# Towards Deep Integration of Electronics and Photonics
## 3 Building Blocks of Hybrid Integrated Circuits
Many other components implementing this principle were suggested, like polarization converters, directional couplers, splitters and ring resonators [50]. Another way to overcome plasmonic losses is to use gain medium and amplifiers. A theoretical explanation of the gain medium mechanism to compensate SPP losses is given in [51]. Performing finite element simulations, the authors demonstrate the possibility of lossless propagation of SPP on a silver surface adjacent to InGaAsP-based medium. SPP amplification is realized experimentally [52] using the Kretschmann scheme, where the silver surface is sustained in contact with optically pumped dye-based gain medium. The achieved value of gain is sufficient to fully compensate losses in silver films. Gain-assisted propagation in plasmonic waveguides is experimentally studied in [53]. A single mode plasmonic waveguide is fabricated using a polymer (polymethylmethacrylate) strip placed on a gold surface. To compensate losses, the waveguide is doped by lead sulfide (PbS) quantum dots. Laser pumping in the central part of the waveguide results in a stimulated emission and increases the propagation length of SPP mode by 27 percent at telecom wavelengths. The direct measurement of SPP gain in a symmetric gold-stripe waveguide was performed in the experiments with optically pumped gain medium, represented by dye molecules in a solution [54]. Mode power gain of 8.55 dB/mm is achieved. By combining gain medium with a plasmonic resonator (both SPP and LSP modes can be used) one obtains SPASER (surface plasmon amplification by stimulated emission of radiation) [55]. Nanoparticles wrapped with a gain medium, for instance, can be used to construct about 20 nm large super-bright light sources useful in biomedical applications [56]. Figure 2: Combining a plasmonic waveguide (**a**), represented by a metallic surface, with a dielectric waveguide (**b**) one can form a hybrid plasmonic waveguide (**c**). A hybrid mode evolution is accompanied by a periodic energy transfer between the dielectric waveguide and the gap. Choosing the right length and distance one can use this effect to perform a smooth conversion of the waveguide mode into plasmon and back. A similar mechanism in successfully used in various models of plasmonic electro–optical modulators. While light sources and waveguides allow to form a photonic network, electro-optical modulators are responsible for the interaction with electronic components of the circuits. From the applied point of view, the major question is how to organize the interaction in the most efficient way: Many theoretical [57; 58; 59; 60; 61; 62] and experimental [63; 64; 65; 66; 67; 68] research works were focused on the models of plasmonic modulators. The HPWG concept and ENZ effect described above are two of the most promising plasmonic solutions which are implemented in different forms and combinations in these models. Different realizations compete for better energy consumption, bandwidth, size and CMOS-compatibility. The idea of a plasmostor is proposed and realized experimentally in [64] (see Figure 3a). In the suggested design, the distance between two silver electrodes is filled by 170 nm thick doped silicon and 10 nm thick insulating layer of SiO\({}_{2}\). Voltage applied to the electrodes influences properties of two available modes, allowing them to interact constructively or destructively at the output. Doped silicon does not support ENZ regime at telecom frequencies, but still may act as an active material, changing its refractive index under the applied voltage. Interference between two modes, thus, provides the modulation effect. There are other prototypes of modulators where the plasmonic slot waveguide configuration is implemented [57] (see Figure 3b). A thin layer of ITO (10 nm) is used in this model as an active material combined with a 50 nm thick layer of SiO\({}_{2}\).