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Physical Review Letters

Multilevel Dark States: Coherent Population Trapping with Elliptically Polarized Incoherent Light

Valery Milner, Yehiam Prior

Published Feb 2, 1998• 42 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.80.940

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European Quantum Electronics Conference (EQEC)•Jan 1, 1996•0 citations
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 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​ $F=2\ensuremath{\leftrightarrow}{F}^{\ensuremath{'}}=1$ 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=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.
Coherent-population trapping, heretofore realized in closed systems, is also possible in highly degenerate open molecular systems. We consider a rovibrational molecular transition, where the ground magnetic m -sublevels are coupled to the corresponding upper states, and are therefore expected to be emptied after a few lifetimes. We show that upon excitation by elliptically polarized light, the population remains trapped in a coherent superposition of ground-state sublevels which does not interact further with the exciting light. Unlike the simple cases of linearly and circularly polarized light and of a closed three-level system ( i.e. atoms), here the trapping level is not the one with | m | = 0, J , but rather a combination of several states, which depends on the ellipticity of the exciting light.
The interaction of laser light of arbitrary polarization with systems of high angular momentum is considered. We show that elliptically polarized light creates an anisotropic spatial distribution of atomic and molecular angular momentum which is qualitatively different from the alignment and orientation induced by light of circular or linear polarization. Multilevel coherent population trapping within a manifold of ground-state magnetic sublevels results in a nonclassical behavior of a high-JJJ molecular rotor. The classical approximation for the angular momentum distribution is compared with the exact quantum calculations, and is shown to fail in cases of long interaction times and high intensities of the exciting light. In these limits, the quantum uncertainty defines the spatial width of the angular distribution. The applicability of the classical treatment is analyzed and found to be different in the cases of $\stackrel{\ensuremath{\rightarrow}}{J}J\ensuremath{-}1$ and $\stackrel{\ensuremath{\rightarrow}}{J}J$ transitions. A biaxial spatial orientation with two preferential axes of rotation is experimentally created in sodium atoms via coherent population trapping by elliptically polarized light. A method for producing an arbitrary orientation of atomic angular momentum by magnetic field assisted coherent population trapping is proposed.
Coherent-population trapping, heretofore realized in closed systems, is also possible in highly degenerate open molecular systems. We consider a rovibrational molecular transition, where the ground magnetic m-sublevels are coupled to the corresponding upper states, and are therefore expected to be emptied after a few lifetimes. We show that upon excitation by elliptically polarized light, the population remains trapped in a coherent superposition of ground-state sublevels which does not interact further with the exciting light. Unlike the simple cases of linearly and circularly polarized light and of a closed three-level system (i.e. atoms), here the trapping level is not the one with |m| = 0,J, but rather a combination of several states, which depends on the ellipticity of the exciting light.