Tracking quasiparticle energies in graphene with near-field optics

Advances in infrared nanoscopy have enabled access to the finite momentum optical conductivity sigma(q,omega). The finite momentum optical conductivity in graphene has a peak at the Dirac fermion quasiparticle energy epsilon(k(F) - q), i.e., at the Fermi momentum minus the incident photon momentum. We find that the peak remains robust even at finite temperature as well as with residual scattering. It can be used to trace out the fermion dispersion curves. However, this effect depends strongly on the linearity of the Dirac dispersion. Should the Dirac fermions acquire a mass, the peak in sigma(q,) shifts to lower energies and broadens as optical spectral weight is redistributed over an energy range of the order of the mass gap energy. Even in this case structures remain in the conductivity, which can be used to describe the excitation spectrum. By contrast, in graphene strained along the armchair direction, the peak remains intact but shifts to a lower value of q determined by the anisotropy induced by the deformation.