## 3 Physical Description of the On-Chip, Optical Wireless Link
### Optical Antennas  
In order to evaluate the propagation and the performances of the wireless links in OWiNoC, some study cases based on different antenna configurations are here considered. Actually, the antenna coupled to the optical waveguide is one of the key elements in the OWiNoC design as it allows to switch from wireless to wired optical propagation and vice versa. At the transmitting section, the antenna radiates in the surrounding medium the optical signal that comes from the input waveguide. The radiated signal propagates wirelessly and it is then transferred to an output waveguide by the receiving antenna. The analysis of propagation and performance of point-to-point optical wireless links carried out in the following sections relies on the two configurations of plasmonic antennas shown in Figure 3a,b, respectively. In particular, Figure 3a outlines the single antenna configuration [9], consisting of a silver plasmonic Vivaldi 1.75 \(\upmu\)m long and 0.57 \(\upmu\)m wide. The antenna is coupled to a silicon waveguide (0.38 \(\upmu\)m \(\times\) 0.22 \(\upmu\)m) through a slotted plasmonic waveguide with length 1.63 \(\upmu\)m. The optical signal (P\({}_{\text{in}}\)) is launched into the Si waveguide and it is vertically coupled to a plasmonic slot waveguide, which is separated from the dielectric one by a dielectric layer. The antenna, obtained by shaping the plasmonic slot waveguide according to a Vivaldi profile, radiates the optical signal in the surrounding medium (P\({}_{\text{r}}\)) in the positive x direction. The behavior is reciprocal when the antenna is used as receiver. The antenna array configuration is instead represented in Figure 3b [37], where multiple Vivaldi antennas are fed by the same Si waveguide. Each antenna is 90\({}^{\circ}\) tilted with respect to the Si waveguide. This geometrical rotation allows a 90\({}^{\circ}\)-tilt of the radiation pattern with respect to the waveguide propagation direction. Moreover, this layout implements a travelling wave excitation of the array, which guarantees compact sizes of the overall radiator (about 3.5 \(\upmu\)m \(\times\) 8.7 \(\upmu\)m for a 5-antennas array). Although the details on the design and on the radiation characteristics of the two antenna configurations are provided in [9, 37] the antennas gain function in the \(\phi\) = 0\({}^{\circ}\) (xz) and in the \(\theta\) = 90\({}^{\circ}\) (xy) planes is plotted in Figure 4a and in Figure 4b for the sake of self-consistency. In particular, the reported curves refer to the single Vivaldi antenna (black continuous), and to the antenna array with N = 3 (blue dotted) and with N = 5 (green dashed) elements. The results shown in Figure 4 were obtained by the FDTD simulations of the antennas radiating in an infinite homogeneous medium with refraction index n = 1.44 (i.e., corresponding to silica, see Table 2), which was implemented in the numerical model by perfectly matched layer boundary conditions. The radiation diagrams were calculated by near-to-far field projections of the fields recorded on a closed box, surrounding the different antenna configurations. Figure 3: Scheme of the single Vivaldi antenna (**a**) and of the tilted Vivaldi antenna array (**b**) coupled to a silicon waveguide. The three antenna layouts correspond to different antenna gain values (G = 9.90 dB, G = 13.45 dB and G = 14.70 dB for the single, three- and five elements configuration, respectively), which take also into account the actual efficiency in the electromagnetic coupling between the silicon waveguide and the transmitting/receiving antenna. These antennas are exploited in the following to assess the impact of different radiation properties on the electromagnetic propagation in the multilayer environment and on the point-to-point link performance.