ACS Photon. (2017)

Light–matter interaction at the nanoscale can be enhanced by exciting plasmons; graphene plasmons, in particular, show tunable properties dependent on doping level or interaction with dielectric materials. However, graphene plasmons are in the mid-infrared region, and although high doping levels and lateral confinement can push the wavelength towards the visible, the coupling between far-field light and graphene will remain weak. Now, de Vega and García de Abajo propose a methodology for visible-plasmon generation in graphene that requires no light at all. Instead, plasmons are generated from tunnelling electrons.

The researchers consider a graphene–hexagonal boron nitride (hBN)–graphene sandwich structure. The hBN layer is 1-nm thick and the two graphene monolayers have different Fermi energy. Applying a bias between the two graphene sheets produces tunnelling electrons through the gap. The researchers find a happy voltage window in which the tunnelling electrons lose energy through the excitation of a propagating optical plasmon rather than dissipate through coupling with the hBN phonons (low bias) or electron–electron interactions (high bias). According to the calculations, using graphene layers with 0.5− and 1.0−eV Fermi energy, the electron-to-plasmon yield can reach unity with a bias lower than 1 V nm−1. Plasmonic devices made in this way, which do not require the mediation of photons, can also be used in reverse as sensors, where a change in the graphene plasmon properties is translated into a voltage readout.