Asymmetric damping of plasmons by electron-phonon instability

Graphene plasmon polaritons (GPP) under a direct current (dc) can be dragged in momentum [1][2] causing a break of optical reciprocity by current flow. We find that under substantial current (~1 mA/um), in addition to the Fizeau drag, there is a drastic change in quality factor of GPP which cannot be explained by regular thermal effects. A noticeable difference in propagation length of GPP is observed under different signs of current flow, revealing a directional perturbance of GPP due to the interplays among electrons, phonons and plasmons. The interdependence of electron-plasmon drag, electron-phonon instability [3] and phonon emission caused plasmon damping are demonstrated by nanoimaging and nearfield photocurrent maps on a dc biased graphene ribbon.