Towards an active biosensing platform in rare-earth ion doped Al₂O₃ microring resonators

Waveguide microring resonators sensors are numerously found in the Si-based technologies. Their operation, however, requires the operation of a tunable laser and the real-time monitoring of the location of their resonance peaks. An active material with gain could omit such operation through the detection of a laser-generated signal. Here, Al₂O₃ is presented as an alternative technology that has the potential for such active sensing applications. Microring resonators of undoped and Er³⁺-doped Al₂O₃ were fabricated and characterized. The undoped microring resonator was used for bulk refractive index sensing with a highest sensitivity of 95 nm/RIU and a limit of detection of 2.5e-6 RIU. After optical pumping the doped devices experienced the compensation of loss through internal gain, but the losses were too high to achieve lasing in the device. This work shows the feasibility of realizing passive sensors on the Al₂O₃ platform and efforts are ongoing to transcend the platform from a passive to an active operation.