  Although the invention of the metamaterials has stimulated the interest of
many researchers and possesses many important applications, the basic design
idea is very simple: composing effective media from many small structured
elements and controlling its artificial EM properties. According to the
effective-media model, the coupling interactions between the elements in
metamaterials are somewhat ignored; therefore, the effective properties of
metamaterials can be viewed as the "averaged effect" of the resonance property
of the individual elements. However, the coupling interaction between elements
should always exist when they are arranged into metamaterials. Sometimes,
especially when the elements are very close, this coupling effect is not
negligible and will have a substantial effect on the metamaterials' properties.
In recent years, it has been shown that the interaction between resonance
elements in metamaterials could lead to some novel phenomena and interesting
applications that do not exist in conventional uncoupled metamaterials. In this
paper, we will give a review of these recent developments in coupled
metamaterials. For the "meta-molecule" composed of several identical
resonators, the coupling between these units produces multiple discrete
resonance modes due to hybridization. In the case of a "meta-crystal"
comprising an infinite number of resonators, these multiple discrete resonances
can be extended to form a continuous frequency band by strong coupling. This
kind of broadband and tunable coupled metamaterial may have interesting
applications. Many novel metamaterials and nanophotonic devices could be
developed from coupled resonator systems in the future.
