Chiral states without planar symmetry breaking in correspondence of Bound States in the Continuum

A comprehensive exploration of the Brillouin zone is essential to achieve a full understanding of the topological complexity of light in photonic crystals (PhCs), such as bound states in the continuum (BICs). C-points have been observed to display a division of the integer topological charge into semi-integer charges, following the breaking of the symmetry of the PhC. In this scenario, the symmetry break serves to differentiate C-point from BICs, as they are linked to resonances with a finite quality factor. We have developed a novel technique to measure EquiFrequencies Surfaces (EFS) in the Brillouin zone, and we have shown that, contrary to previous results, it is not required to break the planar symmetry of a photonic crystal slab in order to create a C-point linked to a BIC. We have also shown that in the absence of chirality in either the materials or the nanostructured PhC, the BIC condition exhibits chiral behaviour, which suggests the presence of a Weyl point. Finally, the study of EFS reveals that it is a type II Weyl point, which is associated with the presence of Fermi arcs, which is in excellent agreement with the theoretical predictions.