Terahertz Metasurface Quantum Cascade Laser
Terahertz (THz) frequency range (0.3–10 THz, 30–1000 μm) is the least explored region in the electromagnetic spectrum, mostly due to a relative lack of convenient, ecient and economical THz sources. However, THz frequency range has the potential for various applications including but not limited to: astrophysics and space science, biological and medical imaging and spectroscopy, and non-destructive evaluation. This Ph.D. research builds upon the growing request for compact and ecient THz sources with both high power and high-quality beam pattern.
Since its invention in 2001, the THz quantum cascade laser (QCL) has emerged as a compact semiconductor THz source capable of delivering milliwatt-level power or higher at various frequencies from 1.2 to 5.6 THz. In the best devices, the operating temperature has reached 200 K in pulsed mode and 129 K in continuous wave (cw) mode. However, the state-of-the-art THz QCLs almost exclusively use sub-wavelength metallic and/or plasmonic waveguides, which leads to highly divergent beams. Achieving high power in combination with an excellent beam pattern for THz QCLs remains a longstanding challenge.
Vertical external-cavity-surface-emitting-laser (VECSEL) has been demonstrated as a very successful approach to achieve high power and good beam pattern for semiconductor lasers in the visible and near-infrared. A typical VECSEL configuration consists of a quantum
well (or dot) semiconductor active medium grown monolithically on a Bragg reflector and an output coupler to form a cavity. Output power is scalable with the active medium area and the cavity can be readily engineered to support only the fundamental Gaussian mode. However, the VECSEL concept has been impossible to implement for QCLs owing to “intersubband selection rule”: the intersubband transitions in cascaded quantum wells — the gain medium of QCLs — are only allowed to interact with and provide gain to the electric field polarized perpendicular to the wells. This is incompatible with the natural polarization for surface incident waves in a VECSEL cavity.
This thesis reports the development of a new class of THz QCL to achieve high-power output with an excellent beam pattern — THz metasurface quantum-cascade VECSEL (QC-VECSEL). The enabling component of QC-VECSEL is an active metasurface reflector composed of a sub-wavelength array of metallic microcavity antennas; each antenna efficiently couples in THz radiation, amplifies it and re-radiates into the free space. Lasing is possible when an active metasurface is paired with an output coupler form a low-loss cavity. This new architecture gives the ability to scale up output power with the metasurface area while maintaining a good beam pattern shaped by the external cavity. Moreover, this approach can leverage ongoing advances in novel metasurfaces to realize versatile functionality for QC-VECSELs, such as engineerable polarization/wavefront, spectral tunability. The demonstration of THz QC-VECSEL marks the two “firsts”: the first VECSEL in the THz frequency range and the first laser built around an active metasurface. The concept of metasurface QC-VECSEL is potentially applicable beyond the terahertz and can be applied to shorter wavelengths.
This thesis describes the theory, design, fabrication, testing and analysis of THz meta- surface QC-VECSELs. Electromagnetic simulations for various active metasurfaces are presented, including the dependence of reflective gain on the antenna size, periodicity and shape, the suppression of metasurface self-lasing. A laser model is laid out for QC-VECSELs to relate the performance metrics with the metasurface and cavity design parameters, providing a tool for metasurface optimization. Numerical methods are also developed to compute the mode profile and diffraction loss of various external cavities. Experimental results and analysis are presented on a wide variety of THz metasurface QC-VECSELs varying in metasurface and cavity designs, which have yielded considerable high-performance results, including high power combined with excellent beam pattern, record-high cw power at >77 K, polarization and wavefront engineering. Finally, prospects are considered for QC-VECSELs with higher power, better temperature performance and capability to generate complex beams and versatile functionality.