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Physical Review A

Multilevel dark states in an inhomogeneously broadened open atomic system

Yifu Zhu, Shijun Wang, Neil Mulchan

Published May 1, 1999• 16 citations
Publisher

Abstract

We report an experimental observation of coherent population trapping in multilevel dark states prepared in the ground hyperfine states of ${}^{87}\mathrm{Rb}$ atoms confined in a room-temperature vapor cell. The dark states are created by a linearly polarized coupling laser that is tuned to the ${}^{87}\mathrm{Rb}$ ${D}_{1}$ $F=2\ensuremath{\leftrightarrow}{F}^{\ensuremath{'}}=1$ or 2 transition and establishes \ensuremath{\Lambda}-type transition chains. The Doppler shift for the \ensuremath{\Lambda}-type transition chains is compensated by the left and right circularly polarized components of the coupling laser. The multilevel dark states that consist of three or two magnetic sublevels among the five degenerate magnetic sublevels in the $F=2$ ground hyperfine state are probed by a weak probe laser scanned across the same ${D}_{1}$ transition and their existence is manifested by a strong dependence of the probe excitation on the probe polarization. Our experiment demonstrates the importance of coherent population trapping in a laser coupled degenerate multilevel open atomic system, which may play an important role in diverse physical phenomena such as atom cooling and trapping, optical pumping, and multiple laser excitation and ionization of atoms and molecules.

DOI
10.1103/physreva.59.4005

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We report our study on coherent population trapping (CPT) in a $\ensuremath{\Lambda}$ configuration coupled by magnetic dipole interactions. The $\ensuremath{\Lambda}$ configuration is formed by bichromatic radio-frequency fields coupling a pair of Zeeman sublevels in one hyperfine level of an alkali-metal atom to another sublevel in the other hyperfine level. The Zeeman sublevel at the apex is coupled to a state in the $nP$ manifold via an optical transition on one of the spectroscopic $D$ lines to induce optical pumping to a dark state. The configuration is closed without leakage to other Zeeman sublevels. We use lithium atoms in an optical trap for our experimental study. The system allows independent control of the main parameters characterizing CPT, which include the upper-state decay rate and the decoherence rate as well as the Rabi frequencies. By turning off the applied fields, the system is frozen so that its quantum state can be measured precisely. We studied the line shapes and dynamics of the CPT system, and measured the phase relation of the dark superposition state to find excellent agreement with theory. The possible application of the scheme as a method to cool optically trapped atoms below the recoil limit in a manner analogous to velocity-selective coherent population trapping is discussed.

