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.
Mid-infrared tuneable diode laser absorption spectroscopy (TDLAS) realized with photonic integrated circuits (PICs) has the potential to create small and portable sensing devices with exceptional sensitivity. Here we present a photonic crystal (PhC) W5 waveguide-based sensor, which provides stronger interaction between the light and the analyte than a free space beam, as well as low scattering and material absorption losses. We demonstrate its performance by analysing the transmission spectra of two CO2 isotopes at 4345 nm.
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