  The physics of electrons, photons, and their plasmonic interactions changes
greatly when one or more dimensions are reduced down to the nanometer scale.
For example, graphene shows unique electrical, optical, and plasmonic
properties, which are tunable through gating or chemical doping. Similarly,
ultrathin metal films (UTMFs) down to atomic thickness can possess new quantum
optical effects, peculiar dielectric properties, and predicted strong plasmons.
However, truly two-dimensional plasmonics in metals has so far elusive because
of the difficulty in producing large areas of sufficiently thin continuous
films. Thanks to a deposition technique that allows percolation even at 1 nm
thickness, we demonstrate plasmons in few-nanometer gold UTMFs, with clear
evidence of new dispersion regimes and large electrical tunability. Resonance
peaks at 1.5-5 micrometer wavelengths are shifted by hundreds of nanometers and
amplitude-modulated by tens of per cent through gating using relatively low
voltages. The results suggest ways to use metals in plasmonic applications,
such as electro-optic modulation, bio-sensing, and smart windows.
