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Physical Review A

Arbitrary orientation of atoms and molecules via coherent population trapping by elliptically polarized light

Valery Milner, B. M. Chernobrod, Yehiam Prior

Published Aug 1, 1999• 19 citations
Publisher

Abstract

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-$J$ 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.

DOI
10.1103/physreva.60.1293

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Physical Review Letters•Feb 2, 1998•42 citations
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 J\ensuremath→J\ensuremath−1\stackrel{\ensuremath{\rightarrow}}{J}J\ensuremath{-}1J\ensuremath→​J\ensuremath−1 and J\ensuremath→J\stackrel{\ensuremath{\rightarrow}}{J}JJ\ensuremath→​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.
We experimentally create and theoretically explain a different type of highly anisotropic atomic angular-momentum distribution with two preferential axes. Such a biaxial spatial orientation is created by optical pumping with elliptically polarized light, and measured by the observation of coherent population trapping in a weak external magnetic field. We probe the angular-momentum distribution by the Hanle configuration, and observe two dark resonances at nonzero magnetic fields---the expected signature of the biaxial spatial geometry. A direct method for preparing arbitrarily oriented atomic ensembles is discussed.
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.
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.
Coherent population trapping (CPT) resonances study in Hanle configuration is reported for different polarizations of the exciting light field, on the D2{D}_{2}D2​ line of Cs atoms. While for linear/circular polarization dip/peak in the fluorescence is registered, in case of elliptical polarization, a complex shape resonance is evidenced experimentally, whose profile strongly depends on the ellipticity of polarization. A theoretical model is proposed and developed, which includes the influence on the resonance of polarization ellipticity, transverse magnetic field, and spatial intensity profile of the laser beam. Good agreement is found between the theoretical and experimental results. The reported results allow one to accomplish more profound than in previous research analysis of the CPT resonances behavior pointing out their sensitivity to the light polarization and power as well as to transverse magnetic fields. The presented study is of general importance for the wide application of the coherent resonances in high resolution spectroscopy and precise measurements. The purity of linear/circular polarization of the light needed in different applications can be estimated implementing the proposed model.
We investigated experimentally and theoretically angular momentum alignment-to-orientation conversion created by the joint interaction of laser radiation and an external magnetic field with atomic rubidium at room temperature. In particular, we were interested in alignment-to-orientation conversion in the atomic ground state. In the experiment the laser frequency was fixed to the hyperfine transitions of the D1{D}_{1}D1​ line of rubidium. To simulate the measured signals we used a theoretical model that takes into account all neighboring hyperfine levels, the mixing of magnetic sublevels in an external magnetic field, the coherence properties of the exciting laser radiation, and the Doppler effect. The experiments were carried out by exciting the atoms with linearly polarized laser radiation. Two orthogonal circularly polarized laser induced fluorescence (LIF) components were detected and afterward their difference was taken. The combined LIF signals originating from the hyperfine magnetic sublevel transitions of 85Rb^{85}\mathrm{Rb}85Rb and 87Rb^{87}\mathrm{Rb}87Rb rubidium isotopes were included. The alignment-to-orientation conversion can be incontrovertibly identified in the difference signals for various laser frequencies, and a change in signal shapes can be observed when the laser power density is increased. We studied the formation of and the underlying physical processes behind the observed signals of the LIF components and their difference by performing an analysis of the influence of incoherent and coherent effects. We performed simulations of theoretical signals that showed the influence of ground-state coherent effects on the LIF difference signal.
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.