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

We uncover a highly non-trivial 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 ${6}^2S_{1/2} \leftrightarrow {6}^2P_{3/2}$ ($\mathrm{D}_2$ line) with unresolved hyperfine structure of the excited state. The light-induced broadening of the SN resonance was found to be strongly different at different slopes of the $\mathrm{D}_2$ line and, generally, varied nonmonotonically with the 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 breaks down completely its ground-state coherence and continuity of the spin precession. Spectral asymmetry of the effect, in this case, is provided by position of the `closed' transition $F = 4 \leftrightarrow F = 5$ at the short-wavelength side of the line. This hypothesis, however, remains to be proven by microscopic calculations.