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.