Femtosecond (fs)–laser-induced micro-modifications in transparent materials have attracted considerable interest and been extensively studied for several decades^[1–3]. Owing to its unique advantages (high accuracy, low thermal effect, and minimum collateral damage), fs laser micromachining is considered and confirmed as one of the most promising ways to modify the micro-properties of transparent materials, such as glasses^[2,4–6], crystals^[3,7], and polymers^[8]. With fs lasers, various micro-properties of materials can be modified, for instance, conversion of single crystal, polycrystal, and amorphous^[9,10], modification of refractive index^[11], change of valence state^[12,13], precipitation of nanoparticles^[14,15], and selective migration of ions^[16]. All these changes induced by a fs laser can be used for fabrication of functional micro-optical devices, such as optical waveguides^[17], directional couplers^[18], three-dimensional (3D) optical data storage media^[19], and 3D micro-fluidic devices^[20,21].

Recently, space-selective manipulation of elements in glass by a fs laser has been an advanced research hotspot^[16,22], since most optical properties of materials such as absorption, luminescence, refractive index, and optical nonlinearity are highly related to element distribution. Numerous interesting phenomena as well as potential applications have been reported by several research groups^[16,23–30]. Microscopic control of the intensity distribution of luminescence^[23] and simultaneous control over the precipitation of multiple crystalline phases and active ion migration^[27] have been realized in some materials. Furthermore, a method for controlling the shape of the elemental distribution with a spatial light modulator has been reported^[28]. According to previous observation^[2,23–25], the relative concentration of glass network modifying ions is usually higher at the periphery and lower in the central area of the laser-affected region, indicating that the network modifying ions tend to diffuse from the vicinity of the focal point to the boundary of the laser modified area. Meanwhile, the relative concentration of glass network forming ions is opposite to that of network modifying ions, migrating from the periphery of the modified region to the center. Generally, diffusion coefficients of different elements and temperature gradients are considered to be the key factors for the formation mechanism of element migration^[16,29]. However, a few experiment results that do not conform with the previous proposition mentioned above have also been reported^[31,32], demonstrating that the mechanism of element migration might be more complicated than what was previously proposed.

In this Letter, we report on an abnormal phenomenon of element migration in oxyfluoride glasses after the irradiation of a 250 kHz fs laser at 800 nm. Electron microprobe analysis shows that the distributions of elements (Ca2+, Yb3+, and Si4+) are highly dependent on laser pulse energy. The glass network modifying ions (Ca2+ and Yb3+) migrate to the center area at lower pulse energy, while they diffuse to the periphery of the modified region at higher pulse energy. The possible mechanism for this diversified migration of elements has been discussed, including the influences of temperature gradient, diffusion coefficient, and laser-induced crystallization. Moreover, the relationship between element distribution and fluorescence intensity of Yb3+has been investigated with the confocal fluorescence spectrum, indicating that space-selective manipulation of element distribution is a promising way to fabricate multifunctional optical devices in glasses.

In this work, a glass with the composition of 60SiO2–20Al2O3–20CaF2–6YbF3–0.2TbF3 (mol%) was chosen for the study. The raw materials (high purity SiO2, Al2O3, CaF2, TbF3, and YbF3) were mixed thoroughly and melted in a covered corundum crucible at 1400°C for 30 min in air. The melt was then cast onto a steel plate and quenched quickly with another plate. The samples were cut and six-facet polished into 5mm×5mm×2mm dimensions for the irradiation experiments.

A high repetition rate fs laser (HRRFS) was employed as the laser source, which emits 800 nm, 70 fs, and 250 kHz mode-locked pulses. The laser beam was focused via a 50× objective lens (numerical aperture=0.8) into the glass sample. The glass sample was put on an XYZ motorized stage with its top facet perpendicular to the laser beam, as shown in Fig. 1. Laser processed lines vertical to the surface of the glass sample were written inside the glass sample with a length of 1.5 mm, using sample translation in the vertical (Z) direction at the speed of 6μm/s. The pulse energy was varied from 2.0 up to 3.52 mJ. After irradiation, the glass sample was cut along the plane perpendicular to the written lines and polished to optical quality so as to expose the cross sections of the laser processed lines for elemental analysis. The elemental distribution of the ions around the focal point in the glass sample was characterized with an electro-probe microanalyzer (EPMA, JEOL, model JSM-6700F). We confirmed that there was no apparent change in the element distribution along the Z direction. The confocal fluorescence spectra were measured by a Raman spectrometer (JOBLN-YVON, T6400) with a 785 nm laser excitation.

