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 11.7 μ m 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 …