On-Chip Mid-Infrared Ge-On-Si Platform for Liquid Sensing

Integrated mid-infrared photonics offers a promising route to label-free chemical sensors. In this work, we present a germanium-on-silicon (Ge-on-Si) platform tailored for liquid-phase analysis, with key components optimized for operation at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$6.9 \mu ~\mathrm{m}$</tex>. We evaluated the sensing performance of this platform for detection of ammonium in aqueous solution resulting in a limit of detection of 108 ppm and a noise floor of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10^{-3} \text{AU}$</tex>. The obtained results demonstrate the Ge-on-Si platform's feasibility for continuous, reagent-free monitoring of ionic species in aqueous media. With further refinement and extension of the operating range to longer wavelengths, this approach holds promise for simultaneous, real-time monitoring of different ions including phosphate, for which no reagent-free sensor currently exists.