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Applied Physics Letters

Polarization rotation under two-photon Raman resonance for magnetometry

Swarupananda Pradhan, Rita Behera, A. K. Das

Published Apr 23, 2012• 18 citations
Publisher

Abstract

The polarization rotation and coherent population trapping signal arising due to two photon process using linearly polarized light are found to be significantly enhanced for a Zeeman degenerate system. The zero crossing of the dispersive profile is found to be shifting proportional to the applied magnetic field, albeit the absorptive profile position remains invariant for a slightly imbalanced orthogonal circular polarization component. It provides an alternative method for precise measurement of vector magnetic field without requirement of a bias field. The use of polarization rotation signal for magnetic field measurement offers added advantage due to improved signal to noise ratio.

DOI
10.1063/1.4705424

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The polarization rotation and coherent population trapping signal arising due to two photon process using linearly polarized light are found to be significantly enhanced for a Zeeman degenerate system. The zero crossing of the dispersive profile is found to be shifting proportional to the applied magnetic field, albeit the absorptive profile position remains invariant for a slightly imbalanced orthogonal circular polarization component. It provides an alternative method for precise measurement of vector magnetic field without requirement of a bias field. The use of polarization rotation signal for magnetic field measurement offers added advantage due to improved signal to noise ratio.
We report the observation of enhanced magneto-optic rotation as the coherent superposition of different hyperfine states is established in an atomic sample. The polarization rotation near the two-photon Raman resonance condition appears to have an analogous characteristic to the well established Faraday rotation observed in the vicinity of a single-photon resonance; however, it contains sharp features arising from coherent population trapping states. The profile of the two-photon rotation signal exhibits interesting features for a slightly imbalanced circular polarization component of the laser field as well as for on and away from the single-photon resonance. The investigation can be used to explore the effect of superposition states generated by coherent population trapping on optical activity. A complete density matrix based numerical simulation that consistently captures all the relevant features of the experiment is presented. The experimental and theoretical investigation can be useful for magnetometry using polarization rotation near two-photon Raman resonance.
Coherent population trapping (CPT) resonances on the degenerate two-level system of the (Fg=2→Fe=1) transition of the 87Rb D1 line were investigated in an uncoated vacuum cell by means of level-crossing type experiments-sweeping a magnetic field around zero value and using a single laser beam. The two-photon resonance manifests itself as fluorescence canceling when the system of levels is degenerate (at zero point of magnetic field). The influence of weak transverse magnetic fields on the amplitude and width of the resonance was investigated. The CPT resonances were observed with linear, circular and elliptical polarized laser light. Transverse magnetic field widens the resonances obtained by linear and circular polarization but splits the resonance created by slightly elliptically polarized laser light. The dependence of this splitting on the elliptical ratio and experimental geometry were investigated. Due to its narrow width and sensitivity to low magnetic fields (< 1 G) this resonance is very promising for small magnetic field measurement.
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.
The nonlinear magneto-optical effect has significantly impacted modern society with prolific applications ranging from precision mapping of the Earth's magnetic field to bio-magnetic sensing. Pioneering works on collisional spin-exchange effects have led to ultra-high magnetic field detection sensitivities at the level of fT/HzfT/\sqrt{Hz}fT/Hz​ using a single linearly-polarized probe light field. Here we demonstrate a nonlinear Zeeman-coherence parametric wave-mixing optical-atomic magnetometer using room temperature rubidium vapor that results in more than a three-order-of-magnitude optical signal-to-noise ratio (SNR) enhancement for extremely weak magnetic field sensing. This unprecedented enhancement was achieved with nearly a two-order-of-magnitude reduction in laser power while preserving the sensitivity of the widely-used single-probe beam optical-atomic magnetometry method. This new method opens a myriad of applications ranging from bio-magnetic imaging to precision measurement of the magnetic properties of subatomic particles.
Abstract We present a magic frequency associated with two-photon processes in room temperature atomic vapour at the coherent population trapping (CPT) condition for an elliptically polarized light. It is realized by combining the symmetries of the lineshape functions arising from the atomic coherence. In practice, the magic frequency is synthesized from a polarimetric measurement involving the reflected and transmitted CPT signal at the polarizing beam splitter, which provides an atomic frequency standard for near zero magnetic fields. We further develop a magnetometer, which concurrently measures the magnetic field and its direction from the enhanced reflected signal with significantly improved performance. Additionally, the magic frequency and its dependence on the polarization content provide a handle to precisely locate the single photon resonance.
The high precision measurement of magnetic field vector based on quantum coherent superposition states due to the interaction of the atomic system with linearly polarized light and magnetic field is described. The coordinate systems are physically orthogonal, which defined by the directions of light polarization and propagation. Because the quantum beat signals resulted by the coherent superposition states are no longer affected by nonlinear Zeeman shift, the measurement has smaller systematic error. The method is suitable for the measurements of vector fields such as the Earth's magnetic field.