Ballistic plasmon polaritons in graphene

Polaritons are hybrid excitations of light and matter that can confine the energy of long-wavelength radiation at the nano-scale. The polaritons can enable many enigmatic quantum effects including condensation and superfluidity, lasing, topological protection, and dipole-forbidden absorption. A necessary condition for realizing these novel phenomena is a long polariton lifetime, which is notoriously difficult to meet. Surface plasmon polaritons in graphene – hybrid modes of Dirac quasiparticles and infrared photons – have emerged as an outstanding platform for exploring light-matter interaction at the nano-scale. However, plasmonic dissipation in graphene has remained significant and its fundamental limits remained undetermined. Here we exploit nano-infrared imaging to investigate propagating plasmons in high mobility graphene at cryogenic temperatures where electrons exhibit ballistic transport. In this regime, plasmons likewise travel over distances limited largely by sample dimensions. Our analysis reveals an intrinsic plasmonic propagation length beyond ten micrometers at liquid nitrogen temperatures that are commonplace in infrared technology.