  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 SiO2 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 femtosecond (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.
