## 3 Surface Plasmons in Photonic Integrated Circuits
### Plasmonic Antennas  
The integration of plasmonic antennas into photonic circuits presents opportunities for mutual benefits. On the one hand, nanoantennas can take advantage of feeding schemes embedded in PICs. These schemes can be interconnected and combined with circuit elements that enable the control of the feed phase [103; 104; 105; 106], facilitating functions such as beam shaping and steering [107]. Integrated coupling methods can also lead to highly effective excitations of the radiating elements. On the other hand, integrating plasmonic nanoantennas into PICs holds potential for short-range on-chip and chip-to-chip wireless communication [108; 109]. The growing demand for high-speed data communications within data centers and high-performance computing platforms, driven by advancements  
Figure 8: Plasmonic nanostructures integrated on top of a waveguide: (**a**) array of metallic pads supporting propagative resonances (SPPs) (according to [98]); (**b**) single bowtie nanoantenna (according to [96]); (**c**) chain of coupled nanocylinders (according to [99]). in electronic devices and multi-core architectures, has led to rapidly increasing speed requirements. To address the limitations of copper-based interconnections [110, 111], such as high power consumption, heat dissipation, latency, signal losses, crosstalk, and bandwidth constraints, the development of parallel optical short-range links has been proposed and pursued [108, 109, 112]. Various types of antennas, such as Vivaldi-type antennas (leveraging propagative plasmons, [113, 114, 115]), Yagi-Uda antennas (acting as resonator-collectors), or LSP chain antennas, can be considered for this purpose. As previously discussed, the energy transfer between the PIC waveguide and the antenna is the key issue for functional efficiency. Figure 10: Scanning electron microscopy image of a 3-period dimer chain integrated on an SOI waveguide. Figure 9: (**a**–**f**) Integrated longitudinal dimer chain for trapping nano-objects [99]. (**a**,**b**,**e**,**f**) FDTD simulation obtained for an injected power of 10 mW. (**a**,**b**) Squared absolute value of the electric field and (**d**,**e**) stiffness along the \(x\)-axis k\({}_{\rm x}\) calculated on the plane of the dimer top facet when the dimer gap g is equal to (**a**) 70 nm and (**b**) 2.5 nm, respectively. (**f**) Stiffness k\({}_{\rm x}\) as a function of the gap g calculated for different values of the radius of the bead interacting with the device. (**c**) Scanning electron microscopy image of the fabricated sample. In all these cases, it is imperative for the plasmonic structure to exhibit radiative behavior, meaning its dispersion curve must fall within the light cone while also effectively coupling with the waveguide modes. In the case of Vivaldi antennas, these properties are obtained by progressively changing the antenna shape and the waveguide structure along the propagation axis. A progressive energy transfer is realized by evanescent coupling, and the excited plasmon radiates at the end of the suspended antenna. The integrated directivity is similar to the free space Vivaldi antenna and enables us to target the receiver, which is itself integrated, as shown in Figure 12.  
Integrated Yagi-Uda antennas are made of a resonant plasmonic \"receiver\", positioned between the \"director\" (made of several resonance-shifted resonators) and a \"reflector\" (wider metallic element). The radiative part of the antenna is the \"receiver\", whereas the other elements play roles of confinement (of the incident signal) or directivity (of the  
Figure 11: (**a**) Optical force (pN) and (**b**) stiffness (fN/nm) along the x-axis, as well as (**c**) optical force (pN) along the z-axis, calculated as a function of the wavelength and position of a spherical biological object. The latter is grazing along the x-axis on the top facet of a plasmonic chain composed of 4 NPs integrated on top of an SOI waveguide. The chain element spacing (center to center) is 150 nm.