| Prior to the invention of the quantum cascade (QC) laser, many applications based on mid-infrared (mid-IR) laser absorption spectroscopy were not be explored. Development of the QC laser provided an inherently compact, semiconductor based, and tunable mid-IR source that could be used for laser absorption spectroscopy. Additionally, QC lasers can be designed to emit at a specific wavelength within a very wide wavelength range from between 3 and 30 mum and can be fabricated to operate single-mode to clearly scan mid-IR absorption "fingerprints" [1]. This allows lasers to be tailored to the exact wavelength of an absorption feature. Two examples of absorption spectroscopy experiments were carried out as part of this dissertation and described herein: C60 in space and dissolved gasses in living tissue. Although QC lasers allow for application development in the mid-IR, they are inefficient and heat dissipation is problematic.; First generation QC lasers relied on either bulky cryogenic cooling systems for continuous wave operation or large, expensive pulse generators [2]. Later, advances in QC laser design, growth, and fabrication led to room-temperature continuous wave operation [3]. These advances promoted additional applications of QC lasers where cryogenic cooling was impossible or highly inconvenient. This dissertation presents comprehensive self-consistent models permitting the optimization of high operating temperature QC lasers. These models employ strategies counter to those used in designing low temperature devices and were used to design, fabricate, and demonstrate high-performance QC lasers.; By self-consistently solving the temperature dependent threshold current density and heat equations, including temperature dependent thermal conductivities, phonon lifetimes, thermal "backfilling," thermionic emission, and energy level broadening, we calculated the effects of doping level, material choice, and waveguide layer thickness on the laser threshold performance [4]. Further expanding this model above threshold operation to roll-over, we performed experiments and derived models for "thermal rollover" and "Stark-effect rollover," the two causes eliciting laser shutoff at high currents [5].; High-conversion efficiency lasers are designed, fabricated, and characterized at room temperature continuous wave operation using these comprehensive models. Various embodiments of these lasers exhibit wall-plug efficiencies of 28% in pulsed mode at 80 K [6] and 4% when operated continuous wave at room-temperature, an order of magnitude higher than before the work of this dissertation. Additional models for high-conversion efficiency lasers are developed to maximize quantum, optical extraction, carrier injection, and current efficiencies. Advanced models and experiments presented in this dissertation support the design and further improvement of high-performance QC lasers. |
