Photonic engineering of CW, ultrabroad gain, aperiodic quantum cascade lasers at terahertz frequencies integrations with 2D materials and study of the optical mode dynamics

The terahertz (THz) frequency range of the electromagnetic spectrum is usually defined in the range between 0.1 THz and 10 THz, corresponding to wavelengths in the interval from 3 mm to 30 µm, lying in-between the infrared and the microwave spectral regimes. In recent years, the progress of THz technology has fostered interdisciplinary research in spectroscopy and tomography to map macroscopic systems, (chemical detection and imaging, amongst others) or microscopic ones, such as nanoparticles and nanowires on either static or dynamic timescales. THz radiation is commonly generated with photoconductive emitters, semiconductor diodes, free-electron lasers, photomixing, and beating of a pump and idler signal from non-linear crystals. These approaches are often bulky, expensive or with limited optical powers. The breakthrough demonstration of quantum cascade lasers operating in the far-infrared, and based on quantum engineered heterostructures, paved the way to the development of much more compact, efficient and powerful semiconductor THz sources. Thanks to the atomic-layer resolution ensured in the heterostructure growth by molecular beam epitaxy (MBE), very accurate designs can be implemented via a proper sequence of quantum barriers and quantum wells. In this way, sharp discontinuities in the conduction and valence bands edges are created, in order to manipulate the electron energy levels and wavefunction localization, and to provide optical intersubband transitions at the desired frequencies. [...]