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Physica Scripta

Oriented atoms in weak magnetic fields

Peter Hannaford

Published Jan 1, 1997• 6 citations
Publisher

Abstract

This article reviews the early investigations of Series on the resonance absorption and fluorescence of light by samples of oriented atoms dagger subjected to static and oscillatory magnetic fields. These investigations include experiments on optical-radiofrequency double resonance, light beats in double resonance, circulation of coherence in optical pumping, and time-biasing in level-crossing spectroscopy. It then describes some selected laser spectroscopy experiments involving the interaction of optically oriented atoms with static magnetic fields that were stimulated by the earlier work of Series, including experiments on hyperfine structure quantum beats, cascade quantum beats, ground-state quantum beats in transmission, and optogalvanic detection of atomic coherences. Finally, it describes some current work which uses static magnetic fields to manipulate the external variables (momentum and position) of laser-cooled oriented atoms, with applications in the rapidly developing field of atom optics and atom interferometry.

DOI
10.1088/0031-8949/1997/t70/018

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•Jan 1, 2008•0 citations
This article reviews the early investigations of Series on the resonance absorption and fluorescence of light by samples of oriented atoms dagger subjected to static and oscillatory magnetic fields. These investigations include experiments on optical-radiofrequency double resonance, light beats in double resonance, circulation of coherence in optical pumping, and time-biasing in level-crossing spectroscopy. It then describes some selected laser spectroscopy experiments involving the interaction of optically oriented atoms with static magnetic fields that were stimulated by the earlier work of Series, including experiments on hyperfine structure quantum beats, cascade quantum beats, ground-state quantum beats in transmission, and optogalvanic detection of atomic coherences. Finally, it describes some current work which uses static magnetic fields to manipulate the external variables (momentum and position) of laser-cooled oriented atoms, with applications in the rapidly developing field of atom optics and atom interferometry.
We discuss experimental and theoretical studies of coherent excitation of magnetic sublevels in nD states of cesium that cross in an external electric field. Crossings of mF magnetic sublevels of hyperfine F levels with &Dgr;mF = ±2 lead to resonances in the linearly polarized laser induced fluorescence, while crossings with &Dgr;mF = ±1 lead to resonances in the circularly polarized laser induced fluorescence. These resonances can be exploited to observe alignment to orientation conversion. From the level crossing signals it is possible to measure atomic properties, such as the tensor polarizability &agr;2 and the hyperfine constant A. Alignment to orientation conversion involves the deformation of the spatial distribution of an atom's angular momentum.
In the optical excitation of atoms with polarized light, producing excited atoms, only some of the Zeeman sublevels of the excited state are actually reached, so that large differences of population can be built up between Zeeman sublevels or between hyperfine structure (hfs) levels. This property can be used to detect radio-frequency resonance in optically excited atomic states. These resonances produce a characteristic change in intensity or in the degree of polarization of the light re-emitted. Zeeman intervals, Stark effects, and hfs intervals can be measured in this manner. (The Stark constant of the 63P1 level of Hg and the electric quadrupole moments of the alkali atoms have been obtained in this way.)The technique of “optical pumping” gives a way to concentrate atoms in some of the Zeeman sublevels of one of the hfs levels of the ground state.Atomic orientation has been obtained with the Na atom, in an atomic beam and in the vapor in equilibrium with the metal. The orientation effects have been studied by detection of radio-frequency resonance signals in the ground state. Orientation can be increased many times by adding a variable pressure of a foreign gas to the pure Na vapor. Because of the coupling between nuclear spin and electron spin, nuclear orientation is produced at the same time as atomic orientation.
This thesis describes theoretical and experimental work concerning radiation forces on atoms, with particular reference to rubidium atoms confined in a magneto-optical trap. After a short history of the field of laser cooling, a review of the semiclassical theory of mechanical interactions between two-level atoms and electromagnetic radiation is given. Different formulations of the semiclassical theory are discussed, including a new formulation in terms of momentum transfer amongst the plane wave modes of the electromagnetic field. Two important applications of light forces on atoms, namely 'optical molasses' and the 'magneto-optical trap', are then described with emphasis on experimental parameters. Three sub-Doppler cooling mechanisms, 'sisyphus cooling', 'motion-induced orientation cooling' and the 'magnetically-assisted sisyphus effect', are described and their role in optical molasses and the magneto-optical trap is discussed. A new study of the polarisation gradients which occur in 3-D monochromatic light fields is presented and quantifies their relative presence in different light field