Analysis and Design of Refractive Index Biosensors Based on Single Silicon Nanobeam Cavity

Single silicon nanobeam photonic crystal cavity based sensors are systematically analyzed and designed. By using perturbation theory and numerical simulations, both dielectric-mode and air-mode cavities are extensively investigated in terms of sensitivity (S), figure of merit (FOM), detection limit (DL), footprint size, and coupling scheme. The analytical study reveals a sensitivity limit of 1176 nm/RIU and a maximum figure of merit of 5070 for nanobeam cavity based sensors, due to the absorption of light near 1550 nm wavelength. when water is used as the carrier fluid. Design of high FOM (> 4800) nanobeam cavities is demonstrated with S of 291 and 232 nm/RIU for air modes and dielectric modes, respectively. The calculation results indicate that on a 220-nm-thick-silicon SOI platform, it is possible to design a nanobeam cavity based biosensor with DL on the order of 4 × 10
^-6
RIU, insertion loss of -30 dB, and cavity length less than 40a (a is the lattice constant). To approach the absorption bounded DL, the presented design is adequate when analyte absorption dominates the loss, regardless if it is for dielectric modes or air modes. These results would be conducive to clarification of the confusion on the priority of air mode and dielectric mode in designing nanobeam cavities based sensors, as well as recent considerable efforts to maximize Sand FOM.