# Integrated Photonics on Glass: A Review of the Ion-Exchange Technology Achivements
## 2 Ion-Exchanged Waveguides  
### Principle and Technology  
Typically, an optical glass is an amorphous material composed by several types of oxides mixed together. According to [PERSON] [10], these oxides can be sorted in three main categories: network formers like SiO2, GeO2, or P2O5 that can create a glass on their own; intermediate network formers (Al2O3, TiO2,... ) that can hardly create a glass alone but can be combined with network formers; finally network modifier oxides like Na2O, K2O, CaO, or BaO that can be inserted in a matrix made by glass formers but are weakly linked to it because of a mismatch between their respective molecular binding structures. The refractive index of a glass depends on its composition through an empirical relation [11]:  
\[n=1+\sum_{m}\frac{a_{m}N_{m}}{V_{0}}=1+\frac{R_{0}}{V_{0}}, \tag{1}\]  
where \(a_{m}\) is the \"refractivity constant\" of the chemical element \"\(m\)\", \(N_{m}\) the number of chemical element \"\(m\)\" by atom of oxygen, \(V_{0}\) and \(R_{0}\) are the glass volume and refractivity by atom of oxygen, respectively. A replacement of a portion of one of the glass components by another one with the same coordination can therefore entail a change of refractive index. Providing that this exchange does not create strong mechanical stresses and does not strongly change the nature of the glass, (1) can be used to link the induced variation of the refractive index to the fraction \(c\) of substituting ions as follows:  
\[\Delta n=\frac{c}{V_{0}}\bigg{(}\Delta R-\frac{\Delta V\ R_{0}}{V_{0}}\bigg{)}, \tag{2}\]  
\(\Delta R\) and \(\Delta V\) are the variation of \(R_{0}\) and \(V_{0}\), respectively, caused by the substitution. From (2), it can easily be deduced that a local change of the glass composition is creating a localized change of refractive index, which can be used to create a waveguide. Since alkali ions are weakly linked to the glass matrix, they are natural candidates for such a process. Indeed, when alkali ions react with silica to form a multicomponent glass, the silica network is maintained because each silicon-oxygen tetrahedron remains linked to at least three other tetrahedra [12]. Therefore, one can exchange one alkali ion to another one without damaging the original glass. Throughout the years, several ion-exchanges have been demonstrated [13; 14] but the topic of this article being integrated glass photonics, we will restrain ourselves on the few ones that have enabled realizing efficient devices. In this case, the ion that is present in the glass is usually Na\({}^{+}\) (sometimes K\({}^{+}\)). It is nowadays mostly exchanged with silver (Ag\({}^{+}\)), more rarely with potassium (K\({}^{+}\)) or thallium (TI\({}^{+}\)). The ion source that allows creating the higher refractive index waveguide's core can be either liquid or solid. The simplest way of performing an ion-exchange is described on Figure 1a. It consists in dipping the glass wafer in a molten salt containing a mixture of both the doping ions B\({}^{+}\) and the glass ones A\({}^{+}\). The salt is usually a nitrate, but sulfates are sometimes used when a temperature higher than 450 \({}^{\circ}\)C is required for the exchange. Although the principle of the process is very simple, it must be kept in mind that ionic diffusion is a process that strongly depends on the temperature; this parameter should hence be homogeneous all other the wafer and consequently in the molten salt. In order to define the parts of the wafer that will be ion-exchanged, a thin-film has previously been deposited and patterned in a clean room environment to define the diffusion apertures. Once the ion-exchange is completed, the masking layer is removed and diffused surface waveguides are obtained. If a more step-like refractive index profile is required, an electric field can be applied to push the doping ions inside the glass, as described in Figure 1b [1].