N. V. Golubev

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.

Non-aqueous sol–gel synthesis of hybrid rare-earth-doped γ-Ga2O3 nanoparticles with multiple organic–inorganic-ionic light-emission features

We present a novel strategy for the synthesis of pure and Eu-doped γ-Ga2O3 nanoparticles with an in situ organic capping resulting from a non-aqueous solution-based benzyl alcohol synthesis route. Photoluminescence spectroscopy highlights the concomitant benzoate-related and γ-Ga2O3 exciton-like Eu3+ excitations in the UV, and a blue emission superimposed onto γ-Ga2O3 donor–acceptor recombination, ascribable to organic moieties different from benzoate.

Broadband infrared light-emitting patterns in optical glass by laser-induced nanostructuring of NiO-doped alkali-gallium germanosilicates.

In this Letter, we show functionalization of NiO-doped 7.5Li(2)O·2.5Na(2)O·20Ga(2)O(3)·35SiO(2)·35GeO(2) glass by space-selective nanocrystallization via exposure to the focused beam of a pulsed copper vapor laser (510.6 and 578.2 nm) at temperature close to the glass transition point (570°C). Irradiated areas drastically change their color, caused by electronic transitions of Ni(2+) dopant ions, without any alteration of the optical quality. Importantly, irradiated regions acquire broadband infrared luminescence (centered at about 1400 nm and possessing 400 nm effective bandwidth) typical of Ni(2+) ions in crystalline environment, and by positive change of refractive index (more than 10(-3)). Spectroscopic and diffractometric data of the irradiated regions indeed resemble those previously observed in thermally nanocrystallized glass, with Ni(2+) ions embedded in γ-Ga(2)O(3) nanocrystals. The results demonstrate the possibility of laser writing nanocrystallized multifunction patterns in germanosilicate glasses for the fabrication of active integrated devices.

Microfluorescence Analysis of Nanostructuring Inhomogeneity in Optical Fibers with Embedded Gallium Oxide Nanocrystals

A spectroscopic protocol is proposed to implement confocal microfluorescence imaging to the analysis of microinhomogeneity in the nanocrystallization of the core of fibers belonging to a new kind of broadband fiber amplifier based on glass with embedded nanocrystals. Nanocrystallization, crucial for achieving an adequate light emission efficiency of transition metal ions in these materials, has to be as homogeneous as possible in the fiber to assure optical amplification. This requirement calls for a sensitive method for monitoring nanostructuring in oxide glasses. Here we show that mapping microfluorescence excited at 633 nm by a He-Ne laser may give a useful tool in this regard, thanks to quasi-resonant excitation of coordination defects typical of germanosilicate materials, such as nonbridging oxygens and charged Ge-O-Ge sites, whose fluorescence are shown to undergo spectral modifications when nanocrystals form into the glass. The method has been positively checked on prototypes of optical fibers--preventively characterized by means of scanning electron microscopy and energy dispersive spectroscopy--fabricated from preforms of Ni-doped Li₂O-Na₂O-Sb₂O₃-Ga₂O₃-GeO₂-SiO₂ glass in silica cladding and subjected to heat treatment to activate gallium oxide nanocrystal growth. The method indeed enables not only the mapping of the crystallization degree but also the identification of drawing-induced defects in the fiber cladding.

On the nature of the second-order optical nonlinearity of nanoinhomogeneous glasses in the Li2O-Nb2O5-SiO2 system

