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Applied Physics Letters

Characterization of 87Rb MEMS vapor cells for miniature atomic magnetometers

Minwei Jiang, Hao Zhai, Chunyu Jiang, Jian Wang, Chen Chen, Qi Zhang, Dongmin Wu, Baoshun Zhang, Zhongming Zeng, Jie Lin, Yiqun Wang, Peng Jin

Published Aug 7, 2023• 11 citations
Publisher

Abstract

Accurate characterization of atomic vapor cells is crucial for enhancing the sensitivity of miniature atomic magnetometers. In this study, a fast and efficient approach is developed to characterize relaxation properties of the inner surface of the 87Rb micro-electro-mechanical systems (MEMS) vapor cell. Based on the zero-field level crossing effect, the transverse relaxation rate was experimentally determined, and the depolarization coefficient of 0.097 for atom-wall collisions in the 87Rb MEMS vapor cell was obtained. The experiments demonstrated that the alkali-wall collision relaxation gradually dominates the depolarization of the 87Rb ensemble as the radius of the vapor cell decreases. This research provides a quantitative method to evaluate the effectiveness of the MEMS vapor cell anti-relaxation coatings and offers the design inspiration for MEMS atomic vapor cells from the perspective of relaxation characterization. It is beneficial for the development of weak magnetic field detections such as biomagnetism detection in ambient environments.

DOI
10.1063/5.0149388

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Accurate characterization of atomic vapor cells is crucial for enhancing the sensitivity of miniature atomic magnetometers. In this study, a fast and efficient approach is developed to characterize relaxation properties of the inner surface of the 87Rb micro-electro-mechanical systems (MEMS) vapor cell. Based on the zero-field level crossing effect, the transverse relaxation rate was experimentally determined, and the depolarization coefficient of 0.097 for atom-wall collisions in the 87Rb MEMS vapor cell was obtained. The experiments demonstrated that the alkali-wall collision relaxation gradually dominates the depolarization of the 87Rb ensemble as the radius of the vapor cell decreases. This research provides a quantitative method to evaluate the effectiveness of the MEMS vapor cell anti-relaxation coatings and offers the design inspiration for MEMS atomic vapor cells from the perspective of relaxation characterization. It is beneficial for the development of weak magnetic field detections such as biomagnetism detection in ambient environments.
Abstract Quantitative assessments of the level and lifetime of alkali metal in the atomic vapor cells are essential for improving the stability of atomic magnetometers. In this paper, a fast and non-destructive approach is proposed to directly characterize the level of 87Rb in a hermetic MEMS vapor cell. The MEMS-compatible 87Rb evaporation technique is developed to dispense 87Rb in the vapor cells with high efficiency. The morphology of the metallic 87Rb in the MEMS vapor cell is visualized by the non-destructive X-ray technique, and the measured contact angle is 43° ± 2°. Combined with the image recognition, the quantitative characterization of the 87Rb is achieved, and the consumption rates of 87Rb in MEMS cells are experimentally investigated. The presented approach is beneficial for the fabrication and performance enhancement of vapor cells for atomic magnetometers.
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.
Sensitive and accurate magnetic-field measurements are essential in many applications, including geophysical studies, biomagnetic signal detection, navigation, event monitoring, and magnetic anomaly detection. In this context, atomic magnetometers are a particularly promising approach, combining high sensitivity and accuracy, and able to rival superconducting quantum interference devices (SQUIDs) without requiring cryogenic cooling. While the best sensitivity levels are achieved in the zero-field regime within magnetic shielding, scalar magnetometers operating in ambient fields also demonstrated remarkable sensitivities. Initiated in the 2000s, the miniaturization of atomic devices, and in particular alkali-vapor cells, paves the way for magnetometers that are both sensitive and compact, well suited to embedded measurements and field applications.This thesis contributes to the development of microfabricated vapor cells for atomic magnetometry. We propose a new filling and sealing method that overcomes some limitations of conventional approaches and could enable the fabrication of purer cells compatible with a wider range of chemical species. This strategy could also facilitate the integration of anti-relaxation coatings, which reduce relaxation caused by collisions with the cell walls. To date, no long-lasting anti-relaxation effects have been demonstrated in microfabricated cells, as traditional methods rely on high temperatures that are incompatible with these coatings.A cell design suited to the deposition of anti-relaxation coatings is presented. We then describe the fabrication and characterization of a multi-axis microfabricated cell, designed for compact atomic devices using multiple optical beams. This multi-axis cell was experimentally validated in a dual-beam magnetometry configuration using an experimental setup developed as part of this thesis.
Spin-exchange relaxation-free (SERF) co-magnetometers exhibit broad application prospects in navigation-grade gyroscopes and fundamental physics experiments. However, their performance is constrained by finite spin relaxation times. To address this challenge, long-relaxation-time vapor cells have been realized through the introduction of dry-film anti-relaxation coatings. Such coatings form a stable hydrophobic layer on the inner walls of the vapor cell, which reduces the adsorption energy of alkali-metal atoms and suppresses wall-induced spin-destruction processes, thereby enhancing polarization retention and extending the coherence times of both electronic and nuclear spins. Nevertheless, a clear quantitative relationship between the coating properties and the resulting anti-relaxation performance has not yet been established. To resolve this issue, the present study introduces the background potential field in the Hamiltonian as an intermediate variable, linking the surface adsorption energy of the coating to atomic spin relaxation processes. From the perspective of microscopic energy structure, a theoretical mapping between coating characteristics and anti-relaxation performance is constructed, thereby filling a critical gap in the theoretical understanding of anti-relaxation coating mechanisms.
We present a microfabricated alkali vapor cell equipped with an anti-relaxation wall coating. The anti-relaxation coating used is octadecyltrichlorosilane and the cell was sealed by thin-film indium-bonding at a low temperature of 140 °C. The cell body is made of silicon and Pyrex and features a double-chamber design. Depolarizing properties due to liquid Rb droplets are avoided by confining the Rb droplets to one chamber only. Optical and microwave spectroscopy performed on this wall-coated cell are used to evaluate the cell's relaxation properties and a potential gas contamination. Double-resonance signals obtained from the cell show an intrinsic linewidth that is significantly lower than the linewidth that would be expected in case the cell had no wall coating but only contained a buffer-gas contamination on the level measured by optical spectroscopy. Combined with further experimental evidence this proves the presence of a working anti-relaxation wall coating in the cell. Such cells are of interest for applications in miniature atomic clocks, magnetometers, and other quantum sensors.
Vapor cells with antirelaxation coating are widely used in modern atomic physics experiments due to the coating's ability to maintain the atoms' spin polarization during wall collisions. We characterize the performance of vapor cells with different coating materials by measuring longitudinal spin relaxation and vapor density at temperatures up to 95 °C. We infer that the spin-projection-noise-limited sensitivity for atomic magnetometers with such cells improves with temperature, which demonstrates the potential of antirelaxation coated cells in applications of future high-sensitivity magnetometers.