  We demonstrate that surface plasmon resonances excited by photon tunneling
through an adjacent dielectric medium enhance photocurrent detected by a
graphene photodetector. The device is created by overlaying a graphene sheet
over an etched gap in a gold film deposited on glass. The detected
photocurrents are compared for five different excitation wavelengths, ranging
from nm to nm. The photocurrent excited with incident p-polarized light (the
case for resonant surface plasmon excitation) is significantly amplified in
comparison with that for s-polarized light (without surface plasmon
resonances). We observe that the photocurrent is greater for shorter
wavelengths (for both s and p-polarizations) due to the increased photothermal
current resulting from higher damping of surface plasmons at shorter wavelength
excitation. Position-dependent Raman spectroscopic analysis of the
optically-excited graphene photodetector indicates the presence of charge
carriers near the metallic edge. In addition, we show that the polarity of
photocurrent switches across the gap as the incident light spot moves across
the gap. Graphene-based photodetectors offer a simple architecture which can be
fabricated on dielectric waveguides to exploit the plasmonic photocurrent
enhancement of the evanescent field for detection. Applications for these
devices include photo-detection, optical sensing and direct plasmonic
detection.
