Refractive index enhancement is crucial in the fields of lithography, imaging, optical communications, solar devices, and many more. We present a review of advancements in the process of designing high refractive index metamaterials, starting from quantum coupling and photonic bandgap materials to metamaterials utilizing deep subwavelength coupling to achieve ever-high values of refractive index. A particular attention is given to experimentally verified schemes in engineering a high index of refraction. The understanding of the evolution of material design from intrinsic electronic states manipulation to meta-atoms design is not only fascinating but also a prerequisite to developing successful devices and applications.
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