Derivation of Absorption from Direct Interband Transitions

The chapter presents step-wise derivation of absorption coefficient from direct interband transitions using different formulas. It begins with an equation for transition rate probabilityfor a transition between two states, written in terms of the quantum mechanical expectation value of the dot product between wave polarization vector and momentum operator. The formula for overall absorption is defined as the difference between the power absorption and the power emission rates. The calculation for the approximation of energies is followed by the derivation of the absorption coefficient.