Vector k center dot p approach for photonic band structures

We point out that k.p treatments of photonic band gap materials based on th e usual master equation must employ not only the physical photonic band sol utions of that equation, but also unphysical solutions, in order to form a complete set. Nonetheless, it is possible to construct correct k.p expressi ons for the group velocity and its dispersion in terms of matrix elements i nvolving only the photonic band solutions.