High quality factor Al2O3 microring resonators for on-chip sensing applications

Microring resonators find many applications for on-chip integrated optical sensors. Their spectral response contains resonance dips that shift due to variations of the optical path length of the microring probed. Numerous examples of such microring resonator sensors in the SOI, Si3N4 and SiON waveguide technologies have been reported for the detection of bulk refractive index variations and the label-free detection of biomarkers. Al2O3 is an alternative waveguide technology that exhibits low optical propagation losses, is transparent over a large spectral range extending from the visible to the mid-IR and permits co-doping with active rare-earth ions, which enables the co-integration of active devices on the chip. In this work an Al2O3 microring resonator sensor was developed for the label-free detection of protein biomarkers. The uncladded microring with a radius of 200 μm had a measured quality factor of 3.2 × 105 at 1550 nm. Submerging the devices in water decreased the quality factor to 45 × 103. This corresponds with propagation losses in the rings of 0.6 dB/cm and 5.7 dB/cm respectively. The bulk refractive index sensitivity of the sensor was determined by flowing NaCl dissolved in water in different concentrations. A sensitivity of 102.3 ± 0.5 nm/RIU with a corresponding limit of detection of 1.6 × 10-6 RIU was demonstrated for TM polarized light. High affinity human monoclonal antibodies mAb S100A4 were immobilized on the sensor to detect the S100A4 protein biomarker down to 12 nM concentrations. These results demonstrate the feasibility of this material for label-free optical biosensors.