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

Abstract

be exploited during the THz QCL operation due to its rapid performance degradation at elevated temperature, [7] which has impeded its practical application in many areas, such as security, astrophysics, and medical treatment. To this date, while the most progressive THz QCL based on phonon-tunneling design could be operated at the maximum temperature of 250 K in pulse mode, [5] there exists a certain distance from room-temperature operation, and its luminous efficiency deteriorates rapidly at elevated temperatures. [1,8] Indeed, we are still far away from achieving high-output THz QCL at room temperature.Regarding this major challenge, numerous researches have been directed to investigating the temperature degradation phenomenon in THz QCL and several limiting factors have been identified accordingly, including degraded population inversion (PI), [9][10][11] reabsorption of broadened states, [12][13][14] and increased parasitic current density at elevated temperature. [3,15,16] Among these factors, the reduction of PI is the most concern, as the peak gain of a laser device is proportional to the degree of PI between its upper laser state and lower level state. In view of this, clarifying the mechanism of the degraded PI is an essential precondition to optimize the elevated temperature 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.

Controllable Synthesis of Highly Symmetrical Streamlined Structure for Wideband Microwave Absorption

“…It also had numerous uniform contacts between air/carbon and carbon/MnO, producing a rich inhibitory interface with positive and negative carrier flow, enhancing the interface polarization. [ 46 ] The MnO off‐axis electron holograph showed a weak air/MnO interface (Figure S11, Supporting Information). Compared with that of pure MnO, the increased interface polarization of the MnO@C core–shell improved the dielectric loss, [ 47 ] which was beneficial for improving the EMA performance.…”