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Journal of Physics B Atomic and Molecular Physics

Conditions for population trapping in a three-level system

S. Swain

Published Oct 14, 1982• 62 citations
Publisher

Abstract

The authors consider population trapping in a three-level system which is driven by two applied fields of arbitrary intensity, frequency and bandwidth. An irreversible loss of population is allowed to take place from the common level. The conditions are investigated under which population trapping can occur when spontaneous emission and finite radiation bandwidths are allowed. For the lambda configuration with monochromatic lasers the authors find that increasing the spontaneous emission rate can increase the amount of population trapped in the system, whereas spontaneous emission destroys trapping in the ladder configuration. For the case of uncorrelated lasers, finite laser bandwidths destroy trapping for either configuration.

DOI
10.1088/0022-3700/15/19/010

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The authors consider population trapping in a three-level system which is driven by two applied fields of arbitrary intensity, frequency and bandwidth. An irreversible loss of population is allowed to take place from the common level. The conditions are investigated under which population trapping can occur when spontaneous emission and finite radiation bandwidths are allowed. For the lambda configuration with monochromatic lasers the authors find that increasing the spontaneous emission rate can increase the amount of population trapped in the system, whereas spontaneous emission destroys trapping in the ladder configuration. For the case of uncorrelated lasers, finite laser bandwidths destroy trapping for either configuration.
Coherent trapping in three level lambda and ladder systems is examined for the case of two fluctuating laser fields with unequal bandwidths, obtained by frequency multiplying methods. The inclusion of uncorrelated laser fields destroys trapping and washes out the narrow coherence hole in the fluorescent intensity. However, if the fields are positively critically cross-correlated, the relevant atomic coherences in lambda systems are much less affected by the laser fluctuations. A significant degree of trapping re-occurs and the narrow coherence hole is largely restored. An enhancement of the destruction of coherent trapping occurs in ladder systems. An application of the theory to molecular photodissociation is discussed.
A three-level system driven by two coherent fields on two-photon resonance leads to the trapping of population in a superposition of initial and final states which is immune to further photoexcitation, manifesting itself in a narrow coherence minimum in the absorption spectrum. The authors analyse the effects of laser field bandwidths and cross-correlations on these phenomena. They find that the inclusion of laser bandwidths dephases atomic coherences which leads to the destruction of population trapping and of the narrow coherence minimum in the absorption spectrum. When the driving fields are not cross-correlated, the atomic coherences are similarly dephased in both lambda and ladder systems. If, however, the driving fields are critically cross-correlated, two-photon coherences in the lambda system are unaffected by laser fluctuations; trapping and the coherence minimum persist. For the ladder system, on the other hand, the cross-correlation does not restore the atomic coherences involved.
We discuss the condition under which a pair of closely lying levels, when coupled to a fourth level via a single-laser field, exhibit population trapping without decay to a lower level. In the discussion, the decay between the pair and the fourth level is taken into account. When the fourth level is lower than the pair, the condition for population trapping in the pair can be satisfied under arbitrary laser intensity by adjusting the laser detuning. When the fourth level is above the pair, population trapping in the pair is impossible when the decay from the fourth level to the pair is included. The influence of the decay between the pair and the fourth level on the spontaneous spectrum from the pair to the lower level is also studied.
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.
We study the steady state in three-level lambda (\ensuremath{\Lambda}) and ladder systems. It is well known that in a \ensuremath{\Lambda} system this steady state is the coherent population trapping state, independent of the presence of spontaneous emission. In contrast, the steady state in a ladder system is in general not stable against radiative decay and exhibits a minimum in the population of the ground state. We show that incoherent population pumping destroys the stability of the coherent population trapping state in the \ensuremath{\Lambda} system and suppresses a previously discovered sharp dip in the steady-state response. In the ladder system the observed minimum disappears in the presence of an incoherent pump on the upper transition.
The author considers the stimulated emission process that occurs at the difference-frequency generation of a bichromatic field interacting with a three-level atom in the Lambda configuration. In the low-intensity limit of both laser fields and at finite detunings, there are two induced excitations whose lifetimes are much longer in comparison to those of excitations spontaneously emitted by the excited states of the atom. Conditions are established under which the two-photon stimulated emission prevails, indicating that significant amplification is likely to occur at the difference-frequency generation.