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ACS Photonics

Space-Variant Polarization Distribution Mitigates AC-Stark Shifts in a SERF Atomic Magnetometer

Zihua Liang, Tengyue Long, Lu Liu, Jinsheng Hu, Peng Zhou, Gen Hu, Yaxiang Wang, Mao Ye

Published Apr 8, 2025• 7 citations
Publisher

Abstract

Emerging biomagnetic imaging requires compact optical pumping magnetometer (OPM) arrays that enable high spatial resolution with remarkable sensitivity. In such devices, atomic pumping fundamentally depends on conventional optical systems, yet realizing diverse pumping modalities persists as a critical challenge─particularly in integrated applications. In this study, a space-variant polarization distribution (SPD) pumping source is achieved and integrated with an atomic magnetometer for the first time. Our method enables modification of optical polarization within a 3 × 3 × 3 mm 3 87 Rb micro-vapor cell to spatially separate the atomic pumping and detection regions, resulting in a 56% mean (75% maximum) reduction in the AC Stark effect compared with the conventional scheme. Experiment results demonstrate a 20.1 fT/Hz 1/2 sensitivity and a 52 Hz bandwidth with a dynamic response range of ±3 nT, which is comparable to the state-of-the-art chip-scale OPMs. By leveraging the SPD pumping method, various atomic devices with a diffusion-pumping scheme are able to achieve chip integration, which further paves the way for the development of high-spatial-resolution biomagnetism imaging and portable atomic sensing applications.

