Observation of a higher-order topological bound state in the continuum

It is commonly thought that spectral isolation is required to identify boundary-localized topological states, as without a bulk bandgap at the same energy as the boundary states, it is not obvious whether these boundary-localized states can remain exponentially localized despite being degenerate with the bulk bands, or if they instead hybridize and lose their localization. Recently, it was predicted that the corner states can still remain localized if additional symmetries are present.

Here, we experimentally realize a higher-order topological bound state in the continuum (BIC) using a two-dimensional array of evanescently-coupled waveguides. First, by injecting light into the corner of the array, we prove that the lattice exhibits a corner-localized mode in its topological phase. Second, by using an auxiliary waveguide to couple into the array, which fixes the effective energy of the initial excitation, we show that this corner-localized mode appears at the center of the spectrum and is degenerate with bulk states of the lattice. Together, these experiments prove that the corner-localized state is a symmetry-protected BIC, and does not hybridize with the bulk bands.

Cerjan et al., in pub. at PRL, arXiv:2006.06524