Microring resonator assisted photothermal spectroscopy for gas sensing: from external excitation to on-chip pump integration

We report on the development of compact gas sensors based on photothermal spectroscopy (PTS) utilizing silicon nitride (SiN) micro-ring resonators (MRRs). We present a comprehensive study detailing two distinct excitation methodologies. First, we demonstrate the fundamental sensing principle using an external pump delivery scheme, where a lensed fiber is positioned vertically over the sensing region. This configuration allowed for the verification of power scaling laws and the optimization of probe wavelength positions relative to the resonator's inflection points. Building on this proof-of-concept, we present an advanced, fully integrated on-chip design incorporating SU-8 polymer waveguides for pump delivery. This integration significantly improves mechanical stability and simplifies alignment. This work outlines a scalable pathway from benchtop characterization to robust, miniaturized photonic gas sensors.