  Bichromatic photonic crystal structures are based on the coexistence of two
different periodicities in the dielectric constant profile. They are realized
starting from a photonic crystal waveguide and modifying the lattice constant
only in the waveguide region. In this work, we numerically investigate the
spectral and topological properties of bichromatic structures. Our calculations
demonstrate that they provide a photonic analog of the integer quantum Hall
state, a well known example of a topological insulator. The nontrivial topology
of the bandstructure is illustrated by the formation of strongly localized,
topologically protected boundary modes when finite-sized bichromatic structures
are embedded in a larger photonic crystal.
