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Laser Physics

Low-Frequency Coherence Excitation in the Ground State Manifold and Saturation of the Quasi-Two-Level Atom by a Resonant Field

R. N. Shakhmuratov, J. Odeurs, Romain Coussement, A. Szabó

Published Jan 1, 2000• 0 citations
OpenAlex

Abstract

The excitation of the low-frequency coherence in the ground state atomic sublevels by a coherent field driving the transition between these sublevels and the remote excited state is considered. The condition of the population trapping in the dark, nonabsorbing, state, which is a particular coherence of the ground state sub-levels, is analyzed. It is shown that the saturated width of the dark resonance is much larger than the line width of the two-level atom saturated by a resonant field

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The excitation of the low-frequency coherence in the ground state atomic sublevels by a coherent field driving the transition between these sublevels and the remote excited state is considered. The condition of the population trapping in the dark, nonabsorbing, state, which is a particular coherence of the ground state sub-levels, is analyzed. It is shown that the saturated width of the dark resonance is much larger than the line width of the two-level atom saturated by a resonant field
The response of a two-level atom in a strong polychromatic field composed of a large number of equidistant frequency components is investigated. We calculate numerically, as well as analytically, the stationary population inversion and show that the saturation of the atomic transition strongly depends on whether or not there is a central (resonant) frequency component in the driving field. We find that, in the presence of the central component, the atom can remain in the ground state even for a strong Rabi frequency of the driving field. In addition, we find that the inversion is sensitive to the relative phase between the frequency components. When the central component is suppressed, the atomic transition saturates with the Rabi frequency independent of the relative phase.
The absorption profile of a four-level ladder atomic system interacting with three driving fields is studied perturbatively and analytical results are presented. Numerical results where the driving field strengths are treated up to all orders are presented. The absorption features are studied in two regimes—(i) the weak middle transition coupling, i.e. Ω 2 ≪ Ω 1,3 and (ii) the strong middle transition coupling Ω 2 ≫ Ω 1,3 . In case (i), it is shown that the ground-state absorption and the saturation characteristics of the population of level |2⟩ reveal deviation due to the presence of upper level couplings. In particular, the saturation curve for the population of level |2⟩ shows a dip for Ω 1 = Ω 3 . While the populations of levels |3⟩ and |4⟩ show a maxima when this resonance condition is satisfied. Thus the resonance condition provides a criterion for maximally populating the upper levels. A second-order perturbation calculation reveals the nature of these minima (maxima). In the second case, I report two important features: (a) filtering of the Aulter–Townes doublet in the three-peak absorption profile of the ground state, which is achieved by detuning only the uppermost coupling field and (b) control of line-width by controlling the strength of the upper coupling fields. This filtering technique coupled with the control of linewidth could prove to be very useful for high resolution studies.
The dynamical evolution of the three-level atom under the influence of a bichromatic excitation is studied in the basis of the dark and bright states.With the help of these states, the analysis can be reduced to the consideration of a two-level system interacting with one effective field.Then, a simple Bloch-vector model is applied to describe the evolution of the initially complicated system.Special attention is paid to the low frequency coherence excitation, nonadiabatic population transfer, and population trapping.
We have detected and analyzed narrow, high contrast coherent population trapping resonances, which appear in transmission of the probe monochromatic light beam under action of the counterpropagating two-frequency laser radiation, on example of the nonclosed three level Λ-system formed by spectral components of the Doppler broadened D2 line of cesium atoms (in the cell with the rarefied Cs vapor). These nontrivial resonances are determined directly by the trapped atomic population on the definite lower level of the Λ-system and may be used in atomic frequency standards, sensitive magnetometers and in ultrahigh resolution laser spectroscopy of atoms and molecules.