  Many critical applications in environmental and biological sensing, standoff
detection, and astronomy rely on devices that operate in the mid-infrared
range. Unfortunately, current mid-infrared devices are costly and have limited
tunability. Nanostructured graphene has been proposed for active mid-infrared
devices via excitation of tunable surface plasmons, but typically present low
efficiencies due to weak coupling with free-space radiation and plasmon
damping. We present an alternative approach, in which graphene plasmons couple
with gold localized plasmons, creating hybridized modes and enhancing the
coupling efficiency. We demonstrate a metasurface in which hybrid plasmons are
excited with transmission modulation rates of 17% under moderate doping (0.35
eV). We also evaluate the metasurface as a mid-infrared modulator, measuring
switching speeds of up to 16 kHz. Finally, we propose a scheme in which we can
excite strongly coupled gold-graphene gap plasmons in the thermal radiation
range, with applications to nonlinear optics, slow light, and sensing.
