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Laser & Photonics Review

Silicon Nitride‐Photonics‐Enabled Optical Pumping for Optically Pumped Magnetometer (Laser Photonics Rev. 19(10)/2025)

Yuting Xu, Zhen Chai, Xiaoqin Meng, Yan Lu Xu, Jie Sun, Peng Zhou

Published May 1, 2025• 0 citations
Publisher

Abstract

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.

DOI
10.1002/lpor.202570039

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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.
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 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.
Quantum optical sensing is a maturing field that offers the capability to achieve extraor- dinary sensitivity. Atomic magnetometers, for example, now have the capability to detect brain signals and to precisely measure Earth’s gravity field. In optical atomic magnetome- ters, the electronic spins of the atoms in vapor form are aligned in the same direction by “pumping” the ensemble using laser light at a select frequency. Any magnetic field alters the spin states, which changes the optical properties of the vapor, such as the transmission of the laser light, providing a way to measure magnetic fields. After reviewing the theory of atomic structure and atom-light interactions relevant to the research presented here, I describe in the first part of this thesis a vector magnetometry method using microwave-assisted optical pumping, in which a microwave field resonant to the ground state hyperfine splitting of 87Rb pumps an ensemble of warm rubidium vapor, in addition to a strong optical pump beam, and a weaker optical probe beam. I present theoretical details as well as preliminary data demonstrating magnetic field measurements of a DC magnetic field. The measurements were taken when the DC field was aligned at various angles relative to the microwave magnetic field alignment. I compare theoretical and experimental differences in the measurements. With new methods for quantum devices comes the need for portability. In the second part of this thesis, we demonstrate the fabrication of millimeter-sized rubidium vapor cells using silicon micro-machining techniques. This method is relatively inexpensive compared to glass-blowing methods and allows rapid, large-scale production. I describe our process flow and demonstrate the presence of rubidium atoms inside our vapor cells. To conclude, suggestions for improvement are discussed.
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.