Multiplexed optical sensing using topological phase singularities in anisotropic borophene ultrathin films

Phase singularities (PSs) of topological darkness have garnered substantial attention in the realms of bio-sensing, single-molecule label-free sensing, and super-resolution imaging, owing to their remarkable characteristics such as ultra-sensitivity, label-free detection capabilities, and robustness against structural defects. Nevertheless, the implementation of multiplexed singular-phase configurations within the topological sensing system is not only essential but also presents formidable challenges, primarily due to the requirement of achieving absorption at specific frequencies. To surmount this obstacle, we conduct a theoretical investigation on a structure comprising a silver substrate, a deposited ultra-thin (<i>h</i> << <i>λ</i>/4<i>n</i>) germanium (Ge) layer, with borophene positioned on top of it. Due to the anisotropic optical property of borophene, <i>p</i>- and <i>s</i>-polarization light beams exhibit different responses at two orthogonal planes of incidence, thereby giving rise to four topological PSs. This structure exhibits the merits of possessing an ultra-thin film, being facile to fabricate, and enabling multi-channel sensing. The analytical and simulation results demonstrate excellent agreement at visible and infrared frequencies. Furthermore, the temporal coupled-mode theory reveals that there are ± π-jumps near the zero-reflection point of the phase of <i>Δ</i>. This study establishes what we believe to be novel multiplexed optical sensing frameworks, thereby showing potential applications in gas detection, biosensor technologies, health monitoring, and pesticide residue analysis.