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Cavity-enhanced infrared absorption spectroscopy
ˇˇ Perhaps the most widely used applied spectroscopic method, infrared (IR) absorption spectroscopy probes the characteristic vibration spectra of molecules, and provides a wealth of structural and chemical information. Traditional IR spectrophotometers, such as Fourier transform infrared (FTIR) spectrophotometers, usually employ a free-space geometry to measure single-pass absorbance. Such a configuration, however, poses a trade-off between sensitivity and instrument footprint, since the sensitivity gain has to be achieved via increasing optical path length.

In this project, we leverage the resonant cavity enhancement effect to resolve this apparent physical limitation. Planar chalcogenide glass micro-resonators are ideally suited for this purpose, because 1) chalcogenide glasses are well-known for their wide infrared transparency; 2) their amorphous nature allows ChG glass film deposition on virtually any substrate using low-cost, large-area techniques such as evaporation and sputtering; 3) sidewall roughness on as-fabricated chalcogenide glass guided wave photonic devices can be effectively removed to achieve low optical loss, a unique advantage over crystalline materials; and 4) compared to F-P cavities and fiber sensors, index-guided planar resonators such as microrings and microdisks do not require complicated optical alignment, and can be mass produced at very low cost via mature planar CMOS microfabrication technologies.

We have demonstrated cavity-enhanced sensing of N-methylaniline solution using chalcogenide glass micro-disks, by detecting its characteristic absorption peak at 1500 nm wavelength. The achieved optical absorption detection limit represents a three-fold improvement compared to a centimeter-long straight waveguide sensor [1], while the physical device length is decreased by 40-fold, an attractive feature for ˇ°sensor-on-a-chipˇ± device applications [2].
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ˇˇ Optical material infrared transparency windows Chalcogenide resonator device under testing
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ˇˇ [1] J. Hu, V. Tarasov, N. Carlie, L. Petit, A. Agarwal, K. Richardson, and L. C. Kimerling, ˇ°Fabrication and Testing of Planar Chalcogenide Waveguide Integrated Microfluidic Sensor,ˇ± Opt. Express 15, 2307-2314 (2007); select for the Virtual Journal of Biomedical Optics 2 (4) and the Virtual Journal of Ultrafast Science 6 (4).
ˇˇ [2] J. Hu, N. Carlie, L. Petit, A. Agarwal, K. Richardson, and L. C. Kimerling, ˇ°Cavity-Enhanced Infrared Absorption in Planar Chalcogenide Glass Microdisk Resonators: Experiment & Analysis,ˇ± to be submitted.