ETD

• Dirac plasmon polaritons
• optical frequency combs
• s-SNOM
• self-mixing interferometry
• terahertz near-field imaging
• terahertz quantum cascade lasers
• topological insulators

This thesis presents the design, fabrication, and characterization of THz quantum cascade laser (QCL) frequency combs and of on-chip dual-comb systems. QCLs are unipolar semiconductor sources based on intersubband transitions that spontaneously generate optical frequency combs through intracavity four-wave mixing. The fabricated devices employ a double-metal waveguide, which provides near-unity modal confinement, and include side absorbers to suppress higher-order lateral modes. The on-chip dual-comb system consists of two QCLs slightly detuned in frequency, by about 0.5 GHz that share the same thermal environment.
This system was implemented in a scattering-type scanning near-field optical microscope (s-SNOM), in which a QCL simultaneously acts as source and phase-sensitive detector via the self-mixing effect.
This configuration was employed to map DPPs propagating on the surface of Bi₂Te₃, a topological insulator whose metallic surface states host massless Dirac fermions protected by time-reversal symmetry. Measurements were performed across the 2–4.3 THz range, achieving sub-micrometer spatial resolution set by the tip apex radius of 40 nm. In this way the full polariton dispersion relation and damping length were reconstructed.