Prediction of hyperbolic exciton-polaritons in monolayer black phosphorus

Fanjie Wang,Yuangang Xie,Guowei Zhang,Chong Wang,Chaoyu Song,Lei Mu,Qiaoxia Xing,Shenyang Huang,Yuchen Lei,Hugen Yan,Andrey Chaves

doi:10.1038/s41467-021-25941-5

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.

Highlights

Hyperbolic polaritons exhibit large photonic density of states and can be collimated in certain propagation directions
Excitons can couple with free photons in the spectral range where the film has a positive imaginary part of the optical conductivity (ImðσÞ > 0) due to the strong exciton resonances, leading to in-plane propagating EPs confined to the two-dimensional (2D) films
The in-plane anisotropic exciton absorption was observed in atomically thin black phosphorous (BP)[24,25]

Summary

Introduction

Hyperbolic polaritons exhibit large photonic density of states and can be collimated in certain propagation directions. 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. Excitons can couple with free photons in the spectral range where the film has a positive imaginary part of the optical conductivity (ImðσÞ > 0) due to the strong exciton resonances, leading to in-plane propagating EPs confined to the two-dimensional (2D) films Such EPs were recently demonstrated in monolayer WS2 encapsulated by hexagonal-boron-nitride[11], where the narrow exciton linewidth and large oscillator strength ensure ImðσÞ > 0 needed for such in-plane polariton modes. The photon energy window for HEPs shrinks with increasing sample thickness and disappears for samples thicker than 4-layer (L)

Methods

Results

Conclusion