Exploiting hidden singularity on the surface of the Poincaré sphere
Jinxing Li, Aloke Jana, Yueyi Yuan, Kuang Zhang, Shah Nawaz Burokur, Patrice Genevet
Abstract
The classical Pancharatnam-Berry phase, a variant of the geometric phase, arises purely from the modulation of the polarization state of a light beam. Due to its dependence on polarization changes, it cannot be effectively utilized for wavefront shaping in systems that require maintaining a constant (co-polarized) polarization state. Here, we present a novel topologically protected phase modulation mechanism capable of achieving anti-symmetric full 2π phase shifts with near-unity efficiency for two orthogonal co-polarized channels. Compatible with -but distinct from- the dynamic phase, this approach exploits phase circulation around a hidden singularity on the surface of the Poincaré sphere. We validate this concept in the microwave regime through the implementation of multi-layer chiral metasurfaces. This new phase modulation mechanism expands the design toolbox of flat optics for light modulation beyond conventional techniques. Arising from singularity on the surface of Poincaré sphere, topology-protected phase for co-polarized light enables 2π phase modulation, which complements the classical cross-polarized Pancharatnam-Berry phase and expands the versatility of flat optics.