A high-power continuous-wave (CW) laser was used to move a steel microsphere through a CaO–Al2O3–SiO2 glass block at room temperature along a trajectory toward the laser source. A compositional analysis revealed that the CaO concentration in the glass decreased at the center of the microsphere’s trajectory but increased in the area adjacent to it; the SiO2 concentration showed an opposite trend while the Al2O3 concentration did not change. Further, the compositional difference between the center and the area adjacent to the microsphere trajectory depends on the velocity of the microsphere, which is controllable by tuning the laser power.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Recently, laser-machining techniques have become effective tools for modifying transparent materials to create various types of functional devices such as optical waveguides [1,2], gratings [3–5], microfluidic channels [6,7], etc. Inorganic glasses are one of the most intriguing materials for such devices because of their wide optical window, various refractive indexes, and mechanical, chemical, and thermal durability. Various micromachining techniques using short-pulse or continuous-wave (CW) lasers with wavelengths ranging from ultraviolet to infrared have been used on inorganic glasses to form unique structures. For example, a femtosecond (fs) laser was used to fabricate nanogratings [3,4] and microholes [6,8], form metal nanoparticles  and nonlinear crystals , and induce changes in the refractive index [1,11,12] and the elemental distribution in a wide variety of glasses [13–15]. In particular, the elemental redistribution phenomenon has become a popular topic of advanced research because this phenomenon can facilitate the three-dimensional modification of not only the optical properties (such as luminescence, absorption, refractive indexes, crystallization behavior) but also the physical and chemical properties. It is currently considered that the elemental redistribution induced by fs-laser irradiation occurs due to thermomigration (the Soret effect) [15,16].
Previously, Hidai et al. developed a technique for manipulating metal particles in glass by using a CW laser [17–21]; this is referred to as the CW-laser induced metal microsphere manipulation (CW-LM3) method. In this technique, a CW laser is used to irradiate to a particle composed of a metal such as austenitic stainless steel, platinum, nickel, etc. contained in silica or Pyrex glass. As the laser irradiation heats the metal particle, the heat is transferred to the surrounding glass and the viscosity of the glass decreases, allowing the metal microsphere to move through it. This migration phenomenon associated with CW-LM3 has been thought to be caused by the gradient of the interfacial tension between the metal and the glass because of the laser-induced temperature profile [18,20]. Optical microscopy observations have revealed the formation of a permanently modified zone [17,18,20] and the precipitation of metal nanoparticles [19,21] around the trajectory of the metal microsphere through silica and Pyrex glasses. Thus, the CW-LM3 method can potentially be used to modify the glass along a continuous, three-dimensional line due to the steep temperature gradient and rapid heating and cooling around the moving metal microsphere.
In this study, we demonstrate the migration of a stainless-steel microsphere through a CaO–Al2O3–SiO2 glass by the CW-LM3 method. We demonstrate and characterize the glass compositional redistribution along the trajectory of the metal microsphere. Moreover, the relationship between the movement of the metal microsphere and the spatial distribution of chemical composition was investigated.
30CaO–10Al2O3–60SiO2 (CAS) glass (in mol%) was prepared by the conventional melt-quenching method. Reagent-grade CaCO3, Al2O3, and SiO2 were weighed and mixed thoroughly with a glass mortar. The glass was prepared by melting the mixed raw materials in a platinum crucible at 1550°C for 1 hour. The glass was then crushed once and re-melted for another 3 hours to improve its homogeneity. After the second melting, the glass melt was poured onto a graphite plate for forming and immediately transferred into an annealing furnace preheated to 760°C. The glass sample was then annealed for 1 hour and subsequently cooled to room temperature by 1°C/min. The glass was cut into blocks and polished to obtain optical-grade surfaces.
The experimental setup for the CW-laser-induced migration of a metal microsphere is shown in Fig. 1. Stainless-steel foil with 10-µm thickness (SUS 304, #753173, Nilaco Corp, Tokyo, Japan) was sandwiched between two Pyrex glass plates (Corning 7440, Corning Inc., Corning, NY, USA) and the CAS glass block was attached to the Pyrex glass. A CW laser (RFL-C020/A/2/A, WuhHan Raycus Fiber Laser Technologies Co., Ltd., Hubei, China) with 1,064 nm wavelength was focused on the foil through the CAS and Pyrex glasses by a lens (NYTL-30-40PY1, Sigma Koki Co., Ltd., Saitama, Japan). It was difficult to introduce a stainless-steel microsphere into the CAS glass by the CW-laser irradiation if the SUS foil was directly attached to the CAS glass because the glass would break due to the thermal shock. Thus, a Pyrex glass with a relatively low coefficient of thermal expansion was used to facilitate the formation of a metal microsphere from the metal thin film in the presence of high-power laser irradiation. The microsphere in the Pyrex glass was observed to move toward the laser source from the Pyrex to the CAS glass upon laser irradiation. The focal point of the laser was adjusted by moving a sample stage.
The sample was observed using a CCD camera at a frame rate of 30 Hz with a metal halide lamp (LS-M250 Sumita optical glass, Inc., Saitama, Japan) as a light source for illumination. A bandpass filter (10BPF10-440, Newport Corp., CA, USA) was inserted in front of the camera to filter out the scattered laser light and the thermal radiation from the metal microsphere.
After the migration of the metal microsphere, the CAS glass was polished to expose the trajectory of the metal microsphere and the compositional distribution in the modified area was analyzed using an electron probe microanalyzer (EPMA; JXA-8200, JEOL).
