Combinatorics

Physics

Nonlinear system

Bound state

Topology (electrical circuits)

Merge (version control)

Diffraction

Photonics

Information retrieval

Mathematics

Computer science

Quantum mechanics

Photonic crystal

Quantum

Reciprocal lattice

Physical Review Letters

Bound states in the continuum (BICs) confine resonances embedded in a continuous spectrum by eliminating radiation loss. Merging multiple BICs provides a promising approach to further reduce the scattering losses caused by fabrication imperfections. However, to date, BIC merging has been limited to only the $\mathrm{\ensuremath{\Gamma}}$ point, which constrains potential application scenarios such as beam steering and directional vector beams. Here, we propose a new scheme to construct merging BICs at almost an arbitrary point in reciprocal space. Our approach utilizes the topological features of BICs on photonic crystal slabs, and we merge a Friedrich-Wintgen BIC and an accidental BIC. The $Q$ factors of the resulting merging BIC are enhanced for a broad wave vector range compared with both the original Friedrich-Wintgen BIC and the accidental BIC. Since Friedrich-Wintgen BICs and accidental BICs are quite common in the band structure, our proposal provides a general approach to realize off-$\mathrm{\ensuremath{\Gamma}}$ merging BICs with superhigh $Q$ factors that can substantially enhance nonlinear and quantum effects and boost the performance of on-chip photonic devices.

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