1. The problem statement, all variables and given/known data I'm attempting to understand better how optical transistions occur in a QW (more specifically quantum cascade lasers) with and without an electric field. If an electron in the second excited state falls to the first, is it simply the transistion matrix between these two equations that determine the transistion rate or is the parity of the hole states also important? Does this matrix affect stimulated emission/absorption or is it only important because of the effect of spontaneous emision? 2. Relevant equations 3. The attempt at a solution http://www.iop.org/EJ/abstract/0022-3719/20/19/010" explains that 'transistions between opposite-parity electron and hole levels are forbidden' which tells me I'm not understanding correctly. I assumed that the electron only recombines with the hole state when it falls to the ground state, am I wrong?