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Physics of Fluids

Investigation of alkali vapor diffusion characteristics through microchannels

Yu‐Chi Chen, Shao-Cheng Fang, Hsiu-Hsuan Lin, Jing-Wu Dong, Yi‐Hsin Chen

Published Jun 20, 2022• 2 citations
Publisher

Abstract

We present Rb vapor transport through micro-scale capillaries on the impact of temperature and capillary inner diameters within the limits of the Knudsen number Kn≫1. Daily absorption spectral measurements were taken over several months to evaluate the dynamics of transport. We provide new insight into the diffusion mechanism and observe a quasi-single-layer coating on the surface based on the analysis of a slowly increasing absorption signal. The dwell time of the atom on the glass is directly derived from the diffusion dynamics at different temperatures. According to the mass flow rate, high vapor temperatures caused a faster transport speed, indicating rapid loading in microchannels. We provide a valuable model for future quantum device implementation through the use of miniaturized structures, such as photonic crystal fibers and optical waveguides.

DOI
10.1063/5.0098421

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We present Rb vapor transport through micro-scale capillaries on the impact of temperature and capillary inner diameters within the limits of the Knudsen number Kn≫1. Daily absorption spectral measurements were taken over several months to evaluate the dynamics of transport. We provide new insight into the diffusion mechanism and observe a quasi-single-layer coating on the surface based on the analysis of a slowly increasing absorption signal. The dwell time of the atom on the glass is directly derived from the diffusion dynamics at different temperatures. According to the mass flow rate, high vapor temperatures caused a faster transport speed, indicating rapid loading in microchannels. We provide a valuable model for future quantum device implementation through the use of miniaturized structures, such as photonic crystal fibers and optical waveguides.
The impact of storage temperature and wall coatings on alkali vapor transport through micron-scale glass capillaries is analyzed. Glass microbore tubing, chromatography vials, and copper tubing are assembled into closed atomic spectroscopy units with varying capillary lengths and inner diameters. Such devices serve as valuable test models for integrated atomic spectroscopy platforms that rely on hollow-core optical waveguides for chip-scale implementation of quantum coherence phenomena such as slow and stopped light. The inside surface of the systems are coated with dimethyldichlorosilane (DMDCS) after which the system is loaded with rubidium vapor and hermetically sealed. The loaded units are stored in a tube furnace at elevated temperatures and tested daily for absorption over several weeks. Both a wall coating of DMDCS and higher storage temperature increases the transport speed of Rb vapor. The limits and implications of these results are discussed and compared to an expected theoretical model. Suggestions for increasing transport speed are given.
This paper presents an approach to quantifying the diffusion rate of rubidium in glass microchannels. These types of microchannels are often used in integrated systems (chip and fiber based) which utilize light and rubidium vapor interactions to produce quantum interference phenomena such as slow light. The test platforms reported here were made using glass capillaries connected to larger glass chambers through glassblowing. Rubidium diffusion was monitored in two different ways. First, fluorescence measurements were taken over time at different points down the length of the capillary. Second, optical absorption measurements were taken through the large glass chambers on either side of the capillaries. Results indicate that reaching measurable atomic densities in microscale geometries can take many days or even weeks.
Abstract Glass cells containing neutral atoms are commonly employed in spectroscopy experiments and have potential applications in metrology. The temperature of the vapor plays a vital role as it directly influences the absorption rate of rubidium atoms. In this research, we study the effect of vapor temperature on the absorption spectrum of rubidium atoms. We observed the broadening of the spectral line and increased absorption, enabling us to determine the absorbance of atoms within the cell. These results provide valuable insights for our future work to characterize absorbance of homemade vapor cells for portable quantum device applications.
The focus of this work is the integration of alkali vapor atomic vapor cells into common silicon wafer microfabrication processes. Such integrated platforms enable the study of quantum coherence effects such as electromagnetically induced transparency, which can in turn be used to demonstrate slow light. Slow and stopped light devices have applications in the optical communications and quantum computing fields. This project uses hollow core anti-resonant reflecting optical waveguides (ARROWs) to build such slow light devices. An explanation of light-matter interactions and the physics of slow light is first provided, as well as a detailed overview of the fabrication process. Following the discovery of a vapor transport issue, a custom capillary-based testing platform is developed to quantify the effect of confinement, temperature, and wall coatings on rubidium transport. A mathematical model is derived from the experimental results and predicts long transport times. A new design methodology is presented that addresses the transport problem by increasing the number of rubidium entry points. This design also improves chip durability and decreases environmental susceptibility through the use of a single copper reservoir and buried channel waveguides (BCWs). New chips are successfully fabricated, loaded, and monitored for rubidium spectra. Absorption is observed in several chips and absorption peaks depths in excess of 10% are reported. The chip lifetime remains comparable to previous designs. This new design can be expanded to a multi-core platform suitable for slow and stopped light experimentation.
Micro-electro-mechanical systems (MEMS)-based (chip-scale) alkali vapor cells are key components in emerging quantum technologies, where device performance critically depends on the atomic number density. Thus, it is important to have an accurate estimate of the atomic number density in MEMS-based alkali vapor cells to optimize light-matter interactions and design efficient quantum sensing systems. Here, a quantitatively validated method is presented for determining the rubidium (Rb) atomic number density in warm vapor using Single-Pass Absorption Spectroscopy (SPAS). The absolute transmission spectra are measured and modeled using the 780.24~nm and 420.29~nm transitions in Rb-filled MEMS vapor. The theoretical model employs a density-matrix formalism within the Lindblad framework and incorporates directly measurable experimental parameters, such as laser beam power, diameter, and cell temperature. The model explicitly accounts for optical pumping, Doppler broadening, and transit-time broadening effects and exhibits quantitative agreement (>99%> 99\%>99%) with experimental spectra over a broad range of temperatures (293-353~K), laser probe powers of approximately 10~μμμW-100~μμμW at the 780.24~nm transition and 8~μμμW-80~μμμW at the 420.29~nm transition, and cell lengths (2--100~mm). This method demonstrates a practical and reliable approach for determining the density of alkali vapor cells for quantum sensing, metrology, and quantum communication applications.
We present the results of a systematic study of Knudsen’s flow of Rb atoms in cylindrical capillary cells coated with a polydimethylsiloxane (PDMS) compound. The purpose of the investigation is to determine the characterization of the coating in terms of the sticking probability and sticking time of Rb on the two types of coating of high and medium viscosities. We report the measurement of the sticking probability of a Rb atom to the coating equal to 4.3 × 10−5, which corresponds to the number of bounces 2.3 × 104 at room temperature. These parameters are the same for the two kinds of PDMS used. We find that at room temperature, the respective sticking times for high-viscosity and medium-viscosity PDMS are 22 ± 3 μs and 49 ± 6 μs. These sticking times are about million times larger than the sticking time derived from the surface Rb atom adsorption energy and temperature of the coating. A tentative explanation of this surprising result is proposed based on the bulk diffusion of the atoms that collide with the surface and penetrate inside the coating. The results can be important in many resonance cell experiments, such as the efficient magnetooptical trapping of rare elements or radioactive isotopes and in experiments on the light-induced drift effect.