DOI
10.1021/acsphotonics.4c02535

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Emerging biomagnetic imaging requires compact optical pumping magnetometer (OPM) arrays that enable high spatial resolution with remarkable sensitivity. In such devices, atomic pumping fundamentally depends on conventional optical systems, yet realizing diverse pumping modalities persists as a critical challenge─particularly in integrated applications. In this study, a space-variant polarization distribution (SPD) pumping source is achieved and integrated with an atomic magnetometer for the first time. Our method enables modification of optical polarization within a 3 × 3 × 3 mm 3 87 Rb micro-vapor cell to spatially separate the atomic pumping and detection regions, resulting in a 56% mean (75% maximum) reduction in the AC Stark effect compared with the conventional scheme. Experiment results demonstrate a 20.1 fT/Hz 1/2 sensitivity and a 52 Hz bandwidth with a dynamic response range of ±3 nT, which is comparable to the state-of-the-art chip-scale OPMs. By leveraging the SPD pumping method, various atomic devices with a diffusion-pumping scheme are able to achieve chip integration, which further paves the way for the development of high-spatial-resolution biomagnetism imaging and portable atomic sensing applications.
The ultrasensitive magnetometer has a vital importance in fundamental research and applications. Currently, the spin-exchange relaxation-free (SERF) atomic magnetometer has been reported with a sensitivity around the level of fT/Hz 1/2 . To enhance the sensitivity, a gradiometer configuration has usually been introduced to cancel the common-mode noise between two separate channels. However, the signal and response from different channels are not the same due to the attenuation of the pump beam. Here, we proposed a counter-propagating optical sideband pumping method to polarize the atoms, using the electro-optic modulator to modulate the single-pump beam, generating two symmetrically red- and blue-detuned sidebands of frequency. This scheme leads to a significant reduction of undesirable effects coming along with the optical pumping, such as light shifts and spatial inhomogeneity in atomic spin polarization. With the help of this pumping scheme, the two channels have the same magnetic response, and we have built a gradiometer atomic magnetometer with a sensitivity of 0.5 fT/Hz 1/2 ranging from 5 to 40 Hz. Our results propose the possibility of creating larger arrays of atomic magnetometers (AMs) with high sensitivity and spatial resolution based on single-vapor cells for magnetocardiography and magnetoencephalography imaging or searching for exotic spin-dependent interactions.
High-performance atomic magnetometers (AMs) rely on the measurement of optical rotation, which requires a set of bulky polarization optics that limit their applications in scenarios where portability and compactness are necessary. In this study, a miniaturized AM is constructed based on a cubic 87 Rb vapor cell and monolithic metalens, which provides an integrated scheme to achieve optical rotation detection induced by the circular birefringence of polarized atoms. The designed metalens achieves polarization splitting with deflection angles of ±10 ∘ and focusing with efficiencies of approximately 30% for orthogonal linear polarizations. The sensitivity of our compact device is ∼30 fT/Hz 1/2 with a dynamic range of around ±1.45 nT. We envision that the presented approach paves the way for the chip integration of emerging atomic devices, which are in demand for applications such as biomagnetic imaging and portable atomic gyroscopes.
Conventional understanding of spin-exchange relaxation-free (SERF) atom ensemble pertains to the common perception that the rapid exchange of atom state finally results in uniform time evolution of the whole ensemble. However, in this study, we demonstrate that by manipulation of pumping polarization in micro-meter level, misalignment between the time evolution of different sub-ensemble can be created within single SERF ensemble with unprecedent independency. A novel pumping system consists of a miniaturized 87^{87}87Rb vapor cell and a space-variant polarization metasurface is developed for the prove of concept. Our method induces position-dependent atomic anisotropy in both pumping and absorption into the thermal atomic ensemble. By constructing calculated Zeeman-sublevel populations in SERF regime, distinct sensing channels are generated with 0.22 V nT−1V\ nT^{-1}V nT−1 average scale factor, which is comparable with single channel generated by single SERF ensemble. Average crosstalk ratio between adjacent channels (between micron scale sub-ensembles) are measured up to 32 dB, through the excitation of fictitious magnetic field (3.5 nT, 30 Hz), which is measured as 20 dB without sub-ensemble isolation in same experimental condition. Our work demonstrates unprecedented spatial resolution in SERF magnetometry which hold new promises for applications including high-spatial resolution neural biomagnetism mapping and portable magnetism measurement device.
Abstract The chip-scale hybrid optical pumping spin-exchange relaxation-free (SERF) atomic magnetometer with a single-beam arrangement has prominent applications in biomagnetic measurements because of its outstanding features, including ultrahigh sensitivity, an enhanced signal-to-noise ratio, homogeneous spin polarization and a much simpler optical configuration than other devices. In this work, a miniaturized single-beam hybrid optical pumping SERF atomic magnetometer based on a microfabricated atomic vapor cell is demonstrated. Although the optically thin Cs atoms are spin-polarized, the dense Rb atoms determine the experimental results. The enhanced signal strength and narrowed resonance linewidth are experimentally proven, which shows the superiority of the proposed magnetometer scheme. By using a differential detection scheme, we effectively suppress optical noise with an approximate five-fold improvement. Moreover, the cell temperature markedly affects the performance of the magnetometer. We systematically investigate the effects of temperature on the magnetometer parameters. The theoretical basis for these effects is explained in detail. The developed miniaturized magnetometer has an optimal magnetic sensitivity of 20 fT/Hz 1/2 . The presented work provides a foundation for the chip-scale integration of ultrahighly sensitive quantum magnetometers that can be used for forward-looking magnetocardiography (MCG) and magnetoencephalography (MEG) applications.
Abstract Emerging miniaturized atomic sensors such as optically pumped magnetometers (OPMs) have attracted widespread interest due to their application in high‐spatial‐resolution biomagnetism imaging. While optical probing systems in conventional OPMs require bulk optical devices including linear polarizers and lenses for polarization conversion and wavefront shaping, which are challenging for chip‐scale integration. In this study, an integrated optical probing scheme based on localized‐interference metasurface for chip‐scale OPM is developed. Our monolithic metasurface allows tailorable linear polarization conversion and wavefront manipulation. Two silicon‐based metasurfaces namely meta‐polarizer and meta‐polarizer‐lens are fabricated and characterized, with maximum transmission efficiency and extinction ratio (ER) of 86.29 % and 14.2 dB for the meta‐polarizer as well as focusing efficiency and ER of 72.79 % and 6.4 dB for the meta‐polarizer‐lens, respectively. A miniaturized vapor cell with 4 × 4 × 4 mm 3 dimension containing 87 Rb and N 2 is combined with the meta‐polarizer to construct a compact zero‐field resonance OPM for proof of concept. The sensitivity of this sensor reaches approximately 9 fT/Hz 1/2 with a dynamic range near zero magnetic field of about ±2.3 nT. This study provides a promising solution for chip‐scale optical probing, which holds potential for the development of chip‐integrated OPMs as well as other advanced atomic devices where the integration of optical probing system is expected.
Abstract Discrete optical system in optically pumped magnetometers (OPMs) presents a significant challenge to their integration into high‐spatial‐resolution magnetic field detection. Here, the electron spin polarization of atoms using a fiber‐coupled silicon nitride () photonic integrated circuit (PIC) is demonstrated, by the way of delivering arrayed elliptically polarized light (EPL) into a vapor cell. Specifically, the photonic integrated circuit (PIC) transforms fiber‐coupled 795 nm pump light via optical waveguides to eight divergent free‐space beams, simultaneously converting linear polarized light (LPL) into EPL with an average polarization conversion efficiency of 71%. The PIC‐enabled OPM has detected the weak magnetic field with an optimal sensitivity of 0.53 pT at 70Hz. This work presents an effective approach for manipulation of alkali atoms using a chip and, crucially, introduces a chip‐based pump solution for OPMs. This approach contributes to the integration of nanophotonics with atomic systems, paving the way for future advancements in quantum sensing.