# Towards Deep Integration of Electronics and Photonics
## 3 Building Blocks of Hybrid Integrated Circuits
Under the applied voltage, additional charges are accumulated in ITO, changing its local permittivity, both real and imaginary parts, and providing the absorption-based modulation. The refractive index of ITO changes much stronger at the applied voltage, compared with Si. In this model the ENZ effect is not used. Further development of the plasmonic slot modulator idea appears in the experiments of [66] (see Figure 3c). The 300 nm space between gold electrodes is filled by ITO, separated from the surface of the metal by a thin layer of Al\({}_{2}\)O\({}_{3}\) insulator. The resulting capacitor allows to accumulate reasonable charges at the ITO/Al\({}_{2}\)O\({}_{3}\) interface at relatively small voltages and reach the ENZ effect in ITO, providing an efficient modulation. In classical photonic circuits, light is assumed to be confined in a dielectric low-loss waveguide. In this case, the application of modulators described above requires the waveguide mode to be transfered into a plasmonic slot mode and back, with unavoidable transformation losses. A fully waveguide-based plasmonic modulator is demonstrated in [68] (see Figure 3d). In this work, the modulating sandwich consisting of layers of ITO (10 nm), SiO\({}_{2}\) (20 nm) and Au is placed on top of a silicon waveguide. A gold layer is used as both an electrical contact and a plasmonic surface. Conversion of a waveguide mode into a hybrid modulator mode (like in Figure 2c) can be made smooth and effective in such a model, by appropriately adjusting the geometry. The applied voltage shifts the frequency of the ENZ point in ITO, effectively absorbing the localized hybrid mode. This structure thus uses both the HPWG and the ENZ effect described above, resulting in a fast and compact model of electro-optical modulator. A family of modulators exists where only the ENZ effect is used to provide switchable absorption with no propagating plasmonic modes involved. Usually, such an approach makes the resulting devices longer in size compared to HPWG based solutions. In the theoretical work [69] (see Figure 3e) an active layer of AZO is incorporated into a single-mode silicon waveguide. In the off-state (when the voltage is applied) an absorbing layer is formed in the middle of the waveguide core, where the intensity of the propagating mode is maximal, providing an efficient modulation. In calculations of [60] (see Figure 3f) and [59] it is suggested to place ITO either around the waveguide or allow it to partially penetrate inside, realizing a similar modulation principle. On the other hand, fully metallic modulators were suggested recently which use only propagating plasmonic modes along with the Mach-Zehnder interference mechanism [63]. To compare with ENZ modulators, metallic solutions usually introduce more optical losses. Instead of fighting these losses, some authors suggest how to use them, for example, allowing SPP modes to come into play only in the off-state of the modulator, where losses work in a positive way [65] More details and an extensive discussion on the performance and characteristics of plasmonic modulators are available in multiple reviews [20; 26; 70]. SPP-based plasmonic modulators are usually strongly polarization-dependent. This is the consequence of the fact that surface plasmons polaritons can be excited only by light polarized perpendicularly to the surface of a metal. For this reason, another important class of auxiliary devices in photonic circuits are polarization converters. Many solutions with different levels of compactness and structural complexity were suggested [71; 72; 73]. Some of these devices may have a structure very similar to the structure of modulators [74] suggesting the idea to combine both devices into one universal unit. In recent experiments, a plasmonic mechanism of conversion is demonstrated [75]. Gold impurity is asymmetrically integrated into the waveguide. The evolution of a hybrid mode is accompanied by the transformation of polarization.