# Towards Deep Integration of Electronics and Photonics
## 1 Introduction  
Further achievements in electronics and photonics imply their better integration. Usage of photons at the level of circuits and single chips has a huge potential to improve modern intelligent systems [1]. Along with the usage of light for communication at small and intermediate scales, photonic circuits may also perform logical and computational operations. This implies that the distribution of functions between photons and electrons may be more advanced and potentially more productive than just communication and computation in the future generation of electro-optical chips. Regarding the future of electronics, it is important to note that the development of modern nanofabrication techniques and mesoscopic physics unavoidably leads to molecular electronics, where basic elements of circuits are represented by single molecules [2]. The idea to use both electrons and photons in future \"molecular chips\", operated by a microscopically small amount of particles, requires the detailed understanding of their quantum properties and interactions. The diffraction limit, unbeatable in classical photonics, keeps the size of light processing components at least an order of magnitude larger than the size of the electrical counterpart, i.e., transistors. The typical size of a transistor in modern processors is of the order of 10 nm, while the telecom wavelength of light is close to 1.5 \(\upmu\)m. In classical photonics, the electro-optical effect enables a modulation of optical signals due to changes in the optical parameters of materials under applied electric field. This effect is typically not as strong to use it on a chip. The mismatch in dimensions and weak light-matter interaction motivates a search for novel materials and subwavelength optics usage [3]. A plasmon is defined as a quantum of collective electron oscillation in a solid. It is also referred as a bulk plasmon (Figure 1a). It may reveal itself in metals, doped semiconductors and other materials (see the next section), where the concentration of charge carriers is sufficient. Charge oscillations near metallic surfaces can be coupled with light, forming surface plasmon polaritons (SPP) [4; 5; 6] as illustrated in Figure 1b. The localized surface plasmons (LSP) are typical for metallic nanoparticles and rods, where they may also be called transverse plasmons (see Figure 1c). The plasmon observations can be done with electron energy loss spectroscopy (EELS) with excitation by electrons [7], in the extinction optical spectra of small particles and films [8; 9], or cathodeluminescence (see [5] and references there). At present time the phenomena of light-matter coupling at the interface between metals and dielectrics is well understood [10]. Traditional applications of LSPs include surface-enhanced Raman spectroscopy (SERS) [11; 12], sensing [13; 14], scanning near-field optical microscopy (SNOM) and tip-enhanced Raman scattering (TERS) (see [4] and references therein). The SPP modes attract an interest from the point of view of communication due to their subwavelength confinement. This property of SPPs can be extremely useful for applications and allows to bring electronics and photonics closer to each other [3; 15; 16; 17; 18; 19]. A key active component in the interaction between electronic and photonic subsystems in mixed circuits is a modulator, i.e., the device which converts an electronic signal to an optical signal or vice versa. The importance of this active element is comparable with the importance of transistors in electronics. While pure electronic or pure optical transistors may perform logical, computational and information processing operations, modulators allow two subsystems talking to each other. Plasmonic modes discussed above can be used to make such devices small and efficient [20]. Various types of electro-optical modulators are discussed below along with other important components required to build fully integrated circuits, like waveguides, light sources, polarization converters and amplifiers.