Electrically Tunable Phase Singularities in Excitonic Two-Dimensional Heterostructures for Active Metasurfaces

We report on gate-tunable excitons in monolayer transition metal dichalcogenides (TMDCs) heterostructures to actively control the spectral position of their phase singularities. Previous analysis on passive metasurfaces revealed that the full-2π phase control of scattered light is associated with a branch cut crossing the real frequency axis on the complex frequency plane. Here, we show how we can dynamically tune the branch cut crossing with moderate voltage values of ±5 V by leveraging the tunability of TMDC exciton resonances. We use the complex frequency plane trajectories to design a metasurface that demonstrates full-2π phase modulation with uniform reflectance, enabling dynamic beam deflection with near-unity diffraction efficiency, and to explore in-plane coupling effects. Our results pave the way for exploiting exciton resonances for designing nanophotonic systems to explore actively tunable topological phenomena.