The submicroscopic structure of lithium niobium silicate glasses of the compositions 2xLiNbO3 · (1 − x)(Li2O · 2SiO2) (x = 0.40, 0.45, 0.50) and 30Li2O · 25Nb2O5 · 45SiO2 in the initial state and after heat treatment for different times at temperatures in the vicinity of the glass transition point Tg are investigated using X-ray powder diffraction, small-angle neutron scattering (SANS), synchrotron small-angle X-ray scattering (SAXS), and electron microscopy. A nanostructure with inhomogeneities ∼40 Å in size is formed in glasses at the initial stages of phase separation at temperatures in the range 600–670°C. This structure is responsible for the appearance of the second-order optical nonlinearity. The SANS, SAXS, and electron microscopic data on the inhomogeneity size are in good agreement with each other. According to the X-ray diffraction, SANS, and SAXS data, the ordering of the glass structure and the difference between the density of inhomogeneities and the density of the matrix increase in the course of heat treatment. At the initial stage of amorphous phase separation, the glass decomposes into regions enriched in SiO2 and regions with an increased content of lithium and niobium. An increase in the temperature or time of heat treatment results in the precipitation of LiNbO3 ferroelectric crystals. The results obtained allow us, for the first time, to make the inference that nanoscale changes in the glass structure lead to considerable changes (by one order of magnitude and more) in the quadratic optical nonlinearity, which can be controlled by heat treatment. The origin of the second-order optical nonlinearity is associated with both the nanosized modulations of the polarizability due to the inhomogeneous glass structure and the polarity of structural nanoinhomogeneities from which the LiNbO3 phase precipitates at the later stages of phase separation.

Rearrangement of optical centers and stimulated radiation of Eu3+ in polycrystalline huntite under optical and electron-beam excitation

Polycrystalline europium huntite EuAl3(BO3)4 has been prepared by solid-phase synthesis. The spectral and kinetic characteristics of its luminescence under the excitation by a xenon lamp, single laser pulse, and electron beam have been studied. It has been established that the laser excitation of the polycrystalline samples in the 7F0 →5L6 transition of Eu3+ ions with the power density P ≥ 5 × 107 W/cm2 leads to the structure rearrangement of the optical centers, which is accompanied by an increase in the probability of the radiation transitions of the activator. The stimulated radiation of the main type of Eu3+ centers in the 5D0→7F1, 7F2, and 7F4 transitions has been obtained under the excitation by the electron beam with an energy of 200 keV and a duration of 2 ns.

Nickel-doped gallium-containing glasses luminescent in the near-infrared spectral range

The heat treatment of glasses synthesized in the M2O-Ga2O3-GeO2-SiO2 (M = Li, Na, K) system results in bulk precipitation of Ga2O3 and LiGa5O8 nanocrystals and, consequently, in luminescence within the bands with maxima at wavelengths of 1500 and 1300 nm, respectively, with the half-width exceeding 300 nm. In their spectral luminescence properties, the gallium germanium silicate glasses synthesized are similar to the gallium silicate glasses. However, the melting temperature of the former glasses is approximately 100°C lower, which considerably facilitates the preparation of glasses of high optical quality.

Formation of Luminescent and Birefringent Microregions in Phosphate Glass Containing Silver

It is shown for glass with the composition (molar content) 40P2O1, 55ZnO, 4Ag2O, and 1Ga2O3 that a femtosecond laser beam can form simultaneously in glass luminescent nanoparticles of silver and micron-size birefringent regions, whose properties depend on the polarization and intensity of the laser beam.

Crystallization and luminescence properties of (SmxY1 – x)2O3–Al2O3–B2O3 glass

Glass and products of solid-phase synthesis in the form of samarium-activated single-phase powders with huntite structure were obtained in the system Y2O3–Al2O3–B2O3. It was shown that the glasses studied crystallize with predominately aluminum borate Al4B2O9 precipitating in the bulk and the solid solution (Y, Sm)Al3(BO3)4 on the surface of the sample. When the crystalline powders (SmxY1 – x)Al3(BO3)4 transitioned into an amorphous state leveling of the Stark structure and broadening of the luminescence bands of Sm3+ was observed, but the luminescence branching ratios changed very little. It was found that lowering the synthesis temperature of such glasses from 1450 to 1250°C increases the BO4 fraction and quantum yield of luminescence with low activator concentration.

Augmented excitation cross section of gadolinium ions in nanostructured glasses

In this Letter, we present detailed absorption and emission data on nanostructured germanosilicate glasses and glass ceramics containing Ga2O3 nanophases and doped with Gd ions. The results show that these systems are suitable hosts for the enhancement of the excitation cross section of rare earth ions via energy transfer from the gallium oxide nanophase with a related quantum yield of 21%. The role of matrix composition and nanostructure morphology on the Gd emission is discussed.