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International Journal of Applied Electromagnetics and Mechanics

Investigation on spin relaxation of microfabricated vapor cells with buffer gas

Runqi Han, Zheng You, Yuejiang Shi, Yong Ruan

Published Sep 4, 2020• 2 citations
Publisher

Abstract

MEMS vapor cells with buffer gas are the core components of chip scale atomic sensors due to the spin precession. We microfabricated rubidium vapor cells filled with neon based on MEMS technology and characterized the performance of MEMS vapor cells by measuring the longitudinal relaxation time. The dependence of spin relaxation time on buffer gas pressure and cell temperature was theoretically and experimentally investigated and the consistency was achieved. This provides a potential simpler approach to evaluate the performance of chip scale atomic sensors, such as atomic magnetometers, based on MEMS vapor cells.

DOI
10.3233/jae-209458

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MEMS vapor cells with buffer gas are the core components of chip scale atomic sensors due to the spin precession. We microfabricated rubidium vapor cells filled with neon based on MEMS technology and characterized the performance of MEMS vapor cells by measuring the longitudinal relaxation time. The dependence of spin relaxation time on buffer gas pressure and cell temperature was theoretically and experimentally investigated and the consistency was achieved. This provides a potential simpler approach to evaluate the performance of chip scale atomic sensors, such as atomic magnetometers, based on MEMS vapor cells.
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.
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.
The microfabricated alkali atomic vapor cells are essential for the chip scale quantum sensing instruments which provide a robust, simple platform for precision measurement for a quantum system. In order to solve the problems of difficult quantitative filling of alkali atoms and poor hermeticity in the fabricated process of the cells, the preparation of microfabricated alkali atomic vapor cells with good hermeticity is studied. The cells are fabricated by the microelectromechanical system (MEMS) technology. Rubidium (Rb) atomic vapor cells with nitrogen (N 2 ) as buffer gas are successfully obtained by photolysis of rubidium azide (RbN 3 ), which is confirmed by the presence of Rb vapor in the cells through absorption spectroscopy for the 5S 1/2 → 5P 3/2 and 5S 1/2 → 5P 1/2 transitions in Rb atoms at 780 nm and 795 nm. The influences of filling amount and baking temperature of RbN 3 solution on the hermeticity of the microfabricated atomic vapor cells are demonstrated. Also, the influences of filling amount of Rb atoms and temperature on the absorption spectrum intensities of Rb vapor in the cells are studied through a theoretical analysis. This research provides a reference for the design and preparation of high quality microfabricated alkali atomic vapor cells.
The dephasing time of spin-polarized atoms in an atomic vapor cell plays an important role in determining the stability of vapor-cell clocks as well as the sensitivity of optically-pumped magnetometers. The presence of a buffer gas can extend the lifetime of these atoms. Many vapor cell systems operate at a fixed (often elevated) temperature. For ambient temperature operation with no temperature control, it is necessary to characterize the temperature dependence as well. We present a spin-polarization lifetime study of Cesium vapor cells with different buffer gas pressures, and find good agreement with expectations based on the combined effects of wall collisions, spin exchange, and spin destruction. For our (7.5 mm diameter) vapor cells, the lifetime can be increased by two orders of magnitude by introducing Ne buffer gas up to 100 Torr. Additionally, the dependence of the lifetime on temperature is measured (25 - 47 oC) and simulated for the first time to our knowledge with reasonable agreement.
We investigate the architecture of microfabricated vapor cells with reflective sidewalls for applications in chip scale atomic sensors. The optical configuration in operation is suitable for both one-beam and two-beam (pump & probe) schemes. In the miniaturized vapor cells, the laser beam is reflected twice by the aluminum reflectors on the wet etched 54.7° sidewalls to prolong the optical length significantly, thus resulting in a return reflectance that is three times that of bare silicon sidewalls. To avoid limitations faced in the fabrication process, a simpler, more universal and less constrained fabrication process of microfabricated vapor cells for chip scale atomic sensors with uncompromised performance is implemented, which also decreases the fabrication costs and procedures. Characterization measurements show that with effective sidewall reflectors, mm³ level volume and feasible hermeticity, the elongated miniature vapor cells demonstrate a linear absorption contrast improvement by 10 times over the conventional micro-electro-mechanical system (MEMS) vapor cells at ~50 °C in the rubidium D1 absorption spectroscopy experiments. At the operating temperature of ~90 °C for chip scale atomic sensors, a 50% linear absorption contrast enhancement is obtained with the reflective cell architecture. This leads to a potential improvement in the clock stability and magnetometer sensitivity. Besides, the coherent population trapping spectroscopy is applied to characterize the microfabricated vacuum cells with 46.3 kHz linewidth in the through cell configuration, demonstrating the effectiveness in chip scale atomic sensors.
An atomic magnetometer based on coherent population trapping (CPT) resonances in microfabricated vapor cells is demonstrated. Fabricated by the micro-electro-mechanical-system (MEMS) technology, the cells are filled with Rb and Ne at a controlled pressure. An experimental apparatus is built for characterizing properties of microfabricated vapor cells via the CPT effects. The typical CPT linewidth is measured to be about 3 kHz (1.46 kHz with approximately zero laser intensity) for the rubidium D1 line at about 90 °C. The effects of pressure, temperature and laser intensity on CPT linewidth are studied experimentally. A closed-loop atomic magnetometer is finally finished with a sensitivity of 210.5 pT/Hz1/2 at 1 Hz bandwidth. This work paves the way for developing an integrated chip-scale atomic magnetometer in the future.