Photorefractivity in Nanostructured Tin-Silicate Glass Ceramics: A Radiation-Induced Nanocluster Size Effect

The possibility of obtaining permanent photoinduced refractive index changes, up to $-{10}^{-3}$, in nanostructured silica-based composites has been demonstrated in ${\mathrm{S}\mathrm{i}\mathrm{O}}_2:{\mathrm{S}\mathrm{n}\mathrm{O}}_2$ optical-grade glass ceramics exposed to ultraviolet radiation. Optical and electron paramagnetic resonance data suggest that the negative refractive index change involves a modification at the surface of the ${\mathrm{S}\mathrm{n}\mathrm{O}}_2$ nanoclusters, leading to a reduction of their crystalline size.

Figure 1

(A) TEM and (B) HRTEM images of ${\mathrm{S}\mathrm{n}\mathrm{O}}_2$ crystalline clusters dispersed in ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ glass in 1.6 mol % ${\mathrm{S}\mathrm{n}\mathrm{O}}_2$-doped silica glass ceramics prepared by the sol-gel method.

Figure 2

(A) Refractive index of ${\mathrm{S}\mathrm{i}\mathrm{O}}_2:{\mathrm{S}\mathrm{n}\mathrm{O}}_2$ glass ceramics vs ${\mathrm{S}\mathrm{n}\mathrm{O}}_2$ volume fraction; (B) refractive index change vs UV laser exposure time (at 266 nm, repetition rate 10 Hz) at a pulse energy density of $150\mathrm{m}\mathrm{J}/{\mathrm{c}\mathrm{m}}^2$ in tin-silicate glass ceramics with 5 mol % ${\mathrm{S}\mathrm{n}\mathrm{O}}_2$ (filled triangles), 2.4 mol % ${\mathrm{S}\mathrm{n}\mathrm{O}}_2$ (filled squares), 1.6 mol % ${\mathrm{S}\mathrm{n}\mathrm{O}}_2$ (filled circles), and in Sn-doped silica glass with 0.5 mol % of Sn at $170\mathrm{m}\mathrm{J}/{\mathrm{c}\mathrm{m}}^2$ per pulse (open triangles) and at $340\mathrm{m}\mathrm{J}/{\mathrm{c}\mathrm{m}}^2$ per pulse (open circles). Curves are obtained from Eqs. (6, 7) (see text). Inset: saturation $\Delta n$ value at $150\mathrm{m}\mathrm{J}/{\mathrm{c}\mathrm{m}}^2$ per pulse vs ${\mathrm{S}\mathrm{n}\mathrm{O}}_2$ mol %.

Figure 3

(A) Effects of isochronal annealing treatment of 30 min on the UV-induced $\Delta n$ in tin-silicate glass ceramics with 5 mol % ${\mathrm{S}\mathrm{n}\mathrm{O}}_2$. Inset: evolution of the UV-induced (pulse energy density $150\mathrm{m}\mathrm{J}/{\mathrm{c}\mathrm{m}}^2$) $\Delta n$ from the starting value (a) through repeated cycles (a)–(d) of annealing treatments (30 min at 700 °C) and UV irradiation, and (e) after an annealing at 900 °C for 30 min followed by a final reirradiation at the same conditions. (B) UV-induced EPR spectra (and relative signal amplitude of the Sn-$E^{\prime }$ resonance at 341 mT, in the inset) during the same annealing-irradiation cycles.

Figure 4

Schematics of the proposed model of glass-ceramics photorefractivity based on nanocluster effects.

Figure 5

(A) PL excitation spectra at 9 K (emission at 2.4 eV) of 5 mol % ${\mathrm{S}\mathrm{n}\mathrm{O}}_2$-doped silica glass ceramics before (curve 1) and after (curve 2) UV irradiation (pulse energy density $150\mathrm{m}\mathrm{J}/{\mathrm{c}\mathrm{m}}^2$); (B) UV-induced change of optical absorption spectra [solid and dashed curves are from Eqs. (8, 9), respectively]. Inset: absorption spectrum before irradiation.