  Two-dimensional (2D) crystals have renewed opportunities in design and
assembly of artificial lattices without the constraints of epitaxy. However,
the lack of thickness control in exfoliated van der Waals (vdW) layers prevents
realization of repeat units with high fidelity. Recent availability of uniform,
wafer-scale samples permits engineering of both electronic and optical
dispersions in stacks of disparate 2D layers with multiple repeating units. We
present optical dispersion engineering in a superlattice structure comprised of
alternating layers of 2D excitonic chalcogenides and dielectric insulators. By
carefully designing the unit cell parameters, we demonstrate > 90 % narrowband
absorption in < 4 nm active layer excitonic absorber medium at room
temperature, concurrently with enhanced photoluminescence in cm2 samples. These
superlattices show evidence of strong light-matter coupling and
exciton-polariton formation with geometry-tunable coupling constants. Our
results demonstrate proof of concept structures with engineered optical
properties and pave the way for a broad class of scalable, designer optical
metamaterials from atomically-thin layers.
