Atomic-layer assembly of ultrathin optical cavities in van der Waals heterostructure metasurfaces

Photonics has been revolutionized by advances in optical metasurfaces, unlocking design and engineering opportunities for flat optical components. Similarly, layered two-dimensional materials have enabled breakthroughs in physics via the deterministic assembly of vertical heterostructures, allowing precise control over the atomic composition of each layer. However, integrating these fields into a single system has remained challenging, limiting progress in atomic-scale optical cavities and metamaterials. Here we demonstrate the concept of van der Waals heterostructure metasurfaces, where ultrathin multilayer van der Waals material stacks are shaped into precisely engineered resonant nanostructures for enhancing light–matter interactions. By leveraging quasi-bound states in the continuum physics, we create intrinsic high-quality-factor resonances originating from WS2 monolayers encapsulated in hexagonal boron nitride at thicknesses below 130 nm, achieving room-temperature strong coupling and polaritonic photoluminescence emission. Furthermore, the metasurface-coupled exciton–polaritons exhibit strong nonlinearities, leading to a saturation of the strong-coupling regime at ultralow fluences of <1 nJ cm–2, three orders of magnitude lower than in previous two-dimensional-material-based cavity systems. Our approach monolithically integrates metasurfaces and van der Waals materials and can be extended to the vast library of existing two-dimensional materials, unlocking new avenues for ambient operation of ultrathin polaritonic devices with atomic-scale precision and control. Ultrathin multilayer van der Waals material stacks are shaped into precisely engineered resonant nanostructures, giving strong nonlinearities at ultralow fluences of <1 nJ cm–2, more than three orders of magnitude smaller than in previous two-dimensional-material-based cavity systems.