## 3 Multi-Nondiffracting SPP-Beam Launching  
The above results of the intensity distribution indicate that some sidelobes exist beside the main lobe of the nondiffracting SPP beam. It is obvious that these sidelobes are induced by the interference of the SPP electric field generated by different subarrays located on different sides of the y-axis. By properly controlling the distance between the two subarrays on different sides of the y-axis and the length of the subarrays, we found that the overlapping region of the SPP beams generated by different subarrays located on different sides of the y-axis can be enlarged and the sidelobes can be converted into a new nondiffracting SPP beam. Therefore, this provides an opportunity to realize multichannel nondiffracting SPP-beam directional launching. To demonstrate this, by reducing the distance between the subarrays on different sides of the y-axis and extending their length appropriately, we realized directional launching of three nondiffracting SPP beams. The FDTD simulation results are shown in Figure 6. In the simulation, the distance between the two subarrays on the different sides of the y-axis is \(\delta=1\)\(\upmu\)m, every column has 25 nano-rectangular slots, and the other parameters are the same as those used in Figure 2a. It can be seen that there are three nondiffracting SPP beams when the incident light is LCP and RCP. Because the distance between the two subarrays on the different sides of the y-axis does not affect the propagating direction of the SPP wave, the nondiffracting SPP  
Figure 5: (**a**): The transversal electric field intensity distributions at \(x=\pm 13\)\(\upmu\)m. The blue and orange lines correspond to the conditions in which the inclined angle is \(-5^{\circ}\) under different polarization states; the green and purple lines correspond to the conditions in which the inclined angle is \(5^{\circ}\) under different polarization states. (**b**) The intensity distribution of the nondiffracting SPP beam in the case in which a cylinder obstacle exists. beam launching direction remains unchanged when adjusting the distance (see Figure 6a). We then further extracted the transversal electric field intensity distributions at \(x=\pm 11\) um, which are shown in Figure 6b. Figure 6c-f shows the results of modulating the three SPP-beam launching directions by changing \(\gamma\). We can observe that when the incidence inclined angle is \(5^{\circ}\) and \(-5^{\circ}\), the inclined angle of the three nondiffracting SPP beams is approximately \(5^{\circ}\). The figure shows that the nondiffracting property is maintained when we regulate the three SPP-beam launching directions by controlling \(\gamma\). The incident of RCP light under an inclined angle \(\gamma=5^{\circ}\) was selected as an example to detect the self-healing ability of the three nondiffracting SPP beams. We placed a gold cylinder obstacle with a radius of 150 nm at the points (10 um, 4.96 um), (11 um, 0.90 um), and (10 um, \(-\)3.33 um) to simulate the SPP intensity distribution. The FDTD simulation results are shown in Figure 7. It can be seen that all three directional launching SPP beams have self-healing ability after the obstacle. Figure 6: (**a**) Three-channel nondiffracting SPP-beam intensity distribution when the incident is RCP light with inclined angle \(0^{\circ}\). (**b**) The transversal electric field intensity distributions at \(x=\pm 11\) μm. The blue and orange lines correspond to the inclined angle of \(-5^{\circ}\) under different polarization states; the green and purple lines correspond to the inclined angle of \(5^{\circ}\) under different polarization states. (**c**–**f**) Three-channel nondiffracting SPP-beam intensity distribution under different incident light polarization states and different inclined angles. Considering that the broadband response of a device is expected in practical applications, we further simulated the response of our multichannel nondiffracting SPP-beam launching device in a broadband wavelength range.