On the ultrastrong vacuum Rabi coupling of an intersubband transition in a semiconductor microcavity

In this invited paper of the 28th International Conference of the Physics of Semiconductors (ICPS-28), we discuss the peculiar quantum electrodynamical properties of a semiconductor microcavity system, in which a cavity photon mode is strongly coupled to an intersubband transition of a doped quantum well system. In this kind of semiconductor system, it is possible to achieve an unprecedented ultrastrong coupling regime, in which the vacuum Rabi frequency is comparable to the electronic transition frequency. We discuss the anomalous quantum properties of the quantum ground state (a squeezed vacuum) and of the intersubband cavity polariton excitations. We address the role of dissipation and point out some future perspectives concerning the investigation of quantum vacuum radiation effects induced by an ultrafast time modulation of the quantum vacuum.