## 2 Materials and Methods  
Silica-zirconia (70%-30%) GC films were synthesized by following a sol-gel and dip-coating procedure, starting from Tetraethyl orthosilicate Si(OC\({}_{2}\)H\({}_{5}\))\({}_{4}\) (TEOS) and zirconium propoxide Zr(OC\({}_{3}\)H\({}_{7}\))\({}_{4}\) (ZPO) precursors, as described in [32]. As usual in sol-gel synthesis, all dopants were calculated and put in addition to this composition: 5 mol% Na was added as sodium acetate for allowing the introduction of Ag by ion exchange; 4 mol% YbNO\({}_{3}\) was added for Yb-doped samples. In short, three solutions were prepared as follows:  
1. TEOS, ethanol (EtOH), H\({}_{2}\)O and HCl (TEOS:HCl:H\({}_{2}\)O:EtOH = 1:0.01:2:25), adding 4 mol% YbNO\({}_{3}\) for Yb-doped samples;
2. ZPO, acetylacetone (Acac), ethanol (ZPO:Acac:EtOH = 1:0.5:50);
3. sodium acetate in methanol (60 mg/mL)  
Solutions (1) and (2) were mixed together, then (3) was added dropwise and left stirring overnight (16 h). Multi-layer films were deposited on fused silica, with each layer annealed in air at 700 \({}^{\circ}\)C for 3 min. After 10 layers, 400 nm total thickness was achieved. A final annealing in air at 1000 \({}^{\circ}\)C for 1 h induced the crystallization of zirconia in the glass-ceramic (GC) matrix.  
Undoped (label GC0) and 4 mol% Yb-doped (label GC4) films were then immersed for 1 h in a molten salt bath (1 mol% AgNO\({}_{3}\) in NaNO\({}_{3}\)) at 350 \({}^{\circ}\)C to obtain Ag\({}^{+}\)\(\leftrightarrow\)Na\({}^{+}\) ion exchange (label GC0A and GC4A). Finally, post-exchange annealing in air at 380 \({}^{\circ}\)C (GC0B and GC4B) and 430 \({}^{\circ}\)C (GC0C and GC4C) were used for controlling Ag migration and aggregation [32].  
The film composition was studied by Rutherford backscattering spectrometry (RBS), by 2.2 MeV \({}^{4}\)He\({}^{+}\) beam at 160\({}^{\circ}\) backscattering angle in IBM geometry, and simulating the experimental spectra by the RUMP code [39]. Areal density (the natural unit of measurement for RBS) to film thicknessconversion was obtained, considering a molar density of the film equal to a weighted average between silica (2.00 g cm\({}^{-3}\)) and zirconia (5.68 g cm\({}^{-3}\)), according to the nominal stoichiometric composition of the matrix (70 SiO\({}_{2}\) - 30 ZrO\({}_{2}\)), which was confirmed by RBS analysis.  
The crystal phase identification was obtained by X-ray diffraction (XRD) at room temperature, with an X'Pert PRO diffractometer (Panalytical) using a Cu anode (K\(\alpha\), \(\lambda\) = 1.54056 A). Diffractograms were collected in Bragg-Brentano geometry in the 20 range 10\({}^{\circ}\)-100\({}^{\circ}\). Nanocrystal size was determined by Line Broadening Analysis (LBA) [40] by the Warren-Averbach method.  
Photoluminescence excitation (PLE) and emission (PL) spectra were recorded by a FLS980 (Edinburgh Instruments). The excitation was provided by a xenon lamp coupled to a double-grating monochromator, while the emission from the sample was analyzed by a double-grating monochromator and recorded by a photon counting R928P (Hamamatsu) cooled at -20 \({}^{\circ}\)C (visible region) or by a photon counting R5509-73 (Hamamatsu) cooled at -80 \({}^{\circ}\)C (NIR region). The excitation wavelength for recording PL emission spectra was 330 nm, which is not absorbed by Yb\({}^{3+}\) ions, while the emission wavelength for acquiring PL excitation spectra was set at 975 nm, the peak wavelength for Yb\({}^{3+}\) ions \({}^{2}\)F\({}_{5/2}\)\(\rightarrow\)\({}^{2}\)F\({}_{7/2}\) transition.