  In this article we report about linear and nonlinear optical properties of
intersubband cavity polariton samples, where the resonant photonic mode is a
defect state in a metallo-dielectric photonic crystal slab. By tuning a single
geometric parameter of the resonator, the cavity Q-factor can reach values as
large as 85, with a consequent large cooperativity for the light-matter
interaction. We show that a device featuring large cooperativity leads to sharp
saturation, or even bistability, of the polariton states. This nonlinear
dynamics occurs at the crossover between the weak and the strong coupling
regimes, where the weak critical coupling concept plays a fundamental role.
