STRUCTURED PLASMONIC WAVEGUIDES AND INTERFACE ROUGHNESS SCATTERING FOR QUANTUM CASCADE LASERS

Abstract

This thesis examines strategies for designing and fabricating improved quantum cascade (QC) lasers, especially at long wavelengths (λ>10 μm). Wavelengths in the range of 8-16 μm are useful for applications such as trace detection of the strongest absorption lines of molecular compounds such as the hazardous uranium hexafluoride. Rapid advancements in QC laser technology have transpired over two decades of research since its first inception. However, room temperature, continuous-wave operation for longer wavelengths has not been achieved in a standalone system.

We present a unique design for a QC laser at λ≈16 μm based on a diagonal optical transition and a one-phonon resonance depletion scheme. This study stems from improving upon design characteristics found in other long wavelength QC lasers. This design has a smaller voltage defect (94 meV) than any other high performance long wavelength laser; we also expect significant voltage tuning of the wavelengths from this design as well.

Additionally, due to phonon resonances at 16 μm in the traditional QC substrate (InP), we present a novel waveguide design using plasmonics to reduce loss and improve growth parameters for our 16 μm QC laser. This design includes a periodic structured periodic air-plasmon waveguide for the top cladding. This method is useful for other mid-infrared QC lasers with long wavelengths.

Finally, we focus on understanding the effects of interface roughness (IFR) scattering in QC lasers. We present a novel QC laser design at ~11 μm based on IFR scattering instead of the traditionally used longitudinal phonon scattering. Our device has upper laser state lifetimes of 6.19 ps and lower laser state lifetimes of 0.14 ps, which is the second-best ratio obtainable for long wavelength QC lasers based on phonon scattering rates. This study and design approach has significant implications for how the QC laser community approaches current and future laser designs.

A good understanding of the intricate details for designing high performance gain material and waveguides for long wavelength mid-infrared QC lasers will result in better QC lasers overall. This in turn will enable QC laser applications to become ubiquitous over the entire mid-infrared wavelength range.