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Localized modification in strontium barium niobate glass doped with Ho3+ under laser irradiation has been carried out. The preliminary samples of this study have been fabricated by the melt quenching method and doped with 2.5% mol of Ho3+. A 3.5W cw multiline Ar-laser has been focused and shifted in a line during laser irradiation. The formation of Strontium Barium Niobate nanocrystals has been confirmed by X-ray diffraction, atomic force microscope image and fluorescence. They have been localized in the irradiation line and change the optical properties of the sample. These nanocrystals have been obtained due to the excitation of the Ho3+ ions which under nonradiative processes produced the heating of the sample. In this work, it has been demonstrated that the diffusion of the Nb5+ ions to the border of the irradiated line controls the growth of the nanocrystals in the sample.
©2010 Optical Society of America
1. Introduction
Laser irradiation of glass materials has received much attention, because this technique has been regarded as a new process for spatially selected structural modification and/or crystallization in glass [1–3]. For many optical applications, it is useful to develop a technique for micromodification of fluorescence properties in glass via laser-induced elemental distribution change combined with the environmental change of fluorescent ions doped in glass. Sato et al [4] have reported that crystal-dots consisting of the Sm2Te6O15 phase are induced by irradiation of a continuous-wave (CW) Nd:YAG laser operating at a wavelength of λ = 1064 nm in RO–Sm2O3–TeO2 (R: Mg, Ba) glasses. Honma et al [5,6] have found that the irradiation of Nd:YAG laser with λ = 1064 nm induces the formation of crystal-dots and lines showing a second harmonic generation (SHG) in Sm2O3–Bi2O3–B2O3 glasses. Recently, the present author’s group has reported the formation of nanocrystals under cw laser irradiation [7–10].
In this work, one of the focuses of our attention is the final distribution of the ions after the laser irradiation. Shimotsuma et al [11] have presented a study of the morphology of the induced structure in the bulk of Ag+- doped silicate glass, fused silica and tellurite glass by Femtosecond laser irradiation. Yin Liu et al [12] have reported micromodification of Eu3+ element distribution in a silicate glass with Femtosecond laser irradiation and demonstrated that the migration of Eu3+ ions have been induced. In this work, it has been studied the possibility of a desvitrification process induced by laser irradiation and the diffusion of the involved ions in the nanocrystals formation.
2. Experimental
The glass, with the following composition in mol%: 2.5 Ho2O3, 11.25 SrO, 11.25 BaO, 22.5 Nb2O5 and 52.5 B2O3, was prepared by a standard melt quenching method [13]. Commercial powders of reagent grade were mixed and melted in a platinum crucible for 1 h in an electric furnace at 1400°C. The melt was poured between two iron plates and the thickness of the obtained sample was 1.5 mm. The sample was polished to obtain a smooth and flat surface in both faces, and this makes sure that the laser does not diverge when irradiates the sample. The glass ceramic (GC) is obtained by thermal treatment of the precursor glass at 620°C for 2 hours. This GC sample has a glassy phase and a crystalline phase of Sr1-xBaxNb2O6 (SBN) nanocrystals with an average diameter of 60 nm. It is used to compare with measurements in the locally damage zone by laser action. A cw multiline Argon Laser was employed to irradiate the original glass sample with a laser power beam fixed at 3.5 W. In order to reach the smallest irradiated area, the laser beam was focused on the sample with a convergent lens of 30 mm. The glass was put on a translator stage and was moved during laser irradiation to construct lines with a speed of 0.5 mm/s.
After the laser irradiation, the emission band of the (5F4, 5S2)→5I7 transition at 750 nm and 5F5 →5I8 transition at 650 nm were measured in confocal setup perpendicularly to the written lines. The scheme of the confocal micro-luminescence setup used to characterize the samples is shown in Fig. 1 . The sample is located at the focal plane of a 20X microscope objective (Mitutoyo, M-Plan NIR, numerical aperture (NA) = 0.26) on a motorized translation stage. The sample is excited using a 532 nm cw laser by means of dichroic mirror while the luminescent emission is detected using an Andor Newton CCD.
