Integrated silicon-based suspended racetrack micro-resonator for biological solution sensing with high-order mode

A biological sensing structure with a high-order mode (\mathrmE_21^y) is designed, which is composed of a suspended racetrack micro-resonator (SRTMR) and a microfluidic channel. The mode characteristics, coupling properties, and sensing performances are simulated by using the finite element method (FEM). To analyze the mode confinement property, the confinement factors in the core and cladding of the suspended waveguide for the \mathrmE_11^x, \mathrmE_11^y, and \mathrmE_21^y are calculated. The simulation results show that the refractive index (RI) sensitivity of the proposed sensing structure can be improved by using the high-order mode (\mathrmE_21^y). The RI sensitivity for the \mathrmE_21^y mode is ～ 201 nm/RIU, which is twice to thrice higher than those for the \mathrmE_11^x mode and the \mathrmE_11^y mode. Considering a commercial spectrometer, the proposed sensing structure based on the SRTMR achieves a limit of detection (LOD) of ～ 4.7×10^-6 RIU. Combined with the microfluidic channel, the SRTMR can possess wide applications in the clinical diagnostic assays and biochemical detections.