Ultrafast Laser-Induced Crystallization of Lead Germanate Glass

Laser-induced space-selective crystallization of glass enabling the growth of continuous crystal-in-glass architectures consisting of non-centrosymmetric phases with functional properties is promising, including single-crystal waveguides for the development of integrated optical circuits. In this study, femtosecond laser direct writing of crystalline lines inside lead germanate glass with the composition close to Pb5Ge3O11 has been demonstrated. The growth of crystalline lines by the .moving focused laser beam required the preliminary growth of a seed crystal by the fixed beam. Confocal Raman spectroscopy revealed the precipitation of ferroelectric Pb5Ge3O11, which, under certain exposure conditions, could be accompanied by precipitation of the metastable lead germanate phase. Depending on the laser beam parameters, either bilateral growth providing split, horseshoe-shaped morphology of the crystal cross-section, or centered growth resulting in elongated, elliptical cross-section shape occurred. The obtained results are of interest for the fabrication of ferroelectric Pb5Ge3O11-based crystal-in-glass waveguides

Effect of Pulse Repetition Rate on Ultrafast Laser-Induced Modification of Sodium Germanate Glass

We report an unexpected pulse repetition rate effect on ultrafast-laser modification of sodium germanate glass with the composition 22Na2O 78GeO2. While at a lower pulse repetition rate (~≤250 kHz), the inscription of nanogratings possessing form birefringence is observed under series of 105–106 pulses, a higher pulse repetition rate launches peripheral microcrystallization with precipitation of the Na2Ge4O9 phase around the laser-exposed area due to the thermal effect of femtosecond pulses via cumulative heating. Depending on the pulse energy, the repetition rate ranges corresponding to nanograting formation and microcrystallization can overlap or be separated from each other. Regardless of crystallization, the unusual growth of optical retardance in the nanogratings with the pulse repetition rate starting from a certain threshold has been revealed instead of a gradual decrease in retardance with the pulse repetition rate earlier reported for some other glasses. The repetition rate threshold of the retardance growth is shown to be inversely related to the pulse energy and to vary from ~70 to 200 kHz in the studied energy range. This effect can be presumably assigned to the chemical composition shift due to the thermal diffusion of sodium cations occurring at higher pulse repetition rates when the thermal effect of the ultrashort laser pulses becomes noticeable

Influence of alumina addition on crystallization and texturing behavior of LaBGeO5 glass

The structure and crystn. behavior of 25La2O3. 25B2O3. 50GeO2 glasses, melted in platinum (P glass) and corundum (A glass) crucibles, were studied by DTA, X-ray diffraction and FTIR spectroscopy. The Al2O3 dissolved from corundum crucible in the A glass was estd. to be in the range 5-7 wt%. This alumina content had almost no influence on glass transition temp. but strongly affected the structure and crystn. behavior of the A glass. In fact, the P glass showed good texture-forming ability: high quality textured glass-ceramic plates based on stillwellite-like LaBGeO5 crystals were easily obtained. On the contrary, the presence of alumina stabilized the A glass from which binary phases crystallize first, and only afterwards they are transformed in stillwellite by secondary crystn.: so in this glass texturing is hindered. Crystn. and texturing behavior of P and A glasses were well related to FTIR data. P glass contained both threefold and fourfold coordinated boron while in the A glass the presence of aluminum forced boron to assume almost exclusively threefold co-ordination. Hence the easier crystn. of stillwellite phase and the good textures obtained from the P glass in comparison to the A glass, can be well understood since all boron atoms have tetrahedral co-ordination in stillwellite LaBGeO5 crystal

Lanthanum borogermanate glass-based active dielectrics

Favorable properties of stillwellite-like LaBGeO5 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 cryst. LaBGeO5 may be realized via bulk or grain-oriented crystn. of lanthanum borogermanate (LBG) glasses. Nucleation, bulk and surface crystn. including grain-oriented crystn. of LBG glasses are briefly reviewed. Dependence of crystn. 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, contg. more than 30 mol% rare-earth oxides, for laser and magnetooptical applications. A wide range of active dielecs. in the LBG system covering nonlinear optical nanostructures, planar wave guides and textures with excellent pyroelec. activity are expected to be developed

DC poled lanthanum boron germanate glasses: Raman study of the poling mechanism

The effect of dc poling on lanthanum boron germanate glasses has been investigated by means of Raman spectroscopy and birefringence experiments. The results show that the dc poling induces in the glass a breakage of its macroscopic centre of inversion and this effect extends to the whole bulk of the poled glasses

