Critical Wavelength Referencing Free Bio-Sensor utilizing Silicon-on-Insulator based Directional Coupler

In this paper, we propose and theoretically analyze a novel sensor structure for bulk refractive index and bio-layer sensing applications. The sensor employs a directional coupler consisting of two silicon-on-insulator waveguides and utilizes the existence of the
critical
wavelength in the transmission spectra of the directional coupler 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 Refractive Index sensitivity (RI) ∼ 30 μm/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.