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IEEE Transactions on Components Packaging and Manufacturing Technology

Improvement of Sensitivity by Using Microfabricated Spherical Alkali Vapor Cells for Chip-Scale Atomic Magnetometers

Yu Ji, Jintang Shang, Qi Gan, Lei Wu, Ching‐Ping Wong

Published Sep 3, 2018• 22 citations
Publisher

Abstract

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.

DOI
10.1109/tcpmt.2018.2868313

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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.
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.
This paper focuses on geometry influence of the micro alkali vapor cell on the sensitivity of the chip-scale atomic magnetometers. For the first time, by using micro spherical alkali vapor cells instead of traditional micro planar alkali vapor cells, we demonstrate a chip-scale atomic magnetometer with a ten-fold improvement in sensitivity experimentally. Besides, results also show that the micro spherical cell has improved anti-relaxation properties, especially at low buffer gas pressure. The geometry of the micro vapor cell might also be important for the performance of other chip-scale atomic devices including chip-scale atomic clocks, vector atomic microwave electrometers and atomic gyroscopes.
Wafer-level integration of micro heater on an alkali vapor cell for chip-scale SERF magnetometers is presented. In order to obtain a suitable resistivity, polysilicon and aluminum are mixed as the target material. Magnetron sputtering is adopted to fabricate the heater on the surface of the alkali vapor cell. Various voltages are applied to the heater, of which the temperatures are measured by thermal sensitive sensors. Spin exchange relaxation free (SERF) atomic magnetometer with the heater integrated on the alkali vapor cell is characterized. Results show that the micro heater is successfully integrated on the alkali vapor cell. With the integrated heater, the rubidium vapor cell is heated up to 120°C by a 445kHz AC current. The sensitivity of the SERF magnetometer is 6.3pT/Hz1/2. Results also indicate that it provides an effective and low-cost solution for the miniaturization of atomic magnetometers.
Micro-fabricated optical magnetometers as the most sensitive magnetic field sensors play a significant role in a variety of areas ranging from health care and nuclear magnetic resonance to fundamental atomic physics research and remote monitoring. Optoelectronic integration is the main barrier for the performance of the micro-fabricated optical magnetometer. In this study, by using micro alkali vapor cell with spherical structure, a novel optoelectronic integration approach with improved sensitivity for micro-fabricated optical magnetometers has been investigated. Firstly, the manufacturing process of the micro alkali vapor cell with spherical structure is presented. Secondly, the optical path length and the buffer gas pressure of the prepared micro alkali cell have been characterized through one optical frequency-locking system. In the end, optoelectronic integration for the micro-fabricated optical magnetometers has been investigated in this study. Results indicate that the micro alkali cell with spherical structure has a greatly improved optical path length and owns the nitrogen buffer gas at the pressure of 2.25amg. Results also show that by using the micro alkali vapor cell with spherical structure, the novel optoelectronic integration approach has greatly improved signal to noise ratio(S/N) and sensitivity for the micro-fabricated optical magnetometers.
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.