Enhanced Sensitivity of Subwavelength Multibox Waveguide Microring Resonator Label-Free Biosensors

In this paper, a novel silicon photonic waveguide for a high sensitivity label-free sensor is presented. We use silicon-on-insulator subwavelength waveguides that consist of five rows of periodic silicon squares with 60 nm gap distances between each square (multibox structure) to construct microring resonators to achieve higher bulk and surface sensitivities than the resonators using conventional strip waveguides. Due to the high surface contact area and low optical confinement of the proposed multibox waveguides, the sensitivity of the microring resonator can be significantly increased. By exposing the multibox microring resonators to ultrapure water and different concentrations of isopropyl alcohol solution, we show a bulk sensitivity of 579.5 nm/RIU, which is an increase compared to recently published subwavelength grating waveguides and strip waveguides in transverse magnetic polarization microring resonators. Finally, a standard biological sandwich assay is employed to demonstrate the multibox sensor's ability, with an enhanced surface sensitivity of 1900 pm/nm.