Decoupled flat-band and high-Q engineering in photonic crystal slabs

Flat photonic bands enable enhanced light–matter interaction but are often accompanied by increased radiative loss or strong angular sensitivity, limiting their practical utility. We report a photonic crystal (PhC) slab combining extreme band flatness with normal-incidence high-Q resonances through two physically cooperative mechanisms. Through effective mass engineering near the Γ point, we realize an ultra-flat dispersion with a relative variation as low as Δλ/λ ≈ 2.4 × 10 -4 , creating a momentum-robust optical mode. Independently, accidental bound states in the continuum (BICs) are manipulated to merge at the zone center, yielding Q-factors exceeding 10 8 . The distinct behaviors of these degenerate modes enable a self-referenced sensing scheme with intrinsic stability against thermal perturbations. This work establishes a unified approach to decoupling dispersion flattening and radiation suppression in planar photonics.