Optical modes exhibiting high quality Q factors and minimal dispersion in momentum space are crucial for enhancing light-matter interactions, including nonlinear optical processes and slow-light phenomena. Bound states in the continuum (BICs) represent a promising photonic platform owing to their theoretically infinite Q factors. However, for conventional individual BICs, the Q factor diminishes exponentially upon deviation from the BIC momentum singularity, and further enhancement of optical nonlinear conversion efficiency remains actively pursued. Here, we merge symmetry-protected and Friedrich-Wintgen BICs within a silicon metasurface to create a super-BIC to boost the Q factors of the associated quasi-BIC mode with ultraflatband dispersion and topological robustness. The ultraflat merging BIC exhibits a near-zero group velocity with a record group index of $5.0\times {10}^8$ and group delay of $8.9\times {10}^5\mathrm{ps}$, surpassing conventional BICs by 2 orders of magnitude, which stems from the near-zero dispersion of the super-BIC-induced flatband. For nonlinear optics, the structure enhances the polarization-sensitive third- and fifth-harmonic generation efficiencies by 6 and 12 orders of magnitude, respectively, achieving conversions efficiencies of $9.6\times {10}^{-3}$ (third-harmonic generation) and $1.8\times {10}^{-10}$ (fifth-harmonic generation) at an ultralow pump density of $43\mathrm{mW}/\mathrm{c}{\mathrm{m}}^2$. Notably, it maintains an average electric-field enhancement of $3.28\times {10}^4$ even under fabrication imperfections. This flatband design establishes a paradigm for robust light-matter interaction control, enabling applications in ultralow-power nonlinear photonics and on-chip slow-light devices.

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• Light-matter interaction
• Near-field optics
• Nonlinear optics
• Photonics
• Strong electromagnetic field effects