# Polarization Effect on the Performance of On-Chip Wireless Optical Point-to-Point Links
## 3 Wireless Propagation in On-Chip Multilayer Structure with TE and TM Polarizations  
For the analysis of the point-to-point on-chip wireless links between two Vivaldi antennas, the multilayer structure shown in Figure 8 was considered in the 3D-FDTD numerical model: (a) yz cross-section and (b) xz cross-section. The multilayer shown in Figure 8 corresponds to commercially available SiN samples with a bottom layer of bulk Si, an overlying layer of SiO\({}_{2}\) of height h\({}_{\text{S}}=2\) um, and a further layer of SiN of thickness t = 300 nm, housing the standard waveguides and the Vivaldi antennas. The antenna layer is then covered with a polymer-based cladding layer. The topmost layer, i.e., air, and the bottom one, i.e., bulk Si, were modeled as infinite in the simulations through Perfectly Matched Layer (PML) boundary conditions. The PML conditions were also applied on all the edges of the computational domain. As numerically and experimentally investigated by the authors in [30,36,37,38] for SOI-based on-chip wireless links, the electromagnetic waves that propagate in the on-chip multilayer structure experience multiple reflections and transmissions at the medium discontinuities. The consequent multipath interference can be constructive or destructive, resulting in increased or fading received signal. Differently from this work, in [37], links between simple taper antennas, which were obtained by tapering standard SOI waveguides, were analyzed, and only the TE polarization was considered for signal excitation. It is worth pointing out that the behavior of the wireless optical signal in a multilayer medium is strongly dependent on the material characteristics, either in terms of refractive indices or in terms of layer thicknesses. These features imply that every specific multilayer structure must be carefully analyzed to improve the link performances. To analyze the effect of the wave polarization on the link performances, different multilayer structures have been studied considering different thicknesses and different refractive indices of the cladding (polymer) layer. The cladding is usually introduced as a passivation layer in integrated optics, mainly to reduce scattering losses. Here, this layer plays another important role due to the presence of multiple reflections. As a case of study, we considered the variation in the thickness of the polymer layer composed of poly-methyl-methacrylate (PMMA), which is characterized by a refractive index value (n\({}_{\text{P}}\) = 1.4925) very near to that of SiO\({}_{2}\) (n\({}_{\text{SiO2}}\) = 1.445). Here, we quantify the performances of the wireless point-to-point links by calculating the transmittance at the receiving (Rx) antenna, as the power coupled into the receiver waveguide divided by that in input at the transmitter (Tx). Figure 9a,b show the transmitances, which were calculated for the TE and the TM polarizations, respectively, as a function of the link distance d\({}_{\text{link}}\) for different values d of the cladding layer thickness. The transmitances, which were obtained through a mode expansion monitor [45], are expressed in dB, and they represent the fraction of power coupled to the mode of the receiving waveguide by the corresponding antenna. Figure 8: Scheme of the on-chip multilayered structure: (a) yz cross-section, and (b) xz cross-section.