Highly Absorbing Monolayer MoS₂ for a Large Reflection Phase Modulation

Manipulation of wavefront lies at the core of next-generation information technologies. Compared to metal and dielectric metasurfaces, atomic 2D materials exhibit excellent prospects toward fulfilling ultra-thin thickness requirements in flat optics in wavefront shaping, with thickness much smaller than those of traditional bulky devices. However, phase manipulation by light propagating through atomic 2D materials is suppressed due to its sub-nanometer thickness. Here, an approach is reported to realize reflection phase singularities by establishing a zero-reflection point in a monolayer MoS₂-based multilayer system, which broadens topological study beyond polarization singularity. This is achieved through the creation of a multilayer Fabry-Perot-type interference, and a pronounced phase change in the reflected light is realized due to the high absorption of monolayer MoS₂ in the studied wavelength range. As an application, a rapid, sensitive, and label-free detection of SARS-CoV-2 (2019-nCov) antigen is demonstrated with a detection limit of 10⁻¹² M L⁻¹ (62 pg ml⁻¹) by using monolayer MoS₂ based optical biosensor. In addition to offering a comprehensive study in phase singularity, efficient wavefront engineering based on the reflective system using materials is presented with atomic thickness which may greatly simplify optical architecture in flat optics, and promote its development toward compactness and integrated functions.