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Physical Review A

Magnetic-field-induced alignment-to-orientation conversion in sodium

X-L Han, G. W. Schinn

Published Jan 1, 1991• 11 citations
Publisher

Abstract

We report a detailed investigation of excited-state alignment-to-orientation conversion in the presence of an external magnetic field. This counterintuitive phenomenon occurs under intermediate-coupling conditions. A weak, linearly polarized, cw laser beam was used to excite and align the Na 3${\mathit{P}}_{3/2}$ state in an atomic beam along the z direction. The degree of circular polarization of the resulting fluorescence was detected along the z direction as a function of magnetic-field strength. The spectrally integrated transitions originating from individual F levels of the 3${\mathit{S}}_{1/2}$ state yield a maximum circular-polarization fraction of \ensuremath{\sim}40%; integrating the circular polarization over all the allowed 3${\mathit{S}}_{1/2}$-3${\mathit{P}}_{3/2}$ transitions gives rise to an \ensuremath{\sim}8% effect. The results are predicted by the Breit formula, which is in excellent agreement with our observations.

DOI
10.1103/physreva.43.266

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We report a detailed investigation of excited-state alignment-to-orientation conversion in the presence of an external magnetic field. This counterintuitive phenomenon occurs under intermediate-coupling conditions. A weak, linearly polarized, cw laser beam was used to excite and align the Na 3P3/2{\mathit{P}}_{3/2}P3/2​ state in an atomic beam along the z direction. The degree of circular polarization of the resulting fluorescence was detected along the z direction as a function of magnetic-field strength. The spectrally integrated transitions originating from individual F levels of the 3S1/2{\mathit{S}}_{1/2}S1/2​ state yield a maximum circular-polarization fraction of \ensuremath{\sim}40%; integrating the circular polarization over all the allowed 3S1/2{\mathit{S}}_{1/2}S1/2​-3P3/2{\mathit{P}}_{3/2}P3/2​ transitions gives rise to an \ensuremath{\sim}8% effect. The results are predicted by the Breit formula, which is in excellent agreement with our observations.
Excited-state angular-momentum alignment--orientation conversion for atoms with hyperfine structure in the presence of an external magnetic field is investigated. Transversal orientation in these conditions is reported. This phenomenon occurs under Paschen Back conditions at intermediate magnetic-field strength. Weak radiation from a linearly polarized diode laser is used to excite Rb atoms in a cell. The laser beam is polarized at an angle of $\ensuremath{\pi}/4$ with respect to the external magnetic-field direction. Ground-state hyperfine levels of the 5S1/2{5S}_{1/2}5S1/2​ state are resolved using laser-induced fluorescence spectroscopy under conditions for which all excited 5P3/2{5P}_{3/2}5P3/2​ state hyperfine components are excited simultaneously. Circularly polarized fluorescence is observed to be emitted in the direction perpendicular to both to the direction of the magnetic field B\mathbf{B}B and direction of the light polarization E.\mathbf{E}.E. The obtained circularity is shown to be in quantitative agreement with theoretical predictions.
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 studied alignment-to-orientation conversion caused by excited-state level crossings in a nonzero magnetic field of both atomic rubidium isotopes. Experimental measurements were performed on the transitions of the D2{D}_{2}D2​ line of rubidium. These measured signals were described by a theoretical model that takes into account all neighboring hyperfine transitions, the mixing of magnetic sublevels in an external magnetic field, the coherence properties of the exciting laser radiation, and the Doppler effect. In the experiments, laser-induced fluorescence components were observed at linearly polarized excitation and their difference was taken afterwards. By observing the two oppositely circularly polarized components, we were able to see structures not visible in the difference graphs, which give deeper insight into the processes responsible for these signals. We studied how these signals are dependent on intensity and how they are affected when the exciting laser is tuned to different hyperfine transitions. The comparison between experiment and theory was carried out fulfilling the nonlinear absorption conditions. The theoretical curves described the experimental measurements satisfactorily, reproducing even small features in the shapes of the curves.
In this study we explored the angular momentum alignment-to-orientation conversion occurring in various alkali metals -- K, Rb, Cs. We used a theoretical model that is based on Optical Bloch equations and uses the density matrix formalism. Our model includes the interaction of 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. Additionally we simulated signals where the ground- or the excited-state coherent processes were switched off allowing us to determine the origins of obtained signals. We also performed experiments on Cs atoms with two laser beams: linearly polarised Cs D1 pump and circularly polarized Cs D2 probe. We used the pump beam to create angular momentum alignment in the ground state and observed the transmission signal of the probe beam as we changed the magnetic field. Full analysis of the experimentally obtained transmission signal from a single circularly polarized probe laser component is provided.
We have experimentally and theoretically investigated the behavior of excited Na atoms under the combined influence of resonant laser light and a weak magnetic field. The use of weak magnetic fields gives another possibility to vary the polarization of the atoms, additional to variation of the polarization of the exciting laser. We calculated in detail the influence of weak magnetic fields on the density matrix describing the excited atoms, using generalized rate equations. We checked the results of our model experimentally, at the Na(3 2 S1/2, Fl=2) →(3 2 P3/2, Fu =3) transition, excited with circularly polarized light. We determined the polarization of the excited atoms, by measuring the fluorescence light at various angles, as a function of both magnetic field strength and laser irradiance. Model and experiment agreed very well. We applied our model to the associative ionization of the excited Na atoms. We measured the ion signal as a function of magnetic field. Analyzing the results, we could determine a specific associative ionization cross section, which is not accessible in our experimental geometry without the use of magnetic fields. Thereby we showed that the application of magnetic fields in polarization dependence studies can provide us with additional information about the process.
We present a detailed theoretical model and experimentally measured signals of alignment to orientation conversion (AOC) in the 7D3/2 and 9D3/2 states of cesium atoms in the presence of an external dc electric field [1]. Alignment produced by linearly polarized light can be converted into orientation by the quadratic Stark effect in an electric field if the angle between the electric field and the polarization vector of the exciting radiation is neither 0 or 90 degrees and if there are magnetic sublevels with Δm = ±1 that cross as the electric field is scanned. The appearance of orientation is verified by the appearance of a net circular polarization in the laser induced fluorescence. The theoretical model is based on the optical Bloch equation and extends models previously developed to solve the rate equations for Zeeman coherences in single-step [2] and double step laser excitation [3] of atoms. The agreement between the simulated and measured signals is excellent.