Topological enhancement of evanescent field localization in all-dielectric metasurfaces

Bound states in a continuum in periodic photonic structures can be described as topological defects in the k -space. These vortices can migrate within the Brillouin zone when the geometrical parameters of the system vary. In this study, we focus on the migration of the polarization vortex across the light line, from the radiation continuum to the waveguide mode domain. We found that during this transition, the far-field polarization vortex transforms into a near-field vortex. This near-field vortex is characterized by an enhanced localization of the evanescent field due to the vanishing fundamental Fourier harmonic of the Bloch amplitude. This mechanism can be used to improve polaritonic devices and optical sensors based on all-dielectric metasurfaces by increasing the overlap between optical modes and 2D excitons or thin layers of bio-analytes.