  Graphene's optical properties in the infrared and terahertz can be tailored
and enhanced by patterning graphene into periodic metamaterials with
sub-wavelength feature sizes. Here we demonstrate polarization sensitive and
gate tunable photodetection in graphene nanoribbon arrays. The long-lived
hybrid plasmon-phonon modes utilized are coupled excitations of electron
density oscillations and substrate (SiO2) surface polar phonons. Their
excitation by s-polarization leads to an in-resonance photocurrent an order of
magnitude larger than the photocurrent observed for p-polarization, which
excites electron-hole pairs. The plasmonic detectors exhibit photoinduced
temperature increases up to four times as large as comparable 2D graphene
detectors. Moreover, the photocurrent sign becomes polarization sensitive in
the narrowest nanoribbon arrays due to differences in decay channels for
photoexcited hybrid plasmon-phonons and electrons. Our work provides a path to
light sensitive and frequency selective photodetectors based on graphene's
plasmonic excitations.
