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Laser & Photonics Review

Scalable Silicon Nitride OPA‐Based Free‐Space Photonic Interface for Chip‐Scale Atomic Magnetometry

Jie Sun, Zhen Chai, Jiaxin Gu, Shaoliang Yu, Yuting Xu, Xiaoqin Meng, Xinran Su

Published Aug 12, 2026• 0 citations
Publisher

Abstract

ABSTRACT Optically pumped atomic magnetometers (AMs) have become powerful tools for biomagnetic sensing owing to their ultrahigh sensitivity. However, their broader deployment increasingly requires compact, scalable, and low‐cost free‐space photonic interfaces capable of delivering high quality beams for light‐atom interaction. Here, we present a photonic chip based on a silicon nitride (SiN) optical phased array (OPA) that serves as a free‐space optical interface for AMs, combining high radiation efficiency, low divergence, and compatibility with batch fabrication. The device generates a stable linearly polarized beam with long propagation capability, a polarization extinction ratio of 26.1 dB, and a degree of linear polarization of 99.5%, with a maximum overall radiation efficiency of 26.5%. In a dual beam atomic magnetometer, the chip emitted beam serves as the probe beam and enables differential detection with a sensitivity of 8 fT/Hz 1/2 . To assess fabrication reliability and reproducibility, we performed a statistical analysis of 168 devices fabricated by deep‐ultraviolet (DUV) lithography, with more than 50% exhibiting radiation efficiencies above 15%. These results demonstrate a low‐cost, scalable photonic interface for chip integrated AMs and highlight its potential for large scale implementation.

DOI
10.1002/lpor.71737

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ABSTRACT Optically pumped atomic magnetometers (AMs) have become powerful tools for biomagnetic sensing owing to their ultrahigh sensitivity. However, their broader deployment increasingly requires compact, scalable, and low‐cost free‐space photonic interfaces capable of delivering high quality beams for light‐atom interaction. Here, we present a photonic chip based on a silicon nitride (SiN) optical phased array (OPA) that serves as a free‐space optical interface for AMs, combining high radiation efficiency, low divergence, and compatibility with batch fabrication. The device generates a stable linearly polarized beam with long propagation capability, a polarization extinction ratio of 26.1 dB, and a degree of linear polarization of 99.5%, with a maximum overall radiation efficiency of 26.5%. In a dual beam atomic magnetometer, the chip emitted beam serves as the probe beam and enables differential detection with a sensitivity of 8 fT/Hz 1/2 . To assess fabrication reliability and reproducibility, we performed a statistical analysis of 168 devices fabricated by deep‐ultraviolet (DUV) lithography, with more than 50% exhibiting radiation efficiencies above 15%. These results demonstrate a low‐cost, scalable photonic interface for chip integrated AMs and highlight its potential for large scale implementation.
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.
ABSTRACT Atomic magnetometers (AMs) are among the most sensitive magnetic field sensors, with uses in magnetic anomaly detection, geosciences, and fundamental physics research. Despite notable progress, developing highly integrated, miniaturized components with high resolution, sensitivity, and efficiency for chip‐scale AMs remains a significant challenge. Here, we present a novel integrated polarization detection scheme based on a bifocal liquid‐crystal polarization metalens (LCPM) for chip‐scale AMs. By leveraging geometric phase and holographic synthesis principles, the LCPM efficiently separates and focuses left‐ and right‐handed circularly polarized light at the rubidium D1 transition wavelength (795 nm), achieving a high focusing efficiency of up to 80% and extinction ratios of 104 and 134 for the two channels. We demonstrate a prototype AM operating in the spin‐exchange relaxation‐free regime, achieving a high magnetic‐field sensitivity of 17 fT/Hz 1/2 and a dynamic range of ±4.8 nT. The proposed LCPM‐based scheme reduces the optical‐path volume by 94% compared to traditional polarization detection systems. Importantly, our method is compatible with large‐scale, cost‐effective production through established liquid‐crystal manufacturing lines, paving the way for high‐throughput fabrication of chip‐scale AMs. This innovation enables compact, affordable polarization detection with enhanced sensitivity of atomic sensors, opening new opportunities for high‐resolution biomagnetic imaging and quantum precision measurements.
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 Nuclear magnetic resonance (NMR) co‐magnetometers are high‐precision atomic sensors, but their practical deployment is hindered by bulky free‐space optics. Here, we propose a circularly polarized light radiation scheme based on a silicon nitride photonic platform and develop a compact, chip‐scale integrated optical pumping module. Distinct from prior on‐chip schemes limited to electron spin polarization, our approach achieves polarization of alkali metal electron spins and noble gas nuclear spins. By integrating a spiral waveguide with angular gratings, the platform emits vertically propagating CPL with a 5.65° divergence angle, and the average degree of circular polarization exceeds 95%. Magnetic field‐dependent measurements reveal clear 129 Xe and 131 Xe precession responses. A 50% increase in SRC pumping power increases the relative nuclear spin polarization of 129 Xe by a factor of 1.72 and that of 131 Xe by a factor of 2.90, verifying efficient nuclear spin polarization. The SRC scheme yields electron and nuclear spin resonance responses in the vapor cell, while the SRC chip itself occupies a compact footprint of only 1 × 2 mm 2 . This work offers a highly integrated photonic solution for atomic polarization, facilitating the miniaturization and practical deployment of high‐performance NMR co‐magnetometers.
Emerging atomic magnetometers (AMs) are among the most advanced sensors for detecting and characterizing magnetic fields. Recently, there has been growing interest in the miniaturization and integration of AMs due to the urgent demand for portability and compactness in various fields such as biomagnetism imaging. While conventional AMs require a bulky setup of optical devices for the pumping and optical readout of atomic spin, here, a novel chip-scale single-beam AM scheme is proposed by leveraging extreme transmissive circular polarization dichroism (TCPD) and geometric phase manipulation of spin-selective interference meta-optics. This is achieved through silicon-based metasurfaces that enable the realization of arbitrary-to-circular polarization conversion and wavefront modulation within a monolithic chip at Rb D1 transition wavelength (λ = 795 nm). Two spin-selective interference metasurfaces, i.e., meta-circular-polarizer (MCP) and meta-circular-polarizer-lens (MCPL), are fabricated and characterized, with a measured TCPD of 0.68 for the MCP as well as focusing efficiency and TCPD of around 70.67% and 0.69 for the MCPL, respectively. As a proof of concept, a 4 × 4 × 4 mm 3 Rb vapor cell is combined with our metasurface to construct a miniaturized single-beam AM. The sensitivity of our compact metasurface-based system is about 15 fT/Hz 1/2, with a dynamic range near zero-field of ± 2.2 nT. We envision that this work could facilitate the development of burgeoning chip-scale quantum sensors, which hold great potential for high-spatial-resolution biomagnetic imaging, on-chip nuclear magnetic resonance, and so forth.
Optical Pumping The cover feature highlights the key elements of the study by Zhen Chai and co-workers outlined in article number 2402292: a silicon nitride photonic integrated circuit (PIC), laser beams used for optical pumping, the spin precession of 87Rb atoms, and the resulting signal. The design visually represents how the PIC transforms fiber-coupled 795 nm pump light into arrayed elliptically polarized beams, effectively manipulating atomic spin states within the vapor cell. This integration of photonic chips with atomic magnetometry opens new possibilities for compact, high-resolution quantum sensing applications.