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Journal of Optics B Quantum and Semiclassical Optics

New method for polarizing complicated atoms

Fucheng Lin, Shouyun Tain, Zhang Guiyan, Fen Li

Published Mar 27, 2001• 0 citations
Publisher

Abstract

In the optical pumping process, spontaneous emission plays a unique role in redistributing the population of the ground level among the magnetic sub-levels. It is clear that the branching ratio of the relevant transition in the optical pumping must be nearly equal to unity in order to avoid the loss of population involved. Unfortunately, most complicated atoms do not have a simple energy level structure and cannot be polarized using optical pumping. We propose here a new polarizing method of a purely laser-driven process that avoids spontaneous emission and can be used in many heavier atoms. For J = 0 ground-state atoms, a complete population transfer to the meta-stable state can be used as efficient laser isotope separation, whereas a coherently prepared superposition state of the non-magnetic ground state and one of the magnetic states of the meta-stable level can be used as a tool to study the decoherent process. For J of the ground level no less than unity, partly removing the population on one of its sub-magnetic states will enhance the magnetic resonance signal significantly.

DOI
10.1088/1464-4266/3/2/359

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In the optical pumping process, spontaneous emission plays a unique role in redistributing the population of the ground level among the magnetic sub-levels. It is clear that the branching ratio of the relevant transition in the optical pumping must be nearly equal to unity in order to avoid the loss of population involved. Unfortunately, most complicated atoms do not have a simple energy level structure and cannot be polarized using optical pumping. We propose here a new polarizing method of a purely laser-driven process that avoids spontaneous emission and can be used in many heavier atoms. For J = 0 ground-state atoms, a complete population transfer to the meta-stable state can be used as efficient laser isotope separation, whereas a coherently prepared superposition state of the non-magnetic ground state and one of the magnetic states of the meta-stable level can be used as a tool to study the decoherent process. For J of the ground level no less than unity, partly removing the population on one of its sub-magnetic states will enhance the magnetic resonance signal significantly.
We propose methods of optical pumping that are applicable to open, high-angular-momentum transitions in atoms and molecules, for which conventional optical pumping would lead to significant population loss. Instead of applying circularly polarized cw light, as in conventional optical pumping, we propose to use techniques for coherent population transfer (e.g., adiabatic fast passage) to arrange the atoms so as to increase the entropy removed from the system with each spontaneous decay from the upper state. This minimizes the number of spontaneous-emission events required to produce a stretched state, thus reducing the population loss due to decay to other states. To produce a stretched state in a manifold with angular momentum JJJ, conventional optical pumping requires about 2J2J2J spontaneous decays per atom; one of our proposed methods reduces this to about log22J{log}_{2}2Jlog2​2J, while another of the methods reduces it to about one spontaneous decay, independent of JJJ.
We have proposed a new technique to obtain highly polarized atoms for improving the sensitivity of the all-optical Cs atomic magnetometer. In a Bell–Bloom magnetometer, the resonance pumping from the ground state F g = 3 to the excited state F e = 4 at the D1 line is used to polarize the Cs atoms and populate the sublevels of the | F g = 4 , m F = 4 ⟩ partly as the transition F g = 4 → F e = 3 remains leaking. Here we utilize a modulation technique to generate a sideband frequency to pump both transitions simultaneously, which drives more atoms to the target state | F g = 4 , m F = 4 ⟩ . This improvement significantly increases the magnetic resonance signal but with little effect on its linewidth. It is very suitable for the miniaturization of the atomic magnetometer in the future.
Anisotropic coherent radiation has been generated from an isotropic medium, in the absence of an external magnetic field, by the spin polarization of an atomic excited state. Lasing on specific hyperfine lines of the 6p2P32→6s2S12 (D2) transition of Cs at 852.1 nm has been realized by photoexciting Cs-rare gas thermal collision pairs with a circularly-polarized (σ+) optical field. Subsequent dissociation of the transient Cs-rare gas B2Σ12+ diatomic molecule selectively populates theF= 4, 5 hyperfine levels of the Cs 6p2P32 state. Not only does electronic spin polarization of the upper laser level yield circularly-polarized coherent emission, but the effective degeneracy (g2) of the 6p2P32 state is altered by the non-statistical hyperfine state population distribution, thereby permitting control of the laser small signal gain with an elliptically-polarized pump optical field. TheD2laser efficiency and output power correlate directly with the molecular orbital structure of the Cs-rare gasB2Σ+state in the region of internuclear separation at which the diatomic complex is born.
The evolution of the hyperfine structure magnetic sublevel populations of 87 Rb and 133 Cs atoms in resonant linearly polarized laser fields is theoretically analyzed. Analytical expressions are obtained for stationary populations of the magnetic sublevels of the ground state 5 2 S 1/2 F g = 1, F f = 2 in 87 Rb and 6 2 S 1/2 F g = 3, F f = 4 in 133 Cs under optical pumping at the F g ↔ F e = F g and F f ↔ F e = F f transitions of the D 2 lines ( n 2 S 1/2 n 2 P 3/2 ) versus the initial populations. The characteristic population stabilization time are obtained by numerical solution for the density matrix equations, depending on the intensity of the laser fields and the detuning of the optical resonance. It is shown that the alternation of the polarization direction of the laser field at the transition F f ↔ F e = F f almost completely transfers atoms to the lower sublevel of the "clock" M1 transition F g M = 0 ↔ F f M = 0 in rubidium and cesium frequency standards, which proportionally increases the signal of the recording system.
Optical pumping makes atom system be partial polarization but magnetic resonance produces depolarization,which is the basic process for investigation of the responsibility to optical pumping for Rb isotope.With improvement on experimental technology,the magnetic resonance signal,also with optical pumping signal,from 85Rb and 87Rb can be completely separated.The relation between detecting signal and physics parameters for optical pumping is drawn out by assuming a model of three energy levels.Results from data analysis indicate the responsibility to optical pumping for 87Rb and 85Rb will be accurately described with unitary relaxation mechanism,respectively.The relaxation time has partly reflected the difference of transition probabilities between energy levels of 87Rb and 85Rb.
Recently the demand for hyperpolarized noble gases is arising to improve NMR spectroscopy resolution for imaging. Hyperpolarization of noble gases is achieved by spin- exchange with alkali metal. atoms that undergo optical pumping by circularly polarized radiation. Optical pumping is usually achieved by high power semiconductor lasers. These sources have a bandwidth very large when compared with the transition bandwidth of the optically active species. This has two serious consequences: (i) the request of high power lasers, (ii) the losses that the unemployed energy delivered to the Rb vapor can provide in the efficiency of the energy transfer. Object of our investigation will be the comparison of two experimental set ups for optical pumping of Rb vapor where pumping is given in one case by a dye laser pumped by a UV nitrogen laser built in our laboratory according to the Blumlein scheme and in the other case by a semiconductor laser. Major elements that will be discussed are (1) the role of stimulated emission of Rb vapor to increase the pumping efficiency, (2) the fate of resonant photons that are emitted during Rb decays, (3) the role of applied magnetic fields in the efficiency of pumping processes.