Ellipticity-induced nonlinear magneto-optical rotation of frequency-modulated light

Ellipticity-induced resonances in the magnetic-field dependence of nonlinear magneto-optical rotation of frequency-modulated light propagating through an atomic vapor are observed and studied. The resonances provide a precision $\textit{in situ}$ method to measure and control the ellipticity of light interacting with the atomic vapor and minimize associated vector light shifts. In these experiments, the light propagation direction is orthogonal to the applied magnetic field $\textbf{B}$ and the major axis of the light polarization ellipse is along $\textbf{B}$. When the light modulation frequency matches the Larmor frequency, elliptically polarized light produces precessing atomic spin orientation transverse to $\textbf{B}$ via synchronous optical pumping. The precessing spin orientation causes optical rotation oscillating at the Larmor frequency by modulating the atomic vapor's circular birefringence. The accuracy with which light ellipticity can be measured exceeds that of existing methods by orders of magnitude.