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arXiv:2006.14647 · Applied Physics Letters

Magnetometry using sodium fluorescence with synchronous modulation of two-photon resonant light fields

Raghwinder Singh Grewal, Mauricio Pulido, G. S. Pati, Renu Tripathi

Published Nov 23, 2020• 8 citations
arXivHTMLPDF

Abstract

We have demonstrated a technique for remote magnetometry using fluorescence measurements in a sodium cell. This technique utilizes two-photon resonance instead of single-photon resonance to generate magnetic resonances with synchronous modulation of two laser fields. A strong magnetic resonance with its dip matching the Larmor frequency, ΩL, is produced when a magnetic field perpendicular to the laser propagation direction is applied. A magnetic field sensitivity of 45 pT/√Hz is measured at a 35% modulation duty cycle where ΩL resonance has a maximum amplitude. We have discussed ways to further improve the sensitivity closer to the shot-noise limit by making changes to our existing setup. An additional magnetic resonance at 3ΩL is also formed due to two-photon resonance, which can be utilized in determining the magnetic field orientation. We have developed a theoretical model based on density matrix equations to study the magnetic resonances under different conditions and also for validating our experimental observations.

DOI
10.1063/5.0030696

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We have demonstrated a technique for remote magnetometry using fluorescence measurements in a sodium cell. This technique utilizes two-photon resonance instead of single-photon resonance to generate magnetic resonances with synchronous modulation of two laser fields. A strong magnetic resonance with its dip matching the Larmor frequency, ΩL, is produced when a magnetic field perpendicular to the laser propagation direction is applied. A magnetic field sensitivity of 45 pT/√Hz is measured at a 35% modulation duty cycle where ΩL resonance has a maximum amplitude. We have discussed ways to further improve the sensitivity closer to the shot-noise limit by making changes to our existing setup. An additional magnetic resonance at 3ΩL is also formed due to two-photon resonance, which can be utilized in determining the magnetic field orientation. We have developed a theoretical model based on density matrix equations to study the magnetic resonances under different conditions and also for validating our experimental observations.
Measurement and monitoring of geomagnetic field in the mesosphere has many potential applications such as detection of oceanic currents, mapping of large-scale magnetic structures, and study of electric-current fluctuations in the ionosphere. Remote measurement of the geomagnetic field can be performed by optical pumping of sodium atoms in the mesosphere with an amplitude modulated (AM) light at the Larmor precession frequency, and observing the magnetic resonance in fluorescence at a ground station. In this context, we have conducted an experiment in the laboratory to demonstrate remote magnetic field measurement using fluorescence signal from a sodium cell. Sodium atoms in the cell are interrogated with AM light produced by an acousto-optic modulator (AOM), introduced in the beam path of a frequency-doubled amplified sodium laser made in our laboratory. Magnetic resonance observed in sodium cell fluorescence is studied as a function of varying magnetic field intensity. Characteristics of the magnetic resonance such as linewidth and contrast (or SNR), and their dependence on optical power density, AM duty cycle, B-field orientation and intensity are studied. Theoretical modeling and experimental measurements are also carried out to determine the sensitivity of the magnetometer for realizing remote magnetometry with mesospheric sodium atoms.
We demonstrate a new mechanism, suitable for performing remote earth field measurement using synchronous modulation of two laser fields, which are two-photon resonant with the sodium atom’s hyperfine ground states. This mechanism relies on coherent population trapping (CPT), and thus, called the synchronous CPT scheme. We have measured the magnetic resonances in fluorescence from a sodium cell to demonstrate suitability of this scheme for remote magnetometry. A strong magnetic resonance with its dip corresponding to the Larmor frequency is produced in the presence of a transverse magnetic field. To estimate the optimal duty cycle of light modulation for attaining high performance, we measured the amplitudes and linewidths of the magnetic resonance as a function of the duty cycle, by keeping the peak intensity in CPT fields constant.
We report on magneto-optical resonances observed in sodium fluorescence from D2 manifold with an intensity-modulated light. Fluorescence resonances are measured in the perpendicular (90°) and backward (180°) directions to the light propagation in laboratory experiments using a sodium cell containing neon buffer gas. Properties of these resonances are studied by varying the magnetic field at fixed-light modulation frequency, and vice-versa. Modulation with low-duty cycle shows higher-harmonic resonances of the modulation frequency and sub-harmonic resonances of the Larmor frequency. A dark resonance with maximum amplitude for laser wavelength closer to the crossover peak is observed. The origin of this dark resonance observed in Na D2 line is discussed using a theoretical model. Present study is aimed towards improving the understanding of magneto-optical resonances for remote magnetometry applications with mesospheric sodium.
Magnetic resonance of sodium fluorescence is studied with varying laser intensity, duty cycle, and field strength. A magnetometer based on a sodium vapor cell filled with He buffer gas is demonstrated, using a single amplitude-modulated laser beam. With a 589 nm laser tuned at the D1 or D2 line, the magnetic field is inferred from the variation of fluorescence. A magnetic field sensitivity of 150 pT/Hz is achieved at the D1 line. The work is an important step toward sensitive remote magnetometry with mesospheric sodium.
Larmor precession of laser-polarized atoms contained in antirelaxation-coated cells, detected via nonlinear magneto-optical rotation (NMOR), is a promising technique for a new generation of ultrasensitive atomic magnetometers. For magnetic fields directed along the light propagation direction, resonances in NMOR appear when linearly polarized light is frequency or amplitude modulated at twice the Larmor frequency. Because the frequency of these resonances depends on the magnitude but not the direction of the field, they are useful for scalar magnetometry. Additional NMOR resonances at the Larmor frequency appear when the magnetic field is tilted away from the light propagation direction in the plane defined by the light propagation and polarization vectors. These resonances, studied both experimentally and with a density matrix calculation in the present work, offer a convenient method of achieving additional information about a direction of the magnetic field.