  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.
