John F. Hulbert

Planar Hollow-Core Waveguide Technology for Atomic Spectroscopy and Quantum Interference in Alkali Vapors

Atomic vapors of alkali metals are widely used to slow and stop light in tabletop experiments. In order to take advantage of the underlying quantum interference effects in future commercial devices, highly reactive alkali atoms must be incorporated into small volumes with integrated optical access. With integration in mind, we describe the development of a hollow-core waveguide technology based on the combination of vapor-filled hollow waveguides and conventional solid-core waveguides on a silicon chip. We discuss the underlying principles of the waveguide design, the development of different approaches to building on-chip vapor cells, the demonstration of linear and nonlinear rubidium spectroscopy on a chip, and the prospects for quantum interference effects such as slow light and giant Kerr nonlinearities using this approach. Ultrasmall active vapor volumes on the order of 100 picoliters with simultaneously high optical density in excess of two illustrate the potential of planar hollow-core waveguides for linear and nonlinear optical spectroscopy of atoms confined on a chip.

Versatile approach to Rb vapor cell construction

A versatile approach to Rb atomic vapor cell construction is proposed and tested. The construction method employs pinch-off copper cold-welds and epoxy to create hermetic seals between dissimilar geometries and materials. Accelerated testing revealed expected lifetimes of 3 days at 90 °C operation and in excess of 1 yr at 25 °C operation. The reaction of Rb with epoxy was determined to be the largest contributor to failure.

Versatile Rb vapor cells with long lifetimes

The authors report on an approach to the construction of long-lasting rubidium atomic vapor cells. The method uses pinch-off copper cold-welds, low temperature solders, and electroplated copper to create long-lasting hermetic seals between containment chambers of dissimilar geometries and materials. High temperature epoxy, eutectic lead/tin solder, and indium solder were considered as sealing materials. These seals were analyzed using accelerated lifetime testing techniques. Vapor cells with epoxy and bare metal solder seals had a decrease in the rubidium atomic density within days after being heated to elevated temperatures. They also exhibited broadened spectra as a result of rubidium reacting with the seals. However, indium solder seals with a passivation coating of electroplated copper did not exhibit a significant decrease in linewidth or atomic density after being held at 95 °C for 30 days. The authors conclude that this particular seal has no rubidium chemical reaction failure mode and when used in...

Sealing techniques for on-chip atomic vapor cells

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.

Quantum interference effects in rubidium vapor on a chip

Harnessing the unique optical quantum interference effects associated with electromagnetically induced transparency (EIT) on a chip promises new opportunities for linear and nonlinear optical devices. Here, we review the status of integrated atomic spectroscopy chips that could replace conventional rubidium spectroscopy cells. Both linear and nonlinear absorption spectroscopy with excellent performance are demonstrated on a chip using a self-contained Rb reservoir and exhibiting a footprint of only 1.5cm x 1cm. In addition, quantum interference effects including V-scheme and Λ-scheme EIT are observed in miniaturized rubidium glass cells whose fabrication is compatible with on-chip integration.

Fabrication methods for compact atomic spectroscopy

Atomic spectroscopy relies on photons to probe the energy states of atoms, typically in a gas state. In addition to providing fundamental scientific information, this technique can be applied to a number of photonic devices including atomic clocks, laser stabilization references, slow light elements, and eventually quantum communications components. Atomic spectroscopy has classically been done using bulk optics and evacuated transparent vapor cells. Recently, a number of methods have been introduced to dramatically decrease the size of atomic spectroscopy systems by integrating optical functionality. We review three of these techniques including: 1) photonic crystal fiber based experiments, 2) wafer bonded mini-cells containing atomic vapors and integrated with lasers and detectors, and 3) hollow waveguides containing atomic vapors fabricated on silicon substrates. In the context of silicon photonics, we will emphasize the hollow waveguide platform. At the heart of these devices is the anti-resonant reflecting optical waveguide (ARROW). ARROW fabrication techniques will be described for both hollow and solid core designs. Solid-core waveguides are necessary to direct light on and off the silicon chip while confining atomic vapors to hollow-core waveguides. We will also discuss the methods and challenges of attaching rubidium vapor reservoirs to the chip. Experimental results for optical spectroscopy of rubidium atoms on a chip will be presented.

Rubidium spectroscopy on a chip

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.

Chip-Scale Platform for Quantum Interference-Based Slow Light in Atoms

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. We will review the development of a chip-scale platform for large quantum interference effects in hot rubidium vapor.

Saturation Absorption Spectroscopy in an Integrated Rubidium Vapor Cell

We demonstrate integrated rubidium vapor cells using hollow-core ARROW waveguides on a silicon chip. Optical mode areas of 8.8 square microns are promising for nonlinear optical applications. Saturation spectroscopy on the Rb-D2 line is demonstrated.