title: Topological nature of optical bound states in the continuum.
abstract: Optical bound states in the continuum (BICs) have recently been realized in photonic crystal slabs, where the disappearance of out-of-plane radiation turns leaky resonances into guided modes with infinite lifetimes. We show that such BICs are vortex centers in the polarization directions of far-field radiation. They carry conserved and quantized topological charges, defined by the winding number of the polarization vectors, which ensure their robust existence and govern their generation, evolution, and annihilation. Our findings connect robust BICs in photonics to a wide range of topological physical phenomena.
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: Bo
authors.initials: B
authors.lastname: Zhen
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA and Department of Physics, Harvard University, 17 Oxford Street, Cambridge, Massachusetts 02138, USA.
authors.forename: Chia Wei
authors.initials: CW
authors.lastname: Hsu
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: Ling
authors.initials: L
authors.lastname: Lu
authors.affiliation: Department of Applied Physics, Yale University, Post Office Box 208284, New Haven, Connecticut 06520, USA.
authors.forename: A Douglas
authors.initials: AD
authors.lastname: Stone
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: Marin
authors.initials: M
authors.lastname: Soljačić