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Journal of Applied Physics

Micro-fabricated caesium vapor cell with 5 mm optical path length

T. Dyer, Stuart J. Ingleby, Camelia C. Dunare, K. Dodds, P. Lomax, Paul F. Griffin, Erling Riis

Published Nov 22, 2022• 12 citations
Publisher

Abstract

Micro-fabricated vapor cells have applications in a number of emerging quantum technology-based devices including miniaturized atomic magnetometers, atomic clocks, and frequency references for laser systems. Increasing the cell optical path length (OPL) and smallest cell dimension are normally desirable to increase the signal to noise ratio (SNR) and minimize the de-polarization rate due to collisions between atomic or molecular species and the cell walls. This paper presents a fully wafer-level scalable fabrication process to manufacture vapor cells with dimensions approaching those of glass-blown cells. The fabrication process is described, and spectroscopic measurements (optical absorption and magnetic resonance) are reported. A magnetic resonance linewidth of 350 Hz is demonstrated, and this is the smallest linewidth reported to date for a micro-fabricated vapor cell.

DOI
10.1063/5.0125490

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Micro-fabricated vapor cells have applications in a number of emerging quantum technology-based devices including miniaturized atomic magnetometers, atomic clocks, and frequency references for laser systems. Increasing the cell optical path length (OPL) and smallest cell dimension are normally desirable to increase the signal to noise ratio (SNR) and minimize the de-polarization rate due to collisions between atomic or molecular species and the cell walls. This paper presents a fully wafer-level scalable fabrication process to manufacture vapor cells with dimensions approaching those of glass-blown cells. The fabrication process is described, and spectroscopic measurements (optical absorption and magnetic resonance) are reported. A magnetic resonance linewidth of 350 Hz is demonstrated, and this is the smallest linewidth reported to date for a micro-fabricated vapor cell.
Atomic clocks represent the most advanced instruments for providing time-frequency standards, with increasing demand for designs that offer high frequency stability while minimizing size. Central to an atomic clock’s function is the atomic vapor cell, which serves as the quantum reference. Compared to traditional cells, wafer-level micro-electro-mechanical systems (MEMS) vapor cells enable cost-effective, scalable production and facilitate integration with silicon-based chips. In this work, we present a wafer-level MEMS vapor cell featuring an innovative silicon-glass-silicon transverse optical path structure. A single wafer is used to fabricate 24 identical atomic vapor cells, each with precise dimensions of 14 mm×14 mm×4.3 mm, ensuring scalability. We demonstrate an optical frequency standard that combines modulation transfer spectroscopy (MTS) with a MEMS vapor cell, featuring a compact design with excellent performance. This frequency standard achieves stability over averaging times of 1–400 s, with short-term stability of 2.6×10 −13 at 1 s and 5.1×10 −14 at 200 s. The laser linewidth is only 3.9 kHz, marking a substantial improvement over existing thermal standards, and opening potential applications in navigation, radar, and precision measurement. This work provides a crucial step toward the development of chip-scale optical clocks.
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.
The article presents the results of the development, fabrication and investigation of miniature cesium vapor cells for quantum frequency standards with optical pumping, working on the effect of coherent population trapping. Microelectromechanical systems technologies were used in the manufacture of cells. For the first time, the dry silicon etching in inductively coupled plasma of sulfur hexafluoride and freon gas in a mixed mode was used to form internal cavities of cells with a thickness of 1.5 mm. Vacuum-tight sealing of cells containing a solid-state source of cesium vapor and buffer gas was carried out by anodic bonding of glass and silicon in a neon atmosphere. The coherent population trapping signal with a linewidth of 750 Hz and a signal-to-noise ratio of 1190 in the 1-Hz bandwidth was observed. Experimental results show that these microcells are appropriate for applications to atomic clock with a relative frequency stability of6⋅10−116\cdot 10^{-11}6⋅10−11at 1 s.
This paper reports the design, fabrication and evaluation of a reflection-type Rb vapor cell with a Glass/Si/Glass structure for chip-scale micro atomic clocks. To miniaturize the cell and obtain a Rb resonance signal with a high sensitivity, long laser light traveling pass on the horizontal plane is required in the vapor cell. The horizontal Rb vapor cell with integrated mirrors are proposed and developed. A (100)-oriented Si wafer with cut off 9.74° toward [011] direction was used to make 45° mirrors by using anisotropic wet etching. Also, 90° mirrors were fabricated by Si deep RIE (Reactive Ion Etching) and H2annealing. Absorption line of Rb was confirmed and CPT (Coherent Population Trapping) resonance was demonstrated in the fabricated devices.
The most appealing features of chip-scale quantum sensors are their capability to maintain extreme sensitivity while enabling large-scale batch manufacturing. This necessitates high-level integration and wafer-level fabrication of atomic vapor cells. In this paper, we describe a micromachining paradigm for wafer-level atomic vapor cells functionalized by CMOS-compatible non-magnetic heaters and temperature sensors and demonstrate several innovative applications. Leveraging standard micro-nanofabrication technology, the integrated vapor cells achieved an ultra-long optical access of 5 mm, nearly four time that of previously microfabricated vapor cells. The feasibility of the integrated atomic vapor cells fabrication process was verified by a consecutive 30-day aging test in a harsh environment (operating temperature of 473 K and vacuum of approximately 1 Pa). Benefiting from the ultra-long optical path, we observed several typical quantum effects, including the saturation absorption and spin fluctuations, a regime previously inaccessible with conventional micromachined vapor cells. Finally, a zero-field quantum magnetometry with an ultra-high magnetic sensitivity of 12 fT/Hz1/2 was also demonstrated. Our achievements broaden the potential applications of microfabricated atomic vapor cells and pave the way for scalable manufacturing of ultrasensitive, chip-scale quantum sensors.
We report our measurement of coherent-population-trapping (CPT) signal linewidths in small vapor cells, which are desirable for chip scale atomic clock (CSAC) applications. The linewidths are measured in both rubidium vapor cells and cesium vapor cells. Using a micro-machined silicon absorption cell (diameter = 1.7 mm, length = 2 mm), we demonstrate a sub-kHz linewidth (FWHM) of the clock transition in the /sup 133/Cs atom (|F = 3, m/sub F/ = 0>/spl harr/|F = 4, m/sub F/ = 0>). The measured intrinsic linewidths (FWHM = 777 Hz for Cs and FWHM = 1496 Hz for Rb) of the cells agree with the calculated results.