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IEEE Electron Device Letters

Microfabricated 3D Optimized Rubidium Vapor Cell Array for Low Cost Atomic Magnetometer

Wenqi Li, Zhongxun Wang, Pengguang Liu, Jintang Shang

Published Apr 1, 2025• 2 citations
Publisher

Abstract

Wearable biomagnetic field mapping with multiple-channel atomic magnetometers (AMs) requires low-cost microfabricated rubidium vapor cell arrays (RVCAs) of high performance. In this letter, an innovative microfabricated 3D RVCA with optimized polarization lifetime is proposed and demonstrated at a wafer level. To characterize the influence of vapor cell geometry on polarization lifetime, the 3D RVCA with four interconnected light-atom interaction chambers of uniform internal buffer gas pressure (0.7 amg) and various surface-to-volume ratios (17.61, 18.15, 20.95, and 27.14 cm-1) are prepared. For the first time, we utilize single-beam configuration to characterize microfabricated 3D RVCA by extrapolating to vanishing light intensity. We measure the intrinsic polarization lifetime of the four chambers within an 3D RVCA. The results indicate that decreasing the surface-to-volume ratio of the chamber in 3D RVCA at 413 K substantially improves the polarization lifetime by 129.3%. Measurements conducted across varying temperature all demonstrate that reductions in surface-to-volume ratios positively influence the enhancement of the polarization lifetime. The proposed RVCA offers critical insights for low-cost and high-performance chip-scale AM array based on microfabricated 3D atomic vapor cells.

DOI
10.1109/led.2025.3556773

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The designs of microfabricated alkali vapor cells play an important role when the atomic magnetometers are developing toward on-chip integration. This paper presents microfabricated spherical alkali vapor cells with improved sensitivity for chip-scale atomic magnetometers. A microfabricated spherical rubidium vapor cell and a traditional microfabricated planar rubidium vapor cell are designed for comparison, which are of the same volume and interconnected by a microchannel to acquire equal inner buffer gas pressure. In chip-scale spin-exchange relaxation-free (SERF) atomic magnetometers, the rubidium polarization lifetimes of three pairs of spherical and planar vapor cells at different buffer gas pressures of 0.35, 0.73, and 2.00 amg are measured, respectively. The sensitivity of SERF atomic magnetometers using the vapor cells at a 0.35-amg buffer gas pressure is also measured. The results show that the transverse polarization lifetime of the spherical vapor cell is 2.46, 1.44, and 1.05 times that of the planar one at 0.35-, 0.73-, and 2.00-amg buffer gas pressures, respectively. Combining the improved polarization lifetime and increased optical path length, chip-scale SERF atomic magnetometer using the microspherical vapor cell has the sensitivity of 60 fT/Hz1/2from 5 to 15 Hz, while that based on the planar one is demonstrated with the sensitivity of 400 fT/Hz1/2from 5 to 15 Hz.
The paper presents a novel design for chip-scale atomic magnetometers using a micro spherical rubidium vapor cell. By modeling and calculation, it was found that compared with the traditional planar rubidium vapor cell with approximately the same internal volume and buffer gas pressure, the micro spherical rubidium vapor cell was demonstrated with improved polarization lifetime and optical length. Experiments show that chip-scale atomic magnetometers based on the micro spherical cell has a significantly improved signal to noise(S/N) and sensitivity for both85Rb and87Rb. The wafer-level micro spherical rubidium vapor cells are also of interests for applications on chip-scale atomic clocks, gyroscope, vector microwave electrometries and other quantum sensors.
This paper reports, for the first time, a systematic optimization of beam characteristics for quantum magnetometers utilizing 3D MEMS rubidium vapor cells. A novel single-beam quantum magnetometer is designed where the beam characteristics can be modified and characterized in situ. The optimized beam diameter exhibits more homogeneous spatial distribution, resulting in an increase of up to 327.3% for the polarization lifetime of the atomic ensemble within 3D MEMS vapor cells. In addition, the optimized beam diameter effectively increases the number of atoms interacting with photons, leading to an improvement of 962.5% in the response slope of the quantum magnetometer. The experimental results provide valuable insights and practical guidelines for the design of high-performance single-beam quantum magnetometers based on 3D MEMS alkali vapor cells.
In this letter, we present a novel microfabricated shaped rubidium vapor cell with 3 cm optical path length for miniaturized atomic magnetometers. The novel shaped rubidium vapor cells are manufactured by a wafer-level fabrication process, and the process consists of the following three steps sequentially: first, wet etching to form oblong shallow cavity arrays; second, anodic bonding, glass reflowing, and wet etching to obtain a glass wafer with shaped structures; third, anodic bonding and chemical reaction to obtain the wafer-level shaped rubidium vapor cells. A miniaturized atomic magnetometer using the novel shaped rubidium vapor cell is characterized in the geomagnetic environment. The performance of the novel shaped rubidium vapor cell is compared with the traditional microfabricated rubidium vapor cell of a “glass–silicon–glass” sandwich structure. Results show that the measured geomagnetic field is 47.85 μT. It is demonstrated that the dispersive signal's slope of the novel shaped rubidium vapor cell is 15 times larger than that of the traditional one. Results also indicate that the miniaturized atomic magnetometer working in the geomagnetic environment has a magnetic noise spectral density of 3.5 pT/Hz1/2from 1 to 2 Hz.
Spin-exchange-relaxation-free (SERF) optical atomic magnetometers are facing the challenges of dramatic decrease of sensitivity when the alkali vapor cells become small. This paper focuses on the sensitivity improvement of the chip-scale atomic magnetometer (CSAM) by changing buffer-gas pressures inside the microfabricated vapor cells. The rubidium vapor cells with three different buffer-gas pressures were tested in the SERF magnetometer. The influences of vapor cell temperature and pumping laser power on the device sensitivity are also characterized for further optimization. Results show that the three atomic vapor cells, with N2buffer-gas pressure of 0.20, 0.88, and 2.35 amg, have sensitivities of 900, 500, and 150 fT/Hz1/2, respectively. Results indicated that when the alkali vapor cells continue to be smaller, high buffer-gas pressure would provide an effective way to reduce the wall collision and improve the performance of CSAMs.
Compared with superconducting quantum interference devices, chip-scale atomic magnetometers using the wafer-level micro alkali vapor cells can be used more conveniently in the applications of geophysical mapping, health care and remote monitoring. Drastic wall collisions and decreased polarization lifetime are the main barriers for the performance of the chip-scale atomic magnetometers. In this study, OTS organic films coated on the inner surface of the micro vapor cells were used to suppress wall collisions and improve polarization lifetime of the alkali atoms in the micro cells. First of all, the fabrication process of wafer-level micro OTS coated vapor cells is introduced, the process is based on anodic bonding technology. Then the prepared wafer-level micro OTS coated vapor cells have been characterized by the water-contact angle test and measured in the micro-machined spin-exchange relaxation free(SERF) atomic magnetometers. Results show that the OTS coating is effective after anodic bonding from the water contact angle test. Results also indicate that the spin exchange relaxation free atomic magnetometer using the micro OTS-coated vapor cell can operate successfully.