Gate-tunable topological singularities in two-dimensional van der Waals heterostructures for active metasurfaces

Analysis of non-Hermitian photonic systems by the spectral positions of their topological singularities plays a central role in their design considerations. Recently, the link between topological singularities and existing schemes for far-field beam shaping revealed that the full-2π phase control of light scattering from metasurfaces is associated with branch cut crossing on the complex frequency plane. Here, we report our study on gate-tunable monolayer transition metal dichalcogenides (TMDCs) to actively change the scattering parameters of our heterostructures by controlling the spectral position of the topological singularities. Specifically, by leveraging the large tunability of TMDC excitonic resonances, we demonstrate an active metasurface based on monolayer TMDC heterostructures that can deflect light to angles up to ±50°. Our analysis reveals that moderate voltage values of ±5 V is sufficient to push the branch cuts to the real frequency axis, enabling 2π phase modulation with uniform reflectance which is crucial for efficient directive deflection. Our results can be extended to explore further effects of excitonic resonances on tunable topological platforms for arbitrary beam shaping and exceptional point-based sensing, contributing to the development of active two-dimensional nanophotonics.