  The surface plasmon polaritons (SPP) of graphene reflect the microscopic
spatial variations of underlying electronic structure and dynamics. Access to
this information requires probing the full SPP response function. We image the
graphene SPP phase and amplitude by combining scanning probe tip coupled
surface plasmon interferometry with phase resolved near-field signal detection.
We show that a simple analytical cavity model can self-consistently describe
the phase and amplitude response both for edge, grain boundary, and defect SPP
reflection and scattering. The derived complex SPP wavevector, damping, and
carrier mobility agree with the results from more complex models. This phase
information opens a new degree of freedom for spatial and spectral graphene SPP
tuning and modulation for opto-electronics applications.
