Observations of Polarization Vortices in Momentum Space

As one of the most fundamental topological excitations in nature, vortices are widely known in hair whorls as the winding of hair strings, in fluid dynamics as the winding of velocities, in superconductors and superfluid as the winding of order parameters. However, vortices have hardly been observed other than those in the real space. Using a home-made polarization-resolved momentum-space imaging spectroscopy, we experimentally observed vortices in momentum space, as the winding of far-field polarization vectors in the first Brillouin zone of periodic plasmonic structures. We completely mapped out the band structure, lifetime and polarization states of all radiative states. The momentum space vortices were experimentally identified by their winding patterns in the polarization-resolved isofrequency contours and their diverging radiative quality factors. Such polarization vortices, leading to high Q factor modes so called bound states in the continuum (BIC), can exist robustly on any periodic systems of vector fields, while they are not captured by the existing topological band theory developed for scaler fields. Our results also inspire that the non-trivial topological effects could be also found in photonic crystals assumed to be topologically trivial in the past.