Laboratory of Advanced Materials, Shanghai Key Lab of Molecular Catalysis and Innovative Materials, Department of Materials Science, Fudan University, Shanghai, 200438, P. R. China.
Joint-Research Center for Computational Materials, Zhejiang Laboratory, Hangzhou, 311100, P. R. China.
Small. 2022 Aug;18(34):e2106943. doi: 10.1002/smll.202106943. Epub 2022 Jul 31.
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.
在高温下，退化的粒子数反转（PI）被认为是太赫兹量子级联激光器（THz QCL）的一个重要温度退化因素，这阻碍了这些器件的广泛应用。在此，从微观角度研究了 PI 温度退化的机制。结果表明，提取系统的有限提取效率主导着 PI 在高温下的降低。具体来说，较高的温度会引起强烈的热激活纵光学声子散射，导致大量电子散射到较低的能级。在这种情况下，共振声子提取系统无法耗尽所有从较低能级的电子。因此，即使共振隧穿注入足以补偿上能级的电子流失，但较低能级的电子密度增加，整体 PI 会降低。此外，还发现高温下的强电子-离子施主分离会导致能级失配，从而打乱注入和提取的最佳条件。同样，随着提取系统需要在几个能级之间进行精确耦合，提取效率也会降低。
