Geometric or Berry phases are fundamental manifestations that appear in many areas of physics. They arise from the geometry of the space describing the properties of multicomponent wave fields. An important example for electromagnetic waves is the spin-redirection Berry phase associated with the evolution of the spin direction. Because this effect has traditionally been studied in isotropic media where the spin is aligned with the ray trajectory, it has become commonly assumed that this spin-redirection Berry phase requires nonplanar rays. Here, we show that a spin-redirection phase can in fact arise along a planar ray if the spin evolves along the ray. We expose this effect through the singular example of a moving unmagnetized plasma, and demonstrate how this behavior can more generally arise from a finite transverse spin. In identifying this new spin-redirection mechanism, our work not only provides the tools to discover additional manifestations of spin-orbit interactions (SOIs) in nature, but also uncovers supplemental degrees of freedom to harness SOIs to control light.

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• Angular momentum of light
• Classical optics
• Electromagnetic waves & oscillations
• Geometric & topological phases
• Geometrical & wave optics
• Light propagation, transmission & absorption
• Plasma waves
• Polarization of light
• Spin-orbit coupling
• Topological effects in photonic systems
• Classical electromagnetism
• Electromagnetic wave theory
• Maxwell's equations
• Perturbative methods
• Phase space methods
• Plane-wave expansion
• Semiclassical methods