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Dual resonances induced by amplitude-modulated light in a spin-exchange relaxation-free magnetometer

Ziqian Yue, Lin Li, Xiangjin Xu, Yuhao Wang, Yueyang Zhai, Junjian Tang

Published Mar 11, 2026• 0 citations

Abstract

Spin-exchange relaxation-free (SERF) magnetometers enable femtotesla-level bio-magnetic imaging, yet their scalability into high-density arrays is impeded by inter-sensor crosstalk arising from conventional magnetic-field modulation. While optical amplitude modulation offers a crosstalk-free alternative, its resonance mechanisms within the SERF regime remain unexplored. Here, we report a dual-resonance mechanism induced by amplitude-modulated light in a SERF magnetometer. Our theoretical model characterizes this behavior as the coexistence of a central zero-field resonance and symmetric parametric resonances at high atomic densities, in excellent agreement with experimental measurements. Systematic temperature and pump-power sweeps reveal the evolution of the dual-resonance profile, demonstrating that high alkali density sharpens and amplifies the zero-field resonance, whereas strong optical pumping restores the dominance of parametric resonances. Exploiting these distinct resonances, we utilize the parametric component to extract the slowing-down factor and spin polarization, which allows in situ coil calibration in both probe and pump channels with errors below 0.5%. In contrast, operation on the zero-field resonance significantly suppresses long-term drifts compared with DC detection and yields a sensitivity of 4.96 fT/Hz 1/2 . Ultimately, these findings elucidate the modulation physics within the SERF regime and establish a crosstalk-free framework that paves the way for high-precision bio-magnetic arrays.

DOI: 10.1364/oe.587988

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Spin-exchange relaxation-free (SERF) magnetometers enable femtotesla-level bio-magnetic imaging, yet their scalability into high-density arrays is impeded by inter-sensor crosstalk arising from conventional magnetic-field modulation. While optical amplitude modulation offers a crosstalk-free alternative, its resonance mechanisms within the SERF regime remain unexplored. Here, we report a dual-resonance mechanism induced by amplitude-modulated light in a SERF magnetometer. Our theoretical model characterizes this behavior as the coexistence of a central zero-field resonance and symmetric parametric resonances at high atomic densities, in excellent agreement with experimental measurements. Systematic temperature and pump-power sweeps reveal the evolution of the dual-resonance profile, demonstrating that high alkali density sharpens and amplifies the zero-field resonance, whereas strong optical pumping restores the dominance of parametric resonances. Exploiting these distinct resonances, we utilize the parametric component to extract the slowing-down factor and spin polarization, which allows in situ coil calibration in both probe and pump channels with errors below 0.5%. In contrast, operation on the zero-field resonance significantly suppresses long-term drifts compared with DC detection and yields a sensitivity of 4.96 fT/Hz 1/2 . Ultimately, these findings elucidate the modulation physics within the SERF regime and establish a crosstalk-free framework that paves the way for high-precision bio-magnetic arrays.

We demonstrate the bandwidth enhancement of an all-optical spin-exchange relaxation-free (SERF) magnetometer based on amplitude-modulated (AM) light. Alkali metal atoms are modulated directly by the pump beam instead of the modulation field or radio frequency field. The first harmonic demodulation of an AM SERF magnetometer with a modulation intensity of 15 kHz results in a high bandwidth of over 11 kHz with a sensitivity of 20 fT/Hz1/2 at 30 Hz and 60 fT/Hz1/2 at 10 kHz. Meanwhile, the AM SERF magnetometer with DC demodulation presents the same sensitivity as a traditional DC SERF magnetometer (6 fT/Hz1/2 at 30 Hz). The presented technique for modulating the amplitude of the pump beam allows AM SERF magnetometers to enter the domain of high-bandwidth magnetometers and opens the door to many areas that are inaccessible to conventional magnetometers.

