  We demonstrate that absorption saturation of a mid-infrared intersubband
transition can be engineered to occur at moderate light intensities of the
order of 10-20 kW$.$cm$^{-2}$ and at room temperature. The structure consists
of an array of metal-semiconductor-metal patches hosting a judiciously designed
253 nm thick GaAs/AlGaAs semiconductor heterostructure. At low incident
intensity the structure operates in the strong light-matter coupling regime and
exhibits two absorption peaks at wavelengths close to 8.9 $\mu$m. Saturation
appears as a transition to the weak coupling regime - and therefore to a
single-peaked absorption - when increasing the incident intensity. Comparison
with a coupled mode theory model explains the data and permits to infer the
relevant system parameters. When the pump laser is tuned at the cavity
frequency, the reflectivity decreases with increasing incident intensity. When
instead the laser is tuned at the polariton frequencies, the reflectivity
non-linearly increases with increasing incident intensity. At those wavelengths
the system therefore mimics the behavior of a saturable absorption mirror
(SESAM) in the mid-IR range, a technology that is currently missing.
