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Optics and Spectroscopy

Influence of transverse configuration of running laser beam on sub-Doppler structure of its absorption spectrum in thin gas cell

А. Ч. Измайлов

Published Jul 1, 2011• 0 citations
Publisher

Abstract

The theoretical investigation is conducted of the strong dependence of sub-Doppler resonances of absorption on the transverse intensity profile of a monochromatic laser beam propagating through a thin cell with rarefied gas medium. The resonances under study correspond to centers of quantum transitions and are caused by the specificity of the optical pumping of atoms during their flight between the walls of a thin cell whose internal thickness is many times smaller than its diameter. The detection of narrow high-contrast sub-Doppler absorption resonances, which are prospective for efficient laser-frequency stabilization, can be facilitated by the proper selection of the radial distribution of the light beam intensity in the cell’s transverse cross section.

DOI
10.1134/s0030400x11070095

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The theoretical investigation is conducted of the strong dependence of sub-Doppler resonances of absorption on the transverse intensity profile of a monochromatic laser beam propagating through a thin cell with rarefied gas medium. The resonances under study correspond to centers of quantum transitions and are caused by the specificity of the optical pumping of atoms during their flight between the walls of a thin cell whose internal thickness is many times smaller than its diameter. The detection of narrow high-contrast sub-Doppler absorption resonances, which are prospective for efficient laser-frequency stabilization, can be facilitated by the proper selection of the radial distribution of the light beam intensity in the cell’s transverse cross section.
This work continues a theoretical investigation of the capabilities of the well-known method based on using a monochromatic probe light beam in combination with optical pumping of atoms (molecules) of a rarefied-gas medium by a broadband radiation in a thin cell the diameter of which is much larger than its internal thickness. In contrast to calculations carried out in the previous publications on this method of spectroscopy, here, we consider the case of arbitrary values of pump intensity and thickness of a cylindrical gas cell. Thus, all the possible mechanisms and specificities of velocity selection of atoms in optically excited levels caused by transit-time relaxation of such atoms in gas cells of this kind are analyzed. Within the framework of this approach, sub-Doppler absorption resonances of the probe light beam corresponding to quantum transitions from the upper level excited by optical pumping are investigated. The obtained results can be used in high-resolution spectroscopy of atoms (molecules), as well as for laser-frequency stabilization to established narrow spectral resonances.
A theoretical investigation is carried out through the interaction of the plane running monochromatic light wave, having an arbitrary intensity, with atoms (molecules) of a rarefied gas in the plane cell (at the normal incidence of the wave). Cases of closed and open resonance transitions from the non-degenerate ground (or metastable) quantum level are considered. Possible sub-Doppler resonances are analysed in the wave absorption, caused by the transient establishment of the optical coherence on the transition, Rabi oscillations between its levels, and optical pumping during the free flights of particles between the walls of the cell. Results of the previous works on given problems are generalized, which were obtained at definite restrictions on the wave intensity and cell length. Moreover, non-trivial sub-Doppler spectral structures, resulting as a consequence of the dependence of the absorption saturation on the transit relaxation of particles, have been established and investigated. Such structures may consist of a number of peaks and dips caused by Rabi oscillations between the transition levels. The results obtained can be used in sub-Doppler spectroscopy and for the stabilization of laser frequencies in thin gas cells.
Abstract A method of sub-Doppler spectroscopy is proposed, based on the peculiarities of narrowing over time of the velocity distribution of optically excited atoms (molecules) of a rarefied gas medium in a thin cell after exposure to a sufficiently short pulse of broadband radiation. Recording of narrow sub-Doppler resonances at the central frequencies of quantum transitions from the atomic state excited by such a pump pulse is carried out by subsequent probe monochromatic light pulses. The optimal conditions for the implementation of this spectroscopy method occur when the radiation decay time of an optically excited atomic level exceeds the characteristic time of flight of atoms between the walls of the thin cell under consideration, the inner thickness of which is many times smaller than its transverse dimensions.
New sub-Doppler resonances at central frequencies of atomic (molecular) transitions that appear in the spectrum of absorption of the probe optical radiation under the influence of optical pumping propagating in the orthogonal direction through a relatively narrow area of a cylindrical cell containing dilute gas medium are discovered and analyzed. These resonances are induced by specific optical pumping of atoms as they fly freely from the inner cell surface through the pumped region toward the probe optical beam. The obtained mathematical relations are used to investigate the dependence of the discussed resonances on the intensity and spatial distribution of the localized optical pumping. The proposed method could allow reducing the Doppler broadening of the detected spectral lines by the factor equal to the ratio of the effective width of the narrow pumped region to the cell radius. The obtained results may find application in high-resolution spectroscopy of atoms (molecules), as well as for laser-frequency stabilization by using the discovered sub- Doppler resonances.
The most narrow sub-Doppler frequency resonances in the linear absorption of monochromatic radiation that propagates in the normal direction through a cell containing a layer of rarefied gas medium with a thickness smaller than or on the order of the wavelength of this radiation are theoretically studied. The calculation is performed using as an example a three-dimensional gas cell shaped like a rectangular parallelepiped. It is shown that the width and amplitude of considered sub-Doppler resonances (in the vicinity of centers of rather weak quantum transitions) significantly depend on the transit relaxation of atomic particles, which is determined by their transit times through the irradiated region of the cell both in longitudinal and in transverse directions. The restrictions of the approximation of the planar one-dimensional cell that was previously used in such calculations are determined. Possible applications of linear absorption resonances in ultrathin (nanometer) gas cells as references for optical frequency standards are discussed.
The method of sub-Doppler spectroscopy is theoretically elaborated, which is based on the specific dynamics of a number of optically excited atomic particles (atoms or molecules) of a rarefied gas medium in a thin cell after the action of the resonance pulse of the monochromatic radiation. Corresponding calculations are carried out on the basis of density matrix equations for the resonance optical transition between Zeeman degenerate ground and excited quantum levels of particles in case of the linear polarization of the laser pulse at its normal incidence on the cell. The situation is considered when the radiative lifetime of the excited level is much more than the characteristic transit time of particles between nearest plane-parallel walls of the cell. Then the distribution of a number of excited particles versus the pulse frequency detuning narrows in the process of particles collisions with walls of the cell after action of the laser pulse. The factor of such a narrowing (in comparison with the Doppler broadening of the spectral line of the resonance transition) may be more than the ratio of the characteristic transverse size of the thin gas cell to its inner thickness. We discuss possible use of given sub-Doppler resonances (of the number of excited particles) in the high-resolution spectroscopy and also in high-selective processes of photo-ionization and photo-dissociation, especially, for isotope (or isomer) separation and detection of rare (in particular single) atoms or molecules of a gas medium.