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For terahertz quantum cascade lasers employing the nonalignment injection scheme via direct-phonon resonance, this study presents the crucial effect of parasitic absorption in the narrow module architecture based on only two quantum wells. The optical gain can be restricted to small (or negative) values even at the low temperature. To address this issue, a suppression strategy by increasing the injection energy is used to isolate the relevant levels. The feasibility of this strategy is experimentally demonstrated by observing lasing at 194 K. [ABSTRACT FROM AUTHOR]

Carrier transport in GaN terahertz (THz) quantum cascade laser (QCL) structures is theoretically investigated using a non-equilibrium Green's function method. Although scattering due to polar optical phonons in GaN is greatly enhanced with respect to GaAs/AlGaAs THz QCLs, the phonon-induced broadening of the laser levels is found to remain much smaller than other sources of broadening arising from impurity and electron-electron scattering. The gain is calculated self-consistently accounting for the correlation effects in level broadening. Three-well based design with resonant-phonon scheme shows a peak gain of 88/cm at 10 K, and 34/cm at 280 K, which remains above the calculated loss of a double metal waveguide. The results suggest that lasing at 6.6 THz, which is beyond the traditional GaAs THz QCLs, is possible up to 280 K. [ABSTRACT FROM AUTHOR]