Transverse‐spin–momentum locking in a single‐crystalline plasmonic gap waveguide with a circular nanohole cavity

Abstract Transverse spin momentum in optical fields enables spin‐dependent routing of light, offering a promising route toward nonreciprocal and chiral photonic functionalities. Here, we demonstrate robust, polarization‐selective waveguiding in a plasmonic gap waveguide, enhanced by a circular nanohole cavity. Our device is fabricated via focused ion beam milling on single‐crystalline gold flakes, yielding atomically smooth interfaces that minimize scattering losses. Under circularly polarized excitation, the nanohole cavity induces transverse spin, thereby locking the propagation direction of surface plasmons polaritons within the waveguide. Experimental measurements confirm strong directional transmission, closely matching theoretical expectations. These findings highlight the utility of single‐crystalline gold for spin–orbit photonic devices and pave the way for integrated optical isolators and other nonreciprocal elements.