Another application of hollow ARROWs (like those used in our optofluidic research) involves filling them with atomic or molecular vapors. We have specifically filled them with rubidium vapor by attaching sealed reservoirs to a silicon chip. Solid rubidium is placed in the reservoirs as shown in Figure 1. This research is done in collaboration with UC Santa Cruz and is supported by the NSF and DARPA.


Figure 1 – (a) Layout showing interconnected hollow-core and solid-core ARROWs, HC-WG and SC-WG respectively, forming two independent vapour cells on a chip, connected by an additional SC-WG. Sealed, rubidium-filled reservoirs are attached at the open ends of the HC-WGs. Fibre-optical access and the beam path across the chip are shown for the lower vapour cell. (b) Cross-section along the HC-WG length showing the ARROW confinement layers (light grey: SiO2; dark grey: SiN), beam path and rubidium atoms inside the HC-WG. (c) Fabricated ARROW-based atomic spectroscopy chip. The optical beam path in the hollow core is 5 mm.

Using the rubidium filled ARROW platforms, we have been able to perform atomic spectroscopy by measuring the optical absorption of the rubidium atoms versus optical wavelength. The on-chip absorption is very comparable to that measured in larger, bulk systems as shown in Figure 2. We were also able to perform saturated absorption spectroscopy on-chip, eliminating Doppler broadening as shown in Figure 3.
Figure 2 – Normalized hyperfine absorption spectrum of natural rubidium D2 line for bulk (top) and integrated ARROW (bottom) cells at 70 ?C. Each peak contains contributions from three transitions that are not resolved due to Doppler broadening. Black lines represent fits with gaussian absorption profiles. Inset: ARROW mode image recorded with a CCD camera.


Figure 3 – Saturation absorption spectroscopy using counter-propagating beams in an integrated ARROW cell. The grey arrows mark the Lamb dips resulting from the elimination of Doppler broadening.

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