Abstract
Compared to the conventional strip waveguide microring resonators, subwavelength grating (SWG) waveguide microring resonators have better sensitivity and lower detection limit due to the enhanced photon-analyte interaction. As sensors, especially biosensors, are usually used in absorptive ambient environment, it is very challenging to further improve the detection limit of the SWG ring resonator by simply increasing the sensitivity. The high sensitivity resulted from larger mode-analyte overlap also brings significant absorption loss, which deteriorates the quality factor of the resonator. To explore the potential of the SWG ring resonator, we theoretically and experimentally optimize an ultrasensitive transverse magnetic mode SWG racetrack resonator to obtain maximum quality factor and thus lowest detection limit. A quality factor of 9800 around 1550 nm and sensitivity of 429.7 ± 0.4nm/RIU in water environment are achieved. It corresponds to a detection limit (λ/S·Q) of 3.71 × 10-4 RIU, which marks a reduction of 32.5% compared to the best value reported for SWG microring sensors.
Highlights
In recent decades, optical resonators based sensors on silicon-on-insulator (SOI) platform have been showing great promise in clinical diagnostic assays and biomarker detection
Various resonance structures have been proposed to optimize the performance of these sensors, including two-dimensional photonic crystal (PhC) micro-cavity resonators [1,2,3,4], one dimensional PhC nanobeam resonators [5,6,7,8], disk resonators [9, 10], and ring resonators [11,12,13]
Subwavelength grating (SWG) waveguide consists of periodic silicon pillars with a period smaller than the operating wavelength, which can increase the interaction between light and the cladding materials on the top and side of the waveguide, and the space between the silicon pillars on the light propagation path [13]
Summary
Optical resonators based sensors on silicon-on-insulator (SOI) platform have been showing great promise in clinical diagnostic assays and biomarker detection. In order to enhance the photon-analyte interaction to increase the sensitivity without decreasing the qualify factor, and to minimize the detection of limit, we propose a silicon-based transverse magnetic (TM) mode SWG racetrack resonator (SWGRTR) operating at around 1550 nm. In order to quantify the photon-analyte interaction and the potential of the proposed SWGRTR structure for sensing applications, we calculate the mode volume overlap integral f between the optical field and the analytes for TM and TE mode while varying the size of pillars. The photonic modes approach the light line of the cladding materials and are subject to more radiation loss resulting in quasi-guided modes when the waveguide width and Si duty cycle are both located in the green dashed region in Fig. 3(a) [26]. The footprint of racetrack resonator is less than that of circular ring resonator with the same circumference
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