Comparative analysis of SOI bulk rectangular and nanowire optical waveguide for label-free lung cancer detection

Silicon-on-Insulator (SOI) waveguides are suitable for photonic integrated design for communication as well as sensing due to its various advantageous features. Initial research on SOI waveguide showed that light as a wave can be confined inside high refractive indexed bulk silicon rectangular waveguide. Later, research reported by researchers working on silicon nanowire waveguide suggests that wave can also be guided in low refractive index (RI) region, mainly inside the gaps between high RI materials. The working phenomena and application for its sensing capacity is related to a number of factors, which need full in-depth understanding. This study started with the comparison of mode field propagation and sensing behaviour of both high refractive indexed guided bulk rectangular waveguide and low refractive indexed guided nanowire waveguide. This study suggests that both, bulk high RI waveguide and nanowire low RI waveguide are supporting hybrid modes of quasi-TE nature. Results indicate that interaction of sensing analyte is significant in nanowire waveguide and negligible in bulk rectangular waveguide. Waveguide responses were analyzed using finite element method-based boundary mode analysis. The result showed that the surface sensing is dominant and to illustrate the role of surface interacting with analyte, the effect of functionalization layer coverage over the surface of silicon wires is investigated using waveguide confinement factor. These results suggest that full cover of functionalization layer is having more interaction towards sensing analyte. On this basis, the concept of functionalization layer over waveguide sensing surface should be taken into account when designing a label-free surface sensing optical device.