Bound states in the continuum (BICs) constitute a class of optical resonances characterized by ultrahigh quality (Q) factors, which promise to significantly enhance light-matter interactions. However, their practical performance is often degraded by the presence of topological defects in momentum space. We propose a “momentum-space purification” scheme for silicon-based metasurfaces. Annihilating oppositely charged accidental BICs eliminates topological singularities around the Γ-point symmetry-protected BIC (SP-BIC), flattening the local Berry-curvature distribution and markedly improving robustness against structural perturbations. This purification eliminates parasitic scattering channels, dramatically boosting third-harmonic generation (THG). Notably, the THG conversion efficiency exhibits a consistent transition from $\eta \propto Q^2$ to $\eta \propto Q^4$ scaling across all postpurification parameters, a signature of a fundamental change driven by topological engineering and a step toward robust nonlinear photonic devices.

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• Bound states in the continuum
• Metasurfaces
• Nonlinear optics
• Topological effects in photonic systems