A unified picture of threshold behaviour in strained quantum well lasers

The focus of this investigation is to provide a unified understanding of the relative impact of compressive and tensile strain on thresholds in III-V separate-confinement-heterostructure single-quantum-well (SCH SQW) lasers. First, a strained-layer laser model for SCH SQW lasers that calculates gain spectra, differential gain, light-current characteristics, and threshold current densities is developed. This model is based on a six valence-band Luttinger-Kohn finite-element dispersion calculation.; Second, an extensive theoretical and experimental study on tensile-strained GaAsP-AlGaAs SCH SQW broad-area stripe lasers is performed to understand a complex interplay of TE and TM gains and modal losses unique to tensile-strained lasers. Threshold current density measurements for sample sets encompassing 10 phosphorus compositions ranging from 0 to 30% and 5 cavity lengths ranging from 300 to 1500 {dollar}mu{dollar}m are reported. The theoretical model is used to replicate detailed features of the experimental data including absolute magnitudes and polarization-switching behaviour. A constant gain contour approach is introduced to explain the dependence of the measured thresholds on strain and cavity length as a result of competition between a TM gain advantage and a TM electromagnetic disadvantage. Tensile strain is shown to have a minimal impact on threshold current densities for GaAsP-AlGaAs lasers.; Third, a comparative analysis of strain effects on laser performance in the InGaAs-GaAs-AlGaAs, GaInAs-GaInAsP and GaInAsP-GaInAsP material systems is presented. Different approaches to analyzing strained-laser performance such as constant-well-width, constant-wavelength and Seki pure strain studies are employed. The constant gain contour approach combined with the pure strain strategy is shown to provide a powerful tool for the understanding of strain effects in 1.3 {dollar}mu{dollar}m GaInAsP-GaInAsP lasers. It is explained that tensile-strain lowers thresholds relative to unstrained and compressively strained lasers only in certain high optical gain regimes and that tensile-strain is expected to provide high differential gain in all regimes of operation for the material systems investigated in this study.