# Towards a Glass New World: The Role of Ion-Exchange in Modern Technology
## 4 Perspectives & Future Insights
### All-Glass Flexible Photonics
In fact, recent analytical studies conducted by [PERSON] and co-workers shed new light on the effects induced by the bending stresses on both the mechanical strength and optical properties (e.g., in terms of refractive index) in the two main silicate glass formulations considered here, the alkali-free and the alkali-based ones [195; 196]. In general, the thinner the glass, the lower the surface stress resulting from a bending action. This allows to reach ever smaller bending radii, within the limits given by the mechanical strength of the material related to the presence of surface flaws. When subjected to extreme curvature, the glass develops high tension stresses that can lead either to its immediate breakage or to the progressive growth over time of micro-cracks, generated both by the effect of the continuous applied tension and by the action of water vapour on surface flaws, resulting in consequent definitive damage of the material. To overcome this problem, the strategy based on alkali-free glasses dictates a complete sealing of their surface, to make it resistant to the deteriorating action of water vapours on any surface flaws. On the other hand, the strategy based on alkali-silicate glasses involves the use of ion-exchange, capable of strengthening the glass by means of a residual tension profile induced on the surface of the sample following the same process. This suggests that the achievement of the surface flaws stability in extreme bending conditions for the glass represents a key factor to be considered. From the mechanical analysis carried out on the two types of samples mentioned above it emerges that, under the same extreme bending conditions, the alkali-based silicate glasses strengthened by ion-exchange generally allow larger thickness, overcoming the limitations imposed on not chemically treated alkali-free or alkali-based glasses. Moreover, from an optical point of view, the response of the two types of glass formulation under the same bending stress appears very different in terms of refractive index. In particular, for glasses strengthened by ion-exchange, the presence of residual surface tensions is associated to a local volume change of the material. This induces a further birefringence phenomenon that well overcomes that coming from the bending action of the glass sample. As a result, a strong refractive index gradient occurs at the surface of these exchanged ultra-thin glasses. This effect must be taken into account when designing all-glass photonic devices on this kind of flexible substrates. From what has been reported up to now it is clear the key role that ion-exchange, together with the glass material engineering, can play in the creation of increasingly performing flexible substrates towards the achievement of an all-glass flexible photonics as valid alternative to that developed on polymeric materials [178]. New Insight towards the Development of Optical Devices Loaded with Noble Metal Ions/Nanoparticles by Ion-Exchange  
Another intriguing prospect of the ion-exchange process that deserves further investigation in the near future is linked to the possibility of realizing new low-cost photonic systems with potentially improved characteristics for a wide application range. Let consider, for instance, the case of whispering gallery mode (WGM) optical microcavities, which continue to gain a growing interest in the field of photonics for their unique and extraordinary properties. In these microstructures, the light, suitably injected into the cavity, propagates grazing the surface of the resonator (few microns in depth) through subsequent total internal reflections along an equatorial plane. Due to their low material losses and high surface quality, these optical microcavities present an extremely high value of their quality factor Q defined as the ratio between the wavelength \(\lambda\) at which a resonance occurs and the linewidth \(\Delta\lambda\) of the resonant wavelength.