Fig. 1. (Color online) (a) Basic four-level system for intersubband lasers and (b) fast LO-phonon scattering process between and in subbands.

Fig. 2. (Color online) (a) Typical mid infrared QCL structure and (b) subband diagram of gain region under applied electric field.

Fig. 3. (Color online) Representative evolution roadmaps for (a) multi quantum wells design (Refs. [2, 5, 8]) and (b) superlattice design (Refs. [3, 6, 11]).

Fig. 4. (Color online) Schematic of (a) a quantum dot cascade laser and (b) energy band structures of quantum wells and quantum dots-based active region.