Abstract
The key challenge for terahertz quantum cascade lasers (THz-QCLs) is to make it operating at room-temperature. The suppression of thermally activated leakages via high lying quantum levels is emphasized recently. In this study, we employ the advanced self-consistent method of non-equilibrium Green’s function, aiming to reveal those kinds of leakages in the commonly used THz-QCL designs based on 2-, 3- and 4-quantum well. At the high temperature of 300 K, if all the confined high lying quantum levels and also the continuums are included within three neighboring periods, leakages indeed possess high fraction of the total current (21%, 30%, 50% for 2-, 3- and 4-quantum well designs, respectively). Ministep concept is introduced to weaken those leakage channels by isolating the desired levels from high lying ones, thus the leakages are well suppressed, with corresponding fractions less than 5% for all three designs.
Highlights
The key challenge for terahertz quantum cascade lasers (THz-QCLs) is to make it operating at room-temperature
The most used optical gain medium for THz-QCLs is gallium arsenide related quantum structures, thanks to its well-established growth technique of molecular beam epitaxy (MBE) which can ensure an atomic-scale tailoring of quantum wells (QWs) structure
The dominant mechanism of thermally degradation was initially ascribed to a nonradiative channel between two laser levels[6,7], that refers to hot electrons in the upper laser level gaining enough kinetic energy, relax down to the lower laser level via longitudinal optical (LO) phonon emission (a nonradiative manner instead of THz photon emission)
Summary
The key challenge for terahertz quantum cascade lasers (THz-QCLs) is to make it operating at room-temperature. The suppression of thermally activated leakages via high lying quantum levels is emphasized recently. Thermal backfilling mechanism is paid attentions in which electrons can be scattered back to the lower laser level and directly reduce the population inversion[13,14] This mechanism is quite sensitive to temperature in the design of bound- tocontinuum[9] mainly because the energy spacing between injection area and laser levels is small. Tall barrier basically can create deep well to confine more high lying quantum levels Those levels may exist as parasitic channels which can be activated when the temperature increases, as a result, reduces the lifetime of upper laser level. It still urgently continue to clarify the leakages in THz-QCLs especially under 300K operation
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