Lanthanum Borogermanate Glasses Research Articles

Ultrafast-Laser-Induced Tailoring of Crystal-in-Glass Waveguides by Precision Partial Remelting

Space-selective laser-induced crystallization of glass enables direct femtosecond laser writing of crystal-in-glass channel waveguides having nearly single-crystal structure and consisting of functional phases with favorable nonlinear optical or electrooptical properties. They are regarded as promising components for novel integrated optical circuits. However, femtosecond-laser-written continuous crystalline tracks typically have an asymmetric and strongly elongated cross-section, which causes a multimode character of light guiding and substantial coupling losses. Here, we investigated the conditions of partial remelting of laser-written LaBGeO5 crystalline tracks in lanthanum borogermanate glass by the same femtosecond laser beam which had been used for their writing. Exposure to femtosecond laser pulses at 200 kHz repetition rate provided cumulative heating of the sample in the vicinity of the beam waist sufficient to provide space-selective melting of crystalline LaBGeO5. To form a smoother temperature field, the beam waist was moved along the helical or flat sinusoidal path along the track. The sinusoidal path was shown to be favorable for tailoring the improved cross-section of the crystalline lines by partial remelting. At optimized laser processing parameters, most of the track was vitrified, and the residual part of the crystalline cross-section had an aspect ratio of about 1:1. Thermal-induced stress emerging during the tailoring procedure was efficiently eliminated by fine post-annealing. The proposed technique suggests a new way to control the morphology of laser-written crystal-in-glass waveguides by tailoring their cross-section, which is expected to improve the mode structure of the guided light.

Early Stages of Crystallization of Lanthanum-Borogermanate Glass by a Femtosecond Laser Beam

Regions in the bulk of lanthanum borogermanate glass modified by a femtosecond laser beam were studied by means of optical microscopy and confocal Raman scattering (RS) spectroscopy. The obtained data show that, at the stages preceding the formation of an LaBGeO5 microcrystal, upon irradiation by a femtosecond beam in a certain interval of laser pulse energy germanium nanocrystals precipitate in the zone of local heating. The correlation between the conditions of laser irradiation leading to the appearance of germanium nanocrystals and the conditions ensuring with high probability the crystallization of phases of the low-temperature modification of lanthanum borate and stillwellite-like lanthanum borogermanate suggest that germanium nanocrystals can serve as nuclei for heterogeneous crystallization of LaBO3 and LaBGeO3.

Direct Laser Writing of LaBGeO5 Crystal-in-Glass Waveguide Enabling Frequency Conversion

We report on a technique of femtosecond laser-induced crystallization of glass, allowing improvement in the homogeneity of continuous crystalline tracks formed in lanthanum borogermanate glass using a focused beam with the elliptical-shaped waist cross section instead of a conventional circular one. Second harmonic generation produced by near-infrared femtosecond laser pulses was demonstrated in the waveguiding mode for the laser written crystalline track. The structure of the fabricated single crystalline waveguides is investigated with high resolution transmission electron microscopy and micro-Raman spectroscopy.

Specifics of the Crystallization of Lanthanum Borogermanate Glass by a Femtosecond Laser Beam

A method of nucleating a crystalline seed by means of a stationary laser beam with increasing pulse energy, which considerably reduces and stabilizes the formation time of microcrystals, is proposed. A significant improvement in the uniformity of the crystalline structures formed by irradiation with a focused femtosecond elliptic laser beam as compared with an ordinary Gaussian beam is demonstrated. It is demonstrated by means of confocal Raman scattering spectroscopy that the crystal channels obtained consist of LaBGeO5 crystals oriented along the direction of laser scanning.

Nanoparticle generation inside Ag-doped LBG glass by femtosecond laser irradiation

Precipitation of silver nanoparticles (Ag-NPs) can occur in a silver-doped lanthanum borogermanate glass matrix, either under thermal annealing or femtosecond direct laser writing (DLW). Macroscopic transmission and localized µ-fluorescence emission spectra provide typical plasmon resonance behavior of Ag-NPs. Annealing leads to the spatially random homogeneous creation of small NPs at low concentrations, while DLW shows higher and broader plasmon peaks, proving highly concentrated 3D localized structured NPs. DLW at high fluencies shows photo-induced dichroism, potentially giving access to 3D dichroic photonic responses.

Structure of lanthanum-borogermanate glass with stillwellite composition according to vibrational spectroscopy data

Dielectric IR spectroscopy and Raman scattering (RS) spectroscopy have been used to confirm the presence in the structure of lanthanum-borogermanate (LBG) glass of triply coordinated boron atoms which are absent in a LaBGe 5 crystal. The grouping ratio [BO 3 ]/[BO 4 ] is independent of the conditions under which the glass is obtained. It is proposed that amorphous nanosize regions of two types with approximately the same chemical composition but different structure are present in the glass. Regions of the first kind contain predominately quadruply coordinated boron atoms and are the basis for the formation of nuclei of stillwellite crystals, while regions of the second kind are more disordered, and a large fraction of the boron atoms in them lie at centers of oxygen triangles.

