# Towards Deep Integration of Electronics and Photonics
## 3 Building Blocks of Hybrid Integrated Circuits  
The building blocks of modern electronic circuits are well established. Components of emerging nanophotonic circuits are usually sorted as the devices that generate light, guide, manipulate, and detect light. Both electronic and photonic subsystems in mixed circuits may contain active elements, like transistors, which are key elements for computations and information processing. A modulator allows manipulation of a photonic signal using an electronic signal or vice versa. Plasmonics is considered here as a promising tool to make the light-matter interaction stronger and corresponding devices compact and fast. Regarding the plasmonic improvements in the area of light emission, it is worth mentioning the technique that enhances the light-emitting efficiency of LEDs [47]. In these experiments, a supplementary silver layer is deposited 10 nm above the InGaN quantum well LED structure. According to the results, coupling between SPP, supported by the metallic layer, and the well increases the density of states in a semiconductor and enhances the intensity of light emission [47]. This effect can be used to construct more efficient solid-state light sources. The interaction between SPPs and semiconductors raises the fundamental question of coupling between excitons and SPPs. There are many applications of this mechanism, including a single photon SPP quantum detector and plasmonic photovoltaics (see below). Plasmonic waveguiding allows to reach a large degree of confinement in comparison with ordinary dielectric diffraction limited waveguides (Figure 2). In the case of a silicon at the typical telecom frequencies, a minimal waveguide cross section supporting a single mode is about 400 nm \(\times\) 200 nm. Plasmonic modes can be confined in layers with thicknesses of the order of tens of nanometers [48]. Thus, subwavelength plasmonic waveguides combine compactness of electronic circuits with a wide band communication capability, typical for photonic networks. The simplest plasmonic waveguide, supporting SPP modes, is just a metal-dielectric interface (Figure 2a). Another convenient structure, especially in quantum plasmonics, is a metallic nanowire [49] (efficient experimental techniques exist to produce thin nanowires with a large degree of homogeneity, see below). Various types of SPP-supporting geometries were suggested in the literature, like metal-insulator-metal and insulator-metal-insulator sandwiches, dielectric loaded plasmonic waveguides, gap waveguides and many others [19]. A convenient feature of the plasmonic waveguiding concept is that metallic surfaces, which already exist in electronic chips, may be used for SPP guiding. For example, metallic contacts in circuits or gate electrodes in transistors can be used as plasmonic waveguides, allowing to naturally incorporate SPP communication into circuits. Losses introduced by metals limit the utility of plasmonic waveguides and complicate the realization of a plasmonic communication network. Nevertheless, for some applications, like, for example, short distance communication between transistors, SPP propagation length provided by ordinary gold/silver surfaces is sufficient. To control plasmonic losses, an elegant solution exists in the form of so-called hybrid plasmonic waveguides (HPWG) [50]. The idea is to combine an ordinary lossless nanoscale photonic waveguide with poor mode confinement and a lossy but compact plasmonic waveguide (Figure 2c). As a result of interaction, a hybrid mode appears which represents a compromise between confinement and losses. By varying the distance between waveguides and the corresponding interaction strength, one may choose a desired proportion. Usually the interaction is strong, which makes the hybrid mode behaviour complicated and requires numerical computations to analyze its propagation. The concept of HPWG is valuable for plasmonic electro-optical modulators, since it allows to make them much smaller (see below).