Steering Chiral Exciton Emission With All Van Der Waals Nonlocal Phase Gradient Metasurfaces

ABSTRACT Monolayer transition metal dichalcogenides with ambient stable excitons have enabled a variety of excitonic light‐emitting devices. As exciton emission is spontaneous emission that is spatiotemporally incoherent, it is challenging to achieve spatially coherent phase control. Here, we present the experimental demonstration of the steering of chiral exciton emission with all‐van der Waals nonlocal phase gradient metasurfaces. By tailoring the in‐plane symmetry of the meta‐atoms, a high‐quality (Q) factor quasi‐bound state in the continuum (quasi‐BIC) resonance appears above the light cone due to the Brillouin zone folding. Such a high‐Q nonlocal optical resonance not only enhances the spontaneous emission of excitons, but also improves the spatiotemporal coherence of the emitted light. By leveraging resonance‐enhanced geometric phase manipulation, we demonstrate controllable beam deflection of exciton emission in a chirality‐dependent manner. This is achieved through tailored spatial phase gradients on metasurfaces with a total device thickness below λ/20. By establishing all‐van der Waals nonlocal phase‐gradient metasurfaces, this work demonstrates a promising avenue for precise directionality control of 2D exciton emission and opens new prospects for designing multifunctional fluorescent emitters.