Transition from strong to ultra-strong coupling regime in mid-infrared   metal-dielectric-metal cavities
  We have investigated the transition from strong to ultra-strong coupling regime between a mid-infrared intersubband excitation and the fundamental mode of a metal-dielectric-metal microcavity. The ultra-strong coupling regime is demonstrated up to room temperature for a wavelength of $11.7 \mu$m by using 260 nm thick cavities, which impose an extreme sub-wavelength confinement. By varying the doping of our structures we show that the experimental signature of the transition to the ultra-strong coupling regime is the opening of a photonic gap in the polariton dispersion. The width of this gap depends quadratically on the ratio between the Rabi and intersubband transition energies. 
Pierre Jouy, Angela Vasanelli, Yanko Todorov, Aymeric Delteil, Giorgio   Biasiol, Lucia Sorba, and Carlo Sirtori
cond-mat.mes-hall