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EIT, Slow light, and Sealing Methods for Embedding Rubidium into the ARROW System

Katherine Hurd

Published Jan 1, 2010• 0 citations
OpenAlex

Abstract

Light-matter interactions are fundamentally based on the quantum mechanical principles that govern photons, electrons and other fundamental particles. One very interesting phenomenon within all of light-matter interactions is Electromagnetically Induced Transparency(EIT). This phenomenon causes an otherwise absorbing atomic transition to stop absorbing through quantum mechanical interference of probability wave functions. Corresponding to that change in absorption, will be a sudden, large change in the index of refraction. This change in the index of refraction leads to another phenomenon in which the group velocity of light can be slowed down dramatically. In the past, many researchers have been able to achieve both EIT and slow light in bulk atomic vapor cells. In an attempt to miniaturize this process and we have been using a platform of Anti Resonant Reflecting Optical Waveguides (ARROW) devices to both guide light and contain the interacting matter. However, the platform creates a whole new set of challenges when integrating rubidium vapor into the hollow waveguides as rubidium is highly reactive and it is difficult to maintain an inert atmosphere for the rubidium vapor. A variety of sealing methods were attempted and their appropriateness and effectiveness was analyzed. Among these sealing methods were PMMA, Crystal Wax, Active Solder, Epoxy, and Indium Solder. PMMA, Crystal Wax and Active Solder each had major faults in one or more of the sealing requirements. We have used a high temperature epoxy with relative success to contain the rubidium vapor. However, the epoxy degrades very quickly at the high temperatures required for EIT testing. Indium solder is the most recent application method. It has high potential although we have yet to fully test its effectiveness. We were able to successfully demonstrate the first EIT and slow light on a chip with our ARROW atomic vapor cell system. In the slow light experiment, we were able to slow light down to 2.5x105m/s. The group velocity of light decreased from the standard 3x108m/s by a factor of 1200. We believe we can achieve even lower group velocities using this same platform through further experimentation.

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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.

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Sealing techniques for on-chip atomic vapor cells

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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.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jan 10, 2008•1 citation
Light-matter interactions are fundamentally based on the quantum mechanical principles that govern photons, electrons and other fundamental particles. One very interesting phenomenon within all of light-matter interactions is Electromagnetically Induced Transparency(EIT). This phenomenon causes an otherwise absorbing atomic transition to stop absorbing through quantum mechanical interference of probability wave functions. Corresponding to that change in absorption, will be a sudden, large change in the index of refraction. This change in the index of refraction leads to another phenomenon in which the group velocity of light can be slowed down dramatically. In the past, many researchers have been able to achieve both EIT and slow light in bulk atomic vapor cells. In an attempt to miniaturize this process and we have been using a platform of Anti Resonant Reflecting Optical Waveguides (ARROW) devices to both guide light and contain the interacting matter. However, the platform creates a whole new set of challenges when integrating rubidium vapor into the hollow waveguides as rubidium is highly reactive and it is difficult to maintain an inert atmosphere for the rubidium vapor. A variety of sealing methods were attempted and their appropriateness and effectiveness was analyzed. Among these sealing methods were PMMA, Crystal Wax, Active Solder, Epoxy, and Indium Solder. PMMA, Crystal Wax and Active Solder each had major faults in one or more of the sealing requirements. We have used a high temperature epoxy with relative success to contain the rubidium vapor. However, the epoxy degrades very quickly at the high temperatures required for EIT testing. Indium solder is the most recent application method. It has high potential although we have yet to fully test its effectiveness. We were able to successfully demonstrate the first EIT and slow light on a chip with our ARROW atomic vapor cell system. In the slow light experiment, we were able to slow light down to 2.5x105m/s. The group velocity of light decreased from the standard 3x108m/s by a factor of 1200. We believe we can achieve even lower group velocities using this same platform through further experimentation.
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.
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.
The demonstration of slow, fast, and stopped light1 has helped generate considerable interest in quantum interference effects. In addition to producing intriguing physical phenomena, these effects will be valuable for future optical communications. Buffers using slow-light modules will be important elements in alloptical data networks. Cryptography systems may also be expected to use quantum interference effects for photon creation and detection. But if slow-light buffers and quantum cryptography are to be practical in real-world applications, wemust create miniaturized, stand-alone quantum interference modules. This is the aim of research involving on-chip atomic vapor cells integrated with optical waveguides Slow-light effects are a product of a medium with large, frequency-dependent gain or absorption features that then give rise to large dispersion characteristics. Our work employs quantum interference effects in atomic vapor to create large dispersion. At present, atomic vapor can provide excellent dispersion characteristics, but it usually requires a large system footprint. In classical optical setups, which beam a laser through a transparent vapor cell, there is also a constant trade-off between focal depth and beamwidth. By using on-chip hollow-core optical antiresonant reflecting optical waveguides (ARROWs), we are able to direct light down vapor-filled cores, maintaining large optical intensities over long distances. This is particularly beneficial for nonlinear optical effects that can be exploited for single-photon generation and detection. In addition, ARROW-based platforms also require only a small footprint (1×1cm). ARROWs are constructed by growing alternating layers of silicon oxide and silicon nitride around a sacrificial core on a silicon wafer. The alternating dielectric materials with specific thicknesses confine and guide light through antiresonance. After etching out the sacrificial core, we are left with a hollow channel into which we can introduce various gases and liquids for optical experiments. Solid waveguides etched into the silicon on Figure 1. A rubidium (Rb)-loaded ARROW atomic vapor cell.
Hollow-core waveguides form the foundation of a new class of atomic spectroscopy chips that allows for large light-matter interactions at ultralow power levels [1-3]. We will review the development of a chip-scale platform for large quantum interference effects in hot rubidium vapor [4]. These include 44% optical transparency and a group index of 1,200. The concomitant reduction in the group velocity of light results in the largest slow light effects observed in waveguide-based environments to date.
The integration of hollow anti-resonant reflecting optical waveguides (ARROWs) with vapor cells on silicon chips provides a compact platform for a number of optical applications, including the study of quantum coherence effects such as electromagnetically induced transparency and single-photon nonlinearities, as well as frequency stabilization standards. The use of hollow waveguides allows for light propagation in low index (vapor) media with compact mode areas. ARROWs make particularly attractive waveguides for this purpose because they can be interfaced with solid core waveguides, microfabricated on a planar substrate, and are effectively single mode. ARROW fabrication utilizes an acidremoved sacrificial core surrounded by alternating plasma deposited dielectric layers, which act as Fabry-Perot reflectors. A demonstration platform consisting of solid and hollow core waveguides integrated with rubidium vapor cells has been constructed. Rubidium was used because it is of particular interest for studying quantum coherence effects. Liquefied rubidium was transferred from a bulk supply into an on-chip vapor cell in an anaerobic atmosphere glovebox. Optical absorption measurements confirmed the presence of rubidium vapor within the hollow waveguide platform. Coherence dephasing in the small dimensions of the ARROW (quantum coherence effect) can be addressed by adding a buffer gas and passivation coatings to the ARROW walls.
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.