Observation of plasmon-polariton modes in two-dimensional electron systems

A manifestation of retardation effects, which were predicted theoretically more than 35 years ago, is revealed for the first time in the plasma excitation spectrum of a two-dimensional electron system with a high electron mobility. It is shown that a significant decrease in the resonant plasma frequency due to a hybridization of the plasma and light modes is observed in zero magnetic field. An unusual dependence of the frequency of the hybrid cyclotron-plasmon mode on the magnetic field has been observed in a perpendicular magnetic field. The experimental results are in good quantitative agreement with the theory.