Quantum Cascade Ring Laser Systems

Abstract

Mid-infrared light possesses the unique property of interacting most strongly with many molecules due to their ro-vibrational resonances, making it a powerful tool for absorption and other types of spectroscopy and sensing. The development of mid-infrared light sources was limited until the advent of quantum cascade (QC) lasers. These lasers, composed of atomically thin heterostructures, offer unparalleled engineerability, inspiring continuous advancement and novel applications over the past three decades. The focus of this dissertation is the exploration of QC ring laser systems. These cavity geometries are at the forefront of research in the field due to their favorable attributes such as low loss, low lasing threshold and high internal optical power. Additionally, the fast gain recovery time of QC lasers enables them, in part, to efficiently generate frequency combs without the need for additional components. Coupled with dual-comb spectroscopy techniques, ring QC lasers have the potential to revolutionize compact and high-precision spectrometers in the mid-infrared range. However, several technological challenges stand in the way of realizing commercial spectrometers from standalone ring QC lasers. To bridge this technological gap, we demonstrate the monolithic integration of ring QC lasers with other optoelectronic components without requiring heterogeneous integration. We develop novel fabrication methods suited to building chip-scale systems on conventional QC laser wafers. Leveraging these methods, we develop active waveguide couplers that enhance the output power of ring lasers by three orders of magnitude compared to standalone devices, in addition to providing precise control over the direction of emission. We demonstrate the successful integration of fast and efficient photodetectors on the same chip eliminating the need for free-space components which allows us to rapidly scale the number of optical outputs on chip. We then expand this system to include two coupled lasers in a photonic molecule arrangement and explore the degeneracies of this highly multimode system focusing on applications to non-Hermitian photonics. We find that degenerate combs of frequencies give rise to two discrete energy bands instead of the typical two-level anti-crossing. The versatility of these active components is such that the same components may perform several functions acting as lasers, detectors, waveguides, amplifiers, filters, switches and frequency combs. Finally, we develop efficient computation methods to study such systems focusing on coherent multimode instabilities in ring lasers and coupled laser arrays. This research advances our understanding of novel laser physics and contributes to the future development of commercial on-chip sensing systems operating in the mid-infrared range.