Highly efficient, ultra-compact, and ultra-broadband bidirectional vertical coupler based on spin-directional locking

An ultra-compact and ultra-broadband polarization-selective bidirectional vertical coupler is proposed. The coupler is designed by coupling unique two-plane chiral π-shaped plasmonic nanoantennas on top of a silicon-on-insulator waveguide. By taking advantage of the spin-directional locking principle and coupled nanoantennas, a circularly polarized incident light can be coupled into a specific direction through the waveguide, depending on its spin. Based on the full-vectorial finite element method, for a beam radius of 0.53 μm, the coupler with a size of 1.07×0.5 μm² provides maximum coupling efficiency and an extinction coefficient of −6.9 dB and 16.9 dB across the telecommunication C to U bands, respectively, with a 2 dB bandwidth of >160 nm. The proposed coupler has potential applications in telecommunications, vertical cross-chip communications, and quantum information processing.