Self-consistent design of strain-compensated InGaAs/InAlAs quantum cascade laser structures: Towards short wavelengths

We designed In_xGa_1-xAs/InyAl_1-yAs quantum cascade laser (QCL) structures, based on the four-quantum well active region operating at lambda ~ 2.8-3.3 mum in terms of an objective function, i.e., z_UL ²(1 - tau_L/tau_UL)tau_U, related to the optical gain, including dipole matrix element (z_UL) and population inversion between electron transitions. For shorter wavelength emission, the higher conduction band discontinuity (DeltaE_C) was achieved by changing the In mole fraction of In_xGa_1-xAs/InyAl_1-yAs strain-compensated layers. The use of strain-compensated In₀.₇₂Ga₀.₂₈As/In₀.₃Al₀.₇As pair (i.e, DeltaE_C = 857 meV) leads to the shortest wavelength up to lambda ~ 2.84 mum with tau₄₃ = 3.96 ps, tau₄ = 1.21 ps, tau₃ = 0.55 ps and z₄₃ = 0.57 nm under an electric field of 94 kV/cm.