title: Ultrastrong coupling regime and plasmon polaritons in parabolic semiconductor quantum wells.
abstract: 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.
authors.affiliation: Institute for Quantum Electronics, ETH Zurich, Wolfgang-Pauli-Strasse 16, 8093 Zurich, Switzerland. mgeiser@ethz.ch
authors.forename: Markus
authors.initials: M
authors.lastname: Geiser
authors.forename: Fabrizio
authors.initials: F
authors.lastname: Castellano
authors.forename: Giacomo
authors.initials: G
authors.lastname: Scalari
authors.forename: Mattias
authors.initials: M
authors.lastname: Beck
authors.forename: Laurent
authors.initials: L
authors.lastname: Nevou
authors.forename: Jérôme
authors.initials: J
authors.lastname: Faist