For this study, we considered a THz QCL, fabricated from the GaAs/Al_0.15Ga_0.85As heterostructure material system. The active region is based on a four-QW design relying on a diagonal transition coupled to a phonon extraction stage (25). It is designed to emit around 3 THz (12 meV, 80 μm). The layer sequence of one period of structure in nanometers is 5.5/11.0/1.8/11.5/3.8/9.4/4.2/18.4 nm. Al_0.15Ga_0.85As barriers are indicated in bold font, and GaAs wells are in normal font. A Si n-doped GaAs layer (n = 2 × 10¹⁶ cm⁻³) is underlined. The QCL was grown on a <100> substrate. The calculated QCL band structure diagram using a Schrödinger solver for electrons and holes states is presented in Fig. 1B. The laser transition occurs between the upper electronic level E5 (blue line) and the lower electronic level E4 (red line). The first band structure (on the left) shows electron and LH confined states, while the second band structure shows electron and HH confined states. The electron-hole dipoles are largest for transitions between first-order confined hole and corresponding electron states, plotted in the same color in Fig. 1B (H1-E1 in green, H3-E3 in orange, H4-E4 in red, and H5-E5 in blue).