  Polaritonic modes in two-dimensional van der Waals materials display short
in-plane wavelengths compared with light in free space. As interesting as this
may look from both fundamental and applied viewpoints, such large confinement
is accompanied by poor in/out optical coupling, which severely limits the
application of polaritons in practical devices. Here, we quantify the coupling
strength between light and 2D polaritons in both homogeneous and anisotropic
films using accurate rigorous analytical methods. In particular, we obtain
universal expressions for the cross sections associated with photon-polariton
coupling by point and line defects, as well as with polariton extinction and
scattering processes. Additionally, we find closed-form constraints that limit
the maximum possible values of these cross sections. Specifically, the maximum
photon-to-plasmon conversion efficiency in graphene is $\sim10^{-6}$ and
$\sim10^{-4}$ for point and line scatterers sitting at its surface,
respectively, when the plasmon and photon energies are comparable in magnitude.
We further show that Mie resonators placed at an optimum distance from the film
can boost light-to-polariton coupling to order unity. Our results bear
fundamental interest for the development of 2D polaritonics and the design of
applications based on these excitations.