The elemental distribution of the ions around the border of the line in the glass sample was characterized with a Dispersive X-Ray Micro-analyzer (Oxford Instruments Microanalysis Group 6699 ATW).
The surface topography was imaged with an atomic force microscope (AFM) operating in tapping mode (Nanoscope IIIa from Digital Instrument/Veeco) in air at room temperature. Etched silicon tips RTESP, 215-254 kHz, 20-80 N/m were used. To minimise the thermal drift, images were taken only after thermal equilibrium was reached.
3. Results and Discussion
After the irradiation process with a cw multiline argon laser a visible modification zone can be observed in the glass matrix and under optical amplification can be detected two parallel lines centered in the border of the irradiation zone (see Fig. 2 ).
Fig. 2 Optical image of the sample after irradiation with and Argon laser and shifted during this process, where A and C corresponds to the borders and B and D inside and outside of the line, respectively.
Under excitation at 532 nm the emission spectra show bands at 650 and 750 nm corresponding to the 5F5 →5I8 and (5F4, 5S2)→5I7 transitions, respectively. In this study confocal microscopy has been used in order to analyze the 5F5 →5I8 transition at 650 nm in different positions of the laser modified zone (see Fig. 3 ). These spectra were obtained perpendicularly to the written lines (see inset of Fig. 3) and they have been normalized for a better comparison and shown in Fig. 3.
Fig. 3 Confocal fluorescence spectra obtained from different positions in the laser modified zone. The inset shows a scheme of the irradiated line, in the same way than Fig. 2. Also a spectrum obtained in a glass ceramics sample (GC) has been included for comparison.
As can be seen in this figure there are important structures changes in the shape of the band depending of the point of the sample. This behaviour is compared with the emission of the glass ceramic sample to conclude that it could be due to a desvitrification process of the irradiated glass sample. This emission has been also included in Fig. 3, where the points A and C correspond to the borders of the line, B inside and D outside the line. As can be seen, the emission spectra corresponding to the points A and C are similar to the obtained ones in the glass ceramic sample. Moreover, the point B, which is located in the middle of the irradiated area, has the same structure than the point D, outside of the irradiated area, where there is only glass. Recently, Honma et al [14] have succeeded in writing nonlinear crystal-lines using YAG laser at 1064 nm in Sm2O3–Bi2O3–B2O3 glasses and suggested that crystal-lines consist of single domain crystalline phase. During the irradiation process a liquid phase is located in the focal region. This liquid phase has a higher thermal expansion coefficient compared with that of the solid-state glassy phase surrounding the liquid phase. Then, the liquid phase would expand to a free surface and swell like a hill due to a surface tension. Finally, crystal nuclei occur at some part of the liquid phase and nanocrystals start to grow [15]. In a previous work [8], it is found a re-amorphous zone due to the gradient of temperature established by the laser action. The amorphization induced under laser irradiation has been reported by other authors also [16–18]. All these works indicate that heating processes could produce diffusion of ions and amorphization of the irradiated zones in the sample. As it is shown below these aspects are very important to explain the experimental results in this work.
Figure 4 shows the intensity of the emission associated with 5F5 →5I8 transition as a function of the distance from the border of the laser modified zone obtained with confocal spectroscopy. These results have been obtained from the spectra obtained perpendicularly to the surface of the sample every 5 steps (6.25 μm), from the border of the line, passing through the irradiated area, to the other border of the line and in the non-irradiated zone. Analyzing this figure, there are some important aspects to comment. When the position moves toward the border of the laser modified zone, the corresponding intensity of this emission changes. The maximum intensity is localized in the borders of the irradiated line whereas the lower intensity point corresponds to places between the borders and outside of the modified zone. As can be seen, the intensity inside the line is similar than that outside of the line, where there is only glass. The intensities of the borders of the line are compared with the glass ceramic emission and it is found that they are similar.
Fig. 4 Emission intensity at 650 nm as a function of the distance from the border of the line (zone C).
These results let us to conclude that the structure of the sample has changed due to the laser irradiation. The Fig. 4 shows clearly the presence of different regions in the sample and the Fig. 3 gives the structural changes in these areas. These results seems to be similar than those obtained for the nanocrystals in the glass ceramic sample.
