  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 electron-photon
collective modes: surface plasmon polaritons (SPPs). Here we show that
polaritonic interference patterns are particularly well suited to unveil the
interactions in Dirac fluids by tracking polaritonic interference in time at
temporal scales commensurate with the electronic scattering. Spacetime SPP
interference patterns recorded in tera-hertz (THz) frequency range provided
unobstructed readouts of the group velocity and lifetime of polariton that can
be directly mapped onto the electronic spectral weight and the relaxation rate.
Our data uncovered prominent departures of the electron dynamics from the
predictions of the conventional Fermi-liquid theory. The deviations are
particularly strong when the densities of electrons and holes are approximately
equal. The proposed spacetime imaging methodology can be broadly applied to
probe the electrodynamics of quantum materials.
