  We present a proof of concept for a spectrally selective near-infrared (NIR)
and short-wavelength infrared (SWIR) photodetector based on nanopatterned
multilayer graphene intercalated with FeCl$_3$ (NPMLG-FeCl$_3$), enabling large
modulation p-doping of graphene. The localized surface plasmons (LSPs) on the
graphene sheets in NPMLG-FeCl$_3$ allow for electrostatic tuning of the
photodetection in the NIR and SWIR regimes from $\lambda =1.3$ $\mu$m to 3
$\mu$m, which is out of range for nanopatterned monolayer graphene (NPG). Most
importantly, the LSPs along with an optical cavity increase the absorbance from
about $N\times 2.6$\% for $N$-layer graphene-FeCl$_3$ (without patterning) to
nearly 100\% for NPMLG-FeCl$_3$, where the strong absorbance occurs locally
inside the graphene sheets only. Our NIR and SWIR detection scheme relies on
the photo-thermoelectric effect induced by asymmetric patterning of the
multi-layer graphene (MLG) sheets. The LSPs on the nanopatterned side create
hot carriers that give rise to Seebeck photodetection at room temperature
achieving a responsivity of $R=6.15\times 10^3$ V/W, a detectivity of
$D^*=2.3\times 10^{9}$ Jones, and an ultrafast response time of the order of
100 ns. Our theoretical results pave the way to graphene-based photodetection,
optical IR communication, IR color displays, and IR spectroscopy in the NIR,
SWIR, mid-wavelength infrared (MWIR), and long-wavelength infrared (LWIR)
regimes.
