Asymmetric plasmon damping and abnormal thermoelectric current by electron-phonon instability

Plasmons polaritons, a collective mode of electron oscillations hybridized with light, assists a plethora of physical processes, such as optical-electrical energy conversion and optical nonreciprocity. Therefore, it is important to understand the interaction between plasmon polaritons and other degrees of freedom, such as phonons and Dirac electron flow. Here we find that highly doped and biased graphene displays unconventional asymmetric plasmon damping and unexpected photocurrent sign-changes. Using cryogenic scanning nearfield microscope, noticeable difference in propagation length of graphene plasmon polaritons is observed at the same launcher for different current directions. Along with this, an abnormal sign-flip of thermoelectric current close to contacts reveals potential new mechanism for tip-based nano-photocurrent generation. Cherenkov emission of acoustic phonons can explain both the asymmetric plasmon damping and the abnormal photocurrent behavior in highly biased graphene. The observations facilitate the understanding of impacts by non-equilibrium phonons on plasmon polaritons and thermoelectric current.