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Journal of the Optical Society of America B

Narrow coherence-induced peaks in Doppler-free spectra of thorium

Brian M. Tissue, Bryan L. Fearey

Published Apr 1, 1994• 3 citations
Publisher

Abstract

We report the observation of narrow peaks in Doppler-free spectra of thorium. The peaks are superimposed upon saturation-broadened line profiles in thorium atomic spectra by use of counterpropagating cw laser beams with resonance ionization mass spectrometry detection. At low laser powers the spectra show a conventional Lamb dip. As the laser power is increased, a narrow peak obscures the dip (for two of four transitions studied). The narrow peak is attributed to coherence effects at high laser powers. To confirm this assignment, we calculated the saturated absorption line shapes by using the theory of Haroche and Hartmann [ Phys. Rev. A6, 1280 ( 1972)]. The spectral simulations reproduce the narrow peaks and show the same dependence with laser power and transition strength as do the data. There are some quantitative discrepancies, particularly in the overall saturation broadening, but given that there were no adjustable parameters in the calculations, the agreement between theory and experiment is considered quite good. The Einstein A coefficients used in the calculations were determined from an analysis of the laser-power dependence of the saturation broadening for each transition.

DOI
10.1364/josab.11.000542

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We report the observation of narrow peaks in Doppler-free spectra of thorium. The peaks are superimposed upon saturation-broadened line profiles in thorium atomic spectra by use of counterpropagating cw laser beams with resonance ionization mass spectrometry detection. At low laser powers the spectra show a conventional Lamb dip. As the laser power is increased, a narrow peak obscures the dip (for two of four transitions studied). The narrow peak is attributed to coherence effects at high laser powers. To confirm this assignment, we calculated the saturated absorption line shapes by using the theory of Haroche and Hartmann [ Phys. Rev. A6, 1280 ( 1972)]. The spectral simulations reproduce the narrow peaks and show the same dependence with laser power and transition strength as do the data. There are some quantitative discrepancies, particularly in the overall saturation broadening, but given that there were no adjustable parameters in the calculations, the agreement between theory and experiment is considered quite good. The Einstein A coefficients used in the calculations were determined from an analysis of the laser-power dependence of the saturation broadening for each transition.
Saturation effects affecting absorption and fluorescence spectra of an atomic vapor confined in an extremely thin cell (cell thickness $L<1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$) are investigated experimentally and theoretically. The study is performed on the D2{D}_{2}D2​ line $(\ensuremath{\lambda}=852\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ of Cs and concentrates on the two situations $L=\ensuremath{\lambda}∕2$ and $L=\ensuremath{\lambda}$, the most contrasted ones with respect to the length dependence of the coherent Dicke narrowing. For $L=\ensuremath{\lambda}∕2$, the Dicke-narrowed absorption profile simply broadens and saturates in amplitude when increasing the light intensity, while for $L=\ensuremath{\lambda}$, sub-Doppler dips of reduced absorption at the line-center appear on the broad absorption profile. For a fluorescence detection at $L=\ensuremath{\lambda}$, saturation induces narrow dips, but only for hyperfine components undergoing a population loss through optical pumping. These experimental results are interpreted with the help of the various existing models and are compared with numerical calculations based upon a two-level modeling that considers both a closed and an open system.
The line shape of a Doppler-broadened transition is dramatically altered by the presence of a laser field resonating with a second Doppler-broadened transition sharing a common level: Two narrow resonances of different widths appear superimposed upon the broad background signal at frequencies symmetrically located about the corresponding line center. The effect has already found application in a number of seemingly different though intimately related studies, including high-resolution hfs and isotope-shift determinations. The theory of the effect is developed with reference to these applications. The treatment is formulated in terms of transition rates induced by two classical fields resonantly interacting with a pair of coupled Doppler-broadened transitions of arbitrary frequencies. The perturbation approach adopted is valid for one field fully saturating its transition; the resulting line-shape expression exhibits important power-broadening effects. This approach is equivalent to the familiar density-matrix formulation, which is also presented. Various features of the resulting expression are discussed in detail as they apply to two precision spectroscopic applications, mode crossing and spontaneous emission line narrowing. The connection with previous work is also discussed.
