| Understanding and harnessing interactions between light and matter have been an enduring endeavor in human history and a centerpiece of modern science and technology developments, and given birth to numerous brilliant inventions such as light emitting diodes and lasers that completely changed the world. Quantum Cascade (QC) lasers as one of the newest achievements in this rank have motivated a broad range of exciting potential applications such as high-sensitivity trace gas sensing, non-invasive glucose monitoring, free-space optical communication, etc. Although the QC laser technology has been undergoing a rapid and steady development phase ever since its invention in 1994, further improvements in aspects such as output power, efficiency, spectral purity and cost-effectiveness are indispensable for large-scale implementations of QC laser based application systems. To meet such ends, we explore in this thesis novel approaches from the device structural design perspective to further improve the overall performance of QC lasers and lower their fabrication cost, while at the same time assess new application possibilities.;Exploiting the extraordinary design flexibility of the QC laser band-structure, we demonstrate a major step forward in the power performance of QC lasers by employing a novel ultra-strong coupling design strategy. A record QC laser wall-plug efficiency of ∼50% is achieved. Such high-performance QC lasers enable our proof-of-concept implementation of a mid-IR backscattering light detection and ranging (LIDAR) system. Moreover, the ultra-strong coupling design strategy is also applied to realizing QC lasers with broad-band optical gain. QC lasers with optical gain spectrum width corresponding to ∼40% of the radiative transition energy are demonstrated.;As single-mode operation of QC lasers is indispensable for most sensing applications, we further explore unconventional laser cavity designs to achieve single-mode QC lasers more cost-effectively. Two fundamentally different approaches, i.e., the monolithic coupled-cavities and the asymmetric Mach-Zehnder interferometer type cavities, are proposed and experimentally verified. Both types of cavities are capable of establishing strong wavelength selectivity and facilitating single-mode operation of QC lasers without the need of sub-wavelength periodic feedback structures, and therefore are much more cost-effective than conventional single-mode QC laser technologies.;Our explorations presented in this thesis widen the territory for the QC laser research field and shed light on new directions for future explorations. |
