  We propose a unified description of intersubband absorption saturation for
quantum wells inserted in a resonator, both in the weak and strong light-matter
coupling regimes. We demonstrate how absorption saturation can be engineered.
In particular we show that the saturation intensity increases linearly with the
doping in the strong coupling regime, while it remains doping independent in
weak coupling. Hence, countering intuition, the most suitable region to exploit
low saturation intensities is not the ultra-strong coupling regime, but is
instead at the onset of the strong light-matter coupling. We further derive
explicit conditions for the emergence of bistability. This work sets the path
towards yet unexisting ultrafast mid-infrared semiconductor saturable
absorption mirrors (SESAMs) and bistable systems. As an example, we show how to
design a mid-infrared SESAM with a three orders of magnitude reduction in
saturation intensity, down to about 5 kW/cm2.
