Emergence of Unpinned Dirac Points in Defected Photonic Crystals

Unpinned Dirac points (DPs) are nodal point degeneracies that occur at generic momentum points lacking high symmetry, often exhibiting characteristics typically forbidden by symmetry. While this phenomenon has been observed in solid-state materials, the formation of unpinned DPs in photonic crystals has not been extensively studied. This study presents a novel approach to achieving unpinned DPs in two-dimensional ``defected photonic crystals"-photonic superlattices created by modifying the size or refractive index of dielectric disks at specific sites. The defected photonic crystal hosts multiple unpinned DPs through accidental band crossings induced by rotational symmetry breaking across the nodal line degeneracies prevalent in the superlattice. Furthermore, the number of unpinned DPs, along with their positions and anisotropic dispersions, can be manipulated by adjusting the size or refractive index of the dielectric disks. This study pioneers the exploration of defected photonic crystals, proposing them as effective means of achieving unpinned DPs and facilitating the investigation of novel relativistic photonic wave phenomena with significant flexibility and control.