# Towards Deep Integration of Electronics and Photonics
## 2 Perspective Materials  
Traditional plasmonic materials are gold and silver due to the fact that their relaxation constants are less than for other metals (approximately 21 meV for silver and 66 meV for gold [21]). The SPP modes are supported at the interface between dielectrics and noble metals due to the large negative real part of the permittivity of the latter [4]. Metals introduce optical losses, caused by the imaginary part of the permittivity. The resulting damping of surface plasmons limits their practical application. Extra problems are related to interband transitions which result in additional losses. These factors stimulate the search for alternative plasmonic materials with improved properties and the extension to the near-infrared (NIR) including the telecom spectral range [22; 23]. Spectral coverage of plasmonic materials were extended also into the deep ultraviolet (UV), where unique plasmon properties have been discovered for ferromagnetic materials as a result of spin-polarization [24]. Alternative plasmonic materials with desired properties applicable in nanophotonics include semiconductors, transparent conductive oxides (TCO) and 2D materials. The excitation of a SPP requires to reach a certain density of charge carriers. Appropriately doped semiconductors, like Si and GaAs may support plasmonic modes. Silicon plasmonics is discussed in the literature as a platform for on-chip photonics with the SPP propagation length of the order of ten micrometers [25; 26]. The advantage of Si is that it is the most common material in electronics with well-developed manufacturing and processing technologies. However, it requires a relatively high level of doping (\(10^{21}\) cm\({}^{-3}\)) to reveal plasmonic properties at telecommunication wavelengths [23]. Another class of materials, commonly referred as transparent conductive oxides, is proven to be useful in plasmonics. Traditional representatives of this class are aluminum-doped zinc oxide (AZO), gallium-doped zinc oxide (GZO) and tin-doped indium oxide (ITO). TCOs show good plasmonic properties in the IR [27]. These materials allow high doping, which makes them conductive [28; 29]. At the same time, a large band gap makes them transparent in the visible range. Materials such as ITO are suitable to produce thin films and are compatible with silicon-based components. The excitation of SPPs in ITO has been demonstrated experimentally in the NIR range [30; 31]. A relatively low external voltage (below 10 V) applied to ITO can induce a refractive index change of about one (\(\Delta n\sim 1\)) at telecom wavelengths, which is a desired property in switches and modulators [32]. According to the Drude model, at optical frequencies \(\omega\) permittivity reads  
\[\varepsilon(\omega)=\varepsilon_{\infty}-\frac{\omega_{p}^{2}}{(\omega^{2}+ \gamma^{2})}+i\frac{\gamma\omega_{p}^{2}}{\omega(\omega^{2}+\gamma^{2})},\]  
where \(\omega_{p}=\sqrt{n_{c}\varepsilon^{2}/(\varepsilon_{0}m)}\) is a plasma frequency, \(\gamma\)--relaxation coefficient, \(\varepsilon_{\infty}\)--asymptotic permittivity at large frequencies, \(\varepsilon\)--electron charge, \(m\)--effective electron mass, \(\varepsilon_{0}\)--free space permittivity and \(n_{c}\) denotes the concentration of charge carriers. The real part of the permittivity may cross zero at certain wavelengths and charge densities. The imaginary part, at the same time, remains finite, which excludes singularities in Maxwell equations. Nevertheless, this epsilon-near-zero (ENZ) point is quite special, since properties of materials vary significantly there [33]. As it was shown experimentally, the Drude model describes ITO quite accurately [34] and the ENZ point is in the IR spectral range. Since the permittivity of ITO depends significantly on the free charge density which enters the plasma frequency, one can locally shift the ENZ wavelength using an external electric field or applying a voltage.