  The ability to manipulate optical fields and the energy flow of light is
central to modern information and communication technologies, as well as
quantum information processing schemes. However, as photons do not possess
charge, controlling them efficiently by electrical means has so far proved
elusive. A promising way to achieve electric control of light could be through
plasmon polaritons - coupled excitations of photons and charge carriers - in
graphene. In this two-dimensional sheet of carbon atoms, it is expected that
plasmon polaritons and their associated optical fields can be readily tuned
electrically by varying the graphene carrier density. While optical graphene
plasmon resonances have recently been investigated spectroscopically, no
experiments so far have directly resolved propagating plasmons in real space.
Here, we launch and detect propagating optical plasmons in tapered graphene
nanostructures using near-field scattering microscopy with infrared excitation
light. We provide real-space images of plasmonic field profiles and find that
the extracted plasmon wavelength is remarkably short - over 40 times smaller
than the wavelength of illumination. We exploit this strong optical field
confinement to turn a graphene nanostructure into a tunable resonant plasmonic
cavity with extremely small mode volume. The cavity resonance is controlled
in-situ by gating the graphene, and in particular, complete switching on and
off of the plasmon modes is demonstrated, thus paving the way towards
graphene-based optical transistors. This successful alliance between
nanoelectronics and nano-optics enables the development of unprecedented active
subwavelength-scale optics and a plethora of novel nano-optoelectronic devices
and functionalities, such as tunable metamaterials, nanoscale optical
processing and strongly enhanced light-matter interactions for quantum devices
and (bio)sensors.
