Electrically Tunable Topological Singularities in Excitonic Two-Dimensional Heterostructures for Wavefront Manipulation

Analysis of non-Hermitian photonic systems by the spectral positions of their topological singularities plays a central role in their design. Recently, the link between topological phase singularities and existing schemes for wavefront manipulation revealed that the full-2π phase control of light scattering from non-Hermitian metasurfaces is associated with branch cut crossing on the complex frequency plane. Here, we report on controlling the spectral positions of topological singularities for electrically tunable monolayer transition metal dichalcogenide heterostructures so as to actively change their scattering parameters. We use the complex frequency plane trajectories to demonstrate full-2π phase modulation with uniform reflectance, enabling dynamic beam deflection with near-unity diffraction efficiency, and to explore in-plane coupling effects at the surface. Our results can be further extended to explore effects of excitonic resonances on tunable topological platforms for arbitrary wavefront manipulation and exceptional point-based sensing as new directions for active two-dimensional nanophotonics.