Ultrafast and Nanoscale Plasmonic Phenomena in Exfoliated
Graphene Revealed by Infrared Pump–Probe Nanoscopy
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Abstract
Pump–probe spectroscopy is
central for exploring ultrafast
dynamics of fundamental excitations, collective modes, and energy
transfer processes. Typically carried out using conventional diffraction-limited
optics, pump–probe experiments inherently average over local
chemical, compositional, and electronic inhomogeneities. Here, we
circumvent this deficiency and introduce pump–probe infrared
spectroscopy with ∼20 nm spatial resolution, far below the
diffraction limit, which is accomplished using a scattering scanning
near-field optical microscope (s-SNOM). This technique allows us to
investigate exfoliated graphene single-layers on SiO<sub>2</sub> at
technologically significant mid-infrared (MIR) frequencies where the
local optical conductivity becomes experimentally accessible through
the excitation of surface plasmons via the s-SNOM tip. Optical pumping
at near-infrared (NIR) frequencies prompts distinct changes in the
plasmonic behavior on 200 fs time scales. The origin of the pump-induced,
enhanced plasmonic response is identified as an increase in the effective
electron temperature up to several thousand Kelvin, as deduced directly
from the Drude weight associated with the plasmonic resonances