Multi topological Weyl points in photonic crystals

Weyl point is a topological point in three dimension momentum space and each Weyl point acts as a monopole of Berry flux. Weyl points with topological charges of the same sign can merge in the momentum space to forma multi Weyl point if certain symmetry conditions are satisfied. Here, we show that Weyl points with topological charges 1 and 2 can be found in a certain class of photonic crystals. Such multi Weyl points are protected by screw rotation symmetry and the physics can be understood using a tight binding model. The sign of topological charges are found to depend on the interaction range. We also find that the topological charge can change sign if the photonic crystal is changed to a “inverse” structure while maintaining the same space group symmetry. The existence of gap less surface states are determined by locations of Weyl points in the Brillouin zone and the direction of the surface orientation when the crystal is truncated to expose surfaces. As the structure of photonic crystal is compatible with standard nano-scale fabrication techniques, the system provide a flexible platform to realize multi Weyl points and the associated topological edge states up to optical frequencies.