Nonlinear magneto-optical rotation with frequency-modulated light in the geophysical field range

Recent work investigating resonant nonlinear magneto-optical rotation (NMOR) related to long-lived (τᵣₑₗ~1s) ground-state atomic coherences has demonstrated potential magnetometric sensitivities exceeding 10^-110.3em0exG∕√Hz for small (10.3em0exμG) magnetic fields. In the present work, NMOR using frequency-modulated light (FM NMOR) is studied in the regime where the longitudinal magnetic field is in the geophysical range(~500mG), of particular interest for many applications. In this regime a splitting of the FM NMOR resonance due to the nonlinear Zeeman effect is observed. At sufficiently high light intensities, there is also a splitting of the FM NMOR resonances due to ac Stark shifts induced by the optical field, as well as evidence of alignment-to-orientation conversion type processes. The consequences of these effects for FM-NMOR-based atomic magnetometry in the geophysical field range are considered.