Sensitivity and intrinsic limit of detection improvement in a photonic refractive-index sensor

In this work, we study the bulk refractive index (RI) sensitivity (S) and intrinsic limit of detection (ILOD) of a micro racetrack resonator (MRR) based RI-sensor. Sensing measurements are performed with solutions of sodium chloride (NaCl)-water. The sensing device includes an array of multiple silicon-oxy-nitride (SiON) MRRs, fibre-coupled to a vertical-cavity surface-emitting laser (VCSEL) operating at 850 nm. We particularly choose 850 nm for sensing measurement, since the water absorption coefficient at this wavelength is three orders of magnitude lower than at 1550 nm (another conventional wavelength in biosensing). Different concentrations of NaCl-water solutions (spanning from 0.0% to 1.8% weight/weight), are flowed on top of the MRR surface in the cladding layer. Measuring of whispering gallery mode (WGM) resonance wavelength shifts in NaCl-water solutions, confirming a S and ILOD of 31.66 nm/RIU and 5.98 x 10(-4) RIU, respectively. Furthermore, with exact engineering of the physical parameters of the system, the best S and ILOD of 325.85 nm/RIU and 6.14 x 10(-5) RIU are found, respectively. The ILOD parameter delineates the minimum amount of RI or correspondingly minimum concentration of a biological solution, which can be detected by a RI-sensor. In this regard, this device can find applications in the exact measurement of NaCl in real biological sample solutions.