Ultraclean graphene at charge neutrality hosts a quantum critical Dirac fluid
of interacting electrons and holes. Interactions profoundly affect the charge
dynamics of graphene, which is encoded in the properties of its collective
modes: surface plasmon polaritons (SPPs). The group velocity and lifetime of
SPPs have a direct correspondence with the reactive and dissipative parts of
the tera-Hertz (THz) conductivity of the Dirac fluid. We succeeded in tracking
the propagation of SPPs over sub-micron distances at femto-second (fs) time
scales. Our experiments uncovered prominent departures from the predictions of
the conventional Fermi-liquid theory. The deviations are particularly strong
when the densities of electrons and holes are approximately equal. Our imaging
methodology can be used to probe the electromagnetics of quantum materials
other than graphene in order to provide fs-scale diagnostics under
near-equilibrium conditions