▎ 摘　　要

Electrons in graphene behave like relativistic Dirac particles which can reduce the velocity of light by two orders of magnitude in the form of plasmon-polaritons. Here we show how these properties lead to a peculiar nonlinear plasmon response in the quasiclassical regime of terahertz frequencies. On the one hand, we show how interband plasmon damping is suppressed by the relativistic Klein tunneling effect. On the other hand, we demonstrate huge enhancement of the nonlinear intraband response when the plasmon velocity approaches the resonance with the electron Fermi velocity. This extreme sensitivity to the plasmon intensity could be used for new terahertz technologies.