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Physical Review Letters

Cross-Relaxation Effects in the Saturation of the 6328-Å Neon-Laser Line

P. W. E. Smith, R. Hänsch

Published Mar 29, 1971• 175 citations
Publisher

Abstract

We report experiments which show that saturation of the inhomogeneously broadened 6328-\AA{} neon-laser line is strongly affected by cross relaxation which tends to redistribute the excited atoms over the Maxwellian velocity distribution and thus leads to partially homogeneous line saturation. Our results for ${\mathrm{Ne}}^{20}$ discharges agree well with theoretical calculations based on a "strong-collision" model. Results for ${\mathrm{He}}^{3}$:${\mathrm{Ne}}^{20}$x mixtures show deviations from this model that we attribute to the "weak" cross relaxation caused by He-Ne collisions.

DOI
10.1103/physrevlett.26.740

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We report experiments which show that saturation of the inhomogeneously broadened 6328-\AA{} neon-laser line is strongly affected by cross relaxation which tends to redistribute the excited atoms over the Maxwellian velocity distribution and thus leads to partially homogeneous line saturation. Our results for Ne20{\mathrm{Ne}}^{20}Ne20 discharges agree well with theoretical calculations based on a "strong-collision" model. Results for He3{\mathrm{He}}^{3}He3:Ne20{\mathrm{Ne}}^{20}Ne20x mixtures show deviations from this model that we attribute to the "weak" cross relaxation caused by He-Ne collisions.
The role of collisional cross relaxation in saturated laser amplification is considered by using a simple strong-collisional model. Expressions for the particle-state density distribution and gain are derived for a four-level system. These are reduced to a simple analytic form, subject to an approximation of wide applicability. The role of collisions in changing an inhomogeneous-broadened line to homogeneous, with a change from rebroadening to narrowing on saturation, is demonstrated. The results are shown to be consistent with the existing limited experimental data.
The interaction of an intense optical-maser field with excited atoms in a gas discharge leads to a considerable change in the population of those levels which are resonant at the maser frequency. This will cause a change in the population of other levels that are connected to the maser levels through collision-induced nonradiative as well as radiative transitions. Using the 1.15-\ensuremath{\mu} optical maser, these effects have been studied in detail for the 2s2s2s levels of neon where the changes are completely due to collisions. By analyzing the intensity changes in spontaneous emission originating from a gas-discharge cell placed within the maser resonator, we have observed the approach to a partial thermalization of two closely spaced levels. In this way we have been able to obtain the atomic-collision cross section for excitation transfer between the 2s22{s}_{2}2s2​ and 2s32{s}_{3}2s3​ levels. In pure neon this has been found to be ${\ensuremath{\sigma}}_{23}=(2.3\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2. For a helium-neon mixture the measured cross section is ${\ensuremath{\sigma}}_{23}=(1.8\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2. The experimental method used to determine these cross sections eliminates the uncertainties caused by the effects of electron-atom collisions and radiative cascade. These techniques also yield the ratio of the Einstein coefficients for the transitions $2{s}_{2}\ensuremath{\rightarrow}2{p}_{10}$ and $2{s}_{3}\ensuremath{\rightarrow}2{p}_{10}$. The measured ratio is $\frac{{A}_{2}}{{A}_{3}}=3.0\ifmmode\pm\else\textpm\fi{}0.4$.
A previously developed diagrammatic method for calculating the nonlinear response of a dilute gas at near resonance with strong classical radiation fields is extended to include velocity-dependent homogeneous and inhomogeneous broadening phenomena. The method is applied to two-level saturated-absorption (Lamb-dip) spectroscopy. Expressions are obtained for the collision-broadened Lamb dip probed by a weak field in the presence of an arbitrarily strong saturating field and are compared to the usual infinite-Doppler-width, weak-saturation expression. We study the effects of velocity and momentum transfer on the line-shape parameters and discuss the effect of phase-changing and strong inelastic collisions on the intensity.
The spectral profile of the strongly resonance-broadened transition lambda 585.2 nm of Ne I has been studied in absorption. A narrow-band tunable dye laser probed a weak discharge in natural neon cooled with liquid nitrogen. The Lorentzian width extrapolated to zero density was found to be 108+or-20 MHz, consistent with the value 114+or-4 MHz calculated from the natural widths of the levels. There is no indication of the excess width found in earlier emission experiments in helium and neon. A preliminary result of (1.7+or-0.2)10 -21 MHz atom -1 m 3 was obtained for the broadening constant of lambda 585.2 nm in absorption at 77K. This is consistent with the value found in emission.
The effect of an intense pump beam on the intensity of the phase-conjugated emission is studied in the case of a three-level atom. We first extend to the case of cross-polarized pump beams the dressed-atom model that we have presented previously [Phys. Rev. A 34, 4008 (1986)] for the case of two-level atoms. The range of validity of our theory is \ensuremath{\Gamma} (natural width) ${\ensuremath{\Omega}}_{1}$ (Rabi frequency of the intense pump) ku (Doppler width). We have studied the line shape in degenerate four-wave mixing, the influence of a static magnetic field, and the resonances that appear in nondegenerate four-wave mixing. Experiments are done in a neon discharge using cw dye lasers. The experimental results are in good agreement with the theoretical predictions. In degenerate four-wave mixing, we have shown that the saturating dip disappears when the weak beams have orthogonal polarizations. In the presence of a static magnetic field and in the nearly degenerate case, narrow resonances corresponding to transitions between energy levels of the dressed atom are observed. The intensity and the width of these resonances are in very good agreement with the predictions of the theoretical model.