Micro-ring resonator-assisted photothermal spectroscopy with integrated pump beam

We demonstrate a compact on-chip photothermal spectroscopy (PTS) system that incorporates the integrated delivery of the pump beam. A silicon nitride (SiN) micro-ring resonator (MRR) serves as the refractive-index transducer, while a SU-8 waveguide delivers the pump beam directly to the sensing region. This configuration eliminates the need for time-consuming alignment of discrete optical components. As a proof of concept, we targeted water (H₂O) vapor absorption at 1395 nm using a distributed feedback (DFB) pump laser, while a telecom wavelength probe beam monitored the resonance shifts of the SiN MRR. The integrated design, coupled via a lensed fiber array and spot-size converters (SSC), achieved a minimum limit of detection (LOD) of 95 ppm and a linear response to varying concentrations. By combining CMOS-compatible photonic integration with established packaging techniques, this work demonstrates a scalable pathway toward robust, miniaturized, and high-sensitivity gas sensors.