Physics of the zero-(n)over-bar photonic gap: fundamentals and latest developments

A short overview is presented on the research works related to the zero-(n) over bar gap, which appears as the volume-averaged refraction index vanishes in photonic structures containing both positive and negative-index materials. After introducing the basic concept of the zero-(n) over bar gap based on both rigorous mathematics and numerical simulations, the unique properties of such a band gap are discussed, including its robustness against weak disorder, wide-incidence-angle operation and scaling invariance, which do not belong to a conventional Bragg gap. We then describe the simulation and experimental verifications on the zero-(n) over bar gap and its extraordinary properties in different frequency domains. After that, the unusual photonic and physical effects discovered based on the zero-(n) over bar gap and their potential applications are reviewed, including beam manipulations and nonlinear effects. Before concluding this review, several interesting ideas inspired from the zero-(n) over bar gap works will be introduced, including the zero-phase gaps, zero-permittivity and zero-permeability gaps, complete band gaps, and zero-refraction-index materials with Dirac-Cone dispersion.