Merging Bound States in the Continuum at Off-high Symmetry Points

Bound states in the continuum (BICs) confine resonances embedded in continuous spectrum by eliminating the radiation loss. Merging multiple BICs provides a promising approach to further reduce the scattering losses caused by the fabrication errors. However, up to now, merging BICs are only limited to /Gamma point which restricts potential applications such as beam steering and directional vector beams. Here we propose a new approach to construct merging BICs at almost an arbitrary point in the reciprocal space. Our approach utilizes the topological feature of the BICs on photonic crystal slab, and we merge a Friedrich-Wintgen BIC and an accidental BIC. The Q factor of the resulting merging BIC decays much slower than both the original Friedrich-Wintgen BIC and accidental BIC. Since Friedrich-Wintgen BICs and accidental BICs are quite common on the band structure, our proposal provides a general approach to realize off-G merging BICs and can boost applications in various directions.