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Advanced optoelectronic Devices with Enhanced QUAntum efficiency at THz frEquencies

Final Report Summary - ADEQUATE (Advanced optoelectronic Devices with Enhanced QUAntum efficiency at THz frEquencies)

Standalone description of the project and its outcomes
The ADEQUATE project (Advanced optoelectronic Devices with Enhanced QUAntum efficiency at THz frEquencies) was devoted to the conception and realisation of efficient terahertz optoelectronic devices. The project aim was to expand the field of optoelectronics by introducing within the context of semiconductor technology some of the concepts that originate from quantum optics. The challenges of this investigation were not in the observation of these quantum effects, well known in atomic physics, but in their exploitation for a specific function, in particular for efficient electrical to optical signal conversion. To achieve our goal, we have conceived and realised devices operating in the “light-matter strong coupling” regime, a physical situation in which light and matter mix to produce new hybrid entities, linear combinations of the two natural components. This occurs, for instance, when a quantum of light (a photon) is confined in a very small optical cavity and as it cannot leave, it exchanges periodically its energy with the excitations of the material by a continuous sequence of emission and absorption. To improve the radiative efficiency of the devices we have studied how the strong-coupling can substantially increase the characteristic rate of spontaneous emission of our quantum structures.

To this end we have first focused on the realisation of “optical cavities” that confine THz photons in very small volumes. We have achieved extremely sub-wavelength confinement using metals that act as mirrors as well as antennas. Thanks to the subwavelength confinement and the high electronic densities in the semiconductors we were able to demonstrate the ultra-strong coupling regime, which occurs when the coupling energy is of the same order of magnitude of the energy of the excitation. This observation was a very important milestone of the project as it allowed us to identify a quantum configuration which is a prerogative of condensed matter physics only, and cannot be reached using the typical densities of atomic clouds.

We have then provided theoretical interpretation of our results and demonstrated that the ultra-strong coupling is a collective effect in which the strength of the coupling is proportional to the density of the electrons that can exchange energy with the cavity photons. We have theoretically analysed the meaning of the strong coupling when the optical cavity is replaced by an electronic circuit resonator where, due to nano-metric dimensions of the capacitor, only few electrons can be coupled to the electric field. We have therefore theoretically studied light-matter interaction in an electronic resonator with an increasing number of electrons from one to many. This has allowed us to understand the details of how the electronic dipoles tie together due to electron-electron interaction and to describe the transition from a few particle system of strong interacting fermions to a bosonic many-body excitation. For very high concentration, electrons solicited by the light have a collective coherent response and oscillate in phase as a density wave.

In the last part of the project we focused on the emission property of our collective excitations in a 2-dimensional electron gas that has a phenomenal coupling with the light. In order to have the best photon extraction from the material thus the highest efficiency, we did not use any optical cavity and coupled directly the excitation with free space radiation. We have measured superradiant emission from our devices with extremely short spontaneous emission lifetimes, in the order of tens of femtoseconds. Moreover, the theoretical description of our light-matter system shows undoubtedly that the matter system enters the strong and ultrastrong coupling regime with free space photons. This implies that mixed light matter states can also be formed with photons propagating at the speed of light away from the material excitation. It is important to notice that for this experiment we have realised transistor-like semiconductor devices, thus proving that these quantum mechanical effects can be engineered in today’s technology components. When current is injected in these devices we obtain a strong signal of electroluminescence in excellent agreement with the quantum features that we have predicted.

Finally we have exploited “light-matter strong interaction” to demonstrate a new device architecture for detectors, with an extremely small effective active area, thus excellent noise properties, that have a strong potential for applications in the THz and mid-infrared spectral region.