Nonlinear magneto-optical rotation with frequency-modulated light
  A magnetometric technique is demonstrated that may be suitable for precision measurements of fields ranging from the sub-microgauss level to above the Earth field. It is based on resonant nonlinear magneto-optical rotation caused by atoms contained in a vapor cell with anti-relaxation wall coating. Linearly polarized, frequency-modulated laser light is used for optical pumping and probing. If the time-dependent optical rotation is measured at the first harmonic of the modulation frequency, ultra-narrow (~a few Hz) resonances are observed at near-zero magnetic fields, and at fields where the Larmor frequency coincides with half the light modulation frequency. Upon optimization, the sensitivity of the technique is expected to exceed 10^{-11} G/sqrt{Hz}. 
D. Budker, D. F. Kimball, V. V. Yashchuk, M. Zolotorev