Microring resonators on silicon-on-insulator substrate have been demonstrated to be promising in sensing applications. We study a microring resonator biosensor based on a novel subwavelength grating (SWG) waveguide structure, which consists of periodic silicon pillars in the propagation direction with a subwavelength period. In this structure, effective sensing region includes not only the top and side of the waveguide, but also the space in between the silicon pillars which is on the path of the propagation mode. This leads to greatly increased bulk refractive index sensitivity as well as extended surface sensing region with constantly high surface sensitivity.
Subwavelength grating (SWG) ring resonators have demonstrated better sensitivity compared to the conventional silicon strip ring resonators due to the enhanced photon-analyte interaction. As the sensors are usually used in absorptive ambient environment, it is extreme challenging to further improve the sensitivity of the SWG ring resonator without deteriorating the quality factor because the coupling strength between the bus waveguide and the circular ring resonator is not sufficient to compensate the loss. To explore the full potential of the SWG ring resonator, we experimentally demonstrate a silicon-based high quality factor and low detection limit transverse magnetic (TM) mode SWG racetrack resonator around 1550 nm. A quality factor of 9800 is achieved in aqueous environment when the coupling length and gap are equal to 6.5 μm and 140 nm, respectively. The bulk sensitivity (S) is ~429.7 nm/RIU (refractive index per unit), and the intrinsic detection limit (iDL) is 3.71×10<sup>-4</sup> RIU reduced by 32.5% compared to the best value reported for SWG microring sensors.