Observation of Diode Behavior and Gate Voltage Control of Hybrid Plasmon-Phonon Polaritons in Graphene-Hexagonal Boron Nitride Heterostructures

TL;DR

This paper demonstrates a diode effect and gate voltage control of hybrid plasmon-phonon polaritons in graphene-hBN heterostructures, enabling unidirectional polariton propagation and potential applications in nano-optic logic devices.

Contribution

It presents the first observation of a polariton diode effect and gate-controlled hybridization of HP3 modes in G-hBN heterostructures, with implications for on-chip nano-optics.

Findings

Unidirectional propagation of HP3 modes observed.

Gate voltage modulates hybridization between plasmons and phonons.

Break in reflection/transmission symmetry demonstrated.

Abstract

Light-matter interaction in two-dimension photonic materials allows for confinement and control of free-space radiation on sub-wavelength scales. Most notably, the van der Waals heterostructure obtained by stacking graphene (G) and hexagonal Boron Nitride (hBN) can provide for hybrid hyperbolic plasmon phonon-polaritons (HP3). Here, we present a polariton diode effect and low-bias control of HP3 modes confined in G-hBN. Using broadband infrared synchrotron radiation coupled to a scattering-type near-field optical microscope, we launch HP3 waves over both hBN Reststrahlen bands and observe the unidirectional propagation of HP3 modes at in-plane heterointerfaces associated with the transition between different substrate dielectrics. By electric gating we further control the HP3 hybridization modifying the coupling between the continuum graphene plasmons and the discrete hyperbolic phonon polaritons of hBN as described by an extended Fano model. This is the first demonstration of unidirectional control of polariton propagation, with break in reflection/transmission symmetry for HP3 modes. G-hBN and related hyperbolic metamaterial nanostructures can therefore provide the basis for novel logic devices of on-chip nano-optics communication and computing.