To determine the element distribution in the laser-affected zone, the EPMA measurement was performed on the samples. Figure 2 shows the relative concentration of glass modifier Ca2+ and Yb3+ ions in the laser modified region with different pulse energy.

It is obvious that the migration tendency of Ca2+ ions changes dramatically with the pulse energy. When the pulse energy is fixed at 2 μJ [Fig. 2(a)], the relative concentration of Ca2+ ions increases at the focal point, which is opposite to the results previously obtained at similar laser conditions (800 nm, 250 kHz)^[2,22,23]. Then, the pulse energy was increased to 2.72 μJ [Fig. 2(b)]. The relative concentration of Ca2+ ions decreases in the central area and increases in the periphery of focal region, forming a ring-shaped enrichment zone. This result is in agreement with previous observations in other glass systems^[2,22,23]. It is noted that the distribution of Ca2+ ions in Fig. 2(a) is nearly opposite to the one in Fig. 2(b), indicating that the pulse energy is a crucial factor in laser-induced element migration. When the pulse energy reaches 3.12 μJ, it is found that the ring-shaped enrichment zone in Fig. 2(c) has a larger diameter than the one in Fig. 2(b). As the pulse energy continues to increase, a more attractive result is observed. In Fig. 2(d), besides the ring-shaped enrichment zone in the periphery of the laser-affected region, another high concentration zone forms at the focal point. The area between the two enrichment zones has a low Ca2+ ions content. Compared with the ion distribution at the pulse energy of 2, 2.72, and 3.12 μJ, this ion distribution is more complex. The dramatic change of ion distribution at different pulse energies provides a possibility that laser-induced micro-modification of element distribution in glass systems can be controlled precisely.

To explore the relationship between the micro-elemental distribution and micro-fluorescence properties, we added Yb3+ ions into the sample. Generally, as the network modifier, the distribution of Yb3+ ions shows a similar variation tendency with Ca2+ ions. However, some unexpected phenomena have been observed. When the pulse energy is 2.72 μJ, the relative concentration of Yb3+ ions decreases at the focal point and increases in the periphery, forming two ring-shaped enrichment zones at the periphery of the laser irradiated area [Fig. 2(f)]. It is noted that the distribution of Ca2+ ions at this pulse energy only has one ring-shaped enrichment zone. When the pulse energy increases to 3.12 μJ, the distribution of Yb3+ ions is no longer similar to the distribution of Ca2+ ions at the same pulse energy. Besides the two ring-shaped enrichment zones, another high concentration zone forms at the focal point [Fig. 2(g)]. We noticed that this distribution of Yb3+ ions is more similar to the distribution of Ca2+ ions at higher pulse energy [Fig. 2(d)]. If the pulse energy increases continuously (up to 3.52 μJ), both the central enrichment zone and the ring-shaped enrichment zones become larger [Fig. 2(h)].

The distribution of network former Si4+ ions was also investigated (Fig. 3). From the EPMA mapping, it can be found that the relative concentration of Si4+ ions slightly decreases in the central area at the pulse energy of 2 μJ [Fig. 3(a)], indicating that Si4+ ions migrate from the focal point to the periphery of the laser-affected zone. However, as the pulse energy increases, the migration of Si4+ ions shows an opposite tendency [Figs. 3(b), 3(c), and 3(d)]. Although the change in the distribution of Si4+ ions is less obvious than that of Ca2+ and Yb3+ ions, the different distributions of Si4+ ions at lower and higher energy are still meaningful. Moreover, it is worth noticing that the migration tendencies of Si4+ and Ca2+ always remain opposite, which are in accordance with the previous studies^[2,22,23].

