  Among its many outstanding properties, graphene supports terahertz surface
plasma waves -- sub-wavelength charge density oscillations connected with
electromagnetic fields that are tightly localized near the surface[1,2]. When
these waves are confined to finite-sized graphene, plasmon resonances emerge
that are characterized by alternating charge accumulation at the opposing edges
of the graphene. The resonant frequency of such a structure depends on both the
size and the surface charge density, and can be electrically tuned throughout
the terahertz range by applying a gate voltage[3,4]. The promise of tunable
graphene THz plasmonics has yet to be fulfilled, however, because most proposed
optoelectronic devices including detectors, filters, and modulators[5-10]
desire near total modulation of the absorption or transmission, and require
electrical contacts to the graphene -- constraints that are difficult to meet
using existing plasmonic structures. We report here a new class of plasmon
resonance that occurs in a hybrid graphene-metal structure. The sub-wavelength
metal contacts form a capacitive grid for accumulating charge, while the narrow
interleaved graphene channels, to first order, serves as a tunable inductive
medium, thereby forming a structure that is resonantly-matched to an incident
terahertz wave. We experimentally demonstrate resonant absorption near the
theoretical maximum in readily-available, large-area graphene, ideal for THz
detectors and tunable absorbers. We further predict that the use of high
mobility graphene will allow resonant THz transmission near 100%, realizing a
tunable THz filter or modulator. The structure is strongly coupled to incident
THz radiation, and solves a fundamental problem of how to incorporate a tunable
plasmonic channel into a device with electrical contacts.
