Photon absorption in a quantum structure is related to a transition between two electronic states of the system. Indeed, the energy of the absorbed photon depends on the energy separation between the two states. However, this is not true at high electronic density, as it can happen in the condensed matter and in particular in semiconductor quantum structures.
In this seminar I will discuss the emergence of collective effects in highly doped semiconductor quantum wells, with several occupied energy levels. I will show that electrons respond to the solicitation of a photon as if they were a whole, with an absorption spectrum presenting a unique resonance at a completely different energy with respect to the transition energies. This unique optical resonance corresponds to a many-body excitation of the system that ties together all the dipoles, thus presenting a phenomenal interaction with light [1,2].
I will give two different manifestations of this cooperative enhancement of the light-matter interaction. In the first system, the electron gas is inserted in a plasmonic microcavity, giving rise to an unprecedented light-matter ultra-strong coupling regime . The second application is a device in which the collective state is electrically excited, resulting in a radiatevely broadened electroluminescence spectrum. The lifetime estimated from both experiments and theoretical model is of the order of 100fs for a transition at = 8m. This time is more than 6 orders of magnitude shorter than the single particle spontaneous lifetime.
 A. Delteil, A. Vasanelli, Y. Todorov, C. Feuillet-Palma, M. Renaudat St-Jean, G. Beaudoin, I. Sagnes and C. Sirtori, Phys. Rev. Lett. 109, 246808 (2012).
 G. Pegolotti, A. Vasanelli, Y. Todorov, and C. Sirtori, Phys. Rev. B 90, 035305 (2014).
 B. Askenazi, A. Vasanelli, A. Delteil, Y. Todorov, L. C. Andreani, G. Beaudoin, I. Sagnes and C. Sirtori, New J. Phys. 16, 043029 (2014).