# Towards a Glass New World: The Role of Ion-Exchange in Modern Technology
## 4 Perspectives & Future Insights
### All-Glass Flexible Photonics
In this context, it will be mandatory to tailor both the ion-exchange parameters and those of the subsequent post-processing treatment to optimize the size of the noble metal nanostructures (e.g., few nanometres in diameter), minimizing losses in the cavity and keeping the quality factor Q for the resulting hybrid active WGM microcavities as high as possible. The considerations made above for bulk microspheres continue to be valid also for coated ones such as, for instance, a silica microsphere with a sol-gel thin film coating of compound glass. Applications in the development of visible laser sources for displays should be expected. The availability of noble metal nanoparticles embedded in a WGM cavity of compound glass could also favour non-linear optical response of the medium that should be further increased by the presence of plasmonic effects. For instance, silver NPs with their large third order nonlinear properties and optical absorption transitions with high oscillator strengths are the main candidates for the development of all-optical signal processing devices such as efficient optical limiters, which require high nonlinear refractive index \(n_{2}\) materials [216, 217, 218]. Also in this case, the general considerations made previously in terms of the optimisation of both process parameters and quality factor Q are still valid. It is important to note that, although the plasmonic absorption peak is generally located in the visible region of the spectrum, the absorption band of glasses with embedded silver NPs presents a tail extending to the near infrared region. This suggests how the local field effect induced by SPR phenomenon would have an impact also in this region of the spectrum [159,219]. This would allow to enhance the visible up-conversion emission in erbium doped glasses with embedded silver NPs. The enhancement in the up-conversion intensity may be also attributed to the local field effect induced by the presence of silver NPs [220]. Figure 10 shows a potential up-conversion emission enhancement in the particular case of an Er\({}^{3+}\)-doped WGM microsphere with embedded silver plasmonic small nanoparticles. It should also be mentioned here that a further important contribution for the achievement of hybrid WGM microcavities should be offered by the research effort towards the development of novel and efficient glass formulations, suitable for ion-exchange process, with the aim to reduce material absorption losses and scattering effects. Another noteworthy application of the ion-exchange technique joined with glass WGM microcavity could be represented by growing noble metal NPs (e.g., metal nanoislands or an array of few ordered and equidistant nanoparticles, working as hot spots and based on localized surface plasmon resonance (LSPR) phenomenon), directly on the surface of an ion-exchanged microresonator--in correspondence with its equatorial plane--by means of a thermal post-process performed with laser irradiation. This methodology could represent a valid alternative to the widely used one, which implies the chemical immobilisation of plasmonic nanoparticles on the surface of the resonant microcavity. In comparison to this technique, the strategy based on laser irradiation reduces the processing times and allows to have nanoparticles partially exposed on the surface, therefore more resistant to the various washing steps generally required in biosensing applications. Figure 11 shows the growth mechanism of plasmonic silver nanoparticles on the surface of an ion-exchanged WGM microsphere by laser beam irradiation technique. Figure 10: (**a**) Scheme of the up-conversion lasing emission in a Er\({}^{3+}\)-doped WGM microsphere with embedded silver nanoparticles. (**b**) Potential emission enhancement process in presence of Ag NPs. In the inset, the expected qualitative trend of the erbium up-conversion signal with (continuous line) or without (dashed line) plasmonic silver nanoparticles.