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arXiv:2102.01679 · Physical Review A

Anomalous light-induced broadening of the spin-noise resonance in cesium vapor

Anatoly A. Fomin, M. Yu. Petrov, G. G. Kozlov, Anton K. Vershovskii, Михаил Михайлович Глазов, Valerii S Zapasskii

Published Apr 29, 2021• 6 citations
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Abstract

We uncover a highly nontrivial dependence of the spin-noise (SN) resonance broadening induced by the intense probe beam. The measurements were performed by probing the cell with cesium vapor at the wavelengths of the transition $6^{2}S_{1/2}\ensuremath{\leftrightarrow}6^{2}P_{3/2}$ (${D}_{2}$ line) with the unresolved hyperfine structure of the excited state. The light-induced broadening of the SN resonance was found to differ strongly at different slopes of the ${D}_{2}$ line and, generally, varied nonmonotonically with light power. We discuss the effect in terms of the phenomenological Bloch equations for the spin fluctuations and demonstrate that the SN broadening behavior strongly depends on the relation between the pumping and excited-level decay rates, the spin precession, and decoherence rates. To reconcile the puzzling experimental results, we propose that the degree of optical perturbation of the spin system is controlled by the route of the excited-state relaxation of the atom or, in other words, that the act of optical excitation of the atom does not necessarily break down completely its ground-state coherence and continuity of the spin precession. Spectral asymmetry of the effect, in this case, is provided by the position of the ``closed'' transition $F=4\ensuremath{\leftrightarrow}{F}^{\ensuremath{'}}=5$ at the short-wavelength side of the line. This hypothesis, however, remains to be proven by microscopic calculations.

