Infrared quantum devices operating in the ultra-strong light-matter coupling regime – THINQE_PINQE

Far infrared optoelectronic devices have an extremely low radiative efficiency.
Intersubband transitions (transitions between confined levels) in semiconductor quantum wells are used to realize emitters in the mid and far infrared. Nevertheless, the typical time of the spontaneous emission is several orders of magnitude longer than the non-radiative lifetime. As a consequence, electroluminescent devices have a very low quantum efficiency, and lasers have low wall plug efficiency. The ultimate goal of this project is to pave the way towards a new generation of optoelectronic devices, with increased quantum efficiency, based on the quantum superposition principle. To this aim, an intersubband excitation and a cavity mode are coupled in order to give rise to mixed light-matter states: the intersubband polaritons. When the coupling energy is of the same order of magnitude as the matter excitation energy, the system enters the so-called ultra-strong coupling regime, in which the usual approximations introduced to describe the interaction with light cannot be used anymore. In this project, we study the physics of the ultra-strong light-matter coupling, we realize devices operating in this regime and we demonstrate new functionalities issued from the control of the interaction with light.

Electronic and photonic engineering for an extreme light-matter coupling and for the realization of electroluminescent devices.
We design, fabricate and characterize novel devices based on semiconductor quantum wells inserted in a cavity. Our cavities are based on metals, and allow an ultra-subwavelength confinement of photons. As a consequence, the thickness of the region in which the interaction with the quantum system takes place can be maximized, in order to achieve very important coupling energies. For the quantum wells design, several strategies are adopted. The quantum wells can be tunnel coupled in order to realize structures in which the polariton states are resonantly populated. Highly doped quantum wells can be used in order to enhance the coupling with the photon mode by means of many-body effects. Once the devices designed, the semiconductor heterostructures are grown and the devices are fabricated in clean room. Electrical and optical characterization, together with time resolved spectroscopy, allows us to verify the device operation and characteristics.