Mechanisms responsible for the ultraviolet photosensitivity ofSnO2-doped silica

Abstract

Tin-doped silica samples were prepared by the sol-gel method. Optical absorption and electron paramagnetic resonance measurements were carried out before and after exposure to 266-nm pulsed radiation from the fourth harmonic of the Nd-YAG ͑yttrium-aluminum-garnet͒ laser. The laser-induced change of refractive index was analyzed at different power densities ͑from 1 to 6ϫ10 7 W/cm 2 , 6-ns pulse duration͒ and exposure times. Positive changes of the refractive index up to 4ϫ10 Ϫ4 were observed. The presence and the consequent laser-induced bleaching of the 5-eV absorption band due to oxygen deficient centers, does not appear relevant, since good positive photosensitivity was observed in samples without detectable absorption at this energy.Exposure of silica-based glasses to ultraviolet ͑UV͒ radiation causes a change of the refractive index of the material. 1 This effect is currently studied for applications in optical devices for fiber-optics telecommunications. 2 Optical filtering and other more complex signal processing functions can be implemented in optical fibers or planar waveguides by exposing the material to uv radiation patterns generating modulated changes of refractive index. It was demonstrated that GeO 2 doping of silica, usually employed to produce the core of commercial optical fibers, has a role in this photosensitivity. 1,2 Other kinds of doping are investigated to enhance this property. 1 In particular, recent works showed that SnO 2 doping can give rise to strong and thermally stable photosensitivity. 1,3,4 However, it is not clear whether the microscopic mechanisms involved are those proposed to be responsible for the photosensitivity of GeO 2 -doped silica photoconversion of optically active defects induced by doping and structural compaction of the doped network. 1,2 Contemporaneously to these works, we investigated SnO 2 -doped silica produced by the sol-gel method in order to find the compositional domain compatible with the introduction of diluted tin atoms in Si sites. 5,6 In this paper, we present the results we have obtained on SnO 2 -doped silica samples from the sol-gel method, where Sn substitutional dilution has been achieved. The analysis of the kinetics of the refractive index change vs power density and exposure time gives evidence to two types of mechanisms in substitutional SnO 2 -doped silica, one which involves two-photon processes.Silica xerogels were prepared with a tin content between 0.4 and 0.5 mol%, by the sol-gel method employing hydrolysis of tetraetyl ortosilicate Si͑OEt͒ 4 and dibutyl tin diacetate. 5 Samples were treated at 1050°C by a very slow thermal program comprising a ramp of 4°C/h in vacuum between 450 and 950°C, obtaining complete densification at 1050°C. Samples were then characterized by means of uv absorption spectroscopy ͑Varian Cary 50͒. The indication of the presence of doping-related oxygen deficient centers ͑ODC's͒ was obtained from the 5-eV absorption band. This band arises from twofold-coordinated -Si-cation sites in pure silica, and -Ge-or -Sn-site...

Interplay between O2 and SnO2: Oxygen Ionosorption and Spectroscopic Evidence for Adsorbed Oxygen

“…Similar findings have also been reported for ZnO [47,93,96] and TiO 2 , [93] where the low-temperature peaks assigned to the superoxide ion are observed only after RT oxygen adsorption on the pre-reduced oxide. * signals with g av % 1.90 attributed to the 1) the unionised deep donor levels, 2) the singly ionised oxygen vacancy, 3) shallow defects; [87,97] * signals with g av (or g iso ) % 1.98 assigned to Sn 3 + ions; [98,99] * isotropic signal with g % g e (2.0023) gives no explanation for SnO 2 . This signal is usually observed on n-type semiconducting oxides; its interpretation is not clear and very controversial: phrases such as "medium polarised electrons", [100] "conduction electrons localised close to adsorbed surface acceptor-like species" [101] are used;…”

Refractive index and absorption spectra changes induced by UV irradiation in lead silicate glasses

“…The maximum photo-induced index change Dn, the index difference before and after UV irradiation, is about 0.01 for hydrogen-loaded germanosilicate glass fibers with limited stability at elevated temperature [6]. Recently, many other silicate glasses, such as tin-doped and lead-doped silicate glasses, were found to have very strong photosensitivity [7][8][9]. Long and Brueck wrote permanent gratings in lead silicate glasses through a phase mask by irradiation with a pulsed KrF excimer laser (248 nm) and a Q-switched YAG laser (266 nm), respectively [7,8].…”

Highly photorefractive Eu3+activated sol-gel SiO 2 -SnO 2 thin film waveguides

“…Even if germanium ions are the most used elements as photosensitive dopants in silica materials [2,3], tin ions represent a promising alternative because many devices, such as Bragg gratings, present a higher thermal stability in this matrix rather than in germanium-silica network [4,5]. Bulk glass materials doped with SnO 2 have been prepared by different fabrication methods, such as melting and/or sol-gel processes, and their photosensitivity response under the UV laser irradiation was well demonstrated together with the possibility to obtain optical devices [6][7][8][9]. Another promising route to fabricate photorefractive materials, principally based on SiO 2 -SnO 2 binary system, is represented by a thin film deposition on a dielectric substrate.…”