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

## TL;DR

This paper presents a compact, high-performance refractive index sensor using a silicon nitride microring resonator for real-time liquid analysis in flow-based systems.

## Contribution

The study introduces a novel comparison of two interrogation methods for microring resonator-based refractive index sensing in dynamic flow conditions.

## Key findings

- The sensor achieved a sensitivity of 113 nm/RIU and a detection limit of 2.3 × 10–6 RIU.
- It successfully resolved transient refractive index peaks in an isocratic sugar separation.
- The device outperformed commercial RI detectors in dynamic range and speed.

## 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.