Emergence of plasmaronic structure in the near-field optical response of graphene

The finite momentum optical response sigma(q, omega) of graphene can be probed with the innovative technique of infrared nanoscopy where midinfrared radiation is confined by an atomic force microscope cantilever tip. In contrast to conventional q similar to 0 optical absorption which primarily involves Dirac fermions with momentum near the Fermi momentum k similar to k(F), for finite q, sigma(q, omega) has the potential to provide information on many-body renormalizations and collective phenomena which have been found at small k < k(F) near the Dirac point in electron-doped graphene. For electron-electron interactions, the low-energy excitation spectrum characterizing the incoherent part of the quasiparticle spectral function of Dirac electrons with k similar to k(F) consists of a flat, small amplitude background which scales with chemical potential and Fermi momentum. However, probing of the states with k near k = 0 will reveal plasmarons, a collective state of a charge carrier and a plasmon. These collective modes in graphene have recently been seen in angle-resolved photoemission spectroscopy and here we describe how they manifest in near-field optics.