  We investigate unusual surface plasmons polariton (SPP) propagation and
light-matter interactions in ultrathin black phosphorus (BP) films, a 2D
material that exhibits exotic electrical and physical properties due to its
extremely anisotropic crystal structure. Recently, it has been speculated that
the ultra-confined surface plasmons supported by BP may present various
topologies of wave propagation bands, ranging from anisotropic elliptic to
hyperbolic, across the mid- and near-infrared regions of the electromagnetic
spectrum. By carefully analyzing the natural nonlocal anisotropic optical
conductivity of BP, derived using the Kubo formalism and an effective
low-energy Hamiltonian, we demonstrate here that the SPP wavenumber cutoff
imposed by nonlocality prohibits that they acquire an arbitrary hyperbolic
topology, forcing operation in the canalization regime. The resulting
nonlocality-induced canalization presents interesting properties, as it is
inherently broadband, enables large light-matter interactions in the very near
field, and allows extreme device miniaturization. We also determine fundamental
bounds to the confinement of BP plasmons, which are significantly weaker than
for graphene, thus allowing a larger local density of states. Our results
confirm the potential of BP as a promising reconfigurable plasmonic platform,
with exciting applications, such as planar hyperlenses, optoelectronic
components, imaging, and communication systems.
