  We report THz-pump / mid-infrared probe near-field studies on Si-doped
GaAs-InGaAs core-shell nanowires utilizing THz radiation from the free-electron
laser FELBE. Upon THz excitation of free carriers, we observe a red shift of
the plasma resonance in both amplitude and phase spectra, which we attribute to
the heating up of electrons in the conduction band. The simulation of heated
electron distributions anticipates a significant electron population in both L-
and X-valleys. The two-temperature model is utilized for a quantitative
analysis of the dynamics of the electron gas temperature under THz pumping at
various power levels.