3. Results and discussion
Figure 2 shows the transmittance-optical images of a metal microsphere with a 42 µm diameter as it moved through the CAS glass under laser irradiation with a power density of 62 kW/cm2 at the focal point. The microsphere immediately began to move in the direction of the laser source upon initiation of the laser irradiation. The microsphere accelerated for less than 0.033 seconds (1 frame) and gradually decelerated thereafter. The trajectory of the microsphere was observed after 0.16 seconds as a dark line as shown in Fig. 2 but there was no optically visible trace line from 0 to 0.16 sec.
Figures 3(a) and 3(b) show a transmission optical microphotograph and the EPMA mapping results from a vertical cross-section of the trajectory of the metal microsphere through the CAS glass, respectively. The trajectory was observed as a dark circle area in Fig. 3(a) while the EMPA mapping around the trajectory, as shown in Fig. 3(b), was used to estimate the elemental distribution. The width of the modified area (dark region) was almost same as the diameter of the microsphere. The EPMA mapping shows that the elemental composition changed around the trajectory of the microsphere movement. The Ca was depleted along the center of the trajectory and concentrated in the area surrounding the trajectory to form ring-shaped region. Fe also concentrated in a ring surrounding the trajectory. In contrast, the Si concentrated along the center of the trajectory. However, no change was observed in the Al concentration distribution.
These compositional changes imply that CaO moved toward the outside edge of the trajectory and SiO2 moved toward the center due to the low-velocity migration of the metal microsphere. It is known that there is a steep temperature gradient around the metal microsphere [17,20]: the temperature at the interface between the metal microsphere and the glass matrix exceeds the melting point while the glass is at room temperature only a few tens of micrometers away from the microsphere . This steep temperature gradient can induce thermomigration (the Soret effect) [16,22], which causes the diffusion of atoms, ions, or molecules.
To further characterize the compositional changes in the glass, the trajectory of the microsphere shown in Fig. 2 was cut lengthwise and quantitatively analyzed by EPMA. Figure 4(a) shows a transmittance-optical microphotograph of the lengthwise cross-section of the trajectory and dashed lines A and B denote the profiles that were analyzed by EPMA. Figure 4(b) shows the compositional profile calculated from elemental concentrations of Ca, Al, and Si and Fig. 4(c) shows the concentration profile of FeO along lines A and B. Along line A, where there were no optically observable changes in the glass, the compositional distributions of CaO and SiO2 were even. On the other hand, along line B, the concentration of SiO2 increased and the concentration of CaO decreased at the center of the trajectory. However, in the area around the trajectory along line B, an opposite trend was observed. The concentration profile of Al2O3 was constant across lines A and B. Moreover, a small amount of FeO, which was derived from the steel microsphere, was detected in the trajectory: the FeO concentration increased at the center of the trajectory at line A but increased in the vicinity of the trajectory at line B. These results indicate that the compositional redistribution may influence whether the changes are optically visible.
Moreover, the influence of the speed of the metal microsphere on the change in the compositional distribution in the glass was investigated. Figure 5 shows the differences between the concentrations at the center of the trajectory and in the region surrounding it as functions of the velocity of the steel microsphere (on a logarithmic scale) by analyzing in the sample shown in Figs. 2 and 4(a). The difference between the compositions of SiO2 and CaO at the center of the trajectory and in the area surrounding the trajectory decreased as the velocity of the steel microsphere increased. The glass composition in the high-velocity region (line A in Fig. 4(a) was almost the same as that of the mother glass. Because the focal point of the laser was fixed in this experiment, the temperature around the metal microsphere decreases over time (or decreasing velocity) as the metal microsphere moves away from the focal point of the laser. Thus, the viscosity of the surrounding glass decreases with increasing microsphere velocity. In silicate glass, the thermomigration can take place within 10 msec , therefore, in the low-velocity region in this experiment, the time is sufficient for forming the compositional gradient under the temperature gradient. The higher temperature should cause a steeper temperature gradient and, thus, enhance the compositional distribution because the compositional gradient due to thermomigration increases with an increase in the temperature gradient. However, the compositional difference was observed to increase with decreasing temperature (velocity). This discrepancy between the predicted and actual behaviors may be because the flow of the molten glass around the microsphere changes from turbulent flow to laminar flow as the Reynolds number, Re ( = ρuL/μ), decreases with decreasing velocity (increasing µ due to the decreasing the temperature while ρ, u, L, and μ are the density of the fluid, the velocity of the fluid, a characteristic linear dimension, and the dynamic viscosity of the fluid, respectively). The mixing of the molten glass under turbulent flow may counteract the compositional redistribution due to the steep temperature gradient around the microsphere caused by the Soret effect.
In this study, the compositional modification in the CAS glass by the CW-LM3 method was investigated. A steel microsphere was formed from a SUS 304 film attached to Pyrex glass by irradiation with a high-power CW laser through the glass. The microsphere was transferred from the Pyrex to the CAS glass under continuous laser irradiation as a sample stage was moved. Compositional analyses revealed that the concentration of CaO decreased along the center of the trajectory of the microsphere and increased in the region adjacent to the trajectory. However, the concentration of SiO2 exhibited an opposite and the concentration of Al2O3 was unchanged. Moreover, we showed that the degree of compositional modification increased with decreasing microsphere velocity, with compositional changes reaching up to approximately 6 mol%. These results imply that the compositional distribution in oxide glass materials can be controlled by changing the velocity of the metal microsphere by adjusting the laser power density using the CW-LM3 method. In the future, this approach can be leveraged to fabricate optical devices, such as three-dimensional optical waveguides, in optical glass materials.
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