Spontaneous PT-symmetry breaking in complex frequency band structures

We demonstrate that the process of band-gap creation in periodic solids such as atomic, photonic, or phononic crystals can be viewed as spontaneous PT-symmetry breaking in the framework of a complex frequency band structure. This allows the use of ordinary artificial structures such as Bragg stacks and simple photonic crystals as suitable test beds for the study of phenomena related to PT-symmetry breaking.