the optimal detuning condition, two‐photon coherences causing adiabtic transfer interfere each other. For a scheme of three degenerate levels, we show a possibility of complete population inversion by the optimal detuning method.
For the purpose of an examination of coherence control, we study the dynamics of population inversion in the four-level λ system where initial populations are arbitrary coherently distributed in two lower levels. It is shown that the coherence control, which causes population transfer between the lower levels to be forbidden, is possible by using the ratio of Rabi frequencies. Under this condition, it is remarkable that the adiabatic following and the optimal detuning methods are useful to invert the four-level system completely, and it is shown that the two-photon coherences of the coupled three-level ladders are dispersive and absorptive in the former and the latter methods, respectively. From the geometrical interpretation, the Bloch vectors, describing the four-level inversions, could be defined as ones in a two-level system.
In a four-level -system interacting with three coherent laser pulses, interference between population inversions in coupled three-level ladders is examined. The interference effect, which affects the process and the completeness of the inversion, is represented by the two-photon coherence. It is shown that, in the adiabatic passage method, the phase and the scaling factor of the two-photon coherence are zero and a constant, respectively, resulting in no interference effect, but, in the optimal detuning method, they are strongly time dependent enough to represent the interference effect to change the process and the amount of inversion significantly. In the latter method, the ratio of Rabi frequencies for the four-level inversion to be completed, which is changed by the interference effect, is obtained.
A precessing vector model has been developed for the geometrical interpretation of the adiabatic following and the optimal detuning methods, which play a decisive role in coherent population transfer in a three-level system. In our model, the Bloch vector defined by a population inversion and two-photon coherence components precesses about the torque vector consisting of an effective Rabi frequency and detuning components. The Bloch vector traces out different routes depending on combinations of the time delay of two laser pulses and detunings from the resonant frequencies. For the extreme cases where the optimal detuning with zero delay and the optimal delay with zero detunings are taken, our vector model with analytical solutions is compared with numerical solutions.
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.
In a three-level atom interacting with nondelayed laser pulses, the inversion dynamics is studied to examine the detunings of the laser frequencies. When two Rabi frequencies are unequal, we show the interesting fact that complete inversion occurs if chirping is used, which causes the effective detuning of a two-photon resonance line to be zero. Because fixed optimal detunings allow an easier experimental approach, it is useful to find a method for obtaining the optimal detunings for realistic ratios of two Rabi frequencies. Using these optimal detunings, the process turns out to be a diadiabatic transition from investigation of the eigenvector and Bloch vector motions, whose behaviours are compared with their behaviour in the case of complete inversion.
We study the correlation between level populations, absorption and two-photon coherence in closed Λ- and V-shaped three-level systems interacting with two lasers of arbitrary intensity. Whereas it is well known that maximum two-photon coherence leads to population trapping in Λ-shaped three-level systems interacting with two equally detuned lasers, we demonstrate that minimum two-photon coherence, under saturation conditions, can lead to equal populations in all the levels of a Λ- or V-shaped system when the lasers are symmetrically detuned. Moreover we show that steady-state population inversion between one of the upper levels of a V-shaped system and the ground state can occur when either the detunings are asymmetrical or the upper states decay at different rates. It is shown that this population inversion does not lead to stimulated emission into the lasing modes. These effects are also correlated with two-photon coherence and are explained physically by a cross three-photon scattering process involving two laser photons of different frequencies.