Lasers based on intersubband transitions in quantum wells

The frequency dependence of the various parameters which determines the feasibility of intersubband lasers based on quantum wells are discussed and the more desirable frequency range in terms of each parameter is described. These parameters include the optical gain, the confinement factor, the relaxation rate, and the optical losses. The advantages and disadvantages of employing both electrical and optical pumping for creating population inversion for intersubband lasers are also discussed. Then, the potential problems concerning the realization of some of the proposed laser structures based on intersubband transitions in quantum wells are discussed and the more promising schemes are determined. Based on the recently published experimental results on resonant tunneling processes in coupled quantum wells as well as the free carrier absorption in doped layers, it is concluded that the realization of proposed electrically pumped lasers at far-infrared frequencies is extremely difficult at the present time. This is due to the high free carrier absorption in the doped layers and the inability to invert the population of the upper and lower subbands (laser states) of the quantum well in the active layer. Optically pumped structures do not necessary rely on the resonant tunneling process for creating population inversion and also doped injector/collector layers may be avoided in these structures and hence they could be more promising for the realization of intersubband lasers at far-infrared frequencies.