On‐Chip Silicon Photonics Gas Sensors: Advances in Raman, Refractive Index, Infrared, and Fluorescence‐Based Approaches

ABSTRACT The growing demand for precise chemical gas detection across environmental, industrial, and healthcare applications has driven the development of miniaturized, cost‐effective optical sensors. Silicon photonics offers a transformative solution by enabling evanescent‐field‐based sensing within sub‐micron waveguides. This platform achieves micro/millimeter‐scale effective interaction lengths while miniaturizing the instrument, eliminating discrete optics, and lowering costs by orders of magnitude. Furthermore, monolithic integration with microfluidics and CMOS electronics facilitates real‐time analyte delivery, signal processing, and IoT connectivity, paving the way for distributed, intelligent gas‐sensing networks. In this Review, we systematically examine the design principles, fabrication strategies, and performance metrics of evanescent‐field on‐chip sensors. We categorize devices according to transduction modality‐refractive‐index, Raman scattering, infrared absorption, and fluorescence spectroscopy‐and benchmark their sensitivity, selectivity, and response time. The fundamental limits and practical challenges associated with each modality are dissected. Finally, we chart a roadmap for next‐generation on‐chip sensor platforms that exploit heterogeneous integration, on‐chip spectroscopy, and deep‐learning‐assisted signal processing to achieve large‐scale, high‐resolution monitoring.