Flow Injection-Based Refractive Index Sensing with a Si₃N₄ Photonic Crystal Nanobeam-Microring Fano Resonator

Abstract

Asymmetric optical resonances shaped by Fano interference can provide enhanced signal contrast and tunable lineshapes in integrated photonics. However, realizing and exploiting these effects in fabricated devices remains challenging, as nanofabrication often reduces asymmetry and slope steepness. In this work, we experimentally demonstrate a hybrid silicon nitride photonic crystal nanobeam-microring resonator (PhCN-MRR) for refractive index (RI) sensing and systematically analyze how Fano-induced line shape asymmetry impacts transduction under realistic flow conditions. Despite exhibiting only moderate asymmetry, the PhCN-MRR shows distinct optical advantages for intensity-based detection, demonstrated using glucose solutions as a model analyte. The sensor was integrated into a custom Si₃N₄ microfluidic platform and tested under both stopped-flow and dynamic flow-injection conditions over concentrations from 0.5 to 10 mg/mL. Time-resolved spectral scans (0.5-1 Hz) enabled dual transduction channels via wavelength shifts (Δλ) and fixed-wavelength intensity changes (ΔI). Compared with a conventional microring resonator (MRR) featuring symmetric Lorentzian responses, the PhCN-MRR exhibited comparable Δλ sensitivities (∼111 to 113 nm/RIU) but delivered enhanced ΔI responsivity and improved contrast at low concentrations (e.g., 2 mg/mL) due to its asymmetric resonance slopes. These results link controlled Fano asymmetry to measurable sensing gains, demonstrating how modestly asymmetric resonances can improve real-time refractometric detection and expand the design space of Si3N4 photonic platforms for lab-on-chip analytical applications.