1：Experimental Design and Method Selection:

The study involves the investigation of a high-temperature three-well GaAs/AlGaAs THz QCL with two upper laser levels. The design is identical to the GaAs/Al

2：21GaAs active region that reached a Tmax of 196 K at a frequency of 8 THz. The QCL is processed into a 2550 μm × 120 μm ridge with the metal-metal waveguide geometry with 15 μm Ni side absorbers added in the processing to suppress higher order lateral modes. Sample Selection and Data Sources:

The active region studied was a high-temperature three-well GaAs/AlGaAs THz QCL with two upper laser levels.

3：List of Experimental Equipment and Materials:

The QCL is processed into a 2550 μm × 120 μm ridge with the metal-metal waveguide geometry with 15 μm Ni side absorbers.

4：Experimental Procedures and Operational Workflow:

The emission spectra for a 3-well terahertz QCL was studied with respect to the two upper laser states |3? and |4?, LT32

5：4 and LT42 8 THz. The individual population and scattering effects for the states were investigated at different temperatures and magnetic fields. Data Analysis Methods:

The emission intensity is dependent on the population of the two levels. This behavior agrees with the calculated band structure, but further studies are required to determine the interactions between the two upper levels, their population, injection, and scattering process.