Martin Wagner, Zhe Fei, Alexander S. McLeod, Aleksandr S. Rodin,Wenzhong Bao, Eric G. Iwinski, Zeng Zhao, Michael Goldflam, Mengkun Liu, Gerardo Dominguez, Mark Thiemens, Michael M. Fogler, Antonio H. Castro Neto, Chun Ning Lau, Sergiu Amarie, Fritz Keilmann and D. N. Basov
Nano Letters 2014 14 (2) pp 894–900 (2014)
Pump–probe spectroscopy is central for exploring ultrafast dynamics of fundamental excitations, collective modes, and energy transfer processes. Typically carried out using conventional diffraction-limited optics, pump–probe experiments inherently average over local chemical, compositional, and electronic inhomogeneities. Here, we circumvent this deficiency and introduce pump–probe infrared spectroscopy with 20 nm spatial resolution, far below the diffraction limit, which is accomplished using a scattering scanning near-field optical microscope (s-SNOM). This technique allows us to investigate exfoliated graphene single-layers on SiO2 at technologically significant mid-infrared (MIR) frequencies where the local optical conductivity becomes experimentally accessible through the excitation of surface plasmons via the s-SNOM tip. Optical pumping at near-infrared (NIR) frequencies prompts distinct changes in the plasmonic behavior on 200 fs time scales. The origin of the pump-induced, enhanced plasmonic response is identified as an increase in the effective electron temperature up to several thousand Kelvin, as deduced directly from the Drude weight associated with the plasmonic resonances.