Ultra-strong light–matter coupling and superradiance using dense electron gases

The physics of the interaction between a dense two-dimensional electron gas and a microcavity photonic mode is reviewed. For high electronic densities, this system enters the ultra-strong coupling regime in which the Rabi energy, which measures the strength of the light–matter coupling, is of the same order of magnitude as the matter excitation. The ultra-strong coupling has been experimentally demonstrated by inserting a highly doped semiconductor layer between two metal plates that produce a microcavity, with extreme sub-wavelength confinement of the electromagnetic field. A record value at room temperature (73%) of the ratio between the Rabi and the matter excitation energies (the relative Rabi energy) has been measured together with a very large photonic gap induced by the polariton splitting. The ultra-strong coupling is a manifestation of a huge cooperative dipole, which is proportional to the number of electrons participating in the interaction. Such a phenomenal interaction with light appears also in the absence of a microcavity and, for a dipole coupled with free space, it gives rise to superradiance.