Nonlinear plasmonic response of doped nanowires observed by infrared nanospectroscopy

We report a strong shift of the plasma resonance in highly doped GaAs/InGaAs core/shell nanowires for intense infrared excitation observed by scattering-type scanning near-field infrared microscopy. The studied nanowires show a sharp plasma resonance at a photon energy of about 125 meV in the case of continuous-wave excitation by a CO₂ laser. Probing the same nanowires with the pulsed free-electron laser with peak electric field strengths up to several 10 kV/cm reveals a power-dependent redshift to about 95 meV and broadening of the plasmonic resonance. We assign this effect to a substantial heating of the electrons in the conduction band and subsequent increase of the effective mass in the nonparabolic Γ-valley.