Can fundamental gain limitations of nanostructure THz lasers be overcome?

We argue that nanostructure based THz lasers of standard design have a principal limitations of gain value. These limitations rise from the obvious necessity to engineer both THz gap and population inversion simultaneously. Typical approach to the gap engineering inherited from midIR lasers utilizes intersubband transitions. However, contrary to midIR range, for THz lasing selective depopulation is problematic. The problem is that the selectivity of both depopulation mechanisms. LO phonon emission and electron - electron scattering, in THz region is substantially weaker than in midIR region. We suggest to use InAs/GaSb coupled quantum wells as a way to overcome this fundamental limitation. This is the only heterostructure where THz lasing can be based not on intersubband but on interband transitions. A proper design of this structure leads to a hybridization gap coming from anti-crossing of the GaSb valence band and InAs conduction band naturally appearing in the THz range. Two more advantages of this design are (i) a large value of the interband dipole matrix element and (ii) W-shaped spectrum leading to a singular density of states. These advantages lead to a gain much higher than for intersubband THz lasing.