Local crystallization of glasses induced by laser irradiation is an efficient technique of fabricating regular structures
(arrays of dots, lines, gratings) which can be used as waveguides and other elements in the novel glass-based devices
of integrated optics and photonics. One of the most important cases is local laser-induced crystallization of nonlinear
optical crystals which can form a base for integrated active elements for optical frequency conversion or
electrooptical modulation. We report a comparative study on laser-induced crystallization of lanthanum
borogermanate glasses with different additives using different types of lasers. A stillwellite-like crystalline LaBGeO5 phase possessing ferroelectric and nonlinear optical properties or its solid solutions with partial substitution of La by
Nd or Sm depending on the additive are shown to precipitate as well-formed crystals a few microns in size near the
glass surface under radiation of the continuous wave Nd:YAG laser (1064 nm), the continuous wave copper vapor
laser (510,6 nm and 578,2 nm). To enhance optical absorption of the basic glass we added Sm2O3 oxide for
irradiation at 1064 nm and Nd2O3 for irradiation at 510,6 nm and 578,2 nm whereas pure glass was subjected to KrF
laser irradiation due to high native absorption at 248 nm. A copper vapor laser which almost has not been applied to
this technique before is shown to be an effective tool for laser-induced crystallization providing short times of crystal
growth and high speed of laser writing.
Silica xerogels doped with Ge(IV), substituting for Si(IV) in the oxide network, are prepared from tetraethylorthosilicate
and germanium-tetraethoxide. The sintering process is carried out in reducing atmosphere at 700 - 900° C by reaction
with H2. Raman spectroscopy and high resolution transmission electron microscopy (TEM) show that reactions with H2 give rise, in the porous silica network, to uncontrolled islands of crystallites of elemental cubic germanium with average size of 50 nm. Sintering process in reducing H2 atmosphere at temperatures just below the phase separation, at about 610°C, gives materials where Ge atoms are dispersed in the matrix in conditions of incipient clustering. Evidences of segregation of germanium nanocrystals are observed with electron irradiation during TEM analysis. Furthermore, the
electron beam induced precipitation leads to the formation of isolated quantum dots-like nanocrystals (5-6 nm in
diameter) and with narrower size dispersion. The ranges of suitable temperature and germanium concentration are analysed, as well as the size dispersion of the resulting Ge nanophases.
Optically transparent nanostructured SiO2 glassceramics containing a high density of monodispersed nanometer-sized
clusters of semiconducting SnO2 have been obtained by phase separation from a sol-gel derived synthesis. Complex
impedance spectroscopy analysis has been performed to get information about the conduction mechanisms to
understand the electrical behavior of the material. Measurements have been taken with applied bias ranging from +40 V
to -40 V and with an alternated voltage signal in the range 20 Hz - 1 MHz and amplitude from 10 mV to 300 mV. An
equivalent circuit, based on a metal-oxide-semiconductor model, comprising nanostructuring contributions, allows
measurements fitting. Capacity-voltage and conductance-voltage curves have been obtained for each component.
We present the spectroscopic study of the mechanisms of excitation transfer between rare earth ions excited by energy transfer from SnO2 nanocrystals in silica. Bulk samples of pure and Er-doped silica with SnO2 nanoparticles were prepared by a sol gel technique and further thermal sintering process. Transmission electron microscopy (TEM) reveals the formation of spherical nanoclusters with a size distribution strongly determined by erbium doping. Small angle neutron scattering (SANS) experiments confirm and detail the TEM data evidencing the existence of a interphase region at the cluster boundaries where a SnOlike phase compensates the structural mismatch between the crystalline lattice in SnO2 nanoparticles and the amorphous silica network. The analysis of the SANS patterns show what kind of modification of the interphase morphology of SnO2 nanoparticles in silica brings to the passivation of interfacial defects. Surface states, which may preclude the exploitation of UV excitonic emission, are reduced after doping by rare earth ions. We demonstrate, by means of transmission-electron-microscopy and small-angle-neutron-scattering data, that a smooth interphase with a non negligible thickness takes the place of the fractal and discontinuous boundary observed in undoped material. The time resolved photoluminescence spectra of erbium in the infrared region show the spectral profile ascribable to ions in a ordered environment. Moreover, the absence of the broad contribution of the radiative decay of erbium ions dispersed in the silica amorphous matrix indicates that the excitation transfer follows paths enveloped in the interphase region. The spectroscopic analysis allows us to conclude that the excitation is transferred from ion to ion within a quasi-crystalline region where each site is surrounded by a different distribution of PL quenching sites which are responsible for the multi-exponential decay kinetics.
