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.

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.

Defect-assisted photocatalytic activity of glass-embedded gallium oxide nanocrystals.

The use of glassceramics in photocatalysis is an attractive option for the realization of smart optical fibers and self-cleaning windows. Here we present the photocatalytic activity of germanosilicate glasses embedding Ga2O3 nanocrystals prepared by batch melting and glass heat treatment. The powdered material is used for UV-assisted degradation of rhodamine in water. The kinetics show changes after repeated experiments. In the first cycle, the apparent rate is governed by a second-order reaction with a Gaussian-like shape, whereas the second cycle follows a first-order reaction. The modification appears to be correlated with perturbations in the defect population. Photoluminescence has been used to monitor the evolution of such defects. Kinetic data on photoreactions and defect formation have been modelled in a combined frame in which the defect concentration determines the photocatalytic activity. The results prove the photocatalytic ability of the studied glassceramics. Moreover, the general validity of the kinetic model can be of interest for other systems in which the photocatalytic response depends on photoreactive species concentration.

Multilevel data writing in nanoporous glass by a few femtosecond laser pulses.

Multidimensional data recording inside nanoporous high-silica glass by a femtosecond laser beam has been investigated. It is shown that three femtosecond laser pulses at pulse repetition rates up to 1 MHz are sufficient for recording 3 bits of information inside nanoporous glass, which is an order of magnitude lower than the number of pulses required for data writing in silica glass and provides a corresponding gain in the data writing speed. Multilayer data recording and reading were demonstrated providing the storage density corresponding to the capacity of 25 GB in the optical compact disc form factor. An outstanding thermal stability of the proposed optical data storage is confirmed by the 24 h long heat treatment at 700°C, which could not damage the recorded data.

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.

Donor-Acceptor Control in Grown-in-Glass Gallium Oxide Nanocrystals by Crystallization-driven Heterovalent Doping.

Incorporation of doping ions in nanocrystals is a strategy for providing nanophases with functions directly related to ion features. At the nanoscale, however, doping can also activate more complex effects mediated by perturbation of the nanophase size and structure. Here, we report a paradigmatic case in which we modify grown-in-glass γ-Ga2 O3 nanophases by nickel or titanium doping of the starting glass, so as to control the concentration of oxygen and gallium vacancies responsible for the light emission. Optical absorption and luminescence show that Ni2+ and Ti4+ ions enter into the nanophase, but differential scanning calorimetry and X-ray diffraction indicate that Ni and Ti also work as modifiers of nanocrystal growth. As a result, doping influences nanocrystal size and concentration, which in turn dictate the number of donors and acceptors per nanocrystal. Finally, the chain of effects turns out to control both the intensity and spectral distribution of the light emission.

Light-emitting Ga-oxide nanocrystals in glass: a new paradigm for low-cost and robust UV-to-visible solar-blind converters and UV emitters.

Wide-bandgap nanocrystals are an inexhaustible source of tuneable functions potentially addressing most of the demand for new light emitting systems. However, the implementation of nanocrystal properties in real devices is not straightforward if a robust and stable optical component is required as a final result. The achievement of efficient light emission from dense dispersions of Ga-oxide nanocrystals in UV-grade glass can be a breakthrough in this regard. Such a result would permit the fabrication of low cost UV-to-visible converters for monitoring UV-emitting events on a large-scale - from invisible hydrogen flames to corona dispersions. From this perspective, γ-Ga2O3 nanocrystals are developed by phase separation in Ga-alkali-germanosilicate glasses, obtaining optical materials based on a UV transparent matrix. Band-to-band UV-excitation of light emission from donor-acceptor pair (DAP) recombination is investigated for the first time in embedded γ-Ga2O3. The analysis of the decay kinetics gives unprecedented evidence that nanosized confinement of DAP recombination can force a nanophase to the efficient response of exactly balanced DAPs. The results, including a proof of concept of UV-to-visible viewer, definitely demonstrate the feasibility of workable glass-based fully inorganic nanostructured materials with emission properties borrowed from Ga2O3 single-crystals and tailored by the nanocrystal size.

Spatially selective Au nanoparticle growth in laser-quality glass controlled by UV-induced phosphate-chain cross-linkage.

Herein we describe how UV excitation of localized electronic states in phosphate glasses can activate structural rearrangements that influence the kinetics of Au nanoparticle (NP) thermal growth in Au-doped glass. The results suggest a novel strategy to address the problem of controlling nano-assembly processes of metal NP patterns in fully inorganic and chemically stable hard materials, such as laser-quality glasses. We show that the mechanism is promoted by opening and subsequent cross-linkage of phosphate chains under UV excitation of non-bridging groups in the amorphous network of the glass, with a consequent modification of Au diffusion and metal NP growth. Importantly, the micro-Raman mapping of the UV-induced modifications demonstrates that the process is restricted within the beam waist region of the focused UV laser beam. This fact is consistent with the need for more than one excitation event, close in time and in space, in order to promote structural cross-linkage and Au diffusion confinement. The stability of the photo-induced modifications makes it possible to design new metal patterning approaches for the fabrication of three-dimensional metal structures in laser-quality materials for high-power nonlinear applications.

Native amorphous nanoheterogeneity in gallium germanosilicates as a tool for driving Ga2O3 nanocrystal formation in glass for optical devices.

Nanoparticles in amorphous oxides are a powerful tool for embedding a wide range of functions in optical glasses, which are still the best solutions in several applications in the ever growing field of photonics. However, the control of the nanoparticle size inside the host material is often a challenging task, even more challenging when detrimental effects on light transmittance have to be avoided. Here we show how the process of phase separation and subsequent nanocrystallization of a Ga-oxide phase can be controlled in germanosilicates - prototypal systems in optical telecommunications - starting from a Ga-modified glass composition designed to favour uniform liquid-liquid phase separation in the melt. Small angle neutron scattering data demonstrate that nanosized structuring occurs in the amorphous as-quenched glass and gives rise to initially smaller nanoparticles, by heating, as in a secondary phase separation. By further heating, the nanophase evolves with an increase of nanoparticle gyration radius, from a few nm to a saturation value of about 10 nm, through an initial growing process followed by an Ostwald ripening mechanism. Nanoparticles finally crystallize, as indicated by transmission electron microscopy and X-ray diffraction, as γ-Ga(2)O(3)- a metastable gallium oxide polymorph. Infrared reflectance and photoluminescence, together with the optical absorption of Ni ions used as a probe, give an indication of the underlying interrelated processes of the structural change in the glass and in the segregated phase. As a result, our data give for the first time a rationale for designing Ga-modified germanosilicates at the nanoscale, with the perspective of a detailed nanostructuring control.

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.