Critical Wavelength Referencing Free Biosensor Using Silicon-on-Insulator-Based Directional Coupler

In this article, we propose and theoretically analyze a novel sensor structure for bulk refractive index (RI) and bio-layer sensing applications. The sensor utilizes a directional coupler (DC) consisting of two silicon-on-insulator (SOI) waveguides and the existence of the critical wavelength in the transmission spectra of the DC to enhance the sensitivity. Optimizing various opto-geometrical parameters, viz., the width of individual waveguides, the separation between them, and the slot width of the upper cladding, we have achieved an extremely high RI sensitivity <inline-formula> <tex-math notation="LaTeX">$\sim 30 \; \mu $ </tex-math></inline-formula>m/RIU for aqueous solutions having RI close to ~1.333, and a surface adlayer sensitivity ~25.5 nm/nm for adsorbed bio-layers of RI 1.45 and thickness 6 nm. The analysis has been performed for the TE-like mode. We also show that the phase-measurement-based sensitivity calculations, despite being relatively complex, offer critical wavelength free sensing, and thus are better suited to avoid false alarms. Owing to the high bulk and surface sensitivities, our results can find application in designing high-precision biosensors.