Sensitive optical magnetometry based on nonlinear magneto-optical rotation with amplitude-modulated light W. Gawlik, M. Gring, M. Kotyrba, S. Pustelny, A. Wojciechowski, and J. Zachorowski Center for Magneto-Optical Research, Marian Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Krakow, Poland D. Budker, A. Cingoz, and N. Leefer Department of Physics, University of California at Berkeley, Berkeley, CA 94720-7300, USA Nonlinear magneto-optical rotation (NMOR), an all-optical technique now finding its use in sensitive magnetometry, is light-intensity-dependent rotation of the polarization plane of linearly polarized light upon its propagation through a medium placed in a magnetic field. For an intense light beam, the dependence of the NMOR angle on the magnetic field can be very strong, which allows sensitivity reaching 3T0" 16 T Hz" 12 [1]. This sensitivity exceeds that obtained with the Superconducting Quantum Interference Devices (SQUIDs). Further advantages of the all-optical approach are low cost and technical simplicity. A common disadvantage of many ultra-sensitive methods is their limitation to near-zero fields. The magnetometers exploiting NMOR overcome this limitation by application of modulated light. With such light, additional NMOR resonances appear at higher magnetic fields B determined by the modulation frequency Q, m : B sa ±M2 m /2g(i B , where g is the Lande factor, h is the Planck's constant, and \i B is the Bohr magneton [1, 2]. Since the widths of the high-field resonances are the same as the width of zero-field resonance and the amplitudes of all three resonances are similar [Fig. 1(a)], appearance of the high-field resonances extends the range of ultra-precise measurements to stronger magnetic fields. We report on a new magneto-optical technique which enables measurements of magnetic field from 0 to 50 uT with the expected sensitivity of 10" 14 T Hz" 1/2 . In our approach a single beam of light is amplitude modulated [2] and it interacts with rubidium vapor contained in a buffer-gas-free, paraffin-coated cell. Application of amplitude-modulated light allows for mitigation of some limiting factors appearing with other modulation techniques. The measurements are performed in the self-oscillating regime where the NMOR signal itself, after appropriate conditioning, drives the modulator. In that way we accomplish the principle of a self-oscillating measurement in which the oscillation frequency is the direct measure of the magnetic field intensity. Fig. 1(b) shows a tracking characteristic of the magnetometer - the light-modulation frequency versus the magnetic field. Fig. 1 (a) Typical raw signal measured in NMOR with light amplitude modulated at 1 kHz (0.1s integration constant); (b) Magnetometer tracking signal measured in NMOR with amplitude modulated light in the self-oscillating arrangement for fields around 10 mG. References 1. D. Budker, D. F. Kimball, S. M. Rochester, V. V. Yashchuk, and M. Zolotorev, "Sensitive Magnetometry based on Nonlinear Magneto-Optical Rotation", Phys. Rev. A 62, 043403 (2000); D. Budker, D. F. Kimball, V. V. Yashchuk, and M. Zolotorev, "Nonlinear magneto-optical rotation with frequency-modulated light", Phys. Rev. A. 65, 055403 (2002). 2. W. Gawlik, L. Krzemieh, S. Pustelny, D. Sangla, J. Zachorowski, M. Graf, A. Sushkov, and D. Budker, "Nonlinear Magneto-Optical Rotation with Amplitude-Modulated Light", Appl. Phys. Lett. 88, 131108 (2006).