Retardation effects for “dark” plasma modes in a two-dimensional electron system

Plasma excitations with different symmetries in a single two-dimensional electron disk are studied by optical detection of resonant microwave absorption. The manifestation of retardation effects for “dark” axisymmetric plasma excitations is studied for the first time. It is shown that the hybridization of these excitations with light is much smaller than the hybridization of dipole active two-dimensional plasmons. An unusual crossing of the magnetic field dispersion of “bright” and dark plasmon-polariton modes is observed in a perpendicular magnetic field.