Can any design support an effective nanostructure lasing for a few THz?

We present here a bird eye view on the basic physical factors limiting the efficiency of nanostructure-based THz lasing. The origin of these limitations is the contradiction between the requirements of THz gaps, high radiative matrix element value, and selective depopulation. Various ways were suggested to go out of these limitations. They include: sophisticated nanostructure layout, the usage of QCLs, and switch to bipolar THz lasing. We present the results of detailed density matrix based calculations comparing these approaches. In theory InAs-GaSb bipolar THz lasers are the most promising.