Exploring hot-electron plasmons in graphene with nano-infrared imaging and spectroscopy

We report a comprehensive nano-infrared imaging and spectroscopy study of the hot-electron plasmons in graphene using the scattering-type scanning near-field optical microscope (s-SNOM). We found that the electrons can be heated up to 1400 K by femtosecond laser fields that are strongly enhanced by the metalized s-SNOM tip. With s-SNOM, we monitored both the plasmon interference fringes and hybrid plasmon-phonon resonance of graphene due to hot electrons. In highly-doped graphene, we found that the plasmon wavelength is smaller and the plasmon-phonon resonance becomes weaker at high electron temperatures. At the charge neutrality point, on the other hand, the plasmon-phonon resonance is stronger when electrons are hotter. Theoretical analysis indicates that the observed hot-electron responses can be understood by the competition by three major factors, namely the drop of chemical potential, the increase of electron scattering rate, and the excitation of thermal carriers at higher electron temperatures. Our work deepens the understanding of plasmonic responses of graphene at elevated electron temperatures and paves the way for future studies of hot-electron plasmons in other plasmonic media.