Black Phosphorus Plasmonics: Anisotropic Elliptical Propagation and Nonlocality-Induced Canalization

TL;DR

This paper explores the unique plasmonic properties of ultrathin black phosphorus, revealing nonlocal effects that enforce canalization over hyperbolic propagation, with implications for miniaturized optoelectronic devices.

Contribution

It demonstrates that nonlocal optical conductivity in black phosphorus prevents hyperbolic plasmons, leading to broadband canalization and larger plasmon confinement than graphene.

Findings

Nonlocality enforces canalization in BP plasmons.

BP supports larger confinement and density of states than graphene.

Potential applications in hyperlenses and optoelectronic devices.

Abstract

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.