Double topological phase singularities in highly absorbing ultra-thin film structures for ultrasensitive humidity sensing

Phase singularities (PSs) in topological darkness-based sensors have received significant attention in optical sensing due to their rapid, ultra-sensitive, and label-free detection capabilities. Here, we present both experimental and theoretical investigations of an ultrasensitive and multiplexed phase-sensitive sensor utilizing dual topological PSs in the visible and near-infrared regions. This sensor uses a simple structure, which consists of an ultra-thin highly absorbing film deposited on a metal substrate. We demonstrate the achievement of dual-polarization darkness points for s- and p-polarizations at different incident angles. Furthermore, we theoretically explain the double topological PSs accompanied by a perfect ±π-jump near a zero-reflection point, based on the temporal coupled-mode formalism. To validate its multifunctional capabilities, humidity sensing tests were carried out. The results demonstrate that the sensor has a detection limit reaching the level of 0.12 ‰. These findings go beyond the scope of conventional interference optical coatings and highlight the potential applications of this technology in gas sensing and biosensing domains.