Ultrastrong coupling regime and plasmon polaritons in parabolic semiconductor quantum wells.

Ultrastrong 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 2π × 670  GHz are observed, suggesting parabolic quantum wells as the system of choice in order to explore the ultrastrong coupling regime.