The surface morphology obtained by a scanning electron microscopy (SEM) reveals the formation of a structure in the border of the line (see Fig. 5 ). From the X-Ray Micro-analyzer mapping the relative concentration of Nb5+ ion has been increased in the border of the line (zone C), while the variation of elemental distribution of O2- showed an opposite tendency (data not showed). The results obtained for the Sr2+ and Ba2+ ion seem to have the same behaviour than the O2-, but due to the lower concentration of these elements in the glass sample it does not let to obtain a good image. In the same way, the proportion of Ho3+ ions in glass is very low, comparing with the other ions, and the image obtained in this case, does not let to distinguish if there is a redistribution of these ions. As can be seen in the Table 1 , there is a similar distribution of the ions located inside the irradiated line and in the original glass matrix. From the glass composition, which is given in the experimental part, the ratio between the constituent ions can be obtained in the glass sample. There is about a factor 4 between the percentage of Nb5+ ions respect to the Ba2+ and Sr2+ ions and factor 10 respects to the Ho3+ ions. As can be seen in the Table 1, the constituent ions keep this ratio inside the irradiated line (zone B). This result confirms the formation of a glassy phase inside the irradiated line.
Fig. 5 (5.1) SEM image and (5.2) X-Ray Micro-analyzer mapping of the Nb5+ ions distribution in the border of the irradiated line.
Table 1. Proportion of the chemical distribution inside the irradiated line, in the border of the line and in the glass sample.
For the border of the irradiated line, the distribution of the ions is different than the glass sample and inside the irradiated line. To explain the changes in the ion distribution on the sample, there is something to consider. At the focal point of the laser, the temperature can reach 1500 °C, which is the melting temperature of the glass, and a sharp temperature distribution is produced with the thermal diffusion process [12]. Because of high ion diffusivity stemming from the induced high temperature, various kinds of ions would diffuse away from the focal point to the cooler periphery region around the focal point, where the diffusivity of those ions is much lower. Thus, the laser-induced temperature gradient appears to be the leading driving force for the elemental redistribution in the glass.
Lifshitz and Slyozov [19] have developed a model (LS model) for the diffusive decomposition of a supersaturated solid solution whose later stages yield a t 1/3 power-law behaviour of the average size (R av: radius);
Where t is the time, D is the diffusion coefficient, σ the interfacial energy, c eq the equilibrium concentration, c α the concentration inside the crystal, k the Boltzmann constant, and T the absolute temperature. According to this model, under the same conditions, the ions with higher diffusion coefficient reach a higher average size of radius. That means they can accumulate at a higher distance from the middle of the irradiated points. The diffusion coefficient depends of the ionic radius of the ion. Consequently, the particles with smaller radius diffuse faster than others due to their small size. In this case, the diffusion coefficient of Nb5+ is larger than Sr2+, Ba2+ or O2- ions; therefore, the border of the line is enriched by Nb5+ ions. At the focal point the decreasing of the relative content of these ions lets to the increase of the percentage of Sr2+, Ba2+ or O2- ions, whose diffusivity is lower [20]. Although, the mapping image of these elements cannot be shown, the Table 1 corroborates the decreasing of the Sr2+, Ba2+ and O2- ions in the border of the line, while the proportion of Nb5+ ions has increased.The fluorescence spectra given in Figs. 3 and 4 predict changes in the final distribution of Ho3+ ions in the border of the laser modified line. There are two possible explanations of this.
The increase of the luminescence intensity implies a higher concentration of Ho3+ ions in this zone. The diffusion coefficient of Ho3+ is larger than the other elements, which could enrich the border of the line, distant of the focal point in the same way than Nb5+ ions. Yin Liu et al have reported the distribution of Eu3+ ions by Femtosecond laser irradiation due to the Eu3+ diffusion coefficient [12] which has the same order than Ho3+ ions. Due to the low concentration of Ho3+ ions in the sample, the X-Ray Micro-analyzer mapping does not let to distinguish if the Ho3+ ions are distributed in this border.
