**3.1 SRS in nanocomposities silica-based materials**

Among the innovative materials for Raman amplification, one of the most interesting classes is oxide glasses, above all silicon dioxide-based glasses due to their compatibility with the current optical fibers technology. To try to improve their SRS efficiency, a useful strategy is to add suitable dopants (heavy metal oxides as Ta2O5, Bi2O3, and Nb2O5) to silica [24–27]. We note that in other systems, such as niobiumphosphate glasses, characterized by a high concentration of niobium, a higher peak Raman gain (but in the best case of ≈ 10 times) and a broadening of the bandwidth with respect to silica glass has been demonstrated [28, 29].

In this paragraph, in order to increase SRS optical features of silica-based glasses, we propose an alternative approach: instead of to investigate new glass compositions, we change the glass arrangement. We note that a glass structural variations can be obtained as a result of an appropriate heat treatments made in the glass transition range, generating glass-ceramics with nanocrystals uniformly dispersed in the glass matrix (glass-crystals nanocomposites) [30]. We consider glasses, belonging to the K2O-Nb2O5-SiO2 (KNS) system, forming transparent and stable glasses and showing interesting non-linear optical properties. For glasses in the class of the KNS glass-forming system, an interesting glass nanostructuring process has been considered. The process contains two partially overlapped processes, namely, phase separation and crystallization [31]. We note that a clear relationship between glass nanostructuring and Raman gain has not been proven yet, although, in our previous paper, a connection between local structure and SRS in bulk

nanostructured 30K2O·30Nb2O5·40SiO2 (KNS 30-30-40) glass was found [31]. It is worth noting that an appropriate choice of the annealing parameters, therefore of the degree of crystallization, can allow to obtain the best compromise between the highest Raman gain and the highest nonlinear coefficients for third order effects.

Moreover, Raman spectroscopy characterization of nanostructured 20K2O·25Nb2O5·55SiO2 (KNS 20-25-55) glasses are also reported. The optical and structural characteristics of the samples have been measured by the Raman set up reported in Ref. [31]. Due to dependence of the intensity of the Raman active modes on both the temperature and the frequency of the vibrational modes, the measured Stokes Raman intensity was reduced according to the procedure also described in our previous papers [28–32]. Then, in order to properly compare the Raman spectra of KNS glasses with the silica glass standard, the measured Raman spectra were modified also for the differences in reflection and angle of collection by using the procedure reported in Refs. [28–32]. Usually, due to the more extended electronic clouds, elements with high atomic number yields highest intensity Raman bands, making the polarizability more sensitive to bonds stretching. Adding niobium oxide to silica-based glasses induces an enhancement of Raman cross section respect to silica glass, being the polarizability of Nb-O bonds higher than Si-O bonds. In the studied glasses, niobium enters in the glass network creating NbO6 octahedra more or less distorted, namely with different NbO bond length, and produces several Raman bands over a wide wavenumbers range. Due to the typical glass disorder, a broadening of all Raman bands occurs and, therefore, Raman spectra of KNS glasses outcome from the strong overlapping of several broad bands.

In **Figure 4** the Raman gain respect to the Raman gain of silica is reported as a function of the Raman bandwidth for different materials. Inter alia, in **Figure 4** is evident that glasses at initial and at different times of heat-treatment show the same bandwidth, but different gain. Hence, the nanostructuring process is nearly complete in glasses at a time between 2 and 10 h and produces nanocrystalline inhomogeneities distributed in the glass matrix [30, 32].
