We propose a new technique of manipulating a metal particle in borosilicate glass. A metal particle that is heated by laser illumination heats the surrounding glass by radiation and conduction. A softened glass enabled metal particle migration. A 1-µm-thick platinum film was deposited on the back surface of a glass plate and irradiated with a green CW laser beam through the glass. As a result, the platinum film was melted and implanted into the glass as a particle. Platinum particles with diameters of 3 to 50 μm migrated at speeds up to 10 mm/s. In addition to platinum particles, nickel and austenitic stainless steel (SUS304) particles can be implanted.
©2010 Optical Society of America
Light has the ability to move matter remotely . Optical tweezers, which are commonly used in life science , control microscale objects, such as cells, in fluids. However, controlled objects must adhere to the surfaces of bases to maintain their position, which restricts three-dimensional applications. If laser manipulation is achieved in transparent solids, for example, a glass, the manipulated matter can be placed wherever in the glass, which is expected to be applied to the fabrication of optical devices and micro-electromechanical systems (MEMSs). Many research studies [3–5] on the fabrication of a waveguide in a glass have been conducted to achieve three-dimensional optical devices. However, such devices must be placed on the surface of the glass, which restricts three-dimensional device and high-density packaging.
It has been reported that microspheres are controlled in solids by local melting, that is, ice plates are melted locally by laser heating, and volume change during phase change generates fluid flow, which moves a polystyrene sphere . The microspheres were controlled only two-dimensionally and the ice must be maintained below 0 °C.
In this report, we propose a new technique of manipulating a metal particle in borosilicate glass. A metal particle that is heated by laser illumination heats and softens the surrounding glass by radiation and conduction. A softened glass enabled metal particle migration. To obtain the metal particle, a metal film or foil was deposited on the back surface of the glass and illuminated with a laser through the glass. This new technique of placing metal particles in a glass expands the variety of the devices and increases the flexibility of design.
2. Experimental procedure
As shown in Fig. 1 , the sample used is borosilicate glass with a thickness of 10 mm (Pyrex®, Corning 7440, Corning Inc.), which is placed on an X-Y-θ stage. A 1-µm-thick platinum - palladium film was deposited by sputtering using a 90 wt.% platinum and 10 wt.% palladium target on the glass sample, unless otherwise noted. Other metals, such as nickel, tin, tantalum, silver, copper and austenitic stainless steel (SUS304), were tested. Tantalum, tin and silver were deposited with a thickness of 1 μm. Copper, nickel and SUS304 foil with a thickness of 10 μm were placed on the glass sample and sandwiched with another glass plate to ensure good contact between the sample and the metal foils. A CW laser beam (Ar ion laser, TSM-20, Coherent Inc.) oscillating with a single line at a wavelength of 514 nm was used to illuminate the film through the glass. The laser beam was focused on the platinum film by a convex lens with a focal length of 170 mm without any relative scanning motion of the laser focus during laser irradiation. Side-view shadowgraph images under white-light illumination were obtained to monitor the process in situ. A blue-IR cut filter (cutoff wavelength: 390 nm) was placed in front of a CCD camera to prevent the detection of scattered laser light and thermal emission.
To analyze the implanted particle, the glass was cut and polished to intercept the particle, then coated with carbon and observed with an energy-dispersive X-ray spectroscopy system (EDX, Link ISIS 300, Oxford Instruments Plc.) installed with a field-emission scanning electron microscopy system (FE-SEM, JSM-6301F, JEOL Ltd.).
3. Results and discussion
After laser irradiation was started, a bright emission was observed at the neighborhood of the platinum film, and then, the emission moved backward (toward the light source). Images of the bright emission are shown in Fig. 2 . Figures 2(a)-2(c) show the bright emission moving backward and a permanent modified zone in the area through which the bright emission passed. After laser irradiation (Fig. 2(d)), a black particle was observed in an area where the bright emission was observed.
To reveal the black particle, the glass was cut, polished and observed with the EDX. Figure 3 shows the spectra of the particle and map results. The white circle observed in the secondary electron image (Fig. 3(a)) is the particle. The EDX spectra of the circle in Fig. 3(a) and the nonirradiated area are shown in Fig. 3(b). No peaks of silicon and oxygen, which are the components of the glass, were observed, but platinum and palladium peaks were observed from the particle. X-ray mapping results (Figs. 3(c) and 3(d)) indicate that the platinum particle and the glass show a clear boundary. Therefore, interestingly, the platinum particle was implanted into the glass. It is noteworthy that EDX analysis detects neither platinum nor palladium from the trajectory of platinum particle migration, that is, the permanent modified zone in Fig. 2. Heating and quenching by laser illumination cause refractive index change ; therefore, the trajectory of platinum particle migration is observed in Fig. 2.
After the particle implantation, the platinum film was observed from the back surface. No platinum film was observed in the area with a diameter of ~300 μm. A part of the removed platinum was believed to be implanted into the glass. The glass was softened at its center with a diameter of ~100 μm.
The particle migration was terminated at a certain point by defocusing and started again by increasing the laser power or focusing. The particle was able to migrate more than 7 mm in the glass by increasing the laser power without the scanning of the focal point. The threshold fluence for platinum particle migration was estimated by changing the laser power. The fluence was calculated from the beam profile and the laser power. As a result, the threshold fluence for the particle migration was almost constant at 0.3 ± 0.06 MW/cm2 . Note that the fluence was the average fluence in the central area with a diameter of 5 μm (same as the diameter of the particle). The average fluence in the laser spot (1/e2 diameter, calculated to be 38 μm at the focus) was calculated to be 0.15 ± 0.03 MW/cm2.
