Some Considerations on Optical Confinement and Free Carrier Confinement of Quantum Cascade Lasers

Calculating the refractive index dispersions considering the influence of doping concentration on refractive indices, we have illustrated the heavily doped contact layers, n = 1 x 10¹⁹cm^-3 for λ ~ 9 μm, and n ~ 1 x 10²⁰cm^-3 for λ = 5μm quantum cascade lasers, to suppress the refractive indices and get sufficient optical confinement. By calculating the transmission of energies through a graded gap superlattice, it is demonstrated that for energies E₁ and E₂ (the two lower states) the electrons can easily get through the graded gap superlattice. While for energy E₃ the graded gap superlattice creates a minigap (the transmission of the energies E₁ and E₂ is 10² times more than that of E₃). In this way, the electrical confinement can be achieved.