## 1 Introduction  
The use of optical signals for the implementation of complex computing, information processing, or communication networks is nowadays essential to meet the growing demand for data exchange. The key features of the all-optical solution include the ability to parallelize optical carriers through spectral multiplexing and specific information encodings. Photonic integrated circuits (PICs) [1; 2; 3] have been developed to generate complete optical systems and to simplify the use of different optical functions from source to modulation and signal detection. Leveraging manufacturing techniques inspired by microelectronics, planar waveguide circuits have achieved two complementary objectives: (1) to control the propagation of the optical signal in a 2D system with very low losses and (2) to facilitate the serial or parallel association of several functions without the use of spatial optical elements. Guided planar optics enables the compact integration of optical elements whose lateral dimension is on the order of the wavelength in the material. However, this dimension remains diffraction limited and cannot be further reduced. Plasmonics has been a heavily researched field since the 2000s due to its unique capability to concentrate light on a sub-wavelength scale [4]. This entails confining light in the form of surface plasmon polaritons (SPPs); these resonant quasiparticles arise from the strong coupling between an electromagnetic wave and an electrical polarization wave carried by the surface conduction electrons of specific metals. Copper, gold, silver, and aluminum exhibit plasmonic responses, with resonance frequencies ranging from near-infrared (IR) to near-ultraviolet (UV). The electromagnetic wave that is \"hooked\" to the surface of the metal by plasmonic interactions has an evanescent wave profile on either side of the interface and contains a very high optical power density. Light is slowed down and concentrated at the subwavelength scale on the metal surface, facilitating enhanced interactions with any entity or material on or near the surface. Plasmons can exist in two primary forms: \"propagative\" on the surface of a metal film and \"localized\" within metal nanostructures [5; 6; 7]. These plasmonic states exhibit several unique properties, resulting from their specific spatial profile and resonance [8]. The integration of plasmonics into photonic circuits holds significant promise for addressing several major issues and challenges in photonics and its related fields. The ability to manipulate light propagation at the subwavelength scale is undeniably of vital importance for future integrated photonic circuits. Plasmonics can facilitate the miniaturization of optical functions [9; 10; 11], enhance non-linear optical interactions [12; 13], display a strong spectral and polarization dependence [14], and enable localized heating [15; 16] and control of the thermally activated phase change materials [17; 18], resulting in a high sensitivity of their resonance to the environment [19]. For instance, by leveraging plasmonic elements, it is possible to strongly reduce the coupling length between waveguides [20] or the position of self-images in hybrid photonic-plasmonic multi-mode interferometers (MMIs) [21], thus reducing the overall footprint of PICs. Finally, the technological manufacture of plasmonic films and structures is compatible with low-cost processes, requiring a reduced number of steps and demonstrating improved tolerance. For all these reasons, plasmonics is expected to lead to the densification of circuits, miniaturization of normal integrated optical functions, and incorporation of novel capabilities. Furthermore, plasmonic components which are expected to be ultrafast and ultrasmall [22; 23] in the future, can be integrated with photonic circuits to perform on-chip signal processing tasks.