• laser feedback interferometry
• self-mixing
• microcavities
• photonic crystals
• terahertz radiation
• quantum cascade lasers

Terahertz radiation is the object of a wide range of technological efforts, in fields as diverse as solid-state fundamental physics, biomedicine and astrophysics. Terahertz light is particularly suitable for sensing, imaging, spectroscopy and communication. In order to unlock the full potential of these applications on a large scale, compact and versatile terahertz sources are needed. A promising candidate is the quantum cascade laser, a compact laser device based on electrically injected semiconductor heterostructures. Quantum cascade lasers operating in the terahertz have already shown high emitted powers and spectral coverage throughout the 1-5 THz range with single mode and broadband devices. However, the research community strives to achieve the performances necessary for the large-scale exploitation of quantum cascade lasers in terahertz technology.
Here, we investigated the possibility to add novel functionalities to terahertz quantum cascade lasers allowing to dynamically control their operation and enlarge their versatility.
One part of the work is dedicated to the photonic engineering of their cavity. We first considered a particular microcavity resonator consisting of two coupled whispering gallery resonators, which shows low-threshold, high efficiency, vertical collimated single mode emission in continuous wave operation. We thus developed this microresonator design allowing the tuning of the laser emission by exploiting different integrated effects. Directly embedding a suspended mechanical resonator, the proposed microcavity concept presents an optomechanical interaction between the confined electromagnetic field and the resonator mechanical motion, which can affect the laser emission frequency. In an optimized design, where self-oscillation in the system is possible, the device can show a dynamical frequency sweep at the mechanical resonance frequency. A second functionality originates from the engineering of the device injection architecture. In a slightly modi- fied version of the same microcavity, a large continuous frequency tuning and an unconventional far-field modification from radial to vertical emission can be obtained by spatially controlling the pumping strength within the device. The system operation for both microcavity designs is shown through finite-element simulations, the corresponding fabrication is described and a preliminary characterization is performed. Exploiting a different concept for the terahertz quantum cascade laser cavity, relying on line defects in a photonic crystal structure, several devices are designed by simulation, fabricated and characterized to finally show features such as in plane directional emission and mode selectivity. Slow-light effects were shown to produce laser current thresh- old reduction with respect to a reference Fabry-Pérot laser with the same active region. Thanks the defect-line ability to waveguide light in the structure, an example of an active platform integrating multiple line-defects to provide single mode emission in different directions is experimentally shown as proof-of-concept of the potential of the line-defect architecture.
Finally, we investigated the modification of quantum cascade laser parameters, such as the emission frequency and laser voltage, in a self-mixing configuration where the intracavity electromagnetic field interferes with the emitted light, reinjected as optical feedback inside the laser cavity, after reflecting on an external element. Specifically, the external element was a mechanical resonator constituted by a suspended silicon-nitride membrane. An optomechanical response with nanometer resolution to membrane mechanical vibrations was observed in the self-mixing signal. The proposed laser feedback interferometry can be viewed as a starting point for more complex schemes for the dynamical control of terahertz quantum cascade laser operation. A promising perspective is represented by the realization of a self-mixing configuration at terahertz frequencies where different lasers could be coupled through the motion of a mechanical resonator driven by radiation pressure.