Anomalous Floquet Topological Disclination States

Abstract: Recently, non-reciprocal two-dimensional unitary scattering networks have gained considerable interest due to the possibility of obtaining robust edge wave propagation in the anomalous Floquet phase. Conversely, zero-dimensional topological states in such networks have been left uncharted. Here, we demonstrate the existence of Floquet disclination states in non-reciprocal scattering networks. The disclination states, characterized by spectral charges, nucleate in the anomalous phase from a resonant rotation-symmetric phase matching condition, and survive until the bandgaps close. Once coupled to the radiation continuum feeding the anomalous chiral edge state, they can induce intriguing topological disclination bound states in the continuum (BICs), associated with an extreme confinement and lifetime. Altogether, anomalous Floquet disclination states and topological disclination BIC broaden the applications of disclination states to microwave, acoustic or optical scattering networks, with new possibilities in chiral topological lasing, robust energy squeezing from topological bound states, and switchable lasing and anti-lasing behavior induced via unidirectional topological coupling.