  Graphene is a privileged 2D platform for hosting confined light-matter
excitations known as surface plasmon-polaritons (SPPs), as it possesses low
intrinsic losses with a high degree of optical confinement. However, the
inherently isotropic optical properties of graphene limit its ability to guide
and focus SPPs, making it less suitable than anisotropic elliptical and
hyperbolic materials as a platform for polaritonic lensing and canalization.
Here, we present the graphene/CrSBr heterostructure as an engineered 2D
interface that hosts highly anisotropic SPP propagation over a wide range of
frequencies in the mid-infrared and terahertz. Using a combination of scanning
tunneling microscopy (STM), scattering-type scanning near-field optical
microscopy (s-SNOM), and first-principles calculations, we demonstrate mutual
doping in excess of 10$^{13}$ cm$^{-2}$ holes/electrons between the interfacial
layers of graphene/CrSBr heterostructures. SPPs in graphene activated by charge
transfer interact with charge-induced anisotropic intra- and interband
transitions in the interfacial doped CrSBr, leading to preferential SPP
propagation along the quasi-1D chains that compose each CrSBr layer. This
multifaceted proximity effect both creates SPPs and endows them with
anisotropic transport and propagation lengths that differ by an
order-of-magnitude between the two in-plane crystallographic axes of CrSBr.
