# Multipolar origin of bound states in the continuum

## TL;DR

This paper introduces a multipolar framework to understand and design bound states in the continuum within dielectric metasurfaces, enabling high-quality resonances for advanced flat-optics applications.

## Contribution

It presents a novel multipole decomposition approach to explain the formation of bound states in the continuum in dielectric metasurfaces, considering symmetry conditions and spatial dispersion.

## Key findings

- Bound states originate from specific multipolar modes.
- Symmetry conditions determine the existence of bound states.
- Metasurfaces can support bound states with wavevectors forming a line in reciprocal space.

## Abstract

Metasurfaces based on resonant subwavelength photonic structures enable novel ways of wavefront control and light focusing, underpinning a new generation of flat-optics devices. Recently emerged all-dielectric metasurfaces exhibit high-quality resonances underpinned by the physics of bound states in the continuum that drives many interesting concepts in photonics. Here we suggest a novel approach to explain the physics of bound photonic states embedded into the radiation continuum. We study dielectric metasurfaces composed of planar periodic arrays of Mie-resonant nanoparticles ("meta-atoms") which support both symmetry protected and accidental bound states in the continuum and employ the multipole decomposition approach to reveal the physical mechanism of the formation of such nonradiating states in terms of multipolar modes generated by isolated meta-atoms. Based on the symmetry of the vector spherical harmonics, we identify the conditions for the existence of bound states in the continuum originating from the symmetries of both the lattice and the unit cell. Using this formalism we predict that metasurfaces with strongly suppressed spatial dispersion can support the bound states in the continuum with the wavevectors forming a line in the reciprocal space. Our results provide a new way of designing high-quality resonant photonic systems based on the physics of bound states in the continuum.