Extraction-Dominated Temperature Degradation of Population Inversion in Terahertz Quantum Cascade Lasers

Degraded population inversion (PI) at elevated temperature, regarded as an important temperature degradation factor in terahertz quantum cascade lasers (THz QCL), has hindered the widespread use of these devices. Herein, the mechanism of the temperature degradation of PI is investigated microscopically. It is demonstrated that the limited extraction efficiency of the extraction system dominates the decrease of PI at elevated temperatures. To be specific, the increased temperature brings about intense thermally activated longitudinal optical phonon scattering, leading to large amounts of electrons scattering to lower level state. In this case, the resonant-phonon extraction system is incapable of depleting all the electrons from lower level states. So even though the resonant-tunneling injection seems efficient enough to compensate the electron runoff at the upper state, the electron density at lower level state increases and the overall PI turns out lower. In addition, it is found that strong electron-ionized donor separation at high temperature can induce level misalignment, which can stagger the optimal conditions of injection and extraction. Also, the extraction efficiency gets lower as the extraction system requires accurate coupling between several energy levels.

Automated summary

This study investigates the mechanism of the temperature degradation of population inversion in terahertz quantum cascade lasers. It is found that the limited extraction efficiency of the extraction system is the main cause for the decrease in population inversion at elevated temperatures. Additionally, the temperature-induced electron-phonon scattering and electron-ionized donor separation also contribute to the degradation of population inversion.