Free-standing thin-film waveguides and slot waveguides offer excellent properties for gas sensing: high air confinement factors resulting in strong light-analyte interaction, reduced material absorption loss, and negligible Fabry-Perot fringes. We demonstrate that such waveguides combined with the sensitive and selective technique of mid-IR laser absorption spectroscopy can detect methane and carbon dioxide down to 300 ppb and 30 ppb levels, respectively. Isotope specific detection of CO2 with excellent 13C isotope ratio accuracy of 0.3‰ has also been shown. The unprecedented sensitivity together with miniature sensor footprint and microlitre sample volumes open new application areas in biology, environmental sensing and industral process monitoring.
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