Formation of crystalline dots and lines in lanthanum borogermanate glass by the low pulse repetition rate femtosecond laser

Femtosecond lasers have become a powerful tool for 3D space-selective crystallization of glasses. A laser-induced cumulative heating effect required for crystal growth is usually considered to take place only at pulse repetition rate over 100 or 200 kHz and 200 kHz is known as the lowest repetition rate at which femtosecond laser-induced crystallization has been reported so far. We for the first time demonstrate precipitation of LaBGeO5 crystals in lanthanum borogermanate glass using a femtosecond laser emitting 1030 nm, 300 fs, 110 µJ pulses with adjustable repetition rate below 100 kHz. For the applied laser, minimal repetition rate enabling nucleation of ferroelectric LaBGeO5 crystals inside the glass was shown to be 9 kHz at maximal pulse energy of 110 µJ and growth of a crystalline line from the formed seed crystal was obtained starting from 8 kHz though smooth homogeneous oriented line which might be regarded as quasi-single-crystalline could be grown at 25 kHz or higher and corresponding pulse energy of 18 µJ. Thus, the pulse repetition rate sufficient for a cumulative heating effect and a stable crystal growth was reduced by an order of magnitude as compared to earlier publications due to relatively high pulse energy. Possibility and efficiency of cumulative heating and crystal growth and average time required for forming the seed crystal have been studied for various combinations of the pulse energy and the repetition rate. Obtained crystalline features have been studied by micro-Raman spectroscopy and Raman mapping which confirmed growth of stillwellite-like LaBGeO5 phase and orientation of its polar axis along the direction of the crystalline line

Low-frequency dynamics of glasses and crystals of similar compositions

Wide range (10–1500 cm-1) dielectric and Raman spectra of crystals and glasses of similar compositions were analysed with the use of both literature (a-quartz, cristobalite, LaBGeO5, Pb5Ge3O11, As2Se3, etc.) and original (Li2Ge7O15, LiBaPO4) data. For all structural types (island, chain, layer or frame like), the boson peak (BP)in the glass spectrum (GS) corresponds to one or several bands in the low frequency (LF) spectrum of the corresponding crystal. In this sense, the LF range (<200 cm-1) is not a specific feature of GS as compared to higher frequency range of stretching and bending modes, where the crystal spectral bands are widened due to disordering and/or vitrification up to their full conversion into a diffuse GS. Therefore the origin of LF bands in GS can be identified taking into account the nature of corresponding bonds in the crystal spectrum by analogy with well known procedure of interpretation of stretching and bending vibrations in GS. The approach proposed is exemplified by different systems. Lead germanate glasses reveal several highly overlapping LF peaks, which originate from optical vibrations of lead atoms by analogy with Pb5Ge3O11 crystal spectrum. The Li2Ge7O15 glass and crystal LF peaks correspond to vibrations of oxygen in layer fragments of the structure. Cristobalite-like structure of silica glass is proposed on the assumption of coincidence of peak frequencies in their Raman spectra at ~50 cm-1, whereas a-quartz has no vibrational modes below 100 cm-1. Thus, the approach proposed allows one to refine and concretise the universal nature of the BP, referring to the short and medium range structure and the vibrational properties of the corresponding crystal

Space-selective modification of Au-doped optical grade glass by the femtosecond laser beam

Femtosecond laser-induced modification of Au-doped phosphate glass at different pulse repetition rates using an Yb femtosecond amplifier emitting pulses of up to 120 μJ energy at 1030 nm with adjustable repetition rate up to 100 kHz is presented. At the repetition rate as low as 1 kHz, only refractive index modification and formation of red color centers are observed. Increasing the average output laser power resulted in increase of color intensity, but this color could be easily erased by the heat treatment at 300ºC for 20 min indicating athermal nature of these laser-induced effects. By contrast, at the repetition rate from 25 kHz to 100 kHz, cumulative heating effect was demonstrated and allowed to realize for the first time one-stage laser-induced growth of Au nanoparticles in the solid glass accompanied by the appearance of a thermally stable red color. Z-scan analysis of nonlinear properties of the studied glass samples is carried out. Details of Au nanoparticles growth and phosphate glass modification by the tightly focused femtosecond laser beam are discussed