V. N. Sigaev

Amorphous nanostructuring in potassium niobium silicate glasses by SANS and SHG: a new mechanism for second-order optical non-linearity of glasses

Abstract Potassium niobium silicate (KNS) glasses xK2OxNb2O5(1−2x)SiO2 with x=0.167; 0.182; 0.200; 0.220 and 0.250 have been subjected to prolonged heat treatments in a wide temperature range above Tg. As a result, glasses exhibiting liquid-type phase separation phenomena have been isolated. Moreover for each glass composition, the temperature zones have been determined to produce transparent, opalescent or opaque materials which have been studied by X-ray diffraction (XRD), small-angle neutron scattering (SANS) and second harmonic generation (SHG) techniques. SANS data unambiguously point at nanostructuring of KNS glasses in the scale of 5–20 nm under appropriate heat treatments near Tg. In contrast to initial KNS glasses, nanostructured glasses exhibit SHG activity. For earliest stages of phase separation SHG-active glasses are characterized by fully amorphous XRD patterns. Further development of phase separation in glasses with increasing of their opalescence leads to diminishing SHG, and subsequently partial crystallization takes place giving opaque materials. Since relative maximum of SHG efficiency corresponds to non-crystalline nanostructured glasses, such new transparent second-order non-linear media may be of both scientific and practical interest. With regard to non-crystalline structure of nano-inhomogeneities, SHG mechanism in the glasses is supposed to be due to a combination of third-order non-linearity with a spatial modulation of linear polarizability.

KTiOPO4 precipitation from potassium titanium phosphate glasses, producing second harmonic generation

Abstract Glass formation in the K 2 O–TiO 2 –P 2 O 5 –SiO 2 system, and the structure and crystallization behavior of glasses having compositions near the stoichiometry of KTiOPO 4 (KTP) have been examined by differential thermal analysis (DTA), X-ray diffraction (XRD), fast Fourier transform infrared (FTIR) spectroscopy and second harmonic generation (SHG). Pure glasses may be obtained adding an appropriate amount of P 2 O 5 as well as of SiO 2 to the KTP composition. Transparent and homogeneous glasses were synthesized for the 56(K 2 O·2TiO 2 ·P 2 O 5 )·44P 2 O 5 (KTP–44P) and the 90(K 2 O·2TiO 2 ·P 2 O 5 )·10SiO 2 (KTP–10Si) molar compositions. In contrast, for the 80(K 2 O·2TiO 2 ·P 2 O 5 )·20SiO 2 (KTP–20Si) and for higher SiO 2 contents, the obtained glasses were partially opaque and phase separated. KTP glasses with lower P 2 O 5 content cannot be obtained as they crystallize during quenching. The KTP–10Si glass shows the only exothermic peak connected to KTP phase crystallization whereas the KTP–44P and the KTP–20Si glasses devitrify in two steps. Firstly the same unidentified phase is formed in both these glasses and the KTP phase is formed only at higher temperatures. Transparent and opalescent KTP glasses exhibiting SHG activity may be produced by careful heat treatments at temperatures just above T g . The origin of SHG in transparent glasses is supposed to be connected with either precipitation of KTiOPO 4 nanocrystallites or very early stages of liquid-type phase separation.

Grain-oriented surface crystallization of lanthanum borosilicate and lanthanum borogermanate glasses

Abstract Under appropriate conditions (high temperature gradient, rapid heating and following prolonged isothermal treatment, suppression of bulk crystallization, polished glass surface, etc.) the La 2 O 3 -B 2 O 3 - X O 2 ( X = Si, Ge) systems glasses are crystallized in preference on the surface with formation of perfect textures based on needle-shaped stillwellite-like crystals of the LaBXO 5 composition. Lanthanum borogermanate (LBG) glasses exhibit more intense texture-forming ability than lanthanum borosilicate (LBS) ones. Glass ceramic textures with a degree of grain orientation up to 0.9 in the LBG system are obtained. The pyroelectric coefficient values of the best of synthesized LBG textures are as high as 0.5–1.5 nC cm −2 K −1 in a wide temperature range. A poor crystallization ability of the LBS glasses at temperatures less than 1000°C supposed to be used in preparing both thin polar crystalline films on the glass surface for nonlinear optical applications and ferroelectric (pyroelectric) glass ceramics by the powder technology.

Stillwellite glass-ceramics with ferroelectric properties

Abstract The process of crystallization of LaBGeO 5 and PrBGeO 5 phases with stillwellite structure from the glassy state is studied. Exotherms on differential thermal analysis curves for lanthanum borogermanate glasses are interpretated by using the X-ray diffraction and the second harmonic generation method data. It is shown that acentric nuclei of the stillwellite phase are formed at the early stages of crystallization. There is not any metastable phase, prior to stillwellite precipitation. Glass-ceramic samples with high stillwellite content were synthesized, and their properties were measured and compared with the properties of a single crystal of LaBGeO 5 .

