Resonant phonon-assisted depopulation in type-I and type-II intersubband laser heterostructures

We show that LO phonon-assisted interband tunneling in type-II intersubband laser heterostructures is more efficient for the fast depopulation of the lower lasing states than the corresponding intersubband process in type-I double quantum wells (DQW). The main peak of the electron-phonon resonance in type-II DQW corresponds to electron transitions from the lowest electron-like subband to the top of the highest heavy-hole subband, which is strongly spin-split and displaced from the center of the Brillouin zone due to the heterostructure asymmetry. Phonon-assisted depopulation can be conveniently employed even when the lower lasing level is designed near the upper edge of the heterostructure leaky window, where direct interband tunneling depopulation becomes inefficient. This design is beneficial for the laser performance providing the highest value of the matrix element for intrawell optical lasing transition and simultaneously preventing thermal backfilling of the lower lasing states. The InAs/GaSb/AlSb material system is very promising for the implementation of high-temperature mid-infrared intersubband lasers covering the 3-5 and 8-12 μm atmospheric windows. Comparing with InGaAs/InAlAs type-I heterostructures, the higher conduction band offset at InAs/AlSb interface allows an extension of the laser operation to shorter wavelengths, while the cross-gap alignment between InAs and GaSb contributes to the better blocking of the injected electrons in the upper lasing states. The lower lasing state depopulation, which is crucial for achieving the inverse population in intersubband lasers, in type-II heterostructures can be favourably accomplished by two efficient processes: the direct interband tunneling through the InAs/GaSb “leaky window” (0<ε <δ) and LO-phonon assisted interband electron transition from the lower electron-like lasing subband c1 into the highest hole-like subband hh1; see Figure 1. In type-II lasers, the direct interband tunneling has always been considered as a basic depopulation mechanism, while the interband LO-phonon assisted process is habitually treated as an inefficient one due to a symmetry difference between the initial (electronlike) and final (hole-like) states involved in the transition. In our previous work [1] we have shown that the efficiency of the direct interband tunneling depopulation, being 443