# Towards a Glass New World: The Role of Ion-Exchange in Modern Technology
## 4 Perspectives & Future Insights
### All-Glass Flexible Photonics
Figure 9 shows photos of bent waveguides fabricated in Willow glass with different thickness. The scattered green light, coming from a laser source working at 532 nm and injected inside the guiding structures by a butt-coupled optical fibre, visually highlights the actual presence of these channel waveguides. By optimising the laser processing parameters in terms of speed and energy pulse, it is possible to obtain single-mode channel waveguides at the working wavelength of 1550 nm with propagation loss as low as 0.11 dB/cm in all flexible Willow glass formulations. In particular, for the selected refractive index change (\(\Delta\)n \(\sim\) 5 \(\times\) 10\({}^{-3}\)), the bending losses of the waveguides are practically negligible for a curvature radius \(\geq\)1.5 cm. Furthermore, the high thermal stability of these optical devices (up to 400 \({}^{\circ}\)C), together with the possibility of writing Bragg grating waveguides, opens up new potential scenarios for the realisation of more complex photonic circuits on these commercial flexible glasses. Due to their alkaline-free content, these glasses find one of their main uses in the development of flat panel displays for high-tech applications [192]. The possibility of further strengthening these ultra-thin glasses could prevent the formation of any surface flaws making the customary handling and processing (e.g., glass cleaning, cutting, and lamination) procedures of these products less critical. Nevertheless, thermal tempering does not represent a suitable solution for strengthening these display glasses due to their thinness and low thermal expansion coefficient. On the other hand, the total absence of alkaline ions (e.g., sodium ions, Na+) in these products would seem to preclude the use of any chemical treatment aimed at further strengthening them. In this context, the interesting work by [PERSON] et al. would seem to offer an elegant solution to the problem, bypassing this impasse through the use of a particular ion-exchange process in alkali-free boroaluminosilicate glass [193]. The idea behind this original method is to replace ions such as Ca2+ and B3+ present inside the pristine glass matrix with Ba2+ ions coming from an ion source where the sample is immersed. Unfortunately, the kinetics of this kind of ion-exchange would be decidedly limited due to the considerable difference in inter-diffusivity between the two ionic species involved in the process, the alkaline and alkali ones, respectively. Moreover, these alkaline salts present high melting point and chemical corrosiveness that could irreversibly damage the glass surface during the exchange process, making their use as ion sources in a standard ion-exchange practically unfeasible. For all these reasons, the new chemical strengthening treatment is carried out in aqueous solutions of alkaline salts at a relative low temperature and under pressure in order to keep the water stable in its liquid state [194]. Proceeding in this way, the authors demonstrated the possibility to reach surface compressive stresses (CS) of the order of 200 MPa and a depth of layer (DoL) of only a few microns (i.e., \(<\)2 um for barium nitrate). This technique can be applied to a wide range of alkali-free glasses, such as ultra-thin  
Figure 9: Photos of bent waveguides written by femtosecond laser in Willow glass strips with different thickness and bending radius (**a**) 100 μm thickness and bending radius 13.5 cm; (**b**) 50 μm thickness and bending radius 2.1 cm; (**c**) 25 μm thickness and bending radius 1.0 cm; (**d**) the same sample reported in (**c**) compared with the size of a small coin. Reprinted with permission from [191] © The Optical Society. Willow glass, contributing to improve mechanical performance and thus helping to make all-glass flexible photonics more feasible. A second promising approach towards the development of all-glass flexible photonics is the one that involves the use of alkali-based glass formulations (i.e., sodium aluminosilicate glass).