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. The Quantum Physics and Devices (QUAD) team at MPQ laboratory (CNRS/Univ. Paris Diderot), in collaboration with ELPHYSE team at LPN (CNRS), demonstrated that the optical properties of a dense electron gas are not related to the energy spectrum, but depend on the Coulomb interaction between electrons in the well. In an article published in Physical Review Letters, the researchers demonstrated that in a highly doped semiconductor quantum well the absorption spectrum does not reflect the electronic transitions. Indeed, the coupling between the transitions gives rise to a sharp optical resonance, concentrating the whole interaction with light (see figure below). As a result, the system can become transparent at the transition energies between electronic states. These results pave the way for a new field of investigation of coherent effects, controlled by the charge density, in electronic devices.

**Figure:**

Measured (black symbols) and simulated (lines) absorption spectrum of the quantum well. The red line has been obtained by taking into account the Coulomb interaction between electronic transitions, while the blue line results of a single particle model. This interaction gives rise to a sharp optical resonance, at a completely different energy with respect to the electronic transitions.

**Contact :
**

Angela Vasanelli.

Carlo Sirtori.

angela.vasanelli@univ-paris-diderot.fr

carlo.sirtori@univ-paris-diderot.fr

**Référence:**

Charge induced coherence between intersubband plasmons in a quantum structure.

A. Delteil, A. Vasanelli, Y. Todorov, C. Feuillet-Palma, M. Renaudat St-Jean, G. Beaudoin, I. Sagnes and C. Sirtori, **Physical Review Letters**, 109, 246808 (2012).