A short overview is presented on the research works related to the zero‐ n 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 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 gap and its extraordinary properties in different frequency domains. After that, the unusual photonic and physical effects discovered based on the zero‐ n 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 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.
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Zhou et al. (2012) studied this question.