Transition from strong to ultrastrong coupling regime in mid-infrared metal-dielectric-metal cavities

We have investigated the transition from strong to ultrastrong coupling regime between a mid-infrared intersubband excitation and the fundamental mode of a metal-dielectric-metal microcavity. The ultrastrong coupling regime is demonstrated up to room temperature for a wavelength of 11.7¼m by using 260 nm thick cavities, which impose an extreme subwavelength confinement. By varying the doping of our structures we show that the experimental signature of the transition to the ultrastrong 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