UCLA

Terahertz (THz) external-cavity lasers based on quantum-cascade (QC) metasurfaces are emerging as widely-tunable, single-mode sources with the potential to cover the 1-6 THz range in discrete bands with watt-level output power. By operating on an ultra-short cavity with a length on the order of the wavelength, the QC vertical-external-cavity surface-emitting-laser (VECSEL) architecture enables continuous, broadband tuning while producing high quality beam patterns and scalable power output. These properties are favorable for spectroscopic applications that can benefit from large bandwidths and high output powers, such as serving as a local-oscillator (LO) for heterodyne receivers in astronomy. THz QC-lasers have garnered much attention in the past decade in the astrophysics community due to the lack of well-established THz LO sources above 3 THz. The QC-VECSEL has potential to fill this technological gap, and provide the power output levels necessary for next-generation heterodyne receiver arrays.

In this thesis, we discuss the methods and challenges for designing the metasurface at various frequencies across the 1-6 THz bandwidth, and establish fundamental rules for VECSEL scaling. We discuss the methods and challenges for designing the metasurface at various frequencies across the THz bandwidth, and demonstrate single-mode lasing up to 5.72 THz. The device is enabled by a reflectarray metasurface composed of sub-wavelength metallic antennas loaded with quantum-cascade gain material. In theory, wavelength-scaling the metasurface is a matter of scaling up or down the geometric parameters proportionally, maintaining the electromagnetic properties of the structure. However, as the QC-VECSEL is scaled below 2 THz, the primary challenges are reduced gain from the QC active region, increased metasurface quality factor and its effect on tunable bandwidth, and larger power consumption due to a correspondingly scaled metasurface area. At frequencies above 4.5 THz, challenges arise from a reduced metasurface quality factor and the excess absorption that occurs from proximity to the Reststrahlen band. Additionally, the effect of different output couplers on device performance across the whole tuning bandwidth is studied, demonstrating a significant trade-off between the slope efficiency and tuning bandwidth.

The second half of this thesis details the first study of self-mixing and optical feedback in the QC-VECSEL. The self-mixing effect has been well explored in THz QCLs over the past couple decades, and has potential to be a highly sensitive, compact, and cost-effective metrological tool with applications in spectroscopy and imaging. In this study, a single-mode 2.80 THz QC-VECSEL operating in continuous-wave is subjected to various optical feedback conditions (i.e. feedback strength, round-trip time, and angular misalignment) while variations in its terminal voltage associated with self-mixing are monitored. Due to its large radiating aperture and near-Gaussian beam shape, we find that the QC-VECSEL is strongly susceptible to optical feedback, which is robust against misalignment of external optics. This, in addition to the use of a high-reflectance flat output coupler, results in high feedback levels associated with multiple round-trips within the external cavity — a phenomenon not typically observed for ridge-waveguide QC-lasers. Thus, a new theoretical model is established to describe self-mixing in the QC-VECSEL. The stability of the device under variable optical feedback conditions is also studied. Any mechanical instabilities of the external cavity (such as vibrations of the output coupler), are enhanced due to feedback and result in low-frequency oscillations of the terminal voltage. The work reveals how the self-mixing response differs for the QC-VECSEL architecture, informs other systems in which optical feedback is unavoidable, and paves the way for QC-VECSEL self-mixing applications.