  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.
