Some of the most sensitive methods of measuring magnetic fields use interactions of resonant light with atomic vapour. Recent developments in this vibrant field have led to improvements in sensitivity and other characteristics of atomic magnetometers, benefiting their traditional applications for measurements of geomagnetic anomalies and magnetic fields in space, and opening many new areas previously accessible only to magnetometers based on superconducting quantum interference devices. We review basic principles of modern optical magnetometers, discuss fundamental limitations on their performance, and describe recently explored applications for dynamical measurements of biomagnetic fields, detecting signals in NMR and MRI, inertial rotation sensing, magnetic microscopy with cold atoms, and tests of fundamental symmetries of nature.

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Optical magnetometry - eScholarship

Summary form only given. It is well-known that nonlinear interference effects in the resonant atom-light interaction can lead to interesting and important phenomena, such as electromagnetically induced transparency (EIT) of atomic medium, coherent population trapping, lasing without inversion, and others. Common to all these phenomena is the appearance of light-induced coherence between atomic levels, which are not coupled by dipole transitions. Recently Akulshin and co-workers (1998-9) have observed subnatural-width resonances in the absorption on the D/sub 2/ line of rubidium vapor under excitation by two copropagating optical waves with variable frequency offset. It is remarkable that, apart from expected EIT-resonances with negative sign, authors detected positive resonances termed as electromagnetically induced absorption (EIA). More recently, similar effects in a (3+5)-state system have been described as a result of constructive interference of the dipole excitation channels. We propose a simple theoretical model for EIA - a four-state N-atom. Remember that three-state /spl Lambda/- and V-systems give the EIT-resonances only.

Electromagnetically induced absorption in a four-state system

This monograph is devoted to the description and interpretation of interference between quantum states of an atom, as manifested in the angular distribution and polarization of spontaneous emission and absorption The basic phenomena are first introduced and explained in terms of the simple classical dipole model A more quantitative description follows, making use of the density matrix formalism and the statistical tensor The theoretical sections are complemented by detailed accounts of experimental methods and results, such that the monograph as a whole gives a comprehensive picture of this field

Interference of atomic states

We report on an all-optical magnetometric technique based on nonlinear magneto-optical rotation with amplitude-modulated light. The method enables sensitive magnetic field measurements in a broad dynamic range. We demonstrate the sensitivity of 4.3×10−9G∕Hz at 10mG and the magnetic field tracking in a range of 40mG. The fundamental limits of the method sensitivity and factors determining current performance of the magnetometer are discussed.

Magnetometry based on nonlinear magneto-optical rotation with amplitude-modulated light

A compact optical fiber magnetic field sensor based on the principle of the Sagnac interferometer is proposed. Different from the conventional ones, a ferrofluid-filled high-birefringence photonic crystal fiber (HB-PCF) is inserted into the Sagnac as a magnetic field sensing element. The refractive index of the ferrofluid filled in the cladding air holes of the HB-PCF will change with respect to the applied magnetic field, and subsequently, the birefringence of the HB-PCF will change, which will affect the shifts of the output interference spectrum in Sagnac. Experiments are carried out to verify the simulation model and the results indicate that the interference spectrum exhibits a red shift with the increment in the magnetic field intensity. The sensitivity of the proposed sensor is up to 0.073 nm/mT for a magnetic field intensity ranging from 10 to 40 mT, while the resolution is 0.001 mT. The proposed magnetic field sensor is attractive due to its compact size, low cost, and immunity to electromagnetic interference beyond what conventional magnetic field sensors can offer.

Magnetic Field Measurement Based on the Sagnac Interferometer With a Ferrofluid-Filled High-Birefringence Photonic Crystal Fiber

Coherent-population-trapping resonances within the degenerate two-level system of the F=2-->F′=1 transition of the 87Rb D1 line were investigated in an uncoated Rb vapor cell by means of level-crossing-type experiments. Tuning over the two-photon resonance is achieved sweeping a magnetic field around zero value. The influence of transverse magnetic fields on the amplitude and the width of the resonances, recorded in fluorescence and absorption, were investigated in the cases of excitation with linear, circular, and elliptical laser light polarization. A theoretical analysis was performed for the case of linearly polarized excitation, the results of which are in good agreement with the experiment.

Polarization-dependent sensitivity of level-crossing, coherent-population-trapping resonances to stray magnetic fields

An investigation is reported of coherent resonance in the degenerate two-level system of the (Fg=2→Fe=1) transition of the 87Rb D 1 line by means of a Hanle effect configuration. In a coherent population trapping experiment, a very narrow (~1-kHz) resonance superimposed upon a broader resonance (~100 kHz) was observed. The dependence of the width and the amplitude of the coherent resonance on the laser power density, on additional constant magnetic fields, on the laser frequency position along the fluorescence profile, and on Rb cells with different dimensions was investigated.

Narrow structure in the coherent population trapping resonance in rubidium.

Coherent population trapping (CPT) resonances on degenerate two-level system of D/sub 1/ line of /sup 87/Rb were investigated with single-frequency laser excitation in Hanle configuration. Their sensitivity to stray magnetic fields was examined for linear, elliptical, and circular laser polarization. A methodology for magnetic field (MF) measurement using the CPT resonance is proposed. The sensitivity of the system to weak magnetic fields was evaluated and a magnetic field of 3 nT was unambiguously detected.

Potential of the single-frequency CPT resonances for magnetic field measurement

The possible use of the nonlinear Faraday effect for optical limitation of the laser power is investigated in a resonant Faraday medium placed between two crossed polarizers. The results are comparable with those obtained at strong magnetic fields as a result of the linear Faraday effect. Advantages of the method are the narrow bandwidth and the wide field of view. The investigations are interesting from the viewpoint of applications for optical sensor protection and automation of the experiment. All measurements are performed at the Fg = 2 → Fe = 1 hyperfine structure transition of the 87Rb D1 line.

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Nonlinear Faraday rotation for optical limitation

The investigation of the coherent-population-trapping CPT resonance on the degenerate two-level system of the Fg=2→Fe=1 transition of the Rb D1 line by means of a Hanle effect configuration in an uncoated vacuum cell Opt. Lett. 28, 1817 2003 has shown that the measured in fluorescence resonance has a complex shape—a very narrow about 1 mG structure superimposed on a broader one about few tens of mG . In this work, the dependence of the width and amplitude of the CPT resonance structures are measured at different laser power densities when the registration is in fluorescence and transmission. While the narrow resonance width does not change within the limits of the accuracy of our measurements, the wide resonance width dependence is complex and is different in fluorescence and transmission. The origins of the observed resonance width narrowing of the CPT resonance structures registered in fluorescence and transmission are discussed.

E. Taskova

Papers

Polarization-dependent sensitivity of level-crossing, coherent-population-trapping resonances to stray magnetic fields

Narrow structure in the coherent population trapping resonance in rubidium.

Potential of the single-frequency CPT resonances for magnetic field measurement

Nonlinear Faraday rotation for optical limitation

Power dependence of the coherent-population-trapping resonances registered in fluorescence and transmission: Resonance-width narrowing effects