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Quantum Electronics and Laser Science Conference

Dark resonance spectroscopy in strong magnetic fields

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

Published Jan 20, 2003• 0 citations
Publisher

Abstract

Summary form only given. We have performed a series of experiments on coherent population trapping (CPT) in the presence of strong magnetic flux densities up to 1 mT. CPT can be observed in /spl Lambda/-systems where two ground states are coupled to a common excited state by two near-resonant light fields. Effective formation of ground state coherences takes place when the frequency difference of the light fields precisely matches the ground state splitting. This process leads to reduced absorption in the medium and thus reduced fluorescence intensity ("dark resonance") at the optical resonance frequency.

DOI
10.1109/qels.1999.807663

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EPJ Web of Conferences•Dec 13, 2016•0 citations
Summary form only given. We have performed a series of experiments on coherent population trapping (CPT) in the presence of strong magnetic flux densities up to 1 mT. CPT can be observed in /spl Lambda/-systems where two ground states are coupled to a common excited state by two near-resonant light fields. Effective formation of ground state coherences takes place when the frequency difference of the light fields precisely matches the ground state splitting. This process leads to reduced absorption in the medium and thus reduced fluorescence intensity ("dark resonance") at the optical resonance frequency.
Summary form only given. The phenomenon of "dark resonances" or coherent population trapping is a well known concept in optics and laser spectroscopy. It can be considered as the basis for such effects as electromagnetically induced transparency and lasing without inversion. We report the observation of the novel spectral features which arise in systems where multiple quantum superposition states are coupled and interact coherently. These novel resonances correspond to the recently predicted "double-dark states". In the experiment, an optically thin ensemble of rubidium-37 atoms in zero magnetic field is excited in a /spl Lambda/ configuration with circularly polarized light from a pair of 789 nm diode lasers.
Summary form only given. We have experimentally investigated the potential of narrow coherent population trapping resonances for precision applications like magnetometry or atomic frequency standards. CPT can be observed in /spl Lambda/ systems where two ground states are coupled to a common excited state by two near-resonant light fields. Effective formation of ground state coherences takes place when the frequency difference of the light fields precisely matches the ground state splitting. This process leads to reduced absorption in the medium (electromagnetically induced transparency) and thus reduced fluorescence intensity (dark resonance) at the optical resonance frequency. Extremely narrow dark resonances can be observed when the two light fields have fixed phase and frequency difference and time-of-flight broadening is suppressed by the use of a buffer gas, making this type of system a promising candidate for precision applications. The /spl Lambda/ systems under study here are the D/sub 2/ lines of Cs, /sup 87/Rb and /sup 85/Rb where the ground state hyperfine splittings are 9.2 GHz, 6.8 GHz, and 3.0 GHz, respectively. Thus in a magnetic field the CPT resonance splits into several Zeeman components whose frequency positions are determined by the Breit-Rabi formula.
24 In recent years interest has been growing into the fascinating properties of dark resonances. Most of the experiments have been carried out in alkali atoms due to their favourable level structure and the commercially available diode lasers, resulting in numerous applications. Teams from Siena and Sofia developed an all-optical magnetometer based on such dark resonances. The dark-state effect was discovered in 1976 by G. Alzetta, A. Gozzini, L. Moi and G. Orriols [1], while they were working on optical-pumping experiments with a multimode dye laser. They wanted to make the observation of multi-photon radio-frequency resonances in sodium atoms easy, i.e., observable by the naked eye. To this end, they introduced a magnetic field (MF) that was constant in time but spatially inhomogeneous along the laser beam, so as to produce Zeeman splitting of groundstate hyperfine levels (Fig.1a, left). In this way, an applied radio-frequency field was resonant only if it matched the Zeeman splitting. As a result, a bright radio-frequency resonance appeared (Fig.1a, right). During these experiments, besides the bright resonances, a new “dark” resonance was observed, which does not require any radio-frequency field. In the introductory illustration only the dark resonance is shown. The new dark state (DS) effect appears in a position along the laser beam where the frequency difference (ω10-ω20) between two optical frequencies is equal to the frequency splitting ω12 between two Zeeman sublevels (Fig.1b, left). This is the resonant condition for the stimulated Raman transitions between the two longliving ground levels. Theoretical studies described the dark resonance as a coherent superposition of two long-living ground levels prepared by the bi-chromatic laser field, and ‘atomic population trapping’ was introduced, leading to the term Coherent Population Trapping (CPT) [2]. l Luigi Moi1, Stefka Cartaleva2 DOI: 10.1051/epn/2012603
We observe coherent population trapping (CPT) in a two-electron atom— 174 Yb—using the transition. CPT is not possible for such a transition according to one-electron theory because the magnetic sublevels form a V-type system, but in a two-electron atom like Yb, the interaction of the electrons transforms the level structure into a Λ -type system, which allows the formation of a dark state and hence the observation of CPT. Since the two levels involved are degenerate, we use a magnetic field to lift the degeneracy. The single fluorescence dip then splits into five dips—the central unshifted one corresponds to coherent population oscillation, while the outer four are due to CPT. The linewidth of the CPT resonance is about 300 kHz and is limited by the natural linewidth of the excited state, which is to be expected because the excited state is involved in the formation of the dark state.
There presented basic information on coherent population trapping (CPT) and related phenomenon of electromagnetically induced transparence, ‘dark’ and ‘bright’ (anomalous absorption) states is presented. Spectrum transformation is considered in probe field spectroscopy for Λ-scheme in various settings. The close relation is revealed between the effect of CPT in the manifold of magnetic sublevels and the phenomenon of optical pumping (optical orientation and optical alignment). CPT is shown to be generalized to the case of several transitions with a common upper level and with the corresponding number of light fields each being in quasiresonance with its ‘own’ transition.