configurations. Polarisation gradient parameters are developed and shown to be directly related to the relativistic spin tensor of the light field. Implications of this polarisation gradient study for laser cooling work are discussed. The design, construction from scratch, operation and testing of a magneto-optical trap for rubidium are described, including novel designs for two vacuum cells. Preliminary experiments to characterise the trap are described and results are presented; they primarily concern the number and distribution of atoms in the trap. Finally. the theory of time domain spectroscopy is reviewed. The construction and testing of a pulsed dye laser for study of coherent transients in samples of laser-cooled atoms and a proposed experiment to measure the temperature of cold atoms using coherent transients are described. Factors expected to influence the shape of coherent transients in cold atoms are discussed.
The history of the discovery, study, and explanation of the effect discovered fifty years ago by the German physicist Hanle and its development and generalization by the French school of physicists led by Kastler are described. The effect is observed optically in an increase in the degree of depolarization of the resonance fluorescence. It is studied in the range of magnetic fields in which the Zeeman splitting of the atomic magnetic sublevels does not exceed the level widths determined by thermal relaxation mechanisms and electromagnetic interaction. It is a direct optical manifestation of interference of degenerate atomic states and is intimately connected with the conservation law for the angular momentum of the photon-atom system. In the review, different applications of the effect to the measurement of mean lifetimes, linewidths, and Stark splitting constants of atoms in excited states are given. The resonance scattering of light by atoms in the ground state in a weak magnetic field is considered in detail—in particular, in the presence of radio-frequency magnetic fields oriented in different ways. The different possible types of parametric resonances in one or two radio-frequency fields for arbitrary orientation of the constant magnetic field are interpreted as the effect of crossing of the levels of an atom that is dressed by the radio-frequency field. Applications of the effect in the quantum magnetometry of ultraweak fields (down to 10-10 Oe) are examined.
Further experiments are reported in which resonance fluorescence of an atomic vapour is excited by modulated light. The system studied was the 3 P 1 level of cadmium excited by the λ 3261 λ intercombination line modulated at 462 kHz. Resonant changes in the amplitude of modulation of the fluorescent light have been observed in both static and time-dependent magnetic fields whenever the modulation frequency matches a frequency separation between the states of the excited atoms. Quantitative investigations of the shape and position of the observed resonances show good agreement between theory and experiment. The application of the technique to the spectroscopy of excited atoms is considered.
Laser-atom interactions create atomic coherence and large nonlinear atomic polarization.\nWe investigate resonant laser-atom interactions to generate large nonlinearities\nand control them using magneto-optical fields. Coherent control of high-order\nsusceptibilities and magneto-optical rotation are demonstrated. Experiments are supported\nby theoretical studies that effectively describe the observed phenomena.\nIt is shown that a new coherent field, with polarization orthogonal to a weak\nsignal field, can be parametrically generated via an all-resonant four-wave-mixing\nprocess. This is demonstrated in a double-ladder system having two intermediate\nstates between a ground and an excited state. It is shown that the parametricgeneration\nprocess can be coherently controlled by coupling lasers and magnetic fields.\nIt is theoretically established that the underlying physics is a resonant three-photon\nprocess with a wide domain of control parameters.\nElectromagnetically induced transparency (EIT), where absorption of a weak\nprobe is suppressed via quantum interference, is demonstrated in a usual three-level\nladder system. It is observed that in contrast with EIT in a usual ladder system,\naddition of a second channel helps to suppress the absorption of two weak probe\nfields in the double-ladder system. The resulting enhancement of transmission in two\ndifferent channels is due to gain caused by three-photon processes.\nCoherent control is strongly limited by coherence lifetime, which is the inverse of\nthe dephasing rate. A lambda-system, having two ground states coupled to a common\nexcited state by lasers, can generate a new eigen (dark)-state that is transparent to incoming fields and hence suppresses fluorescence. However, ground-state dephasing\nperturbs the dark state. A new method for measuring the ground-state dephasing\nrate from fluorescence signals is proposed and a proof-of-principle experiment demonstrated.\nWhile two laser fields in a lambda-system are resonant with their respective\ntransitions, the atomic polarizations are very sensitive to an applied magnetic field.\nThis effect can be used for optical magnetometry. The degree of sensitivity of the\nmagnetometer is determined by two competing parameters–atomic density and laser\nintensity. It is shown experimentally that the optimal sensitivity reaches saturation,\nwhich is contrary to the idea that sensitivity increases indefinitely with an increase\nin the above parameters.