• photonic resonators
• quasicrystal resonators
• random lasers
• Terahertz QCL

In the last decades the fields of photonics and nanotechnology have led to some impressive scientific and technological achievements. Among them, the exploration of yet unexploited spectral regions, such as the Terahertz (THz) range, i.e. wavelengths of 60micron-300 micron, has been a major breakthrough.
On the one hand, this was possible after the novel theoretical concept of light amplification in multiple quantum wells and superlattices was proposed in the Seventies, introducing the groundbreaking quantum cascade laser (QCL) idea. On the other hand, the development of new nanofabrication technologies and crystal growth techniques, such as the molecular beam epitaxy (MBE), allowed an unprecedented control over the material structure, down to the deposition of nanometer-thick semiconductor layers. This paved the way to the practical realization of electrically pumped multi-stage gain media, the QCL, and to the successful demonstrations of their operation in a broad frequency range, from the mid-IR to the far-infrared.
Apart from the purely scientific interest, Terahertz photonics has now a fundamental role in many applications, like metrology, spectroscopy, biomedical and pharmaceutical imaging, quality and process control, communications and security.
Nowadays, a lot of effort is made to improve the performance of Terahertz QCL in terms of optical power, efficiency, beam pattern, frequency control and thermal management. Some of these crucial issues can be addressed by the use of photonic structures, i.e. specially designed patterns of dielectric scatterers superimposed to the active region. Such structures can be implemented in one- (1D), two (2D)- or three (3D)-dimensional architectures, to provide a tight control of the frequencies and far-field emission pattern of the laser.
Periodic photonic crystals have been studied for long time, providing intriguing insights. More recently, aperiodic patterns have attracted increasing attention due to their greater flexibility and the possibility to study and explore novel physical phenomena.
The aim of the present thesis is to design, fabricate and investigate the transport and optical behavior of THz QCLs exploiting distributed feedback, achieved through the use of 2D quasi-periodic and random resonators. The main goal is to demonstrate multimode emission over a broad frequency bandwidth, centered around 3.1 THz.
Unlike perfect photonic crystals, quasi-crystal geometries do not possess discrete translational invariance, yet they do possess long-range order which gives rise to a rich spectrum. After developing a simulation code based on the generation algorithm called "Generalized Dual Method", we designed the following quasi-crystal geometries:
i) a 7-fold pattern with a perfect symmetry under 2π/7 rotations around a central axis,
ii) an imperfect 7-fold geometry where small defect points are introduced.
This allowed to compare the effects of introducing a small amount of disorder in the design of the photonic structures. In order to understand the effect of a further increase of disorder, a third type of random structures was also studied, whose scatterers positions were extracted from a uniform pseudo-random distribution.
We then simulated these photonic structures using the numerical approach of finite elements analysis, to understand how light propagation is affected by the size, the number and the arrangement of the scatterrers.
A set of devices for each geometry was selected among those with the largest number of electromagnetic modes with predicted high quality factors Q. They were then nano-fabricated with the same QCL active region in a cleanroom facility, using a combination of UV optical lithography, plasma-assisted etching, metal deposition, chemical processes and ultrasonic wedge bonding.
Finally, all lasers were characterized electrically and optically to study how the different physical and geometrical parameters affect the lasing threshold, the slope efficiency, the emitted power and the far-field intensity profile. The emission spectra were probed via Fourier Transform Infrared Spectroscopy (FT-IR), demonstrating the predicted multimode emission in most devices.
In a future perspective, such multimode emission could be used to mode-lock radiation in a THz QCL, for example using passive optical components. An interesting possibility is the future integration of graphene in QCL to exploit its saturable absorption in the THz region.
To this end, the transmission of THz radiation through a few layers of graphene transferred on an intrinsic silicon substrate was measured, reporting saturable absorption in the THz.