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International Conference on Electrical Engineering and Photonics (EExPolytech)

Fabrication of Cesium Vapor Cells for Chip-Scale Atomic Clock Based on Coherent Population Trapping

Alexey Kazakin, Yu. D. Akul'shin, Yakov Enns, R Kleimanov, Ivan Komarevtsev, Andrey V. Korshunov

Published Oct 20, 2022• 1 citation

Abstract

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 of$6\cdot 10^{-11}$at 1 s.

DOI: 10.1109/eexpolytech56308.2022.9950855

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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 of

6\cdot 10^{-11}

at 1 s.

A universal method for fabrication of miniature cells for frequency standards and quantum magnetometers containing 87Rb atoms in the atmosphere of inert gas neon based on integrated technologies is considered. The results of experimental studies of coherent population trapping signals observed for a series of cells which provided recovery of vapors of an alkali metal from the rubidium dichromate salt with the help of laser radiation are presented. The coherent population trapping signals with a typical linewidth of 2–3 kHz and a signal-to-noise ratio of 1500 in the 1-Hz bandwidth were observed, which allows one to provide a relative frequency stability of atomic clock of 10−11 at 100 s.

This paper describes the fabrication of chip-scale alkali atom vapor cells, for use in highly miniaturized atomic frequency references, using silicon micromachining and anodic bonding technology. The cells consist of silicon cavities with internal volume ranging from a few mm/sup 3/ to less than 1 mm/sup 3/. The cells were filled with cesium and nitrogen buffer gas either by chemical reaction of cesium chloride and barium azide, or by direct injection of elemental cesium within a controlled anaerobic environment. Cesium optical absorption spectra were obtained from the cells, and coherent population trapping resonances with linewidths of about 1 kHz were measured.

We describe the fabrication of chip-sized alkali atom vapor cells using silicon micromachining and anodic bonding technology. Such cells may find use in highly miniaturized atomic frequency references or magnetometers. The cells consist of cavities etched in silicon, with internal volumes as small as 1 mm3. Two techniques for introducing cesium and a buffer gas into the cells are described: one based on chemical reaction between cesium chloride and barium azide, and the other based on direct injection of elemental cesium within a controlled anaerobic environment. Cesium optical absorption and coherent population trapping resonances were measured in the cells.

We report the fabrication of a micro vapor cell which has Rb and Ne inside with internal volumes of several millimeters, which is the key component of chip-scale atomic devices like clocks and magnetometers. The cells were filled with Ne at a controlled pressure as the buffer gas by using glass-silicon anodic bonding techniques and dispenser pills of SAES Getters. And a special two-step bonding method was taken to avoid breakdown caused by the buffer gas and to secure the quality of the bonding. We also built an optical measuring system and measured the coherent population trapping (CPT) resonance of85Rb D1 line with a linewidth below 2.0 kHz and a contrast about 1.1%. Zeeman spectrum for CPT resonance was also detected in these microcells. These results show that these vapor cells are capable to be used in atomic clocks or magnetometers.

Herein, a microfabricated millimeter-level vapor alkali cell with a high hermeticity is fabricated through a wet etching and single-chip anodic bonding process. The vapor cell, containing Rb and N2, was investigated in a coherent population trapping (CPT) setup for the application of a chip-scale atomic clock (CSAC). The contrast of CPT resonance is up to 1.1% within the only 1 mm length of light interacting with atom. The effects of some critical external parameters on the CPT resonance, such as laser intensity, cell temperature, and buffer gas pressure, are thoroughly studied and optimized. The improved microfabricated vapor cell also exhibited great potential for other chip-scale atomic devices.

A method for filling alkali vapor cells with cesium from a dispensing paste is proposed and its compliance with miniature atomic clock applications is evaluated. The paste is an organic-inorganic composition of cesium molybdate, zirconium-aluminum powder, and a hybrid organic-inorganic binder. It is compatible with collective deposition processes such as micro-drop dispensing, which can be done under ambient atmosphere at the wafer-level. After deposition and sealing by anodic bonding, cesium is released from the consolidated paste through local heating with a high power laser. Linear absorption signals have been observed over one year in several cells, showing a stable atomic density. For further validation of this technology for clock applications, one cell has been implemented in a coherent population trapping clock setup to monitor its frequency stability. A fractional frequency aging rate around –4.4 × 10−12 per day has been observed, which is compliant with a clock frequency instability below 1 × 10−11 at one day integration time. This filling method can drastically reduce the cost and the complexity of alkali vapor cell fabrication.