Prediction of hyperbolic exciton-polaritons in monolayer black phosphorus

TL;DR

This paper predicts that monolayer black phosphorus naturally supports hyperbolic exciton-polaritons due to its anisotropic optical properties and strong exciton resonances, offering a new platform for studying natural hyperbolic materials.

Contribution

It introduces the prediction of natural hyperbolic exciton-polaritons in monolayer black phosphorus based on optical spectroscopy analysis.

Findings

Determined exciton binding energy of ~452 meV in monolayer black phosphorus.

Observed strong and sharp ground state exciton peak in reflection spectrum.

Predicted natural hyperbolic exciton-polaritons due to in-plane anisotropy.

Abstract

Hyperbolic polaritons exhibit large photonic density of states and can be collimated in certain propagation directions. The majority of hyperbolic polaritons are sustained in man-made metamaterials. However, natural-occurring hyperbolic materials also exist. Particularly, natural in-plane hyperbolic polaritons in layered materials have been demonstrated in MoO3 and WTe2, which are based on phonon and plasmon resonances respectively. Here, by determining the anisotropic optical conductivity (dielectric function) through optical spectroscopy, we predict that monolayer black phosphorus naturally hosts hyperbolic exciton-polaritons due to the pronounced in-plane anisotropy and strong exciton resonances. We simultaneously observe a strong and sharp ground state exciton peak and weaker excited states in high quality monolayer samples in the reflection spectrum, which enables us to determine the exciton binding energy of ~452 meV. Our work provides another appealing platform for the in-plane natural hyperbolic polaritons, which is based on excitons rather than phonons or plasmons.