Charge-Transfer Hyperbolic Polaritons in $\alpha$-MoO$_3$/graphene heterostructures

TL;DR

This paper demonstrates direct charge transfer and hyperbolic polaritons in epitaxially grown $ ext{MoO}_3$/graphene heterostructures, revealing new opportunities for nano-optical and energy devices.

Contribution

It provides the first direct observation of charge transfer hyperbolic polaritons in pure polaritonic heterostructures, with detailed analysis of the interface and charge transfer effects.

Findings

Charge transfer hyperbolic polaritons observed with wavelength elongation up to 150%

Fermi energy of 0.6 eV for graphene extracted via nano-imaging

Charge transfer attributed to pristine heterointerface in epitaxial vdW heterostructures

Abstract

Charge transfer is a fundamental interface process that can be harnessed for light detection, photovoltaics, and photosynthesis. Recently, charge transfer was exploited in nanophotonics to alter plasmon polaritons by involving additional non-polaritonic materials to activate the charge transfer. Yet, direct charge transfer between polaritonic materials hasn't been demonstrated. We report the direct charge transfer in pure polaritonic van der Waals (vdW) heterostructures of $\alpha$-MoO$_3$/graphene. We extracted the Fermi energy of 0.6 eV for graphene by infrared nano-imaging of charge transfer hyperbolic polaritons in the vdW heterostructure. This unusually high Fermi energy is attributed to the charge transfer between graphene and $\alpha$-MoO$_3$. Moreover, we have observed charge transfer hyperbolic polaritons in multiple energy-momentum dispersion branches with a wavelength elongation of up to 150%. With support from the DFT calculation, we find that the charge transfer between graphene and $\alpha$-MoO$_3$, absent in mechanically assembled vdW heterostructures, is attributed to the relatively pristine heterointerface preserved in the epitaxially grown vdW heterostructure. The direct charge transfer and charge transfer hyperbolic polaritons demonstrated in our work hold great promise for developing nano-optical circuits, computational devices, communication systems, and light and energy manipulation devices.