  Two-dimensional materials give access to the ultimate physical limits of
Photonics with appealing properties for ultracompact optical components such as
waveguides and modulators. Specifically, in monolayer semiconductors, a strong
excitonic resonance leads to a sharp oscillation in permittivity; at energies
close to an exciton, the real part of the permittivity can reach high positive
values or even become negative. This extreme optical response enables surface
exciton-polaritons to guide visible light bound to an atomically thin layer.
However, such ultrathin waveguides support a transverse electric (TE) mode with
low confinement and a transverse magnetic (TM) mode with short propagation.
Here, we propose that realistic semiconductor-insulator-semiconductor
superlattices consisting of monolayer WS$_2$ and hexagonal boron nitride (hBN)
can improve the properties of both TE and TM modes. Compared to a single
monolayer, a heterostructure with a 1-nm hBN spacer improves the confinement of
the TE mode from 1.2 to around 0.5 $\mu$m, whereas the out-of-plane extension
of the TM mode increases from 25 to 50 nm. We propose two simple additivity
rules for mode confinement valid in the ultrathin film approximation for
heterostructures with increasing spacer thickness. Stacking additional WS2
monolayers into superlattices further enhances the waveguiding properties. Our
results underscore the potential of monolayer superlattices as a platform for
visible nanophotonics with promising optical, electrical, and magnetic
tunability
