Si₃N₄ Microring Resonator-Based Refractive Index Sensing for Liquid Samples: Comparing Wavelength Scanning and Fixed-Wavelength Probing

Abstract

Measuring refractive index (RI) changes of liquid samples is central to many sensing applications including flow injection analysis, liquid chromatography, biosensing and photothermal spectroscopy. Commercial refractive index detectors optimized for liquid chromatography suffer from a limited linear range and measurement rate, restricting their use largely to separation sciences. In contrast, microring resonators (MRR) integrated with low-volume microfluidics, offer enhanced performance by minimizing sample dilution during flow-through RI measurements and increased dynamic range. MRRs realized by modern photonic integrated circuitry (PIC) technology also have the potential to be used as transducers in more advanced sensing schemes. Here, we demonstrate a silicon nitride (Si3N4) MRR integrated into a low-volume microfluidic system as a compact, chip-scale RI detector capable of real-time operation under dynamic flow conditions. Two interrogation modalities were experimentally compared for flow-through liquid sensing using the same MRR for the first time: resonance wavelength scanning for wide-range refractive index detection, and fixed-wavelength probing on the resonance slope for high-speed measurements. Using glucose solutions as test samples, the device was benchmarked against a commercial RI detector, achieving a sensitivity of 113 nm/RIU and a sLOD of 2.3 × 10−6 RIU (0.014 g/L glucose). To demonstrate the applicability of the developed RI-sensor for resolving transient RI peaks in realistic chromatographic flow conditions we also report its successful use in an isocratic separation of four sugars (sorbitol, fructose, glucose, and sucrose). These results highlight the potential of integrated Si3N4 MRRs as versatile, miniaturized transducers for quantitative, high-speed RI sensing in flow-based analytical systems.