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

Area of Science:

• Integrated photonics
• Nanophotonics
• Optical sensing

Background:

• Fano interference creates asymmetric optical resonances for enhanced signal contrast in integrated photonics.
• Fabrication challenges often reduce asymmetry and slope steepness in nanodevices.
• Silicon nitride (Si3N4) photonic platforms offer potential for lab-on-chip applications.

Purpose of the Study:

• To experimentally demonstrate a hybrid silicon nitride photonic crystal nanobeam-microring resonator (PhCN-MRR) for refractive index (RI) sensing.
• To analyze the impact of Fano-induced line shape asymmetry on sensing performance under flow conditions.
• To compare the sensing capabilities of the PhCN-MRR with conventional microring resonators (MRRs).

Main Methods:

• Fabrication of a hybrid PhCN-MRR sensor integrated into a Si3N4 microfluidic platform.
• Refractive index sensing experiments using glucose solutions (0.5–10 mg/mL) under stopped-flow and dynamic flow conditions.
• Time-resolved spectral scans (0.5–1 Hz) to measure both wavelength shifts (Δλ) and intensity changes (ΔI).

Main Results:

• The PhCN-MRR exhibited moderate asymmetry but showed distinct optical advantages for intensity-based detection.
• Comparable wavelength shift sensitivities (∼111–113 nm/RIU) were observed compared to conventional MRRs.
• Enhanced intensity responsivity and improved contrast at low concentrations were achieved due to asymmetric resonance slopes.

Conclusions:

• Controlled Fano asymmetry in PhCN-MRRs leads to measurable sensing gains.
• Modestly asymmetric resonances can improve real-time refractometric detection.
• The PhCN-MRR expands the design space for Si3N4 photonic platforms in lab-on-chip analytical applications.