Silica glass with SnO2 nanocrystals, obtained from sol-gel synthesis and thermal densification at 1100 °C, was
poled by means of a two-step process consisting of infrared 1064 nm laser irradiation followed by 532 nm laser
exposure in high-voltage static electric field. Maker fringe experiments were then carried out at 1064 nm.
The results show the formation of second-order nonlinearity with macroscopic nonlinear thickness (about 1
mm) and nonlinear susceptibility comparable with thermally poled silica (about 0.1 pm/V). Photoluminescence
measurements suggest that mechanisms for this process should involve the activation and anisotropic ionization of defects at the interface between nanocrystals and glass.
Optically transparent nanostructured SiO2 glassceramics containing a high density of monodispersed, nanometer sized clusters of SnO2 were obtained by phase separation from sol-gel synthesized xerogel. This material was produced either as bulk samples or as thin films on silicon and it can be easily doped with rare earth ions during the sol-gel synthesis. Spectroscopic measurements were carried out on bulk samples, demonstrating an effective energy-transfer between nanoclusters and rare earth ions. A particular sol-gel derived synthesis was employed, producing silica-based films with relatively low electrical resistivity and thickness ranging between 300 and 1000 nm. Suitable sol features allowed thick single-step deposition by spin-coating techniques, assuring the homogeneous nanosized clustering of the semiconducting SnO2 phase. Refractive index and thickness were analyzed in films produced in different conditions of synthesis and thermal treatments of densification. The current-voltage response and the electro-induced optical emission in this material were investigated, suggesting potential technological applications in photonics and UV-emitting devices.
Wide-band-gap semiconductor-doped-glasses were obtained by synthesizing SnO2:SiO2 nanostructured glassceramics. In this binary system, comprising two chemically compatible oxides, crystalline SnO2 nanoclusters were embedded in a pure silica matrix in a controlled way, by setting appropriate thermochemical parameters, up to 10% of volume fraction of the semiconductor crystalline phase. Measurements of third order non-linearity were carried out by means of z-scan technique at 1064 nm finding a non linear refractive index comparable with that of glasses doped with Cd chalcogenides. Optical spectroscopy, micro-Raman scattering and electron microscopy indicated good optical and nano-structural features, suitable for stable optical applications, both in bulk and film samples.
Tin(IV) doped SiO2 xerogel and SnO2/SiO2 glass-ceramics were synthetised by sol-gel route. Synthesis and physical properties of those materials will be disclosed in this paper. Particularly, the solubility behaviour of tin(IV) in SiO2 and the oversaturation condition in glass and xerogel intermediate were investigated. Both materials (glass and glass ceramics) are useful in photonics, and different application will be proposed in this work.
Tin-doped silica glass has been recently investigated as photosensitive optical material for optoelectronic device applications. The mechanisms responsible for the material photosensitivity and the optical activity induced by Sn doping are presented. Studies performed on perform slides and on sol-gel bulk samples show that the refractive index change can be ascribed to structural rearrangements induced by photochemical reactions. Photoluminescence measurements indicate that tin atoms are embedded in Sn-substituted Si sites of the silica network. The modified structure shows extremely high stability, and gratings written in fibers exhibit a negligible erasure in 30 minutes below 600 °C. At high UV radiation fluences the refractive index modulation saturates and does not exhibit any decrease. Optical measurements and electron paramagnetic resonance data show that different processes contribute to the refractive index change. The comparison between samples with and without optical absorption at the UV laser wavelength shows that the presence and the consequent laser-induced bleaching of the 5 eV absorption band due to oxygen deficient centers does not appear crucial for photosensitivity. In fact a refractive index change is also observed in samples without detectable absorption at this energy.
There is considerable interest in Er-doped silica-based materials with photosensitive properties for lasing and photorefractive applications. Tin doping has been recently found to induce strong and stable photosensitivity in silica glass. In this work, Er co-doped Sn-doped silica samples are obtained by sol-gel method by hydrolysis and condensation of TEOS and tin dibutyl diacetate, Er(NO3)3, or Er(CH3COO)3 as dopant precursors. Samples with Er content ranging between 0.1 and 1 mol% are investigated. Sn doping (0.4 mol%) is employed to induce photorefractivity properties in the glassy host. Time resolved photoluminescence, optical absorption and refractive index measurements are carried out and analyzed as a function of the Er content. The specific effects of Er content and Sn doping on the rare earth emission efficiency and photosensitivity of the glass are discussed. In particular it is shown that the Er emission shows the typical features of Er doped sol gel silica and the strong photosensitivity induced by tin doping is not affected by rare earth ions.
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