  Photonics has been revolutionized by breakthroughs in optical metasurfaces
and layered two-dimensional materials. Yet, integrating these two fields in a
singular system has remained challenging. Here, we introduce the concept of van
der Waals (vdW) heterostructure metasurfaces, where ultrathin multilayer vdW
material stacks are shaped into precisely engineered resonant nanostructures
for boosting light-matter interactions. By leveraging quasi-bound states in the
continuum to create intrinsic cavities from WS$_2$ monolayers encapsulated in
hexagonal boron nitride, we observe room-temperature strong coupling and
polaritonic luminescence, which further unveils a saturation of the
strong-coupling regime at ultralow fluences <1 nJ/cm2, more than three orders
of magnitude smaller than in previous 2D-cavity systems. Our approach,
seamlessly merging metasurfaces and vdW materials, unlocks new avenues for
ultrathin optical devices with atomic-scale precision and control.
