Paraxial spin-induced vortex generation in epsilon-near-zero materials

Spin–orbit interactions (SOI) of light have emerged as a prominent area of research in nanophotonics, whereas epsilon-near-zero (ENZ) materials are gaining attention for their ability to interact with electromagnetic fields peculiarly. Integrating these two, we show that homogeneous ENZ thin films enable efficient spin-to-vortex conversion even under paraxial illumination. Using a generalized angular-spectrum formalism and experimental verification, we demonstrate that circularly polarized light incident on an indium-tin-oxide slab generates a second-order vortex in the cross-circular channel. Moreover, for elliptically deformed Gaussian excitation, the spin-induced vortex undergoes astigmatic splitting into two generic vortices. In contrast, a Bessel–Gaussian annulus with phase asymmetry from a tilted axicon produces a similar vortex splitting accompanied by a spin-dependent rotation of the double-core vortex pattern. These findings, combined with the intrinsic nonlinearity of ENZ materials, highlight their potential as a versatile platform to explore SOI effects across linear and nonlinear regimes.