Other possibility to explain the increase of the luminescence of the Ho3+ ions is that these ions are influenced by a different local environment. The optical spectroscopy and SEM measurements seem to confirm this explanation. In the Fig. 3, there are many similitudes between the spectra in the border of the line and the glass ceramic sample. The presence of nanocrystals in this area could explain this increase of the luminescence. To corroborate the formation of nanocrystals, the surface topography has been characterized with an atomic force microscope (AFM) (see Fig. 6 ). Inside the line (Zone B), there is not any construction or formation of nanocrystal particles, the AFM images shows a glassy state due to the higher temperatures reaches in this area, in similar way that the results obtained for the glass samples (Fig. 6.1). The surface is formed by particles around 20 nm long and ranging 1-2 nm high giving a roughness of 1.15 nm. This result with the obtained spectra (Fig. 3) and the chemical proportion (shown in Table 1) let us to conclude that this zone has been re-vitrified due to the laser action, where the temperature reaches the melting temperature of the glass.
Fig. 6 AFM images, tapping mode, 1µm x 1µm, obtained on different zones of the sample: (6.1) glass, zone D; (6.2) inside zone C, right border of the line; (6.3) inside zone C, left border of the line; and (6.4) glass ceramic
When the analysis is carried out along the C zone, it is found different particles size. Near of the border of the C zone, close to the D (glass), the AFM images shown nano-particles size with 33 nm wide and 3 nm high with a surface roughness of 2.84 nm (Fig. 6.2). Whereas, in the other border of the zone C, close to B, the surface shows a strong topographic change (Fig. 6.3). The particle size has increased to 60-120 nm long and 40-80 nm high giving a surface roughness around 22.44 nm, i.e. one order of magnitude greater than that corresponding to Fig. 6.2.
The AFM image of the glass ceramic sample, obtained by a thermal treatment (Fig. 6.4) shows particles with a size of 70-100 nm long and around 30-33 nm high, with a roughness of 2.33 nm. It is know that the size of the particles can be controlled by the thermal treatment of the sample [21]. In this way, the particle’s size in the border of the irradiated line could be explained by the temperature reached in the sample. This temperature depends of the distance to the center of the irradiated line. Therefore, it is possible to obtain particles with different sizes along the zone C (from the right to the left border).
The laser energies absorbed by Ho3+ ions are transferred to thermal energies through a non-radiative relaxation process, and thus the surroundings of Ho3+ ions are heated, consequently inducing effectively crystallization in the glasses. It is known, the temperature of Ar:Laser irradiated parts in glass depends on the amount of the active ions, laser power, laser irradiation time, specific heat and thermal conductivity of a given glass matrix [22]. The growth of crystal in laser-induces crystallizations would be controlled by the diffusion of constituent ions [15] as it is demonstrated in this work.
In order to corroborate the nature of induced nanocrystals, measurements of X-Ray Diffraction (XRD) have been carried out. In the Fig. 7 , it is shown the XRD pattern in the border of the irradiated line which confirms the formation of nanocrystalline structure. The average size of the precipitated nanocrystals has been estimated by using the Sherrer’s equation [23], with a size around 50 nm. By comparing the X-ray diffraction pattern of the irradiated area with the glass ceramic (see Fig. 7), it is found that are similar indicating that both process growth SBN nanocrystals with a Sr0.75Ba0.27Nb2O6 crystalline structure.
4. Conclusions
Localized modification in strontium barium niobate glass doped with Ho3+ under laser irradiation was carried out. The precursor glass of this study were fabricated by the melt quenching method and doped with 2.5% mol of Ho3+. A cw Ar-laser was focused and moved during laser irradiation. The formation of SBN nanocrystals was confirmed by XRD, AFM image and spectroscopy and they were localized in the borders of the irradiated line and affected to the optical properties of the sample. These SBN nanocrystals were obtained due to the excitation of the Ho3+ ions which produced the heating of the sample. The diffusion of the constituent ions is fundamental in order to explain a distribution of two parallel lines (formed by SBN nanocrystals) to the irradiated line in the irradiated samples.
Acknowledgment
We would like to thank Comisión Interministerial de Ciencia y Technología (MAT 2007-63319, MAT 2007-65990-C03-02 and CTQ2008-06017/BQU), Malta Consolider-Ingenio 2010 (CSD2007-0045) and FPI of Gobierno de Canarias, for financial support.
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