Bipolar THz laser: The way out of fundamental gain limitations

All nanostructure based THz lasers of standard design have principal limitations of gain value. The origin of these limitations is the necessity to engineer both THz gap and population inversion simultaneously. We present here estimations for these limitations and suggest to use InAs/GaSb coupled quantum wells as a way to overcome them. 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 orders of magnitude higher than for intersubband THz lasing.