High-order magnetic resonance in the presence of strong longitudinal radio-frequency magnetic field based on atomic alignment states

A longitudinal radio-frequency (rf) magnetic field resonating at the Larmor frequency along the ̂ \^z direction in the presence of a static transverse magnetic field s investigated. Optical-radio-frequency double resonance in ⁸⁵Rb characterizes the strength of an external magnetic field through the precession frequency of ⁸⁵Rb atoms. The measurements are carried out in a paraffin-coated ⁸⁵Rb cell with a spin alignment configuration. An intriguing discovery of magnetic resonance peaks is observed at subharmonics of the Larmor frequency, accompanied by the emergence of nonlinear effects in the vicinity of the resonance. Notably, when the Larmor frequency reaches zero, the optical rotation signals induced by the rf magnetic field demonstrate a superposition of high-order harmonics of the rf frequency. Based on the simplified density matrix formalism and perturbation treatment, analytical expressions for the optical rotation are consistent with experimental outcomes. These findings hold the potential of extending our understanding on unveiling the dynamics of population and coherence among Zeeman sublevels, thereby advancing our knowledge in the field of alignment magnetometry.