Uniaxial plasmon polaritons $\textit{via}$ charge transfer at the graphene/CrSBr interface

TL;DR

This paper demonstrates that a graphene/CrSBr heterostructure supports highly anisotropic surface plasmon-polaritons in the mid-infrared and terahertz range, enabled by charge transfer-induced doping and mutual interactions at the interface.

Contribution

It introduces the graphene/CrSBr heterostructure as a new platform for anisotropic SPP propagation, combining experimental and theoretical methods to reveal charge transfer effects.

Findings

Mutual doping exceeds 10^13 cm^-2 in the heterostructure.

SPPs exhibit anisotropic propagation lengths along different CrSBr axes.

Charge transfer activates and guides SPPs in the heterostructure.

Abstract

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.