Nickel-assisted growth and selective doping of spinel-like gallium oxide nanocrystals in germano-silicate glasses for infrared broadband light emission

The target of taking advantage of the near-infrared light-emission properties of nickel ions in crystals for the design of novel broadband optical amplifiers requires the identification of suitable nanostructured glasses able to embed Ni-doped nanocrystals and to preserve the workability of a glass. Here we show that Ni doping of Li(2)O-Na(2)O-Ga(2)O(3)-GeO(2)-SiO(2) glass (with composition 7.5:2.5:20:35:35 and melting temperature 1480 °C, sensibly lower than in Ge-free silicates) enables the selective embedding of nickel ions in thermally grown nanocrystals of spinel-like gallium oxide. The analysis of transmission electron microscopy and x-ray diffraction data as a function of Ni-content (from 0.01 to 1 mol%) indicates that Ni ions promote the nanophase crystallization without affecting nanoparticle size (~6 nm) and concentration (~4 × 10(18) cm(-3)). Importantly, as shown by optical absorption spectra, all nickel ions enter into the nanophase, with a number of ions per nanocrystal that depends on the nanocrystal concentration and ranges from 1 to 10(2). Photoluminescence data indicate that fast non-radiative decay processes become relevant only at mean ion-ion distances shorter than 1.4 nm, which enables the incorporation of a few Ni ions per nanoparticle without too large a worsening of the light-emission efficiency. Indeed, at 0.1 mol% nickel, the room temperature quantum yield is 9%, with an effective bandwidth of 320 nm.

Low-frequency band at 50 cm-1 in the Raman spectrum of cristobalite : identification of similar structural motifs in glasses and crystals of similar composition

Abstract Raman spectra of high-temperature α-cristobalite ceramics and silica glass have been obtained in a wide frequency range from 10 to 1500 cm −1 . A low-frequency maximum at 50 cm −1 in the spectrum of cristobalite has been observed to be similar to the well-known band at 52 cm −1 in the Raman spectra of silica glasses. The combination of literature reports on inelastic neutron scattering, low-temperature heat capacity, X-ray and neutron scattering in the region of the first sharp diffraction peak (FSDP) and our data on Raman and far IR spectroscopies indicate a cristobalite-like medium-range order in silica glasses. The comparison of the obtained spectra with the Raman spectrum of α-quartz, as well as the comparison of far IR spectra of some multicomponent glasses with the ones of corresponding crystals, demonstrates that by means of low-frequency spectroscopic data it is possible to identify the crystalline polymorphs reflected by the glass.

Local crystallization of glasses in the La2O3-B2O3-GeO2 system under laser irradiation

A method is proposed for local crystallization of glasses under laser irradiation. This method makes it possible to nucleate and grow microcrystals with a size distribution similar to a monodisperse distribution for several fractions of a second in any glass region chosen in advance. It is demonstrated using glasses in the La2O3-B2O3-GeO2 system as an example that the crystallization of the stillwellite-like phase LaBGeO5 with the composition close to the composition of the initial glass is observed in the glass under irradiation with the copper vapor laser operating in the high-speed pulse modulation mode. Strips (up to ∼300 μm) produced at a specified depth from the glass surface contain extended regions consisting of uniformly distributed crystals, which have almost identical sizes, exhibit a pronounced faceting, and are identified using X-ray diffraction. The size and the number of crystals can be changed over a wide range by varying laser treatment conditions. This opens up the way to the design of new glass-ceramic materials in which the location of the crystalline phase in the glass bulk is controlled by a developer.

Direct writing of birefringent elements by ultrafast laser nanostructuring in multicomponent glass

Self-assembled nanostructures created by femtosecond laserirradiation are demonstrated in alkali-free aluminoborosilicate glass. The growth of the induced retardance associated with the nanograting formation is three orders of magnitude slower than in silicaglass and is observed only within a narrow range of pulse energies. However, the strength of retardance asymptotically approaches the value typically measured in pure silicaglass, which is attractive for practical applications. A similar intensity threshold for nanograting formation of about 1 TW/cm2 is observed for all glasses studied. The radially polarized vortex beam micro-converter designed as a space-variant quarter-wave retarder for the near-infrared spectral range is imprinted in commercial Schott AF32 glass.

Surface and bulk stillwellite textures in glasses of the La2O3–B2O3–GeO2 system

Abstract Both surface and bulk textures penetrating the whole volume of the glass may be formed in glasses of the La 2 O 3 –B 2 O 3 –GeO 2 system near the stoichiometry of LaBGeO 5 stillwellite. These textures cause non-linear optic, ferroelectric and pyroelectric properties of crystallized glasses. The present work represents SEM study of surface and bulk grain-oriented crystallization of stillwellite glasses for different conditions of synthesis of both planar surface (non-linear optic) and bulk (ferro/pyroelectric) textures based on oriented needle-shaped crystals of LaBGeO 5 stillwellite.

Vibrational spectroscopy of LaBSiO5 glass and glass–crystal composites

Abstract Infrared (IR) reflectivity and Raman scattering spectra of LaBSiO 5 glass and glass–crystal composites were studied in the temperature range 25–260 °C. Using an analogy with the LaBGeO 5 crystal it was possible to assign the main spectral features of the LaBSiO 5 glass and glass–crystal composites. The BO 4 chain arrangement and the bending vibrations of SiO 4 are influenced by the loss of the long-range order in the glass whereas the stretching vibrations of the SiO 4 groups are practically unaffected. The structural disorder in LaBSiO 5 crystallites is caused by rotation of BO 4 tetrahedra.