Integrated polymer ring resonator sensor for environmental monitoring

In this paper, we investigate the properties of polymer ring resonators for environmental monitoring. Optical sensors provide low-cost and real-time gas sensing to monitor air quality. Polymers in combination with whispering gallery mode resonators offer high sensitivity and a broad selection of materials that can be tailored for specific needs. We used SU-8, which is a low-cost, common photoresist, and we tested its sensitivity to relative humidity and temperature. To excite the resonances, we used a 760 nm tunable laser. Our results show that it is important to optimize the gap between the resonator and the waveguide to improve coupling efficiency, which increases the Q factor and, consequently, the sensitivity. However, the resonator width is not crucial. By applying a PMMA coating, we demonstrated that functionalized coatings can improve the selectivity of the sensor.