The complex refractive index components, n and k, have been studied for thin films of several common dielectric materials with a low to medium refractive index as functions of wavelength and stoichiometry for mid-infrared (MIR) wavelengths within the range 1.54-14.29 μm (700-6500 cm(-1)). The materials silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, and titanium oxide are prepared using room temperature reactive sputter deposition and are characterized using MIR variable angle spectroscopic ellipsometry. The investigation shows how sensitive the refractive index functions are to the O2 and N2 flow rates, and for which growth conditions the materials deposit homogeneously. It also allows conclusions to be drawn on the degree of amorphousness and roughness. To facilitate comparison of the materials deposited in this work with others, the index of refraction was also determined and provided for the near-IR and visible ranges of the spectrum. The results presented here should serve as a useful information base for designing optical coatings for the MIR part of the electromagnetic spectrum. The results are parameterized to allow them to be easily used for coating design.

When conventionally calculating carrier leakage for state-of-the-art quantum cascade lasers (QCLs), that is, LO-phonon-assisted leakage from the upper laser level via electron thermal excitation to high-energy active-region (AR) states, followed by relaxation to low-energy AR states, ∼18%-wide gaps were recently found between calculated and experimentally measured internal efficiency values. We incorporate elastic scattering [i.e., interface-roughness (IFR) and alloy-disorder scattering] into the carrier-leakage process and consider carrier leakage from key injector states as well. In addition, the expressions for LO-phonon and IFR-triggered carrier-leakage currents take into account the large percentage of thermally excited electrons that return back to initial states via both inelastic and elastic scattering. As a result, we find that the gaps between theoretical and experimental internal efficiency values are essentially bridged. Another finding is that, for the investigated state-of-the-art structures, IFR scattering causes the total carrier leakage to reach values as much as an order of magnitude higher than conventional inelastic scattering-only leakage. The developed formalism opens the way to significantly increase the internal efficiency (i.e., to more than 80%) via IFR-scattering engineering, such that maximum wall-plug efficiencies close to projected fundamental, both-facets values (e.g., 42% at λ = 4.6 μm) can be achieved. By employing this formalism, we reached a 4.6 μm-emitting-QCL preliminary design for suppressing IFR-triggered carrier leakage, which provides an internal efficiency of 86% as well as a projected single-facet wall-plug efficiency value of 36% at a heatsink temperature of 300 K.

No abstract

ULL (level 3) to Injector (level 1) LO-phonon scattering time.

We present a so-called "split-well direct-phonon" active region design for terahertz quantum cascade lasers (THz-QCLs). Lasers based on this scheme profit from both elimination of high-lying parasitic bound states and resonant-depopulation of the lower laser level. Negative differential resistance is observed at room temperature, which indicates that each module behaves as a clean 3-level system. We further use this design to investigate the impact of temperature on the dephasing time of GaAs/AlGaAs THz-QCLs.