The existence of a novel resonance in the modulation of the fluorescence light emitted by an atom or a molecule when coherently excited by a rapidly rotating linearly polarized light beam is predicted. Such resonances are experimentally observed due to a transfer of coherence from light to atoms or molecules, via either stimulated emission or absorption of light, without having to shift the positions of the levels by an external perturbation. These resonances directly permit the measurement of energy level lifetimes, even in the case where usual magnetic resonance methods would require the use of large magnetic fields.
A precise, accurate, and relatively straightforward in situ method to measure and control the ellipticity of light resonantly interacting with an atomic vapor is described. The technique can be used to minimize vector light shifts. The method involves measurement of ellipticity-induced resonances in the magnetic-field dependence of nonlinear magneto-optical rotation of frequency-modulated light. The light propagation direction is orthogonal to the applied magnetic field
B and the major axis of the light polarization ellipse is along
B. When the light modulation frequency matches the Larmor frequency, elliptically polarized light produces a precessing atomic spin orientation transverse to
B via synchronous optical pumping. The precessing spin orientation causes optical rotation oscillating at the Larmor frequency by modulating the atomic vapor's circular birefringence. Based on this technique's precision, in situ nature (which avoids systematic errors arising from optical interfaces) and independent control of the most important systematic errors, it is shown that the accuracy of light ellipticity measurements achievable with this technique can exceed that of existing methods by orders of magnitude.
We report on investigations of nonlinear magneto-optical rotation (NMOR) in rubidium vapor subjected to a modulated magnetic field and continuous-wave (cw) laser-light illumination. By superimposing modulation and a static (dc) magnetic field, we demonstrate the appearance of resonances at both small and large (compared to the ground-state relaxation rate) values of the static field. Since in conventional NMOR there is no rotation at high fields, this suggests an existence of an alternative mechanism generating anisotropy in the considered case, which we identify as parametric resonance. The experiments are performed using light of small ellipticity, and rotation signals are significantly enhanced by combining atom-induced polarization rotation with a passive rotation induced with a wave plate. All the observations are supported with theoretical simulations. The density-matrix formalism and angular-momentum probability surfaces are used to provide an intuitive explanation of the observed signals.