Broadband gate-tunable THz plasmons in graphene heterostructures

TL;DR

This paper demonstrates a novel all-optical method to generate and control tunable THz plasmons in graphene heterostructures, achieving octave frequency tunability and high efficiency through nonlinear processes and gate control.

Contribution

It introduces a counter-pumped all-optical difference frequency process in graphene for coherent THz plasmon generation with broad tunability and high efficiency, supported by theoretical modeling.

Findings

Achieved octave tunability of THz plasmons from 9.4 THz to 4.7 THz.

Demonstrated gate-tunable control of THz plasmons in graphene heterostructures.

Supported experimental results with theoretical modeling of graphene plasmon physics.

Abstract

Graphene, a unique two-dimensional material of carbon in a honeycomb lattice, has brought remarkable breakthroughs across the domains of electronics, mechanics, and thermal transport, driven by the quasiparticle Dirac fermions obeying a linear dispersion. Here we demonstrate a counter-pumped all-optical difference frequency process to coherently generate and control THz plasmons in atomic layer graphene with an octave tunability and high efficiency. We leverage the inherent surface asymmetry of graphene for a strong second-order nonlinear polarizability chi(2), which together with tight plasmon field confinement, enables a robust difference frequency signal at THz frequencies. The counter-pumped resonant process on graphene uniquely achieves both energy and momentum conservation. Consequently we demonstrate a dual-layer graphene heterostructure that achieves the charge- and gate-tunability of the THz plasmons over an octave, from 9.4 THz to 4.7 THz, bounded only by the pump amplifier optical bandwidth. Theoretical modeling supports our single-volt-level gate tuning and optical-bandwidth-bounded 4.7 THz phase-matching measurements, through the random phase approximation with phonon coupling, saturable absorption, and below the Landau damping, to predict and understand the graphene carrier plasmon physics.