Impurity bands in photonic insulators

A chain of impurity cells in a photonic insulator introduces impurity modes of the electromagnetic held 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 degrees. We also point out the possibility of using impurity bands in photonic insulators to study wave propagation along an effectively one-dimensional disordered chain. [S0163-1829(98)11919-7].