Observation of Spin-polarized Directive Coupling of Light at Bound States in the Continuum

Spin-polarized directive coupling of light, associated to the photonic quantum spin-Hall effect (QSHE), is a nanoscale phenomenon based on strong spin-orbit interaction that has recently raised large attention. Herein, we discuss the experimental manifestation of QSHE in the Bloch's waves associated to a bound state in the continuum (BIC) of a dielectric photonic crystal metasurface (PhCM). We numerically show that BICs in nanoscale PhCMs have photonic spin angular momentum density transverse to the orbital momentum not only at the interfaces but also inside the confining dielectric medium. Then, we experimentally demonstrate that the fundamental Bloch's waves of the BIC mode, macroscopically amplified on resonance, propagate along the symmetry axes of the PhCM obeying spin-momentum locking also at normal incidence, i.e. with no symmetry breaking. This BIC-enhanced spin-directive coupling of light may enable versatile implementations of spin-optical structures, paving the way to novel strategies for photonic spin multiplatform devices.