  It is well known that optical absorption saturation of intersubband
transitions in doped semiconductor quantum wells is independent of the
introduced doping in the absence of a cavity. When inserting the system in a
resonator, we show that this remains valid only in the weak light-matter
coupling regime. In the strong light-matter coupling regime instead, we
demonstrate that absorption saturation is no more doping independent and it is
instead tailorable. Based on this unified formalism for saturation in weak and
strong coupling, we provide designs for semiconductor saturable absorption
(SESAM) mirrors and bistable systems operating in the mid-infrared range of the
electromagnetic spectrum and with extremely low saturation intensities.
Countering intuition, we show that the most suitable region to exploit low
saturation intensities is not the ultra-strong coupling regime, but is instead
at the onset of strong light-matter coupling.
