Abnormal elemental redistribution in silicate glasses irradiated by ultrafast laser
Introduction
In recent years, high repetition rate femtosecond (fs) laser induced element redistribution in glasses has become an advanced research hotspot since this technique can control glass composition three-dimensionally [1], [2], [3]. The composition of glass is an important parameter that affects many glass properties such as luminescence, absorption, refractive index, crystallization temperature and other physical and chemical properties. Some interesting results of this phenomenon as well as potential applications have been reported by various research groups [3], [4]. Temperature gradients caused by thermo-diffusion and diffusion coefficient of various elements are considered as two key factors for the formation mechanism of element redistribution [5]. According to previous observation, the elemental migration in silicate glasses can be summarized as follows: the relative concentrations of the glass network former ions, e.g. Si4+, are higher in the central area of the laser focal point, while the glass network modifier ions, e.g. Ca2+, migrate to the outside to form ring-shaped regions. While in tellurite glass, the relative glass composition distribution remain almost unchanged compared with the un-irradiated area [6]. The extraordinary glass network structure, consist of small structure units, rather than large multi-membered structure, is responsible for this counterexample. Except for common circular patterns which are due to the circular temperature distribution, shape-controlled of element distribution can also be formed flexibly by simultaneous irradiation at multiple spots inside a glass using a spatial light modulator [7].
LiNbO3 single crystal is one of the most important nonlinear optical materials because of its excellent physical properties and wide transparency. It has been used in various devices such as phase modulator waveguides and surface acoustic devices [8]. So it is attractive to precipitate orientation nonlinear crystal LiNbO3 in glass. Size-controllable LiNbO3 crystals have been space-selectively precipitated in glass using fs laser irradiation [9]. However, the research of elemental migration is still vacant and needs investigation. In this letter, we not only investigate the element distribution on the cross section of laser lines, but also along the laser writing direction. Unlike related results obtained by Yoshinori et al. [10], there is no apparent composition variation for both glass network former and modifier ions in the laser irradiated region. It may be interpreted by a balance between the thermo-migration induced by temperature gradient and elemental diffusion for nucleation and crystal growth.
Section snippets
Experimental
A glass with the composition of 33Li2O-33Nb2O5-34SiO2 (mol. %) was chosen for the study. Reagent-grade Li2CO3, Nb2O5, and SiO2 were used as starting materials. A well-mixed batch was melted in a Pt crucible at 1430 °C for approximately 2 h. Then the obtained melt was poured onto a steel plate heat-treated at 500 °C, and then quickly transferred to another electric furnace preset at 540 °C cooling down to room temperature. The transparent glass was annealed to release the stress at the
Results and discussion
On the cross-section: After irradiation, the sample was firstly cut along the plane (YZ-plane) perpendicular to the written lines, polished to optical quality and etched in 2% hydrogen fluoride (HF) acid lasting 30 s. Fig. 1 shows a series of scanning-electron microscope (SEM) images on the cross section of laser tracks and corresponding element distributions. The elliptical shaped laser traces were observed due to a balance between the nonlinear increase of refractive index and the defocusing
Conclusions
In summary, we systematically studied the element distribution in silicate glass by high repetition rate fs laser irradiation. Chemical analysis has taken on the cross section of laser tracks and also along the laser writing direction. EDX, WDS and nuclear microprobe results implied that no elemental migration occurred after fs laser irradiation. A balance between the thermo-migration under high temperature gradient and the elemental diffusion for nucleation and crystal growth is considered as
Acknowledgements
This research work was financially supported by the National Natural Science Foundation of China (51572202) and the Nanotechnology Program of Suzhou (ZXG201438), the frame of FLAG (Femtosecond Laser Application in Glasses) consortium project with the support of several organisations: the Agence Nationale pour la Recherche (ANR-09-BLAN-0172-01), the RTRA Triangle de la Physique (2008-056T)and the FP7-PEOPLE-IIF FemtoNano 908582.
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