Helically Magnetized Plasma: From Photonic Fermi-Arc Metal to Chirality-Free Uniaxial Medium.

Fermi-arc metals, unconventional semi-metals featuring cylindrical Fermi surfaces formed by Fermi arcs, have recently attracted extensive attention for realizing a novel metallic phase that retains chiral anomaly responses yet suppresses quantum oscillations. Although it was proposed that spatially twisting a superlattice of thin Weyl metals can form a Fermi-arc metal, previous local-approximation analyses are valid only for slowly varying systems and cannot capture all rich physics in such systems. Here, we report an optical realization of such a phase in a natural magnetized plasma subjected to a helically modulated magnetic field. Unlike previous studies on artificial heterostructure platforms, we admit a fully analytical, nonperturbative treatment that tracks a complete evolution of Weyl node. In the slowly varying regime, our platform faithfully realizes and manipulates the Fermi-arc metal state in a real system. As the modulation rate increases, Fermi-arcs inheriting opposite chiralities start to hybridize. Remarkably, in the deep nonperturbative limit, chirality vanishes, not through the conventional Weyl point annihilations, but via Fermi arc recombination, resulting in a chirality-free uniaxial optical medium. These findings unveil global topological transitions in nonuniform Weyl systems and open routes toward photonic devices based on engineered Fermi-arc dynamics.