A High-Performance Gas Sensor Employing a Spacer-Based Cavity With Distributed Bragg Reflectors

A spacer-based cavity with distributed Bragg reflectors (DBRs) is employed in high-performance gas sensing utilizing wavelength interrogation. The DBR is formed by a 1-D photonic crystal (1DPhC) comprising six bilayers of TiO2/SiO2 with a termination layer of TiO2, and the spacer is a Ti layer deposited partially on one of the DBRs. The reflectance spectra of the sensor are measured in visible and near-infrared spectral regions at a near-normal incidence of light for humid air, and narrow dips in the 1DPhC bandgap due to the cavity modes are resolved. Two thicknesses of the spacer are considered, and it is revealed that the spacer thickness change is accompanied by nearly fivefold sensitivity enhancement. The sensitivity to relative humidity (RH) of humid air, a figure of merit (FOM), and the limit of detection reached 0.444 nm/%RH, 0.178%RH−1, and 0.022%RH, respectively. The theoretical analysis showed that the RH response of the sensor is due to the porous termination layer of the 1DPhC, and the strong dependence of the sensor sensitivity and FOM on both the spacer thickness and the position of the resonance dip in the bandgap was confirmed. A sensor employing a spacer-based cavity with DBRs thus represents an effective alternative with advantages such as sensing at normal incidence of light with high sensitivity and FOM.