Physical Review A•Mar 10, 2014•7 citations
We report an experimental observation of coherent population trapping in multilevel dark states prepared in the ground hyperfine states of 87Rb{}^{87}\mathrm{Rb}87Rb atoms confined in a room-temperature vapor cell. The dark states are created by a linearly polarized coupling laser that is tuned to the 87Rb{}^{87}\mathrm{Rb}87Rb D1{D}_{1}D1​ F=2\ensuremath↔F\ensuremath′=1F=2\ensuremath{\leftrightarrow}{F}^{\ensuremath{'}}=1F=2\ensuremath↔F\ensuremath′=1 or 2 transition and establishes \ensuremath{\Lambda}-type transition chains. The Doppler shift for the \ensuremath{\Lambda}-type transition chains is compensated by the left and right circularly polarized components of the coupling laser. The multilevel dark states that consist of three or two magnetic sublevels among the five degenerate magnetic sublevels in the F=2F=2F=2 ground hyperfine state are probed by a weak probe laser scanned across the same D1{D}_{1}D1​ transition and their existence is manifested by a strong dependence of the probe excitation on the probe polarization. Our experiment demonstrates the importance of coherent population trapping in a laser coupled degenerate multilevel open atomic system, which may play an important role in diverse physical phenomena such as atom cooling and trapping, optical pumping, and multiple laser excitation and ionization of atoms and molecules.
We report an experimental study of coherent population trapping and four-wave mixing via multilevel dark states prepared in the ground hyperfine states of 87Rb{}^{87}\mathrm{Rb}87Rb atoms. The dark states are created by \ensuremath{\Lambda}-type coupling chains driven by a linearly polarized pump laser tuned to the D1{D}_{1}D1​ $F=2\ensuremath{\leftrightarrow}{F}^{\ensuremath{'}}=1$ transition. The Doppler shift in the coupling chains is compensated by the left- and right-circularly polarized components of the pump laser. The multilevel dark states are explored by a probe laser and their existence is manifested by a strong dependence of the probe excitation and four-wave-mixing emission on the probe polarization. Our experiment shows that coherent population trapping in multilevel dark states of a degenerate atomic system should be useful in the coherence control of optical nonlinearity.
We present the first experimental proof for the existence of elliptical dark states, the multilevel analog of the well known three-level dark states. An ensemble of multilevel atoms, when prepared by elliptically polarized light, becomes transparent to probe light of the same polarization. The effect stems from laser-induced coherences between many ground-state magnetic sublevels which reflect the polarization of the light that had created the dark state. The novelty and essential character of the dark states are elucidated by the experimental demonstration of their creation by incoherent light. It is anticipated that elliptical dark states will play an important role in the laser cooling and manipulation of molecules.
Summary form only given.We present here the first experimental proof for the existence of elliptical dark states (EDS), the multilevel analog of the well known three-level dark states. We demonstrate that an ensemble of multilevel atoms, when prepared by elliptically polarized light, becomes transparent to probe light of the same polarization. The effect stems from laser-induced coherences between many ground-state magnetic sublevels that reflect the polarization of the light that had created the dark state. The novelty and essential character of the dark states are elucidated by the experimental demonstration of their creation by incoherent light. The experiments were performed in a Na atomic beam.
We present a general formalism for studying the interaction between laser fields and degenerate-Zeeman-sublevel atoms correct up to the first order in the weak probe field. We derive from this theory the equations of motion for a \ensuremath{\Lambda}-type system involving S1/2{\mathit{S}}_{1/2}S1/2​, F=2\ensuremath{\leftrightarrows}P1/2{\mathit{P}}_{1/2}P1/2​, F\ensuremath{'}=1\ensuremath{\leftrightarrows}S1/2{\mathit{S}}_{1/2}S1/2​, F=1 transitions in 87Rb^{87}\mathrm{Rb}87Rb atoms. These equations are used to numerically investigate the coherent population trapping (CPT) schemes in the S1/2{\mathit{S}}_{1/2}S1/2​, F=2\ensuremath{\leftrightarrows}P1/2{\mathit{P}}_{1/2}P1/2​, F\ensuremath{'}=1 transition induced by a linearly polarized pumping field and the electromagnetically induced transparency exhibited in the weak probe spectrum in the S1/2{\mathit{S}}_{1/2}S1/2​, F=1\ensuremath{\leftrightarrows}P1/2{\mathit{P}}_{1/2}P1/2​, F\ensuremath{'}=1 transition. We discuss the effects of the CPT on the probe spectrum with and without the Doppler broadenings by comparing the probe spectrum derived from the real system with that derived from an ideal \ensuremath{\Lambda}-type system. We show that the atoms in the CPT states are shielded from interacting with the probe field while those in the absorbent states make contributions to the weak probe by the Raman anti-Stokes process, and the effect of the real system on the probe field becomes equivalent to the ideal \ensuremath{\Lambda} system only in the strong CPT parameter regime. \textcopyright{} 1996 The American Physical Society.
Nonlinear magneto-optical resonances on the hyperfine transitions belonging to the D2{D}_{2}D2​ line of rubidium were changed from bright to dark resonances by changing the laser power density of the single exciting laser field or by changing the vapor temperature in the cell. In one set of experiments atoms were excited by linearly polarized light from an extended cavity diode laser with polarization vector perpendicular to the light's propagation direction and magnetic field, and laser-induced fluorescence was observed along the direction of the magnetic field, which was scanned. A low-contrast bright resonance was observed at low laser power densities when the laser was tuned to the ${F}_{g}=2\ensuremath{\rightarrow}{F}_{e}=3$ transition of 87{}^{87}87Rb and near to the ${F}_{g}=3\ensuremath{\rightarrow}{F}_{e}=4$ transition of 85{}^{85}85Rb. The bright resonance became dark as the laser power density was increased above 0.6 mW/cm2{}^{2}2 or 0.8 mW/cm2{}^{2}2, respectively. When the ${F}_{g}=2\ensuremath{\rightarrow}{F}_{e}=3$ transition of 87{}^{87}87Rb was excited with circularly polarized light in a second set of experiments, a bright resonance was observed, which became dark when the temperature was increased to around 50${}^{\phantom{\rule{0.16em}{0ex}}\ensuremath{\circ}}$C. The experimental observations at room temperature could be reproduced with good agreement by calculations based on a theoretical model, although the theoretical model was not able to describe measurements at elevated temperatures, where reabsorption was thought to play a decisive role. The model was derived from the optical Bloch equations and included all nearby hyperfine components, averaging over the Doppler profile, mixing of magnetic sublevels in the external magnetic field, and a treatment of the coherence properties of the exciting radiation field.
We report our study on coherent population trapping (CPT) in a $\ensuremath{\Lambda}$ configuration coupled by magnetic dipole interactions. The $\ensuremath{\Lambda}$ configuration is formed by bichromatic radio-frequency fields coupling a pair of Zeeman sublevels in one hyperfine level of an alkali-metal atom to another sublevel in the other hyperfine level. The Zeeman sublevel at the apex is coupled to a state in the nPnPnP manifold via an optical transition on one of the spectroscopic DDD lines to induce optical pumping to a dark state. The configuration is closed without leakage to other Zeeman sublevels. We use lithium atoms in an optical trap for our experimental study. The system allows independent control of the main parameters characterizing CPT, which include the upper-state decay rate and the decoherence rate as well as the Rabi frequencies. By turning off the applied fields, the system is frozen so that its quantum state can be measured precisely. We studied the line shapes and dynamics of the CPT system, and measured the phase relation of the dark superposition state to find excellent agreement with theory. The possible application of the scheme as a method to cool optically trapped atoms below the recoil limit in a manner analogous to velocity-selective coherent population trapping is discussed.