On-demand steering of hyperbolic chiral polaritons

Controlling light polarization and propagation in sub-wavelength architectures is foundational to nanophotonic technologies. A frontier direction is leveraging strong optical spin-orbit interactions to realize polarization-selective light steering. Hyperbolic plasmon polaritons offer a compelling platform as they offer large optical spin-orbit coupling from strong confinement and dielectric anisotropy, as well as ray-like propagation. Despite theoretical predictions, however, the hyperbolic spin Hall effect - helicity-locked propagation of hyperbolic polaritons - remains elusive in natural materials. Here, we demonstrate the hyperbolic spin Hall effect in the visible to near-infrared range in the natural van der Waals metal MoOCl₂. Enabling this finding is a far-field pump-probe microscope that images hyperbolic polaritons with high sensitivity through interference with a high-momentum reference field, overcoming a key barrier to far-field and polarization-selective interrogation of hyperbolic materials. We show that both hyperbolic and surface plasmons in MoOCl₂ display chiral fields, and that their propagation direction can be completely switched upon light helicity reversal. Our results demonstrate on-demand steering of chiral plasmons, establishing natural hyperbolic materials as ideal components for reconfigurable nanophotonics and chiral light-matter coupling.