Subwavelength Mode (De)multiplexer Based on Sodium-Assisted Hybrid Plasmonics

To enable high-density multimode photonic integration, we propose and numerically demonstrate a plasmonic hybrid mode division multiplexer–demultiplexer with a sodium coating on a silicon-on-insulator (SOI) platform. The device enables simultaneous mode division multiplexing (MDM) and demultiplexing of the transverse magnetic modes TM0 and TM1 at λ = 1550 nm. Full-wave three-dimensional finite-difference time-domain (3D-FDTD) simulations confirm an insertion loss of 1.2 dB, an inter-modal crosstalk of −20.3 dB, and a compact footprint of 5 μm × 16 μm. Compared with representative SOI-based mode-selective couplers with longer conversion regions, the proposed design substantially reduces the coupling length and device footprint. This numerical study provides a potential route toward compact, high-density multimode photonic integration based on sodium-assisted hybrid plasmonics.