Intersubband Polaritons in the Electrical Dipole Gauge
  We provide a theoretical description for the coupling between the intersubband excitations of a bi-dimensional electron gas with the electromagnetic field. This description, based on the electrical dipole gauge, applies to an arbitrary quantum heterostructure embedded in a general multilayered waveguide or a microcavity. We show that the dipole gauge Hamiltonian automatically takes into account the Coulomb interactions in this system. Furthermore, it can be conveniently expressed in terms of the many-body collective plasmon modes, which interact both with each other and with the light field. The dipole gauge therefore provides a suitable framework for the study of solid state Quantum Electrodynamics (QED) phenomena, such as the ultra-strong light-matter interaction regime, occurring at very high electronic densities. 
Yanko Todorov and Carlo Sirtori
cond-mat.quant-gas