This work focuses on the design, the realization and the characterization of devices, based on semiconductor quantum wells and operating in the strong or ultra-strong coupling regime between an intersubband excitation and a microcavity mode. The mixed states issued from this coupling are the so-called intersubband polaritons. In the first part of this work, we demonstrate an electroluminescent device working in the strong coupling regime, in which the upper polariton branch is populated at a voltage dependent energy. Furthermore, we observe a relaxation from the upper to the lower branch by means of the emission of an optical phonon. Thanks to this efficient process, the occupancy of the lower branch reaches about 15 % and could lead to stimulated emission of polaritons. By increasing the electronic density in the well, it is possible to reach the ultra-strong coupling regime, characterized by a Rabi energy of the same order of magnitude as the intersubband transition energy. For this reason, the second part of this work is devoted to the study of highly doped quantum wells, with several occupied subbands. In particular, we theoretically and experimentally investigate the Coulomb interaction between the intersubband plasmons associated to the different optically active transitions. We present a device based on a quantum well with two occupied subbands, in which a gate voltage controlling the electronic density modifies the interaction between plasmons and thus the optical response. For the highest densities the oscillator strength is redistributed into the high energy peak. Owing to to this phenomenon, we demonstrate that the optical response of a quantum well with at least three occupied subbands presents a single sharp resonance that corresponds to a collective excitation associating in phase all the intersubband transitions. This collective excitation is observed both in absorption and electroluminescence measurements. The ultra-strong coupling regime is demonstrated by inserting the quantum well in both a planar and a metal - dielectric - metal microcavity. Furthermore, we demonstrate that the Rabi energy increases monotonously with the electronic density in the well. This work shows that the light-matter interaction in highly doped quantum wells has to be considered as a purely collective process driven by charge induced coherence.
