A microscopic approach for THz intersubband challenges

The main candidate to be a ubiquitous high power THz source is the intersubband-based quantum cascade laser (qcl). In this review we start by discussing current challenges for THz intersubband qcl development from a microscopic point of view. Next we evolve to a discussion about the search for new mechanisms and structure designs that can lead to intersubband gain without population inversion. This is a very important topic of current research, since is both an extremely elegant phenomenon from the basic physics of view and crucial for effective lasing in the THz range. The reason is that scattering phenomena can lead to level broadenings of the same order of magnitude of the lasing transitions, making population inversion by carrier injection in upper lasing subbands extremely difficult. Previous work in the literature is compared and contrasted with a new scheme that may lead to high temperature lasing by engineering the nonequilibrium population inversion with a combination of band structure and many body effects mediated by a k-space filter.