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Applied Optics

Sub-Doppler spectroscopy of metastable atomic particles in ultrathin gas cells

А. Ч. Измайлов, Chingiz O. Qajar, R. A. Karamaliev

Published Dec 2, 2011• 2 citations
Publisher

Abstract

On the basis of density matrix equations, we theoretically investigated narrow sub-Doppler resonances in the spectral distribution of a number of atomic particles (atoms or molecules) optically excited in a metastable state in an ultrathin gas cell with the inner thickness less than or of the order of the wavelength λ ≤ 1 μm of the monochromatic light wave incident in the normal direction. Essential dependence of these resonances (on the center of the optical transition) was analyzed versus the inner thickness and transversal size of such a gas cell and also versus the intensity of the incident wave. Investigated resonances may be applied as references in high-accuracy optical frequency standards and for high precision measurements.

DOI
10.1364/ao.50.006424

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On the basis of density matrix equations, we theoretically investigated narrow sub-Doppler resonances in the spectral distribution of a number of atomic particles (atoms or molecules) optically excited in a metastable state in an ultrathin gas cell with the inner thickness less than or of the order of the wavelength λ ≤ 1 μm of the monochromatic light wave incident in the normal direction. Essential dependence of these resonances (on the center of the optical transition) was analyzed versus the inner thickness and transversal size of such a gas cell and also versus the intensity of the incident wave. Investigated resonances may be applied as references in high-accuracy optical frequency standards and for high precision measurements.
Theoretical studies have been conducted on sub-Doppler resonances of fluorescence of an ultrathin gas layer in a cell with an internal weakness less than or on the order of the wavelength of a monochromatic light beam incident on the given cell in the normal direction. The features of these resonances are analyzed both under steady-state action of the incident radiation and immediately following abrupt termination of its action. The essential dependence of these resonances (at the centers of optical transitions with sufficiently small radiation spectral linewidths) on the transversal dimensions of the gas cell (using the example of a rectangular parallelepiped shape). The main focus is on analyzing the linear optics case when the examined resonances are narrowest. The possibility is noted of using such sub-Doppler fluorescence resonances in nanometer gas cells as references for optical frequency standards.
Abstract The nonlinear optical effect of free polarization decay (FPD) in an ultrathin gas cell, the inner thickness of which is less than or on the order of the wavelength of the exciting monochromatic laser radiation propagating orthogonally to the plane-parallel walls of this cell, is studied theoretically. The considered coherent FPD signal is emitted immediately after abrupt termination of this comparatively weak stationary radiation. The sub-Doppler resonances appearing at the central frequencies of the studied quantum transitions in the FPD intensity and energy spectra due to the transit-time relaxation of atoms in the gas cell are detected and studied. It is shown that these resonances and the FPD dynamics strongly depend on the ratio between the inner thickness of the gas cell and the wavelength of the exciting monochromatic radiation. The narrow high–contrast sub-Doppler resonances of FPD observed and studied in a linear optical regime can be used in atomic (molecular) high-resolution spectroscopy and as references for compact frequency standards.
The paper theoretically establishes and studies the sub-Doppler linear optical Ramsey resonances that arise under certain conditions near centers of optical transitions in the absorption of a sufficiently weak monochromatic light beam during its stationary propagation in the normal direction through an ultrathin gas cell whose internal thickness is less than or of the order of the wavelength of this radiation. We consider a situation when the cross section of this beam consists of two coaxial spatially separated regions, with the absorption signal being detected in a comparatively narrow central part of the beam. The paper studies the significant dependence of the linear optical Ramsey resonances, that arise in such an absorption spectrum, on the distance between these regions, as well as on the phase difference between them. In particular, it is shown that if this phase difference is close to pi, then instead of absorption, amplification of the central part of the incident beam can occur. The Ramsey resonances under consideration are most clearly manifested when the internal thickness of the gas cell is equal to a small halfinteger number of wavelengths of the resonant radiation. However, these resonances do not arise if this thickness is equal to an integer number of such waves. The established Ramsey resonances, under certain conditions, can find application in ultrahigh resolution atomic (molecular) spectroscopy, as well as effective references in compact optical frequency standards.
Abstract Excitation of atoms from the ground quantum level to a metastable state by traveling monochromatic radiation in a thin gas cell, the internal thickness of which is much smaller than its radius, is studied theoretically. Sub-Doppler resonances are detected and analyzed that arise in the population of a metastable atomic level due to light-induced Rabi oscillations on a given optical transition and specific features of transit-time relaxation of atoms in a considered cell with a rarified gas upon scanning the radiation frequency. Such nontrivial resonances can be recorded in fluorescence of atoms with the help of an additional probe light beam. A method is proposed for a direct measurement of the Rabi frequency from spectral intervals between detected sub-Doppler resonances. The results obtained can be applied in atomic spectroscopy for the precision determination of components of dipole moment matrix elements, as well as for the Einstein coefficients and oscillator strengths related to these components, for quantum transitions between the ground and metastable levels of atoms.
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 degen- erate 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 par- ticles collisions with walls of the cell after action of the laser pulse. The factor of such a narrowing (in compar- ison 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.
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.