High-Accuracy Equifrequency Surfaces Measurement Unlocks Photonic Crystal Topology

In photonics and solid‐state physics, topology is characterized by how the dispersion relation behaves in reciprocal space. Dispersion surfaces (DS) are key to exploring bound states in the continuum (BICs) within photonic crystals (PhCs), as they reveal how the shape of the energy-momentum relationship gives rise to topological charges that manifest as polarization singularities in the far-field emission. Accurate measurement of these dispersion features—especially near the Γ-point—is vital, since even minimal geometric changes can dramatically alter the placement of topological charges and their associated polarization patterns. For these reasons, we introduce a new method for mapping the DS of PhC slabs with exceptional precision—angular resolution reaching 3 × 10⁻⁴ radians and spectral resolution down to 0.2 nm—across an enlarged portion of the Brillouin zone. Uniquely, our approach avoids diffraction and optical aberrations by relying solely on spectral measurements rather than imaging. Using this technique, we demonstrate its reliability by differentiating between anisotropic BICs aligned along a particular symmetry axis and isotropic BICs near the Γ-point. Furthermore, the detailed DS data obtained experimentally provide the necessary groundwork for identifying and studying super-BICs.