Plasmonic Doppler Effect in Graphene

High mobility two-dimensional electron gases reveal an intriguing phenomenon of the plasmonic Doppler shift. The plasmonic response is altered when direct current (DC) is applied provided the drift velocity of electrons reaches a substantial fraction of the Fermi velocity. When plasmons are coupled with light, surface plasmon polaritons (SPP) are predicted to acquire a quasi-relativistic Doppler effect [D.S. Borgnia and L.Levitov, arXiv: 1512.09044]. Here we utilize cryogenic nano-imaging technique to search for the current-induced Doppler effect in the SPP dynamics in graphene. Directional carrier flow breaks time-reversal symmetry and causes non-reciprocal plasmonic responses in infrared frequencies. Changes of SPP wavelength in real space are attributable to the Doppler effect. SPP imaging data inform us of the behavior of hybrid quasiparticles under current flow.