We have studied the effects of optical pumping and near-resonant radiation pressure that compete with the saturation effect on the formation of the line shapes of Doppler-free saturation absorption spectra of alkali atoms. We have measured the reverse of the sign of F g =3 to F e =4 saturated absorption of the rubidium D 2 line and the change in its asymmetry as the optical pumping effect is gradually turned on compared with the saturation effect. The experimentally observed behaviors of optically pumped Lamb dips and crossover resonances in rubidium show good agreement with the calculation using the phenomenological model previously introduced for cesium.
The method of Doppler – free comb – spectroscopy for dipole transitions was proposed. The numerical calculations for susceptibility spectrum for two-level system driving by strong counter propagating combs were made. The narrow peaks with homogeneous width arise on the background of Doppler counter. The contrast of these peaks is large for largest amplitudes of comb-components. Power broadening is increasing with increasing of field amplitudes. The amplitudes of peaks depend on the phase difference between carrier frequencies of combs. The spectral range of absorption spectrum is determined by the spectral range of comb generator and all homogeneous lines arise simultaneously. The spectral resolution is determined by the width of homogeneously –broadening lines. The physical nature of narrow peaks is in the existing of multi-photon transitions between manifolds of quasi-energy levels arising for different groups of atoms moving with velocities that satisfy to the resonant conditions 2kv= (n+l)Ω⁄j, where n, l, j-are integers and Ω - frequency difference between comb teeth.
The interaction of two closely spaced resonances in degenerate four-wave-mixing spectra is investigated theoretically. Degenerate four-wave-mixing spectra for the phase-conjugate geometry (counterpropagating pump beams) are calculated by integrating the time-dependent density-matrix equations at numerous grid points along the phase-matching axis and summing the polarization contribution from each of these grid points. Both purely homogeneously broadened resonances and resonances that are both collision and Doppler broadened are considered. The homogeneously broadened results are found to agree with the results of the widely used Abrams–Lind expression for the case of stationary absorbers exposed to a steady-state excitation. We examine the effects of power broadening, pressure broadening, transition dipole moment, and population on the spectra of two neighboring homogeneously broadened resonances. As the laser power is increased, the real parts of the resonance susceptibilities destructively interfere with each other in the region between the lines, resulting in a significant and persistent dip in reflectivity. When Doppler broadening is included in the calculations, similar interference effects are observed, although at first the interference dip begins to disappear until the homogeneous part of the line shapes is broadened significantly by saturation. Using this numerical approach, we obtain good agreement with experimental results of primarily Doppler-broadened neighboring resonances.
We have studied theoretically nontrivial specific features of sub-Doppler resonances of saturated absorption in a multilayer gas cell with a rarefied gas medium, which is a compact analog of a large number of plane-parallel atomic beams. Spatially separated saturating and probing monochromatic laser beams that co- and counterpropagate (at the same frequency) in this cell have been considered. In this situation, the action of the light radiation of the saturating beam on the probing beam is determined by optically pumped atoms that fly between the beams under conditions that the longitudinal components of their velocities experience selection due to a specific geometry of the multilayer cell. Such a selection leads to a narrowing of the sub-Doppler resonance in the absorption of the probing beam and decreases the difference between the structures of this resonance for the cases of co- and counterpropagation of the saturating radiation. We have found that, in the considered multilayer cell, the effective width of the sub-Doppler resonance can be smaller (by a factor of about 1.5) than the extremely narrow characteristic width of the well-known Lamb dip in spectroscopy of saturated absorption in the standard gas cell. Results of this study can be used in atomic spectroscopy of ultrahigh resolution and for the laser-frequency stabilization.