High-sensitivity silicon nitride microring resonator opto-fluidic sensor

TL;DR

This paper demonstrates a high-sensitivity, scalable opto-fluidic sensor based on silicon nitride microring resonators integrated on a photonic chip. It reports a mean refractive-index sensitivity of 579 nm/RIU in the C-band using a foundry-fabricated device and validates performance with IPA solutions, while examining the trade-off between Q-factor and evanescent-field overlap. Modelling shows radius-dependent sensitivity linked to field overlap, supporting the experimental findings and informing design for improved detection limits. The work argues for CMOS-compatible, scalable sensor platforms with functionalisation potential for environmental and biomedical applications, and outlines avenues to further enhance sensitivity through device and microfluidic optimisations.

Abstract

Photonic integrated circuit devices can be used as refractometric opto-fluidic sensors to detect the presence of analytes in solution at low concentrations. In this work, we investigate the refractive index sensitivity of silicon nitride microring resonator based photonic integrated circuit fluidic sensors. The performance of a foundry fabricated sensor is measured over the C-band in the presence of liquid samples achieving a mean sensitivity of 579 nanometres per refractive index unit. This demonstration of a scalable, high-sensitivity opto-fluidic sensor, compatible with recognition marker surface functionalisation, opens the way to applications in environmental and bio-sensing.

Figures (3)

• Figure 1: Experimental setup. (a) Schematic of the experimental setup used to characterise our SiN MRRs. (b) SEM micrograph of a fabricated 40µ m radius MRR. (c) Schematic of the SiN MRR immersed in analyte. (d) Schematic cross section of the SiN sample tray showing how analyte solutions are delivered to the sensing microring. (e) Camera image of the SiN photonic chip with etched channel shown.
• Figure 2: Optical characterisation. (a) Transmission spectra of a 40µ m radius SiN microring resonator with a mean FSR of 4.83nm. The orange trace corresponds to the 'though port' and the blue trace corresponds to the 'drop port'. (b) Experimental characterisation of a 40µ m SiN microring in the presence of varying concentrations of IPA. The predicted progression of the resonance modes is shown. (c) IPA solution refractive index as a function of $\%$ (w/w) concentration. (d) Resonance wavelength shift as a function of IPA refractive index, from which we calculate the sensor's sensitivity.
• Figure 3: Simulation results of a 40µ m SiN microring resonator. (a) Schematic showing the simulation setup. (b) Transmission spectra of the 40µ m microring showing the first three cladding refractive indices. (c) Resonance wavelength shift as a function of the cladding refractive index for different microring radii. We chose the optical mode at $\approx$1526nm with a cladding index of 1.330 as the starting point to measure subsequent shifts in the resonance wavelength. (d) Microring sensitivity and loaded Q-factor as a function of microring radius.