To achieve lasing in any system, a number of requirements must be fulfilled. Each of these basic principles will be revisited later in the specific case of the quantum cascade laser.
There are three basic photon-carrier processes involved in the operation of the laser. Figure 1 depicts these in a simple two level system. An incident photon with an energy larger than that of the energy gap between the two levels has a high probability of being absorbed and exciting a carrier. This is stimulated absorption, Figure 1(a). Carriers will reside in this excited state for some finite time before decaying to a state lower in energy. In doing so, a photon is emitted with an energy equal to that of the energy gap. This is spontaneous emission, Figure 1(b). The principle photon creation process in the laser is stimulated emission (Figure 1(c)) where an incident photon stimulates the aforementioned decay to produce a further photon. Importantly, the new photon has the same energy and phase as the one that stimulated it. These photons then continue to further stimulated emission, resulting in gain in the system and an amplification of the photon count, hence the term LASER - Light Amplification by Stimulated Emission of Radiation.