Ultrafast optical bleaching of intersubband cavity polaritons

We report on the transition from the strong to the weak light-matter coupling regime between an intersubband excitation and a photonic crystal resonance in a nanostructured semiconductor membrane. Such a transition is induced by varying the intensity of an ultrafast light pulse, which is employed for pumping and probing the system eigenmodes. The phenomenon is interpreted in terms of the saturation of the intersubband transition due to the large number of photoexcited electrons in the quantum well, as confirmed by a thorough analysis performed both in frequency and time domain.