RESUMO
Per the fluctuation-dissipation theorem, the information obtained from spin fluctuation studies in thermal equilibrium is necessarily constrained by the system's linear response functions. However, by including weak radio frequency magnetic fields, we demonstrate that intrinsic and random spin fluctuations even in strictly unpolarized ensembles can reveal underlying patterns of correlation and coupling beyond linear response, and can be used to study nonequilibrium and even multiphoton coherent spin phenomena. We demonstrate this capability in a classical vapor of (41)K alkali atoms, where spin fluctuations alone directly reveal Rabi splittings, the formation of Mollow triplets and Autler-Townes doublets, ac Zeeman shifts, and even nonlinear multiphoton coherences.
RESUMO
'Spin noise spectroscopy' is an optical technique for probing electron and hole spin dynamics that is based on detecting their intrinsic fluctuations while in thermal equilibrium. Here we show that fluctuation correlations can be further exploited in multi-probe noise studies to reveal information that in general cannot be accessed by conventional linear optical spectroscopy, such as the underlying homogeneous linewidths of individual constituents within inhomogeneously broadened systems. This is demonstrated in singly charged (In,Ga)As quantum-dot ensembles using two weak probe lasers: When the lasers have similar wavelengths, they probe the same quantum dots in the ensemble and show correlated spin fluctuations. In contrast, mutually detuned probe lasers measure different subsets of quantum dots, giving uncorrelated fluctuations. The noise correlation versus laser detuning directly reveals the quantum dot homogeneous linewidth even in the presence of a strong inhomogeneous broadening. Such noise-based correlation techniques are not limited to semiconductor spin systems, but are applicable to any system with measurable intrinsic fluctuations.
