Design of strain-compensated In_xGa_1-xAs/In_yAl_1-yAs quantum cascade laser structures towards the shorter wavelengths

摘要

We design the In _x Ga_1-x As/In_yAl_1-yAs quantum cascade laser (QCL) structures, based on the four-quantum well active region with vertical transition with strain compensation towards the shorter wavelengths, operating at λ ~2.93–6.6μm. Considering the objective function related to the optical gain, i.e., z₄₃2 (1-t₃ /t₄₃)t₄{z_{43}^2 (1-tau_3 /tau_{43})tau_4}, and the escape energy related to the carrier leakage to continuum, the design of QCLs is carried out in double infinite feedback loops. Each barrier/well layer in the active region is optimized towards the lasing wavelength as short as possible under a properly designed injector. For efficient short wavelength emission, the larger conduction band discontinuity (ΔE _c ) is achieved by changing the indium mole fractions of strain-compensated In _x Ga_1-x As/In_yAl_1-y As layers. The experimentally verified injectors are properly modified for the stronger coupling between the wavefunctions of the ground state in injector and the upper state in active region. Thus the use of strain-compensated In_0.72Ga_0.28As/In_0.3Al_0.7As structure (i.e., ΔE _c = 857 meV) leads to the shortest wavelength up to λ ~2.93μm with τ ₄₃ = 2.13 ps, τ ₄ = 0.87 ps, τ ₃ = 1.43 ps and z ₄₃ = 1.03 nm under an electric field of 96 kV/cm.