Evolution of topological polarization singularities in honeycomb photonic crystal slabs

Area of Science:

• Photonics
• Condensed Matter Physics
• Optical Engineering

Background:

• Dielectric photonic crystal slabs with honeycomb lattices exhibit unique optical properties.
• Polarization singularities, such as vortex points, are crucial features in wave phenomena.
• Bound states in the continuum (BICs) offer high-quality factors and potential applications.

Purpose of the Study:

• To study the evolution of polarization singularities in photonic crystal slabs.
• To analyze the transformation of symmetry-protected BICs under reduced symmetry.
• To understand the topological charge changes associated with these transformations.

Main Methods:

• Theoretical investigation of dielectric photonic crystal slabs with ring-shaped clusters of rectangular holes.
• Analysis of symmetry-protected BICs at the Brillouin zone center (Γ point).
• Perturbation analysis by breaking C2 symmetry while preserving C3 symmetry to observe BIC evolution.

Main Results:

• Symmetry-protected BICs with topological charge q=-2 were identified as vortex singularities (V points).
• Breaking C2 symmetry transformed the high-order BIC into a low-order BIC (q=+1).
• Six circularly polarized states (C points, q=-1/2) emerged, shifting and rotating with increased symmetry breaking.

Conclusions:

• The study demonstrates a clear pathway for the evolution of polarization singularities in photonic crystals.
• Symmetry breaking provides a mechanism to tune topological charges and control BIC properties.
• These findings are relevant for designing advanced optical devices and understanding topological phenomena in photonic systems.