High-accuracy measurement of equifrequency surfaces in photonic crystals: applications to bound-states in the continuum and self-collimation

The optical characterization of photonic crystals (PhCs) enables the prediction of diverse optical applications. However, traditional approaches often focus on high-symmetry band landscapes, leaving much of the Brillouin zone unexplored. In this work, we introduce a novel technique to measure and analyze Equifrequency Surfaces (EFS) of photonic crystal slabs with exceptional accuracy, achieving angular precision up to 10⁻⁷ radians. By performing simultaneous spectral measurements along polar and azimuthal directions, we transform real-space data into reciprocal space for each wavelength, generating comprehensive equifrequency contours. This approach grants access to the entire Brillouin zone, unveiling unique optical properties of PhCs. Notably, we distinguish unidirectional bound states in the continuum (BIC) from isotropic states around the Γ-point and fully validate the self-collimation effect, demonstrating the technique's robust capability to characterize and understand advanced photonic phenomena.