Fig. 1. Schematic diagram of the temporal evolution process of plasmonic vortices of the same topological charge generated by different couplers. Sample 1 introduces only the geometric phase through varying the orientation angles of the slit resonators. (a-c) Show the SP intensity fields evolution of the generated plasmonic vortex, from moment t₁ when the excited SPs begin to concentrate to the center, with an interval of Δt, where SP fields reach and decay simultaneously at the target orbit. Sample 2 introduces both geometric phase and propagation phase through varying the radial position of slit-pairs. (d-f) Show the SP intensity fields evolution for Sample 2, from moment t₂, with an interval of Δt. The height of projection represents the relative intensity of SP field.

Fig. 2. Structure scheme of the plasmonic vortex coupler and illustration of numerical calculation. (a) Top views of the plasmonic vortex coupler composed by m slit-pairs arranged in an Archimedean spiral-shape and zoomed-in view of a single slit-pair. (b) Schematics of SPs excited by a single slit resonator orientated by an angle of θ with respect to the x-axis. (c) The component of incident RCP pulse in x direction (the blue line) and y direction (the red line) with a relative time delay Δ. (d) Slit resonators orientated by different angles and their corresponding radiation waveforms. (e) From top to bottom, the orientation angles correspond to 0, π/2 and θ.

Fig. 3. Schematics of designed structures and corresponding numerical investigation results. (a, d) Schematics of the plasmonic vortex couplers. The red dotted squares represent the calculated area. (b, e) and (c, f) Show the corresponding SP intensity fields and phase distributions, respectively, in the xy-plane under the RCP incidence. The inset white circles at the top-right corner denote the spin direction of corresponding incidence, similar hereinafter. (g, i) Snapshots of the SP normalized amplitude field distribution in the xy-plane, respectively, of Sample 1 and Sample 2, with a temporal interval of 1 ps. The black dotted circles in (g4–g6) and (i5–i7) represent the target orbits. (h, j) The absolute amplitude value of SPs, which are extracted on the target orbit corresponding to the Bessel radius of fourth-order vortex, in the same temporal dimension with (g, i), respectively. These absolute amplitudes are normalized and the five circles radially outward from the center represent values 0, 0.25, 0.5, 0.75 and 1 in turn, shown in the first picture of (h). The azimuthal angles of the whole circle cover 0 to 2π along the counterclockwise direction.

Fig. 4. Schematic diagram of the experiment setup for detecting the vertical component of the SP field E_z. In the transmitter modules, the spilt laser beam is focused onto the photoconductive antenna to excite electron-hole pairs, which are accelerated by the applied bias voltage SWPS (the square wave power supply) and formed transient currents to generate broadband terahertz radiation. CDA, LIA and DAQ represent the current dumping amplifier, the lock-in amplifier and the data acquisition card, respectively. The inset shows the detailed sample progress.

Fig. 5. Microscopic images of the plasmonic vortex couplers and experimental results. (a, d) Microscope photos of the fabricated Sample 1 and Sample 2. The red dotted square whose center coincides with the sample center represents the scanned range in our experiment. (b, e) and (c, f) are the measured intensity distributions and phase distributions of the SP fields in the plane at 75 μm above the sample at the air side under the RCP incidence. (g, i) Snapshots of the SP normalized amplitude field distribution in the xy-plane, respectively, of Sample 1 and Sample 2. The black dotted circles in g4–g6 and i5–i7 represent the target orbits. (h, j) The absolute amplitude value of SPs, which are extracted on the target orbit, in the same temporal dimension with (g, i), respectively.