Excitons Enabled Topological Phase Singularity in a Single Atomic Layer.

The nontrivial and rigorous Heaviside phase jump behavior of phase singularities (PSs) empowers exotic topological modes and widely divergent nature compared to neighboring points, which has attracted great attention in condensed matter physics as well as applications in photonics and ultrasensitive sensors. Here we demonstrate the universal existence of a family of topologically protected PSs generated from exciton resonances of single-atom layers. We obtain the PSs by coating the transition metal dichalcogenide (TMDC) monolayers on a nonabsorptive semi-infinite substrate without surface plasmon effect or other assisted resonators, which exploits the benefits of both exciton-dominated enhancement and peculiarities of the singular phase. We show that a refractive indices matched transparent substrate enables TMDC monolayers to exhibit topologically protected zero reflection accompanied by a perfect Heaviside π-phase jump at strong light adsorptions, which can be utilized to radically reduce the thickness of PS-based devices to a single atomic layer. By using the TMDC monolayer-based PSs for refractive index biosensors, we demonstrate its superior phase sensitivity at a level of 10 degrees per refractive index unit and detection of bioactive bacteria, respectively, which is comparable to the cutting-edge surface plasmon and Fabry-Perot resonance sensors. Our proof-of-concept results offer experimental and theoretical insights into a single atomic playground for flat singular optics and label-free biosensing technologies.