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Physical Review A

Influence of excited-state hyperfine structure on ground-state coherence

A. Nagel, C. Affolderbach, Svenja A. Knappe, R. Wynands

Published Dec 10, 1999• 18 citations
Publisher

Abstract

In a three-level $\ensuremath{\Lambda}$ system, a bichromatic light field can induce a ground-state coherence that leads to vanishing absorption of the bichromatic field. Although this coherence is between ground states only, in a real atom it can nevertheless be influenced by the hyperfine structure of the excited state. We present experimental results which show that for suitable detunings of the bichromatic field from optical resonance, the specific contribution of each individual hyperfine level of the excited state can be distinguished through increased amplitude or width of the ground-state coherence.

DOI
10.1103/physreva.61.012504

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In a three-level \ensuremathΛ\ensuremath{\Lambda}\ensuremathΛ system, a bichromatic light field can induce a ground-state coherence that leads to vanishing absorption of the bichromatic field. Although this coherence is between ground states only, in a real atom it can nevertheless be influenced by the hyperfine structure of the excited state. We present experimental results which show that for suitable detunings of the bichromatic field from optical resonance, the specific contribution of each individual hyperfine level of the excited state can be distinguished through increased amplitude or width of the ground-state coherence.
We use the phenomenon of electromagnetically induced transparency in a three-level atomic system for hyperfine spectroscopy of upper states that are not directly coupled to the ground state. The three levels form a ladder system: the probe laser couples the ground state to the lower excited state, while the control laser couples the two upper states. As the frequency of the control laser is scanned, the probe absorption shows transparency peaks whenever the control laser is resonant with a hyperfine level of the upper state. As an illustration of the technique, we measure hyperfine structure in the 7 S 1/2 states of 85 Rb and 87 Rb, and obtain an improvement of more than an order of magnitude over previous values.
We have investigated the interaction between bichromatic optical fields and atomic transitions. We have considered both the interaction of a saturating bichromatic field in the form of a 100% amplitude-modulated field resonantly interacting with a single atomic transition as well as the interaction of a pair of 100% amplitude-modulated fields with two coupled atomic transitions. In our studies of a single atomic transition resonantly excited by a 100% amplitude-modulated field we have used a fully quantum-electrodynamical analysis to model the Autler-Townes effect induced by a bichromatic optical field. We find that the effect induced by a bichromatic field is markedly different from the effect produced by a monochromatic exciting field. We have derived analytic expressions for the bichromatic Autler-Townes absorption spectrum. The absorption spectrum consists of equally spaced lines spaced by the modulation frequency. The linewidths of neighboring lines alternate as functions of the field strength. Experimentally, we observed this spectrum for both resonant and detuned 100% amplitude-modulated driving fields by monitoring the absorption of a weak monochromatic field as it was tuned about the resonance frequency of the 3²P₃/₂(F = 3, mF = 3) ↔ 4²D₅/₂(F = 4, mF = 4) transition while driving the 3²S₁/₂(F = 2, mF = 2) ↔ 3²P₃/₂(F = 3, mF = 3) transition with a saturating 100% amplitude-modulated field. We found excellent agreement between our predicted and measured spectra. We then investigated the resonant interaction between a pair of 100% amplitude-modulated fields and two atomic transitions sharing a common atomic level. We considered both a three-level atomic cascade system and a three-level atomic lambda system. We examined the effect that the relative phase between the two modulations has on the rate at which the modulated fields are absorbed. We find that the saturated time-averaged field absorption exhibits parametric resonances in both the cascade and lambda systems. The time-averaged absorption is strongly dependent upon the relative phase of modulation between the two fields and for certain phase differences the time-averaged absorption can be enhanced over the maximum saturated absorption experienced by cw fields of the same time-averaged intensities. We also find that the stationary atomic response exhibits periodic oscillations and for the cascade system periodic two-photon inversions of up to 40% can be achieved. We further find dark absorption resonances as a function of phase in the lambda system. We analyze the results from the point of view of semi-classical adiabatic dressed states.
An ``amplitude model'' of the laser-rf double-resonance process has been quantitatively verified, providing a complete explanation of the anomalous line shapes observed in studies of Na2{\mathrm{Na}}_{2}Na2​ hyperfine structure. We find excellent agreement with new laser-rf double-resonance measurements in low-J'' rotational levels of the X1{X}^{1}X1${\ensuremath{\Sigma}}_{g}^{+}$(v''=0) ground state of Na2{\mathrm{Na}}_{2}Na2​ as well as with earlier data for J''=28. These results confirm the idea that absorption and stimulated-emission Rabi cycles in the presence of the laser field create coherence between the ground-state hyperfine levels when these levels are optically unresolved. A novel ground-state ``quantum beat'' experiment is proposed.
We present a theoretical study of quantum coherent effects in a Λ-three-level system with a strong bichromatic coupling field and a weak probe field. When one component of the strong bichromatic coupling field is resonant with a corresponding transition and the other is detuning with an integer fraction of the Rabi frequency of the resonant field, the absorption spectrum exhibits a series of symmetrical doublets. While two frequencies of the strong bichromatic coupling field are symmetrically detuned from the transition, the position and the relative intensity of the absorption peak are both affected by the coupling field intensity and detuning. An explanation of the spectrum is given in term of the dressed-state formalism.
Electromagnetically induced transparency(EIT) with extremely narrow spectral width is obtained in a well-studied lambda three level system where a coupling field and a probing field interact with two optical transitions.By tuning coupling field,it excites the transitions between two ground state hyperfine levels and excited state level.In this paper,both EIT and spontaneous radiation induced coherent dip can be obtained,and even the gain is observed to superpose to the EIT window.By discussing the behaviors of probing field absorption profiles under the effect of the tuned coupling field,it can be seen that there are quantum interference and coherence between EIT and spontaneous radiation induced coherent dip,and the coherence becomes remarkable when the excitation frequency of the coupling field satisfies certain conditions.
The interference spectra arising from the competition between a spontaneous process and an induced one by two laser fields, which interact on two-photon resonance with a three-level atom in the Lambda configuration have been considered. In the low intensity limit of both laser fields, the frequency profiles of the two peaks, which arise from the spontaneous and induced processes, respectively, cancel each other out completely at the centre of the line for zero detuning and at the finite values of the detuning as well. At those frequencies, the fluorescent light totally disappears. The interference spectra arising from the absorption of a laser photon by the ground state of the atom consists of two peaks describing the induced processes of absorption and stimulated emission, respectively.