Discovery of Superluminal Two-Dimensional Plasma Waves

We report the discovery of "superluminal" electromagnetic 2D plasma waves in the electromagnetic response of a high-quality GaAs/AlGaAs two-dimensional electron system on a dielectric substrate. We measure the plasma wave spectrum on samples with different electron density. It is established that at large two-dimensional densities, there is a strong hybridization between the plasma and the Fabry-Perot light modes. In the presence of a perpendicular magnetic field, the plasma resonance is shown to split into two modes, each corresponding to a particular sense of circular polarization.