It is noteworthy that the particle migrated during the laser illumination, even after laser illumination was stopped. Figure 4 shows the particle migration observed in between laser illuminations. The particle stopped at a certain point after laser illumination was stopped (Fig. 4(a). Laser illumination was started again (Fig. 4(b) and the particle migrated again (Figs. 4(c) and 4(d)). No changes, except for a small bend, were observed at the point where the particle stopped, as indicated by an arrow in Fig. 4(e).
In addition, the direction of the particle migration was controlled by changing the direction of the laser beam, and the migration was always directed toward the light source. Therefore, the particle location was controlled by adjusting the beam fluence and direction. Figure 5 shows bent and curved modified zones formed in the trajectories of the platinum particles. The direction of the laser beam was changed after the illumination was stopped, and the illumination was started again. The direction of the particle migration was changed and a bent modified zone was formed (Fig. 5(a)). The direction of the migration was curved (Fig. 5(b)) by simultaneously increasing the laser power and gradually changing the direction of the laser beam.
The speed of the particle migration at the focal point, which was measured by changing the laser fluence, is plotted in Fig. 6 . The migration speed increased with the fluence, and the maximum speed was ~10 mm/s. When the fluence was higher than ~0.95 MW/cm2, the glass itself absorbed the light, which is the same as the phenomenon indicated in references  and , and the particles did not migrate.
The diameter of the particles was controlled by changing the film thickness. The films with thicknesses of 0.1 μm and 5 μm were illuminated with the laser. The implanted particle is shown in Fig. 7 . The diameters of the particles were ~3 μm and ~50 μm when the thicknesses of the deposited films were 0.1 μm and 5 μm, respectively.
Migration force was considered. Optical trapping is known as the force caused by light, and the trapping force is generated as the counterforce of the light . A platinum particle reflects laser light. Therefore, the force is applied in the direction of the light progression, which is the counter direction of the particle migration. Photophoretic force also induces the migration of particles, and its direction depends on the particle size. When the wavelength is about twice as large as the particle diameter, the direction is against the light progression, because the excitation of surface plasmons heats the back surface more . In our case, the particle with a diameter of 3 µm - 50 µm migrates toward the light source, which cannot be explained by photophoresis. On the other hand, fluid flow in ice with local melting by laser illumination and freezing has been reported. The water flow in molten ice in the direction of laser scanning is generated by the volume change induced by repetitive melting and freezing . In contrast, when gel is used  instead of ice, the fluid flow is directed against the movement of the heated spot, because the gel expands upon heating. In our case, the specific volume of the glass is increased by heating. Hence, the flow direction is against the movement of heated platinum particles. This direction is the counter direction of the particle migration; therefore, the migration force cannot be explained on the basis of fluid flow. The moving force for the particle migration has not been identified.
Other kinds of metals, such as nickel, tin, tantalum, silver, copper and austenitic stainless steel (SUS304), were tested. As a result, nickel and SUS304 were implanted in the same manner as platinum. However, tantalum, tin, silver and copper were not implanted. Figure 8 shows the thermal conductivities and melting points of the implanted metals, which are below 1 W/cm·K and ranged from 1500 to 2200 K, respectively.
We suppose that the temperature at the laser spot governs the difference, because the implanted metals have similar melting points and thermal conductivities. Hence, numerical calculations were performed to estimate the temperature. To describe a two-dimensional model of the temperature increase in the cylindrical coordinates r and z in a rectangular area, we employ the heat conduction Eq. (1) in the following forms :
Figure 9 shows that the calculated temperatures increase at the center of the laser spot. The temperatures of platinum, nickel, SUS304, tantalum and tin exceeded their melting points within 0.01 s, whereas those of silver and copper did not exceed their melting points even after 1s.
When a platinum film was used, the results were categorized into the following three types in terms of an increase in laser power. i) No change was observed in the glass. ii) Metal particles were implanted. iii) The glass itself absorbed the laser beam and was modified in the same manner as that indicated in references  and . The threshold fluences in the 1/e2 spot for ii) and iii) were ~0.15 MW/cm2 and ~0.2 MW/cm2, respectively. When tantalum, silver and copper films/foils were used, only glass absorption was observed with increasing laser power. Compared with the platinum film, the metals with higher thermal conductivities, such as silver and copper, required higher fluences to melt. As a result, the threshold fluence for metal implantation was higher than that for glass absorption; therefore, glass absorption was believed to occur before metal implantation. Tantalum was not implanted, although it was heated at temperatures higher than its melting point. The absorption of glass increased with temperature ; therefore, the threshold fluence for glass absorption was reduced to less than the threshold for the implantation at the melting point of tantalum. The tin film was evaporated and no changes were observed. This result indicates that the tin melted before glass softening and absorption.
We have shown that platinum, nickel and SUS304 particles can be implanted in borosilicate glass by laser irradiation. The metal particles heated by laser illumination softened the surrounding glass. Glass elements were not detected from the metal particles and no metals were detected in the trajectory of the migration. The platinum particles with diameters of ~3 to ~50 μm were implanted at speeds up to 10 mm/s. The threshold fluence required for platinum particle migration was 0.3 MW/cm2. Further studies are necessary to clarify the driving force for the particle migration.
Support by the Japan Society for the Promotion of Science under a Grant-in-Aid for Scientific Research (B, 20360065) is gratefully acknowledged.
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