The development of terahertz (THz) optoelectronics faces two major challenges: first, a need for miniaturization of the existing radiative sources, and second, an improvement of their performances. Amongst the current sources of THz radiation, quantum cascade lasers (QCLs) represent to date the best candidates to match these two requirements. The integration of compact sources necessarily results in decreased optical output power and laser beam directionality. Therefore, a considerable amelioration of the active region performances must be achieved in parallel with the miniaturization of the dimensions of the photonic cavity. Because the latter are subject to the diffraction limit, which imposes on at least one dimension to be of the order of the effective half wavelength, further miniaturization of photonic devices requires a new approach. In this manuscript, a new class of metal-semiconductor-metal (M-SC-M) THz resonators is presented, both theoretically and experimentally. These devices, inspired by electronic LC resonators, allow to achieve a record effective volume Veff =LxLyLz/λeff =5.10-6, where Lx,y,z are the cavity dimensions and λeff is the effective wavelength resonance inside of the resonator core (GaAs). These devices are intrinsically free from the diffraction limit in the three spatial dimensions, and present the typical functionalities which are usually found only in a resonant electronic circuit. In order to demonstrate that their photonic properties are preserved, these devices have been successfully applied to THz intersubband polariton, demonstrating at the same time that they can be used for strong light-matter coupling. In parallel to this work, the feasibility of a THz QCL operating at λ=100 microns with an extremely thin active region is demonstrated experimentally. A systematic study of the laser characteristics for different thicknesses of the active region (Lz ) resulted in the reduction of Lz = 10 microns (≈λeff/2.7) down to the record value of Lz = 1.75 microns (≈λeff/13) in a M-SC-M Fabry-Perot waveguide. Despite a strong increase in optical losses, lasing is maintained above liquid nitrogen temperature (78 K) in the device with thinnest active region. This unexpected behavior is attributed to the existence of a large fraction of the current flowing through the active region at laser threshold being non-radiative. These results are very promising for future developments of efficient THz QCL active regions, as well as for fabrication of microcavity lasers with extremely low effective volumes. The perspectives of this work extend from cavity quantum electrodynamics to the development of a nanolasers. Potential applications of hybrid resonators can span over a broad range, depending on the chosen configuration. They can be used as passive elements for detection, as well as active elements such as antennas. Finally, the use of a thin active region in combination with an optimized version of these hybrid resonators should allow for the realization of an ultra-compact THz QCL free from the diffraction limit, with the possibility of fine tuning the laser frequency by adapting the equivalent complex impedance combination of the LC elements.
