Optical absorption

Transfer of energy from an optical electromagnetic wave into matter. Electronic absorption depends on transition matrix elements, occupied and empty states, and the joint density of states. Bound excitons can create resonances below an independent-particle continuum edge.

For allowed direct transitions between three-dimensional parabolic bands with a slowly varying dipole matrix element, the joint state density grows as $\sqrt{\hslash \omega -E_g}$. Linear absorption is proportional to that factor divided by $\omega$, before Coulomb enhancement and broadening are included.