  The plasmon damping has been investigated using resonant microwave absorption
of two-dimensional electrons in disks with different diameters. We have found
an unexpected drastic reduction of the plasmon damping in the regime of strong
retardation. This finding implies large delocalization of retarded plasmon
field outside the plane of the two-dimensional electron system. A universal
relation between the damping of plasmon polariton waves and retardation
parameter is reported.
