# Towards a Glass New World: The Role of Ion-Exchange in Modern Technology
## 4 Perspectives & Future Insights
### All-Glass Flexible Photonics
Moreover, their small mode volumes and long storage lifetimes for the photons guarantee high energy density levels and very narrow linewidths for the resonant modes sustained by these resonant microstructures. All these properties make such optical devices of great interest for applications in lasing, non-linear optics, optical communications and sensing [197; 198; 199]. If fused silica represents one of the basic materials for the realisation of WGM microresonators with extremely high Q factor thanks to its excellent chemical durability and transparency in a wide range of wavelengths [200], soft glasses are now gaining an increasing interest as a bulk material for these WGM microcavities due to their better versatility in terms of material properties, low phonon energy, and high rare earth solubility [201; 202; 203; 204; 205]. Based on this promising background in terms of materials for WGM microresonators, the basic idea could be to apply ion-exchange process to load the soft glass material constituting these microresonators with noble metal ions and/or nanoparticles in order to create hybrid devices, able to make the most of the intrinsic characteristics of the WGMs joined with those of the ion-exchanged material. For example, let's consider the case of a lasing emission by up-conversion phenomenon in WGM glass microspheres doped with rare earth ions [206; 207; 208; 209; 210]. The possibility to nucleate and grow noble metal nanostructures (i.e. : in the form of dimer clusters and/or metal nanoparticles with few nanometre sizes) within the active medium of the cavity following ion-exchange and thermal post-process may enhance the luminescent efficiency of the rare earth element and its emission by surface plasmon resonance (SPR) and/or energy transfer (ET) mechanisms, as observed for the planar substrates [211; 212; 213]. Moreover, an increase in the up-conversion signal comes from the same WGM microcavity which, supporting high recirculating light energy densities, allows the photons to interact several times both with the rare earth ions and metal nanostructures generally located at a distance of few tens/hundreds of nanometres below the cavity surface within the WGMs action field. In particular, if these nanostructures are represented by small plasmonic NPs (e.g., few nanometres in size), then a combination between the two resonant phenomena--the one linked to the host WGM microcavity and the plasmonic one of the nanoparticles--may occur generating a further significant signal enhancement. Obviously, the ion-exchange process with the consequent presence of noble metal nanostructures may increase the losses in the cavity reducing its Q factor value. In this regard, it should be noted that the thermal annealing process in air may offer a double advantage: formation of noble metal nanoparticles embedded in the glass and smoothening of the WGM microcavity surface as a result of temperature and surface tension. As proof of this statement, [PERSON] and co-workers recently demonstrated how the quality factor Q of a silver iodide phosphate glass microsphere remains almost unchanged (\(\sim\)10\({}^{4}\)) before and after the formation of silver nanoparticles following a thermal annealing treatment in air. The size of silver nanoclusters so obtained was around 350 nm [214]. This encouraging result suggests that the achievement of hybrid WGM microcavities, both in passive and in active glasses, by means of the ion-exchange technique may not be such a distant possibility. Currently, one of the most promising glass formulations for the realisation of active WGM microresonators is represented by phosphate glasses able to guarantee high Q factor values, above 10\({}^{6}\), for these optical devices both in bulk and coated configuration [201; 215]. Hence, these host materials may represent good candidates for the realisation of hybrid active WGM microresonators by ion-exchange process.