Guided-mode resonance based humidity sensing using a multilayer dielectric structure

We report on a highly sensitive measurement of the relative humidity of air, which utilizes a guided-mode resonance (GMR) of a multilayer dielectric structure (MDS) and the spectral interference of s- and p-polarized waves reflected from the MDS. We employ the MDS represented by four bilayers of TiO2/SiO2 with a termination layer of TiO2 and demonstrate that the GMR shows up as a shallow and asymmetric dip. The GMR enables us to measure the relative humidity (RH) of air with sensitivities of 0.031-0.114 nm/%R.H. In addition, by employing a birefringent crystal of mica, which modifies the phase difference between the polarized waves, the GMR is transformed into the resonance with a sharp dip, and the measured sensitivity is enhanced to 0.120 nm/%RH at 81 %RH. We also determined the sensitivity to the refractive index and the figure of merit as high as 8000 nm/refractive index unit (RIU) and 702 RIU-1, respectively. The results demonstrate that the GMR based sensor employing the MDS and the spectral interference of polarized waves with their phase difference appropriately adjusted enables a highly sensitive, hysteresis-free humidity measurement, characterized by a high FOM. Humidity sensors employing dielectric multilayers thus represent an effective alternative to available sensors, with advantages such as better mechanical and chemical stability. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement