Dielectric-Loaded Sb ₂ S ₃ Fishbone Structures for Actively Switchable Unidirectional Surface Plasmon Excitation

Actively switchable directional excitation of surface plasmon polaritons (SPPs) is important for developing plasmonic optical nanocircuit devices for applications that require precise optical signal routing, dynamic reconfigurability, and miniaturization features. Here, we propose an actively switchable dielectric-loaded plasmonic fishbone-like metasurface on a thin gold film using a phase-change material (PCM), antimony trisulfide (Sb2S3), for the polarization-dependent unidirectional launching of SPPs at 1.55 μm. Sb2S3 is a versatile semiconducting material known for its high refractive index and low optical loss in the red to near-infrared regime. It also exhibits large refractive index contrast and fast, reversible phase switching capability via thermal or laser irradiation between its amorphous and crystalline states. We show by numerical simulations that Sb2S3 dielectric-loaded plasmonic fishbone metasurfaces can excite highly unidirectional SPPs with a propagation direction depending on the polarization state of the excitation. We present rational designs for the reversible launching direction of SPPs and a switchable dual-function metasurface, i.e., unidirectional or bidirectional, by controlling the phase of Sb2S3, using circularly polarized excitation. The switchable unidirectional SPP launching is also achievable upon linearly polarized excitation. The directionality stems from the interference of two 1D arrays of Sb2S3 nanobars with opposite planar asymmetries arranged in a fishbone configuration. We explain the working principle and illustrate the design rules.