Spin–orbit interaction mediated by a nonseparable twist phase of light

Spin–orbit interaction (SOI) of light provides a fundamental mechanism for controlling optical angular momentum, with applications ranging from light manipulation to quantum information processing. Conventionally, SOI is regarded as an intrinsic property of vector vortex beams arising from the coupling between vortex phase and polarization but is generally weakened in low-coherence optical fields. Here, we demonstrate a distinct form of SOI in tightly focused partially coherent vortex beams, mediated by an intrinsic nonseparable twist phase. We reveal that twist and vortex phases represent two independent mechanisms for orbital angular momentum (OAM) generation, whose interplay fundamentally reshapes angular momentum conversion processes. In orbital-to-spin conversion, the chirality of twist phase determines the sign of the longitudinal spin angular momentum (SAM) density, while the spatial coherence width governs the effectiveness of the conversion, enabling on-demand generation of localized SAM. In spin-to-orbital conversion, OAM induced by the twist and vortex phases undergoes constructive or destructive coupling, yielding tunable longitudinal spectral density distributions ranging from Gaussian-like to doughnut-like profiles. These results establish partial coherence engineering as a versatile platform for tailoring SOI of light, opening avenues for advanced structured light generation and nanoscale optical manipulation.