Vector k small middle dotp approach for photonic band structures
We point out that k small middle dotp treatments of photonic band gap materials based on the usual master equation must employ not only the physical photonic band solutions of that equation, but also unphysical solutions, in order to form a complete set. Nonetheless, it is possible to construct correct k small middle dotp expressions for the group velocity and its dispersion in terms of matrix elements involving only the photonic band solutions.