Terahertz quantum cascade lasing in disordered photonic systems

Scattering, which is created by dust, clouds, white paints or imperfections of artificial material such as photonic crystal, is normally undesirable in the field of photonics. Unequivocally, a deep understanding of disorder can help to overcome the issues arising from scattering. On the other hand, it is also possible to utilize the disorder to create useful photonic devices. A most well-known example is the random laser, in which lasing action is sustained by multiple scattering of light within the disordered media. Disordered structures are also proposed for the applications including wavelength filters, optical switches, sensors and cavity quantum electrodynamics. Most of the research work on disordered structures has been focused on short wavelength range. Several years ago, this tide spreads to mid-infrared range with the experimental demonstration and theoretical optimization of mid-infrared random laser at ~10 μm. However, in the technically important terahertz (THz) frequency range, the disordered photonics are least explored. This thesis investigates disordered photonic systems based on the double-metal (DM) waveguide of THz quantum cascade laser (QCL) and explores the emission properties with the assistance of gain. As the first demonstration, I design and fabricate one-dimensional disordered grating structures on top of the DM waveguide of the THz QCLs. Disorder is introduced into the waveguide by dislocating the position of each aperture from a periodic configuration. Numerical calculation indicates that the introduced disorder creates multiple microcavities with small optical losses near the bandgap frequency window of the underlying long-range order. With electrical pumping, 8 emission peaks at around 3.2 THz are observed, which are located near the bandgap region of the periodic counterparts. Simulation work and experimental measurements provide strong evidence for spatial localization of light. Instead of utilizing long-range order for creating microcavities, I also investigate two-dimensional disordered structures consisting of randomly-distributed pillar scatterers fabricated from a quantum cascade gain medium, in which long-range order does not exist. Due to the strong scattering efficiency of a high-index dielectric pillar for transverse-magnetic (TM) polarized wave, it is shown that such structures can achieve multimode lasing, with strongly localized modes at THz frequencies. This is a typical feature of random lasers. The weak short-range order induced by the pillar distribution is sufficient to ensure high quality-factor modes that have a large overlap with the active material. It is also found that the emission spectrum can be easily tuned by tailoring the size and filling fraction of the scatterers. While the dielectric pillar scatterer is beneficial for better performance of multimode lasing (lower threshold, more lasing modes etc.) due to high scattering efficiency, it also limits the lasing to the Mie resonance frequency range, making the broadband random lasing applications difficult. The non-resonant scatterers feature a weaker scattering strength nonetheless a relatively frequency-insensitive scattering efficiency. Inspired by this, I further develop metallic pillars, a class of non-resonant scatterers with high scattering efficiency in the THz frequency range, enabling possible a broadband random laser. It is demonstrated that such random lasers, with metallic pillars embedded in THz QCL gain medium, outperform their dielectric-counterparts in terms of lasing spectral range, the number of lasing mode and laser threshold. Doctor of Philosophy (EEE)