# Plasmonic and Dielectric Metasurfaces: Design, Fabrication and Applications
## 2 Plasmonic Metasurfaces
### N-Fold OAM Multicasting Using V-Shaped Antenna Array [120]  
Design: We design a V-shaped antenna array to realize on-chip multicasting from a single Gaussian beam to four OAM beams. Figure 9 illustrates the concept and principle of N-fold multicasting of OAM beams using V-shaped antennas. At the OAM multicasting side, an input Gaussian beam (\(l=0\)) with a planar phase front at its beam waist is modulated by a V-shaped antenna array, which is specially designed to be equivalent to a complex multi-OAM phase pattern to generate collinearly-superimposed  
Figure 8: Schematic diagram of the fabrication process of the proposed metasurfaces to generate broadband OAM-carrying vector beams. EBL, electron-beam lithography; EBE, electron-beam evaporation. Figure 7: (**a**) Wavelength-dependent extinction ratio for three OAM-carrying vector beams; (**b**) the dependence of extinction ratio on the initial orientation angle \(\alpha_{20}\) (second ring); (**c**) spatial distributions of phase, power components and polarization. Reproduced with permission from [119], Copyright The Optical Society, 2013.  
multiple OAM beams. After the modulation, the signal data carried by the input Gaussian beam are duplicated and delivered to N OAM beams, which are distinguishable from each other owing to their different charge values. The phase singularity of OAM beams contributes to the zero intensity distribution at the center of superimposed multiple OAM beams. At the OAM demultiplexing side, those N OAM beams (..., \(l_{i-1}\), \(l_{i}\), \(l_{i+1}\),...) are distributed to N end users (..., \(\text{User}_{i-1}\), \(\text{User}_{i}\), \(\text{User}_{i+1}\),...), respectively. For each end user, an inverse spiral phase pattern is used to remove the spiral phase front of the desired OAM beam, leading to a bright spot at the center, which can be separated from other OAM beams by spatial filtering. To enable on-chip OAM multicasting, a pattern search-assisted iterative (PSI) algorithm is employed to prepare a specific phase pattern for the simultaneous generation of multiple collinearly superimposed OAM beams. Figure 10a displays the calculated phase pattern using the PSI algorithm to generate four collinearly superimposed OAM beams with charge values of \(l=\) 1, 4, 7, 10, respectively. In view of the discrete characteristic of the V-shaped antenna, it is necessary to replace the calculated continuous phase pattern with a discrete one. The right image of Figure 10a plots the discrete phase pattern, which discretizes the continuous phase pattern into 32 values along the azimuthal direction. To evaluate the performance degradation caused by the discrete phase pattern on the generated collinearly-superimposed OAM beams, power distributions of OAM channels (i.e., OAM spectra) generated by both continuous and discrete phase patterns are calculated, compared and depicted in Figure 10b. Here we define the crosstalk for all OAM channels by the power ratio of the desired OAM channel (e.g., \(l=\) 4) to its neighboring ones (e.g., \(l=\) 3 and \(l=\) 5). A slight difference is observed for the target multicast OAM channels when using the discrete phase pattern. The crosstalk degradation for all four OAM channels after transmitting through the discrete phase pattern is less than 2 dB. By replacing the discrete phase pattern with the corresponding V-shaped antennas, we design the V-shaped antenna array to facilitate multicasting from a single Gaussian beam to four OAM beams, as shown in Figure 10c,d. Figure 11a depicts the calculated far-field intensity distribution of the collinearly-superimposed multiple OAM beams generated by the designed V-shaped antenna array, which has a triangular dark center. After demultiplexing, the intensity distributions of OAM channels are shown in Figure 11b,c. For multicasting OAM channels, there appears a bright spot at the beam center (Figure 11c), while for undesired channels, the beam center remains a dark region (Figure 11b).