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Journal of Physics B Atomic and Molecular Physics

Subnatural linewidth effects in non-stationary velocity-selective optical pumping

Wojciech Gawlik, J. Kowalski, Frank N. Trager, Michael Vollmer

Published Mar 14, 1987• 5 citations
Publisher

Abstract

Signals with subnatural linewidths observed in a polarisation spectroscopy experiment on sodium have been investigated under various experimental conditions. The authors find that the mechanism responsible for the subnatural structures is based on non-stationary velocity-selective optical pumping. It is discussed in the light of the presented measurements and a theoretical analysis. Possible spectroscopic applications are considered.

DOI
10.1088/0022-3700/20/5/018

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We have observed that spectral lines obtained in sodium vapor can be strongly distroted when using two-photon spectroscopy with a resonant intermediate state. In some cases, the spectral lines appear to be split into two components. We demonstrate that these effects are due to velocity-selective optical pumping of the atomic ground state; the line shapes reflect the velocity distribution of atoms in the excited intermediate state. We have analyzed the effects using the time-dependent solutions of the rate equations which describe the time evolution of optical pumping for the various atomic velocity groups, and we obtain good agreement with experiment. Optical-pumping problems are often analyzed with the use of steady-state rate equations which incorporate phenomenological relaxation; such approaches are not capable of describing our observations. Velocity-changing collisions are shown to suppress the effects and they explain why these effects were not previously observed; the previous experiments were carried out with slight amounts of a buffer gas in the vapor cell. The effects reported here must be accounted for when carrying out laser-induced line-narrowing experiments in atoms or molecules that can be optically pumped; they can also be used as a new and more sensitive probe of optical pumping in vapors.

Physical Review A•Nov 1, 1982•32 citations
Signals with subnatural linewidths observed in a polarisation spectroscopy experiment on sodium have been investigated under various experimental conditions. The authors find that the mechanism responsible for the subnatural structures is based on non-stationary velocity-selective optical pumping. It is discussed in the light of the presented measurements and a theoretical analysis. Possible spectroscopic applications are considered.
Optical resonances with subnatural linewidth as narrow as 2.6 MHz have been observed with excellent signal-to-noise ratio in the Na D1{D}_{1}D1​ line (natural width 10 MHz) in a polarization spectroscopy experiment. The new, subnatural resonances arise from Zeeman coherences which are produced by a strong probe beam and destroyed by a velocity-selective optical Hanle effect.
Time-dependent analysis of nonstationary effects in velocity-selective optical pumping is performed with a four-level model for weak light intensities. No phenomenological relaxation constants are introduced and the nonstationary effects are obtained as the consequence of averaging the time evolution of individual atoms in a vapor phase moving in the light fields when typical pumping times are comparable with the finite laser-beam transit times. Under appropriate conditions strong deformation of the population distribution of the long-lived lower state occurs, which in the case of polarization spectroscopy is responsible for the appearance of a dip (possibly narrower than the homogeneous linewidth) in the center of a Doppler-free resonance. The theoretical results are in a good qualitative agreement with the recent experimental observations.
Theoretical analysis of D 1 lines of most stable alkali atoms in velocity selective optical pumping (VSOP) spectroscopy is presented by using the approximation of a single cycle of optical pumping in a four-level system. VSOP spectroscopy is clearly compared with polarization spectroscopy and then it is shown that the results obtained for polarization spectroscopy can be applied to the theory in VSOP spectroscopy. The relative signal intensities of Na D 1 line in polarization spectroscopy have been theoretically calculated and experimentally confirmed to be in good agreement with those in VSOP spectroscopy. The first-order theory in VSOP spectroscopy can be applied to the cases of the D 1 lines of the other alkali atoms. It is expected that the theoretical analysis can be useful for future experiments of the Doppler-free D lines in the alkali atoms.
We have observed that spectral lines obtained in sodium vapor can be strongly distroted when using two-photon spectroscopy with a resonant intermediate state. In some cases, the spectral lines appear to be split into two components. We demonstrate that these effects are due to velocity-selective optical pumping of the atomic ground state; the line shapes reflect the velocity distribution of atoms in the excited intermediate state. We have analyzed the effects using the time-dependent solutions of the rate equations which describe the time evolution of optical pumping for the various atomic velocity groups, and we obtain good agreement with experiment. Optical-pumping problems are often analyzed with the use of steady-state rate equations which incorporate phenomenological relaxation; such approaches are not capable of describing our observations. Velocity-changing collisions are shown to suppress the effects and they explain why these effects were not previously observed; the previous experiments were carried out with slight amounts of a buffer gas in the vapor cell. The effects reported here must be accounted for when carrying out laser-induced line-narrowing experiments in atoms or molecules that can be optically pumped; they can also be used as a new and more sensitive probe of optical pumping in vapors.