Superluminal propagation of Dirac-cone modes in photonic crystal slabs

Apparent superluminal propagation characterized by a divergent group velocity is shown for the Dirac-cone modes in photonic crystal slabs by deriving their dispersion relation in the presence of diffraction loss. An analytical expression connecting the energy velocity to the group velocity is presented. It is shown that the former remains smaller than the speed of light even when the latter diverges