Ultrafast plasmonic behavior of graphene probed by infrared nanoscopy

Recent experiments using near-field spectroscopy (s-SNOM) have revealed the spectroscopic (Z. Fei et al., Nano Lett. 11, 4701 (2011)) and real-space characteristics (Z. Fei et al., Nature 487, 82 (2012)) of graphene plasmons and show that this technique is ideal for their investigation. Here, we discuss the time-dependent plasmonic behavior of graphene. Combining s-SNOM with ultrafast laser excitation we were able to perform near-infrared pump mid-infrared probe spectroscopy beyond the diffraction limit on exfoliated samples. We show picosecond ultrafast plasmon modulation by optical means with an efficiency comparable to electrostatic gating and also to other plasmonic materials such as metals. Modeling of our results reveals that pump-induced heating of carriers is responsible for the ultrafast change in Drude weight that s-SNOM is probing.