Symmetry-Protected Photonic Chiral Spin Textures by Spin–Orbit Coupling

Abstract

Chiral spin textures are widely researched in condensed matter systems and have shown potential for use in spintronics and storage applications. Photonic counterparts of these textures are observed in various optical systems with broken inversion symmetry. Unfortunately, the resemblances are only phenomenological. This work proposes a theoretical framework based on the variational theorem to show that the photonic chiral spin textures in an optical interface originate from the system's symmetry and relativity. Analysis of the rotational symmetry in optical systems indicates that the conservation of the total angular momentum is preserved from the local variations of spin vectors. Specifically, although the integral spin momentum does not carry net energy, the local spin momentum distribution, which determines the local subluminal energy transport and minimization variation of the square of the total angular momentum, results in the chiral twisting of the local spin vectors. The findings of this study deepen the understanding of the symmetries, conservative laws, and energy transport in optical systems, reveal the similarities in the formation mechanisms and the geometries of photonic and condensed-matter chiral spin textures, and have potential applications in chiral and spin photonics.