Integrated photonics circuits as transducers for refractive index sensing

Abstract

On chip microring resonators (MRRs) have sparked widespread interest for refractive index sensing applications in recent years. In our work we demonstrate the sensing capabilities of an on-chip Si3N4 MRR on silicon substrate with an oxide layer. We incorporate the chip with low-volume PDMS microfluidics to allow for rapid and continuous sensing of refractive index changes of the fluid on top of the MRR. The setup is temperature controlled and housed to allow more robust and sensitive measurements. A commercial HPLC system was employed to provide a steady flow with glucose solutions of varying concentrations used as test samples. The changes in refractive index were measured continuously and the results were compared to the performance of a commercial refractive index detector. For the measurement two principal methods were evaluated: wavelength-shift based detection and intensity-change at a fixed wavelength. In the wavelength shift method the laser was swept across the resonance of the MRR repeatedly and the position of the resonance was used to monitor refractive index changes. On the other hand, for the intensity change method, the laser was tuned to the inflection point of the resonance and the intensity changes caused by the resonance shifts were measured. The comparison showed distinct differences in the linear range and sensitivity of the two methods. While the sweeping method has lower sensitivity it has a higher linear range, outcompeting that of commercial refractive index detectors.