Coupling between Meta-atoms: a new degree of freedom in metasurfaces manipulating electromagnetic waves

Nanophotonic systems have attracted tremendous attention due to their exotic abilities to freely control electromagnetic (EM) waves. In particular, much attention has been given to metasurfaces consisting of multiple plasmonic/dielectric meta-atoms coupled in different ways. Compared to simple systems containing only one type of resonator, coupled photonic systems exhibit more fascinating capabilities to manipulate EM waves. However, despite the great advances already achieved in experimental conditions, theoretical understandings of these complex systems are far from satisfactory. In this article, we summarize the theorized tools for developing nanophotonic systems including both coupled resonators and periodic metasurfaces. We aim to understand the EM properties in closed and open systems, and introduce methods of employing them to design new functional metasurfaces for various applications. We will mainly focus on works done in our own group and we hope that this short review can provide useful guidance and act as a reference for researchers in related fields.