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Bound States in Continuum and Zero-Index Metamaterials: A Review

Bound states in the continuum (BICs) are waves that remain localized even though they coexist with a continuous spectrum of radiating waves that can carry energy away. Their very existence defies conventional wisdom. Although BICs were first proposed in quantum mechanics, they are a general wave phenomenon and have since been identified in electromagnetic waves, acoustic waves in air, water waves and elastic waves in solids. These states have been studied in a wide range of material systems, such as piezoelectric materials, dielectric photonic crystals, optical waveguides and fibers, quantum dots, graphene and topological insulators. In this Review, we describe recent developments in this field with an emphasis on the physical mechanisms that lead to BICs across seemingly very different materials and types of waves. We also discuss experimental realizations, existing applications and directions for future work. At last, we present our recent effort to design a novel type of silicon (Si) based mu-Near-Zero (uNZ) metamaterials that reduces radiative loss through destructive interference of multiple loss channels, resulting in a bound state in the continuum. This design consists of Si pillars array. By adjusting the unit-cell dimensions including radius, pitch and height, we can eliminate the out-of-plane radiation.