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IEEE International Conference on Micro Electro Mechanical Systems (MEMS)

Rubidium vapor cellwith integrated nonmetallic multilayer reflectors

Maximillian A. Perez, Uyen Nguyen, Svenja A. Knappe, Elizabeth A. Donley, John E. Kitching, Andrei M. Shkel

Published Jan 1, 2008• 9 citations
Publisher

Abstract

This paper reports on a method for improving the optical efficiency of micromachined reflectors integrated in rubidium vapor cells. A hybrid bulk micromachining and multilayer thin film process is used to form the reflectors on angled sidewalls, which redirect laser light through the vapor cell and back toward the plane of the source with reduced optical power loss. The optical return efficiency of two paired dielectric reflectors is shown to be improved by as much as eight times over silicon reflectors alone. The D1absorption line in a one cubic millimeter miniature alkali 87 Kb vapor cell by use of two integrated thin film reflectors is experimentally demonstrated.

DOI
10.1109/memsys.2008.4443775

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This paper reports on a method for improving the optical efficiency of micromachined reflectors integrated in rubidium vapor cells. A hybrid bulk micromachining and multilayer thin film process is used to form the reflectors on angled sidewalls, which redirect laser light through the vapor cell and back toward the plane of the source with reduced optical power loss. The optical return efficiency of two paired dielectric reflectors is shown to be improved by as much as eight times over silicon reflectors alone. The D1absorption line in a one cubic millimeter miniature alkali 87 Kb vapor cell by use of two integrated thin film reflectors is experimentally demonstrated.
This paper reports on the design and implementation of thin film multilayer dielectric to form Bragg reflectors on the sidewalls of micromachined atomic cells. Due to deposition shadowing, significant variations in the thicknesses of the thin films are encountered when the layers are deposited using PECVD. These gradients in thickness may limit optical performance of the reflector in atomic cells. An optimized design procedure is described to maximize the performance of the reflector at a wavelength of 795 nm under the variations in material thickness. In addition, an optical design based on two shifted quarter wave Bragg reflectors in series is used to form a reflector with extended optical bandwidth for added robustness. The extended reflectance range maintains high reflection at the D1 absorption wavelength of 87 Rb despite variations in uniformity greater than 70 %. The developed reflector technology is ideally suited for use in miniature rubidium vapor cells. We demonstrate less than 1.5 dB of return loss with circular polarization ellipticity maintained to ±2 ◦ is demonstrated, as
We investigate the architecture of microfabricated vapor cells with reflective sidewalls for applications in chip scale atomic sensors. The optical configuration in operation is suitable for both one-beam and two-beam (pump & probe) schemes. In the miniaturized vapor cells, the laser beam is reflected twice by the aluminum reflectors on the wet etched 54.7° sidewalls to prolong the optical length significantly, thus resulting in a return reflectance that is three times that of bare silicon sidewalls. To avoid limitations faced in the fabrication process, a simpler, more universal and less constrained fabrication process of microfabricated vapor cells for chip scale atomic sensors with uncompromised performance is implemented, which also decreases the fabrication costs and procedures. Characterization measurements show that with effective sidewall reflectors, mm³ level volume and feasible hermeticity, the elongated miniature vapor cells demonstrate a linear absorption contrast improvement by 10 times over the conventional micro-electro-mechanical system (MEMS) vapor cells at ~50 °C in the rubidium D1 absorption spectroscopy experiments. At the operating temperature of ~90 °C for chip scale atomic sensors, a 50% linear absorption contrast enhancement is obtained with the reflective cell architecture. This leads to a potential improvement in the clock stability and magnetometer sensitivity. Besides, the coherent population trapping spectroscopy is applied to characterize the microfabricated vacuum cells with 46.3 kHz linewidth in the through cell configuration, demonstrating the effectiveness in chip scale atomic sensors.
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.
This paper reports on an original architecture of microfabricated alkali vapor cell designed for miniature atomic clocks. The cell combines diffraction gratings with anisotropically etched single-crystalline silicon sidewalls to route a normally-incident beam in a cavity oriented along the substrate plane. Gratings have been specifically designed to diffract circularly polarized light in the first order, the latter having an angle of diffraction matching the (111) sidewalls orientation. Then, the length of the cavity where light interacts with alkali atoms can be extended. We demonstrate that a longer cell allows to reduce the beam diameter, while preserving the clock performances. As the cavity depth and the beam diameter are reduced, collimation can be performed in a tighter space. This solution relaxes the constraints on the device packaging and is suitable for wafer-level assembly. Several cells have been fabricated and characterized in a clock setup using coherent population trapping spectroscopy. The measured signals exhibit null power linewidths down to 2.23 kHz and high transmission contrasts up to 17%. A high contrast-to-linewidth ratio is found at a linewidth of 4.17 kHz and a contrast of 5.2% in a 7-mm-long cell despite a beam diameter reduced to 600 μm.
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.
Miniature (<few cm3) vapor-cell based devices using optical pumping of alkali atoms, such as atomic clocks and magnetometers, today mostly employ vertical-cavity surface-emitting lasers as pump light sources. Here, we report on the demonstration of optical pumping in a microfabricated alkali vapor resonance cell using (1) a microfabricated Rb discharge lamp light source, as well as (2) a conventional glass-blown Rb discharge lamp. The microfabricated Rb lamp cell is a dielectric barrier discharge (DBD) light source, having the same inner cell volume of around 40 mm3 as that of the resonance cell, both filled with suitable buffer gases. A miniature (∼2 cm3 volume) test setup based on the Mz magnetometer interrogation technique was used for observation of optical-radiofrequency double-resonance signals, proving the suitability of the microfabricated discharge lamp to introduce efficient optical pumping. The pumping ability of this light source was found to be comparable to or even better than that of a conventional glass-blown lamp. The reported results indicate that the micro-fabricated DBD discharge lamp has a high potential for the development of a new class of miniature atomic clocks, magnetometers, and quantum sensors.