Impurity bands in photonic insulators

A chain of impurity cells in a photonic insulator introduces impurity modes of the electromagnetic field over a narrow band of frequencies. We introduce a model of this band in the manner of a tight-binding description of impurity bands in semiconductors, and use it to describe waveguiding along the chain, and, in particular, across a corner of $90°$. We also point out the possibility of using impurity bands in photonic insulators to study wave propagation along an effectively one-dimensional disordered chain.