Mid-IR photonic platform for trace gas detection

Mid-Infrared laser spectroscopy is a well-established technique for trace gas detection; It is excellent in terms of sensitivity and selectivity, and, unlike e.g.mass-spectroscopy, it is non-invasive and calibration-free. However, current mid-infrared laser spectrometers are almost exclusively realized using freespace optics, and therefore remain large, heavy, and costly. The aim of our work is to develop a platform that will bring this powerful technique on a photonic chip, replacing the traditional multi-pass cells and associated beam handling optics by tightly patterned low-loss optical waveguides. Despite significant footprint reduction, our photonic sensors will maintain the same selectivity as today's high-end spectrometers. Moreover, sensitivity in the subppm concentration range is pursued, which is vital for a large number of environmental, medical, and industrial application. Such sensitivity is about three orders of magnitude higher than what is currently achievable with integrated chip-based sensors. In the presentation, we will introduce the sensing concept and present first experimental results obtained with a new type of free-standing tantalum pentoxide waveguides, optimized specifically for the task of methane detection. The waveguide design targets (i) low propagation loss to maximize the optical pathlength, and (ii) very large evanescent field confinement in the air (50-70%) to allow for efficient interaction between the optical mode and the surrounding environment. The design allows us to pattern more than 10 cm long waveguides on a footprint smaller than 1 cm2, capable of e.g. detection of atmospheric methane down to approximately 200 ppb, or measurement of isotope specific detection of carbon dioxide in human breath with 18O/16O and 13C/12C ratios with a precision better than 1%