Integrated SU-8 ring resonator for rapid humidity sensing

We demonstrate an integrated on-chip whispering gallery mode (WGM) ring resonator tested for humidity sensing. When developed, SU-8 is chemically and mechanically stable, as well as optically transparent above 400 nm wavelength and it has a high refractive index. Therefore, it is a suitable material for optical WGM resonators. When light is coupled in the resonator, it circulates along the surface for prolonged periods of time interacting with the surrounding environment. Resonance wavelength depends on the refractive index and/or the radius of the resonator. Polymers, including SU-8, are sensitive to gas and temperature changes in the environment. We tested the ring resonators in changing relative humidity (RH). Due to changes in RH, the refractive index of SU-8 changes, and we observed a shift in the resonance wavelength. While the sensitivity was average compared to similar studies using other materials or geometries, the ring geometry showed excellent response and recovery times. This property is important in many areas, such as industrial production and environmental monitoring.