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Journal of Vacuum Science & Technology A Vacuum Surfaces and Films

Temperature and wall coating dependence of alkali vapor transport speed in micron-scale capillaries

Matthieu Giraud-Carrier, Trevor K. Decker, Joshua S. McClellan, Linsey Bennett, Aaron R. Hawkins, Jennifer A. Black, Soren Almquist, Holger Schmidt

Published Mar 20, 2017• 5 citations
Publisher

Abstract

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.

DOI
10.1116/1.4978888

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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.
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 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.
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.
We have recently reported atomic spectroscopy using on-chip rubidium vapor cells based on hollow core waveguides. To make the cells more robust and capable of multiple temperature cycles, we examined several techniques for Rb introduction and sealing. To date the most successful sealing technique has been pinching off the end of a short length of copper tubing. This technique not only hermetically seals the cells, but also allows them to be evacuated to a desired pressure. We have been able to evacuate glass prototype Rb vapor cells to a pressure as low as 80 mTorr and as high as 2 Torr and successfully observe the Rb optical absorption spectrum. Along with our testing of sealing techniques we have been observing the effects of different epoxies and inert gas atmospheres on the robustness of vapor cells. With optimal parameters we have successfully observed the Rb optical absorption spectrum through multiple temperature cycles. These new Rb introduction and sealing methods will be applied to on-chip cells containing integrated hollow waveguides which can be used for a number of different optical applications, such as electromagnetically induced transparency, single-photon nonlinearities, and slow light.
We conducted a long-term aging test at high temperature on cubic glass rubidium vapor cells. The inner surface of the cubic glass cell was coated with Al 2 O 3 using electron beam deposition, resulting in a thickness of 26.9 nm on the side walls and 63.8 nm on the bottom surface. The rubidium number density was monitored by measuring the absorption spectrum of the D 1 line. The measured spectrum was compared with a simulated profile based on the Beer–Lambert law. The rubidium number density in the Al 2 O 3 -coated rubidium vapor cell remained constant for nearly four times longer and then decreased 4.6 times more slowly than in the uncoated rubidium vapor cell.
We review the current status of integrating optical quantum interference effects such as electromagnetically induced transparency (EIT), slow light, and highly efficient nonlinear processes on a semiconductor chip. A necessary prerequisite for combining effects such as slow light and related phenomena with the convenience of integrated optics is the development of integrated alkali vapor cells. Here, we describe the development of integrated rubidium cells based on hollow-core antiresonant reflecting optical waveguides (ARROWs). Hollow-core waveguides were fabricated on a silicon platform using conventional microfabrication and filled with rubidium vapor using different methods. Rubidium absorption through the waveguides was successfully observed which opens the way to integrated atomic and molecular on a chip. The realization of quantum coherence effects requires additional surface treatment of the waveguide walls, and the effects of the surface coating on the waveguide properties are presented.