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

TL;DR

This paper investigates the complex, nonmonotonic broadening of spin-noise resonance in cesium vapor under intense light, revealing dependence on excitation pathways and proposing a novel explanation involving excited-state relaxation.

Contribution

It introduces a new understanding of light-induced spin-noise broadening, emphasizing the role of excited-state relaxation pathways and their impact on ground-state coherence.

Findings

Spin-noise broadening varies nonmonotonically with light power.

The effect depends on the relation between decay, precession, and decoherence rates.

Spectral asymmetry is linked to the position of specific hyperfine transitions.

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 ${6}^2S_{1/2} \leftrightarrow {6}^2P_{3/2}$ ($\mathrm{D}_2$ 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 $\mathrm{D}_2$ 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 $F = 4 \leftrightarrow F' = 5$ at the short-wavelength side of the line. This hypothesis, however, remains to be proven by microscopic calculations.