Zero-field optically pumped magnetometers operating in the spin-exchange relaxation-free (SERF) regime have been extensively studied, and usually depend on zeroth-order parametric resonance to measure the magnetic field. However, the studies conducted on this topic lack thorough analyses and in-depth discussion of nonzero-order magnetic resonances in single-beam SERF magnetometers. In this paper, we analyzed the nonzero-order resonance, especially the first-order resonance, based on a single-beam SERF magnetometer, and discussed its various applications. A comprehensive theoretical analysis and experiments were conducted with respect to multiple functions, including nonzero finite magnetic field measurements, spin polarization measurement, and in situ coil constant calibration. The results showed that first-order resonance can be utilized for nonzerofinite magnetic field measurements, and the spin polarization of alkali-metal atoms can be determined by measuring the slowing-down factor using the resonance condition. Furthermore, acquiring the first-order resonance point at an equivalent zero pump light power through fitting offers an approach for quick and precise in situ coil constant calibration. This study contributes to the applications of SERF magnetometers in nonzero finite magnetic fields.

The light shift in a spin-exchange relaxation-free (SERF) atomic magnetometer (AM) generated by detuning of circularly polarized pump light, results in deviations of orthogonality measurement and attenuation of magnetometer response. In this paper, we propose and systematically investigate a dual-axis method to suppress light shift based on a self-developed single-beam compact SERF AM capable of measuring magnetic field components along two orthogonal sensitive axes. By sequentially observing the non-sensitive axis response and applying a magnetic field along the pump light direction, the virtual magnetic field induced by light shift is compensated and is rigorously demonstrated as a dominant component in this direction. Experimental results show a significant reduction in crosstalk between the dual sensitive axes, improving accuracy in resonant frequency determination of87Rb atoms and orthogonality evaluation of triaxial coils. Furthermore, the performance of dual-axis measurement in the SERF AM is tested, showing a more notable enhancement than that of single-axis measurement.

Atomic magnetometers operated in spin-exchange relaxation-free (SERF) have the potential to replace superconducting quantum interference devices (SQUIDs) in the field of biomagnetism. Here, we demonstrate the dual-axis closed-loop control of a single-beam SERF magnetometer that allows sensing of two orthogonal magnetic fields simultaneously and independently with suppressed cross-talk effect, achieved by applying a rotating modulation field and using of the negative feedback. Operating in the dual-axis closed-loop mode with a miniature 4 × 4 × 4 mm 87Rb cell, we achieve magnetic field sensitivity of 20 fT/Hz1/2 and a bandwidth of 1.8 kHz. In addition to the experimental study of the sensitivity and bandwidth, we perform a theoretical analysis to the dual-axis response and develop an effective decoupling method. The implementation of closed-loop control enables the magnetometer to be robust against the disturbance of the ambient field, which could enlarge the dynamic range, enhance the systematic stability, and suppress the cross-talk effect. Such a simple, yet high-performance SERF magnetometer could be suitable as a promising candidate in the field of biomagnetism, allowing for a denser sensor array and opening up new possibilities for the noninvasive biomagnetic imaging.

Atomic magnetometer (AM) operated in a spin-exchange relaxation-free (SERF) regime features superior sensitivity and non-cryogenic operation, and thus is competitive with the best superconducting quantum interference devices. Previously, SERF AM with fT/Hz 1/2 level sensitivity commonly acted as a single-axis sensor. Here we demonstrate a dual-axis SERF AM capable of simultaneously and independently detecting x - and y -field components with a sensitivity of 20 fT/Hz 1/2 . As there is no necessity to worry about the cross-talk effects arising from field modulations, the dual-axis scheme proposed here is of particular interest to AM array and hence the biomagnetic applications.

Abstract A single-beam spin-exchange relaxation-free (SERF) atomic magnetometer can extract vector magnetic field information by detecting the transmission intensity of a resonant circularly polarized pumping beam, which depends sensitively on the atomic density of the alkali metal. We present a novel scheme to determine atomic density based on zero-field magnetic resonance. The resonance linewidth under different transverse DC magnetic fields is fitted by means of a quadratic function. The atomic density can be extracted from the quadratic coefficients of the fitted function. The experimental results indicate that the deviation of measured density is less than two times compared with the theoretical values between 378 K and 403 K. Furthermore, the influence of modulation field on resonance linewidth is investigated experimentally and theoretically. A miniature single-beam SERF atomic magnetometer with a sensor head volume of only 16.2 cm 3 and the measuring sensitivity of 40 fT/ Hz 1/2 has been achieved. These results should be beneficial for guiding the development of a chip-scale atomic magnetometer with high sensitivity and spatial resolution for bio-magnetic field imaging applications.