Nanoscale infrared imaging and spectroscopy of hot-electron plasmons in graphene

Abstract

We report a nanoscale infrared (IR) imaging and spectroscopy study of hot-electron plasmons in graphene, which are excited by the sharp metallic probe of the scattering-type scanning near-field optical microscope (s-SNOM) illuminated with a mid-IR femtosecond pulsed laser. We found the average electron temperature $({T}_{e})$ can reach as high as $\ensuremath{\sim}1400\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ within the pulse duration, which can be controlled by tuning the laser power. With s-SNOM, we monitored both the plasmon interference fringes and the hybrid plasmon-phonon resonances of graphene. When graphene is heavily doped, a higher ${T}_{e}$ leads to a smaller plasmon wavelength and a weaker plasmon-phonon resonance intensity. At the charge neutrality point, on the other hand, the plasmon-phonon resonance intensity is enhanced when ${T}_{e}$ increases. With quantitative modeling and theoretical analysis, we concluded that the observed plasmonic responses of hot electrons are governed by the temperature dependencies of chemical potential, electron scattering, and thermal carrier generation. The competition of these factors leads to distinct ${T}_{e}$ dependence of graphene plasmons at various doping levels.