Digital and analytical functions performed by today’s semiconductor devices are governed by the electronic transport across an engineered material system with a well-defined electronic structure. Even if a multitude of electrons are concerned in the device operation, the device fundamental characteristics arise from properties inherent to single electrons. For instance, photon emission is related to transitions between electronic states of the system and for optoelectronic devices operating in the mid and far infrared wavelength range is characterized by an extremely long spontaneous emission time (>100ns), which hinders the realization of efficient light emitting diodes. In this project we plan to realize novel optoelectronic devices, whose performances do not belong to single electron properties, but rather depend on the ensemble of the interacting carriers. We recently demonstrated that the optical properties of a dense electron gas do not reflect the energy spectrum, but depend on the Coulomb interaction between electrons. The absorption spectrum of a semiconductor quantum well with several occupied energy levels presents a single absorption peak at an energy completely different from the single particle transition energies. This unique optical resonance, concentrating the whole interaction with light, corresponds to a many-body excitation of the system, the “multisubband plasmon”, in which the dipole-dipole Coulomb interaction locks in phase the optically allowed transitions between confined states. In this project, the peculiar properties of multi-subband plasmons will be exploited for mid and far infrared optoelectronics. The first property is the fact that, as the permittivity of multisubband plasmons depends on the doping level and on the size of the quantum well, semiconductor layers with ad hoc dielectric properties (hence metamaterials) can be realized. As a first application we will design all-dielectric waveguides in the mid and far infrared for quantum cascade lasers. A second application will be the design of engineered infrared absorbers. The second part of the project is based on another fundamental property of collective electronic excitations: their superradiant nature. Indeed the multisubband plasmon is the bright state issued from the coherent superposition of several intersubband excitations. As a superradiant state can be visualized as one in which a macroscopic polarization is established over a region of space, a very interesting way to characterize this state will be its observation by using Electron Energy Loss Spectroscopy. The superradiant nature of multisubband plasmons results in a radiative lifetime of the order of few hundreds fs, thus much shorter than the typical intersubband spontaneous emission lifetime. We will exploit this property to conceive and realize two different classes of optoelectronic infrared emitters based on many-body excitations: - Quasi-monochromatic fast and tunable incandescent sources - Quantum engineered superradiant emitters The first kind of devices is based on the same geometry as a field effect transistor: the electron gas is excited by a source – drain current, while the electronic density can be controlled by a gate voltage. This point will be also studied in collaboration with STMicroelectronics, which will provide FDSOI and CMOS devices, in order to observe far-infrared optical signals in state-of-the-art electronic devices. In order to fully take advantage of the superradiant character of multisubband plasmons, another generation of devices will also be conceived, realized and characterized, using quantum engineering for resonant excitation. We will design a device based on vertical transport through the electron gas, a plasmon assisted tunnelling device. More selective injection mechanisms will also be investigated, by exploiting the dipole-dipole interaction in systems of tunnel coupled quantum wells.