Princeton University

School of Engineering & Applied Science

High Performance Quantum Cascade Lasers: Loss, Beam Stability, and Gain Engineering

Pierre Bouzi
Engineering Quadrangle J401
Thursday, December 11, 2014 - 11:30am to 1:00pm


Quantum Cascade (QC) lasers are semiconductor devices emitting in the mid-infrared (3-30 μm) and terahertz (3-300 μm) regions of the electromagnetic spectrum. Since their first demonstration by Jerome Faist et. al. in 19941, they have evolved very quickly into high performance devices. In this thesis, we investigate a further increase of the performance of QC devices and, through meticulous device modeling and characterizations, gain a deeper understanding of several of their unique characteristics.
First, in our quest to achieve higher performance, we investigate the effect of growth asymmetries on device transport characteristics. Through a symmetric active core design, we find that interface roughness and ionized impurity scattering induced by dopant migration play a significant role in carrier transport through the device.
Understanding how interface roughness affects intersubband scattering, in turn, we engineer the gain in QC devices by placing monolayer barriers at specific locations within the device band structure. Preliminary measurement results from modified devices reveal a 50% decrease in the emission broadening compared to the control structures, which should lead to a two-fold increase in gain.
A special class of so-called "strong coupling" QC lasers recently emerged with high power efficiency at cryogenic temperatures. However their performances decay rather rapidly with temperature in both pulsed and continuous wave modes. Through detailed measurements and analysis, we investigate several possible causes of this shortcoming and propose design modifications for temperature performance improvement.
While the strong coupling devices are efficient and powerful, they often suffer from beam steering at high power. Here, we first identify the root of this pointing instability to be from non-linear interactions between multiple transverse modes. Then, we employ focused ion beam (FIB) milling to etch small lateral constrictions on top of the devices and fill them with metal. This has the effect of greatly reducing the intensity of higher order transverse modes as they propagate through the cavity.
A good grasp of the microscopic details involved in QC device operations will result in better lasers, with high beam quality. This, in turn, will enable new applications, such as the detection of SO2 isotopologues in outer space.