Fig. 1. Controlling strategy of the dual-function switchable SP device with a GST metasurface. (a) When GST is in the amorphous state, the device works as a spin-dependent directional plane-wave SP coupler. The lower right inset shows the geometry of the composed SRs; the upper right inset shows the schematic of the device unit cell. (b) When GST is in the crystalline state by an annealing process, the device works as a spin-dependent directional focusing SP coupler. (c) When tuning GST back to the amorphous state by a nanosecond laser pump, the device function is switched back to that in (a). (d) Schematic of the device design based on FZP concept, where the GST patches are placed at the even zones. (e) Microscopy images of the device, with the right showing the overall image (taken after annealing and nanosecond illumination) and the left displaying a zoomed-in view of the section (taken just after fabrication) marked by a red frame in the overall image. The darker color of GST patches in the overall image is related to the material property of our fabricated film upon stimuli. Notice that the dual-function switching ability of the device still holds.

Fig. 2. Passive validation of the controlling strategy. (a) Schematic of the NSTM system. (b), (e) Microscopy images of sample 1 and sample 2. (c), (f) Measured normalized SP intensity distributions of sample 1 and sample 2 under RCP and LCP incidences, respectively. (d), (g) Simulated results corresponding to those in (c) and (f).

Fig. 3. Characterization of GST. (a), (b) Measured amplitude transmission spectra and real-part conductivity of a 100-nm-thick GST film on a silicon substrate after fabrication (green dashed-dotted line, termed as unheated), under increasing annealing temperatures of 150°C, 200°C, 260°C, 280°C, 290°C, 300°C, 320°C, 340°C (solid lines), and after nanosecond laser illumination (purple dashed line, termed as illuminated), respectively. (c) Simulated amplitude transmission spectra corresponding to those in (a). (d) Measured and simulated transmission amplitudes as well as measured real-part conductivities of the GST film at 0.75 THz extracted from (a) to (c), respectively. Notice that the scatterers representing unheated and illuminated are plotted before 150°C just for clear comparison, which does not mean they are annealed at corresponding temperatures.

Fig. 4. Characterization of the dual-function switchable SP device. (a), (c) Measured normalized SP intensity distributions under RCP and LCP incidences, respectively. (b), (d) Simulated normalized SP intensity distributions under RCP and LCP incidences corresponding to those in (a) and (c). The labels at the right-bottom corners represent the processing to the GST, which are initially unheated, with 150°C to 300°C annealing temperatures, and 120  mJ/cm2 nanosecond laser illumination from top to bottom, respectively.

Fig. 5. SP intensity profiles at the focal lines of the device. (a), (c) Measured normalized results extracted from Figs. 4(a) and 4(c). (b), (d) Simulated normalized results extracted from Figs. 4(b) and 4(d).