A microscopic approach to THz intersubband challenges

The main candidate to be a practical and low cost high power THz source is the intersubband-based quantum cascade laser, which can have a tremendous impact in many practical applications, including last mile and indoor telecommunication systems. In this review we discuss current challenges for THz intersubband device development from a microscopic point of view. Next summarize 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.