Artificial double-zero-index materials

Materials with near-zero refractive index have attracted much attention over the past decade due to the fascinating phenomena they enabled, such as energy squeezing in thin waveguides, engineering of wavefronts, and “photonic doping”. These materials are not directly available in nature, but can be realized in periodic artificial structures. Among near-zero refractive index materials, double-zero-index materials are a special type with both constitutive parameters vanishing simultaneously, leading to intriguing applications including arbitrarily shaped high-transmission waveguides, cloaking of inclusions, nonlinear enhancement, and directional emissions. This perspective focuses on the recent developments on double-zero-index materials, including their fundamental physics, design principles, experimental realizations, and potential applications.