Cooperative downconversion and near-infrared luminescence of Tb3+–Yb3+ codoped lanthanum borogermanate glasses

In the present paper, we investigate the near-infrared (NIR) luminescence of Tb3+–Yb3+ codoped lanthanum borogermanate (LBG) glasses under visible and ultraviolet light excitation. The results indicate that NIR quantum cutting occurs through cooperative energy transfer from Tb3+ to Yb3+ ions when only 4f8 levels of Tb3+ ions are excited in the wavelength region of 300–490 nm. The highest quantum efficiency under the excitation 5D4 level of Tb3+ at 484 nm is 146%. Ultraviolet excitation that populates the charge transfer band (CTB) of Yb3+ near 270 nm does not result in quantum cutting as the fast nonradiative decay from CTB to 2F5/2 level dominates. These materials are expected to be used as a converting layer for silicon solar cells to enhance their efficiency by splitting each high-energy photon into two NIR photons.

Cooperative downconversion in Yb^3+–RE^3+ (RE=Tm or Pr) codoped lanthanum borogermanate glasses

We report on cooperative downconversion in Yb(3+)-RE(3+) (RE=Tm or Pr) codoped lanthanum borogermanate glasses (LBG), which are capable of splitting a visible photon absorbed by Tm(3+) or Pr(3+) ions into two near-infrared photons. The results indicate that Pr(3+)-Yb(3+) is a more efficient ion couple than Tm(3+)-Yb(3+) in terms of cooperative downconversion. We have obtained a highest quantum yield of 165% and 138% for Pr(3+)-Yb(3+) and Tm(3+)-Yb(3+) codoped LBG glasses under 468 nm excitation, respectively. However, ultraviolet light excitation to the charge transfer band of Yb(3+) does not result in quantum splitting as rapid relaxation from the charge transfer band to 4f(13) levels of Yb(3+) dominates.

In situ Raman spectroscopy of pressure-induced changes inLaBGeO5 glass: hysteresis and plastic deformation

In situ micro-Raman spectroscopy was performed on lanthanum borogermanate (LBG)glasses, compressed in a diamond anvil cell at ambient temperature. Up to 5.6 GPa thestructural changes are reversible, whereas experiments performed at 10 GPa and higher arecharacterized by hysteresis loops. A noticeable change of evolution of the main Raman band at800 cm−1 has been evidenced around 8 GPa. Indeed, at such a pressure, this Raman band is shifted inthe opposite direction while the pressure is still increasing. This change of slopes may bethe sign of a pressure-induced coordination number change. Upon decompression theRaman shift of this band follows a different path from the one during compression. Whenthe sample is returned to ambient pressure, it shows a shifted and lightly modified Ramanspectrum, suggesting that a new amorphous phase for LBG glass is reached under highpressure and still exists at atmospheric pressure. However, a comparison withLaBGeO5 crystals with the same composition shows that this material has a full elastic behaviour inthe same pressure range.

Lanthanum borogermanate glass-based active dielectrics

Favorable properties of stillwellite-like LaBGeO 5 stimulated many attempts to realize them on an industrial scale. However, the growth of stillwellite single crystals is problematic and, because of strong structural anisotropy, dense stillwellite ceramics were not synthesized to date. Unique properties of crystalline LaBGeO 5 may be realized via bulk or grain-oriented crystallization of lanthanum borogermanate (LBG) glasses. Nucleation, bulk and surface crystallization including grain-oriented crystallization of LBG glasses are briefly reviewed. Dependence of crystallization behavior of LBG glasses on the synthesis conditions was found to be closely linked with alumina impurities that suppress texture forming but improve glass-forming ability. The latter offers an opportunity to obtain LBG-based glasses of optical quality, containing more than 30 mol% rare-earth oxides, for laser and magneto-optical applications. A wide range of active dielectrics in the LBG system covering non-linear optical nano-structures, planar wave guides and textures with excellent pyroelectric activity are expected to be developed.

Anisotropy in extruded lanthanum borogermanate glasses? Structural study by Raman spectroscopy

Polarized Raman spectra of lanthanum borogermanate (LBG) glasses of different compositions, as well as of LBG glasses obtained by the hot-extrusion technique, have been obtained. In addition, the crystallization behaviour of LBG glasses of different compositions has been studied by differential thermal analysis and X-ray diffraction. Both the Raman spectra profile and the crystallization behaviour depend on the lanthanum–boron ratio. Polarized Raman spectra of hot-extruded samples show differences from the unextruded samples. For the first time, structural changes in glasses under hot-extrusion treatment were observed by vibrational spectroscopy.

Non-isothermal crystallization of La2O3·B2O3·2GeO2 glasses

Abstract The non-isothermal crystallization of lanthanum borogermanate glasses, of stoichiometry LaBGeO 5 has been studied by differential thermal analysis. The phase that crystallizes during the DTA runs was identified by X-ray diffraction as the stillwellite-like LaBGeO 5 . In the glasses studied, crystallization from surface nuclei was found to be dominant. The activation energy of crystal growth, evaluated from DTA curves, was about 465 kJ mol −1 .

Stillwellite glass-ceramics with ferroelectric properties

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.