Ultra strong coupling regime and plasmon-polaritons in parabolic   semiconductor quantum wells
  Ultra strong coupling is studied in a modulation-doped parabolic potential well coupled to an inductance-capacitance resonant circuit. In this system, in accordance to Kohn's theorem, strong reduction of the energy level separation caused by the electron-electron interaction compensates the depolarization shift. As a result, a very large ratio of 27% of the Rabi frequency to the center resonance frequency as well as a polariton gap of width 2? ? 670GHz are observed, suggesting parabolic quantum wells as the system of choice in order to explore the ultra-strong coupling regime. 
Markus Geiser, Fabrizio Castellano, Giacomo Scalari, Mattias Beck,   Laurent Nevou, J\'er\^ome Faist
cond-mat.mes-hall