Basic Physics of Intersubband Radiative and Nonradiative Processes

Quantum cascade lasers are based on Intersubband transitions between quantum confined states in semiconductor heterostructures. The origin of these states is briefly described in this chapter starting with linear combination of atomic orbitals and then proceeding to the k.P theory. The relations between the interband and Intersubband transitions including their oscillator strength and selection rules are established. It is shown that “giant” Intersubband dipole owes its existence to the confinement induced band mixing. Aside from the radiative Intersubband transitions investigated in this chapter, nonradiative transitions also play important roles in QCL operation, hence most relevant of these processes: electron phonon, electron-electron, interface roughness and alloy disorder are also described in detail.