Dipole approximation in solids

I have seen the use of dipole approximation (where the extent of the wave function is assumed to be much smaller than the wavelengths of the transitions or that of the electromagnetic field applied) for both the case of an atom and an atomic crystal.

Since the extent of the wave function in atomic crystals is much larger than that of an atom, is the dipole approximation still valid? In short, is the energy level spacing still such that the corresponding wavelength is much larger than the length scale of the wave function ?

• I'm curious about this--could you give reference for usage of the dipole approximation in a crystal, if you have one handy? – Sam Bader Dec 18 '14 at 21:16
• Hi,there is one usage here for semiconductors where a periodic potential has been assumed as the wavefunction is in Bloch form optics.unm.edu/sbahae/publications/NLR-IEEE-JQE91.pdf – user56199 Dec 18 '14 at 21:34