DOI
10.1103/physreva.103.042820

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We uncover a highly nontrivial dependence of the spin-noise (SN) resonance broadening induced by the intense probe beam. The measurements were performed by probing the cell with cesium vapor at the wavelengths of the transition 62S1/2\ensuremath↔62P3/26^{2}S_{1/2}\ensuremath{\leftrightarrow}6^{2}P_{3/2}62S1/2​\ensuremath↔62P3/2​ (D2{D}_{2}D2​ line) with the unresolved hyperfine structure of the excited state. The light-induced broadening of the SN resonance was found to differ strongly at different slopes of the D2{D}_{2}D2​ line and, generally, varied nonmonotonically with light power. We discuss the effect in terms of the phenomenological Bloch equations for the spin fluctuations and demonstrate that the SN broadening behavior strongly depends on the relation between the pumping and excited-level decay rates, the spin precession, and decoherence rates. To reconcile the puzzling experimental results, we propose that the degree of optical perturbation of the spin system is controlled by the route of the excited-state relaxation of the atom or, in other words, that the act of optical excitation of the atom does not necessarily break down completely its ground-state coherence and continuity of the spin precession. Spectral asymmetry of the effect, in this case, is provided by the position of the ``closed'' transition F=4\ensuremath↔F\ensuremath′=5F=4\ensuremath{\leftrightarrow}{F}^{\ensuremath{'}}=5F=4\ensuremath↔F\ensuremath′=5 at the short-wavelength side of the line. This hypothesis, however, remains to be proven by microscopic calculations.
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The effect of optically driven spin precession discovered by Bell and Bloom [W. E. Bell and A. L. Bloom, Phys. Rev. Lett. 6, 280 (1961)] is widely used nowadays as a basis for numerous experiments in fundamental physics and for diverse applications. In this paper we consider a much less popular version of the light-induced spin precession that does not imply coherent precession of the spin-system magnetization and is excited by linearly (rather than circularly) polarized light. Pump-probe measurements performed on the D2{D}_{2}D2​ line of cesium vapor show that the magnitude of the signal of the optically driven spin-alignment precession, in ``vacuum'' cells (with no buffer gas) is close to that of classical spin-orientation precession. In the presence of buffer gas, however, the signal of spin-alignment precession appears to be strongly suppressed. The discovered effect is ascribed to spin mixing of excited states of cesium atoms in the cycle of optical pumping.
Highly sensitive Faraday rotation spectroscopy is used to measure the fluctuating magnetization noise of noninteracting rubidium atoms under resonant and nonresonant optical probing conditions. The spin-noise frequency spectra, in conjunction with the probe light detuning with respect to the D2{D}_{2}D2​ transition, reveal clear signatures of coherent coupling of the participating electronic levels. The results are explained by extended Bloch equations, including homogeneous and inhomogeneous broadening mechanisms. Our measurements further indicate that spin noise originating from excited states is governed at high intensities by collective effects.
A new and unusual phenomenon which we call light narrowing is reported and discussed in this paper. We discovered this effect in dense, spin-polarized cesium vapor optically pumped with a cw blue dye laser beam tuned to the second resonance D1{D}_{1}D1​ line (4593 \AA{}). We observe a significant narrowing of the radio-frequency power-broadened magnetic resonance lines (linewidths narrow by as much as a factor of 2.5) when the intensity of the circularly polarized incident dye laser beam is increased by either focusing the beam or by the removal of attenuating filters from the focused beam. The magnetic resonance linewidths in spin-polarized cesium vapor were measured over a wide range of cesium number densities (5\ifmmode\times\else\texttimes\fi{}1012{10}^{12}1012 ${\mathrm{cm}}^{\ensuremath{-}3}$ \ensuremath{\le}[Cs]\ensuremath{\le}1\ifmmode\times\else\texttimes\fi{}1016{10}^{16}1016 ${\mathrm{cm}}^{\ensuremath{-}3}$). This corresponds to cesium spinexchange rates of 4.5\ifmmode\times\else\texttimes\fi{}103{10}^{3}103 to 9\ifmmode\times\else\texttimes\fi{}106{10}^{6}106 ${\mathrm{sec}}^{\ensuremath{-}1}$. For low cesium number densities (5\ifmmode\times\else\texttimes\fi{}1012{10}^{12}1012 [Cs]3\ifmmode\times\else\texttimes\fi{}1014{10}^{14}1014 ${\mathrm{cm}}^{\ensuremath{-}3}$) the light-narrowing effect is large (a factor of 2.5) and independent of [Cs]. In the region of 3\ifmmode\times\else\texttimes\fi{}1014{10}^{14}1014 to 1\ifmmode\times\else\texttimes\fi{}1015{10}^{15}1015 ${\mathrm{cm}}^{\ensuremath{-}3}$ in number densities, the light-narrowing effect decreases with increasing [Cs]. At high cesium number densities ([Cs]> 1\ifmmode\times\else\texttimes\fi{}1015{10}^{15}1015 ${\mathrm{cm}}^{\ensuremath{-}3}$) this light-narrowing effect almost completely disappears. In the limit of low-radio-frequency power the magnetic resonance linewidths for focused and unfocused dye laser beam are nearly the same. Experimental observations on this new effect are presented in detail. In the latter part of this paper a self-contained theoretical treatment of the light-narrowing effect is developed. Using Bloch equations in the presence of optical pumping, spin relaxation (diffusion, electron randomization), rapid spin exchange, and radio-frequency magnetic field, expressions for magnetic resonance line shapes are derived. In general, we find good agreement between our experimental results and the theory.
Bright and dark nonlinear magneto-optical resonances associated with the ground state Hanle effect have been studied experimentally and theoretically for D1{D}_{1}D1​ excitation of atomic cesium. This system offers the advantage that the separation between the different hyperfine levels exceeds the Doppler width, and hence transitions between individual levels can be studied separately. At the same time, the system retains the advantages offered by ordinary glass cells, including simplicity and subnatural width Hanle resonances. Experimental measurements for various laser power densities and transit relaxation times are compared with a model based on the optical Bloch equations, which averages over the Doppler contour of the absorption line and simultaneously takes into account all hyperfine levels, as well as mixing of magnetic sublevels in an external magnetic field. In contrast to previous studies, which could not resolve the hyperfine transitions because of Doppler broadening, in this study there is excellent agreement between experiment and theory regarding the sign (bright or dark), contrast and width of the resonance. The results support the traditional theoretical interpretation, according to which these effects are related to the relative strengths of transition probabilities between different magnetic sublevels in a given hyperfine transition.
The relaxation of atomic polarization in buffer-gas-free, paraffin-coated cesium vapor cells is studied using a variation on Franzen's technique of ``relaxation in the dark'' [Franzen, Phys. Rev. 115 850 (1959)]. In the present experiment, narrow-band, circularly polarized pump light, resonant with the Cs D2 transition, orients atoms along a longitudinal magnetic field, and time-dependent optical rotation of linearly polarized probe light is measured to determine the relaxation rates of the atomic orientation of a particular hyperfine level. The change in relaxation rates during light-induced atomic desorption (LIAD) is studied. No significant change in the spin relaxation rate during LIAD is found beyond that expected from the faster rate of spin-exchange collisions due to the increase in Cs density.