Intensity-related optical susceptibility and 'active' noise spectroscopy

In this paper, we consider informative potentialities of the 'active' optical noise spectroscopy, under which we understand, generally, the spectroscopy of the response of a multilevel quantum system to a resonant optical field with its intensity (or polarization) modulated by a broadband ('white') noise. We show that, in the linear approximation, such a response can be treated most conveniently by introducing the notion of Stokes susceptibility (SS) whose spectrum is determined by a Laplace transform of the response to a small step-wise change of the light beam Stokes vector. The results of the calculations performed for a specific four-level energy diagram, typical for certain low-dimensional quantum systems, show that the low-frequency SS spectrum may provide information not only about the ground-state structure of the system (like conventional 'passive' spin noise spectroscopy), but also about the properties of the optical transitions (including nutation frequencies in the applied optical field). The considered version of the spin noise spectroscopy allows one, on the one hand, to take advantage of the highly efficient methods of data acquisition developed nowadays in this field of research and, on the other, to considerably broaden potentialities of the noise spectroscopy. (paper)