When an HRRFS is focused inside a glass sample, the temperature at the focal point can reach higher than several thousand K (Kelvin) because of the heat accumulation, which leads to localized heating. The subsequent thermal diffusion process will result in sharp temperature and pressure gradients, which drive diffusion of ions in the laser-affected region^[2,16]. The chemical diffusion coefficient of elements can be expressed by the equation D=D0exp(−Q/RT),(1)where Q is the activation energy, R is the gas constant, and T is the local temperature^[2,32]. Previous studies have proven that glass network modifying ions will migrate to the periphery of the laser focal region due to their higher diffusion coefficient, while glass network forming ions migrate to the central area, filling the vacancies resulted from migration of other ions, since they have a stronger binding energy with oxygen or fluorine than the network modifying ions. However, in the present study, the migration of ions does not follow such principles as stated above, indicating that the elemental migration mechanism is more complex than previously thought. It is probable that some influences besides the temperature and pressure gradients take place in laser-induced element migration.

Actually, numerous interesting phenomena may take place in the affected zone of HRRFS, for instance, laser-induced crystallization in glass^[33]. At the focal point of an HRRFS, the power density is high enough to bring about the nonlinear absorption, resulting in the generation of high-density electron plasma. Subsequently, the excess energy releases from the plasma into the surrounding media, heating it up. If the glass is heated to the specific temperature and holds for some time, the crystalline phase will precipitate in the glass. Laser-induced space-selective precipitation of crystals has been observed in many glass systems, such as silicate glasses^[33] and oxyfluoride glass^[34]. Aluminosilicate-based CaF2–Al2O3–SiO2 oxyfluoride glasses are considered to be the most frequently used glass system for the preparation of glass ceramics with CaF2 nanocrystals^[34]. It has been reported that CaF2 nanocrystals can be precipitated in SiO2–Al2O3–CaF2–YbF3–TbF3 glass^[35], which is the same glass system as we used in our experiment. Therefore, it is reasonable to assume that CaF2 crystallization may happen in the present experiment.

In principle, the crystallization of glass takes two steps: nucleation and crystal growth^[36]. Firstly, nucleation occurs in the region where the temperature is lower than the crystallization temperature. Then, the growth of crystal begins. Since the temperature is the highest at the laser focal point and decreases from the center to the outside, the nucleation usually takes place at a certain distance from the focal point, and the crystal grows toward the inside. Previous studies proposed that the nucleation and crystal growth process may attract the component element of crystals to the crystallization region^[36], providing a driving force for element migration. Moreover, it is proved that Yb3+ ions can be incorporated into CaF2 nanocrystals through crystallization^[35].

We put forward a tentative explanation on the abnormal element migration in our experiment, including the temperature and pressure gradients, the attraction of CaF2 crystallization, and the incorporation of Yb3+ ions caused by crystallization.

For Ca2+ ions, when pulse energy is low (2 μJ), the temperature near the laser focal point is just located in the range that is suitable for crystallization. CaF2 crystallization takes place in the center of the laser-affected zone. As a component element for CaF2 crystal, Ca2+ ions could be attracted and diffused towards the central area. Meanwhile, the influence of temperature and pressure gradients is limited by the low pulse energy. In this stage, the attraction of CaF2 crystallization dominates. Ca2+ ions show relatively high concentration in the focal region [Fig. 2(a)]. As the pulse energy increases, the temperature near the laser focal point rises. The central area is no longer the right place for crystallization. Nucleation takes place at a certain distance from the focal point, and the CaF2 crystal grows toward the inside, forming a ring-shaped crystallization area^[33]. Ca2+ ions could be attracted to the new crystallization region away from the center. Simultaneously, heat accumulation at the laser focal point becomes much more intense that results in sharp temperature and pressure gradients, driving the Ca2+ ions to migrate outward. In this stage, the attraction of CaF2 crystallization and temperature and pressure gradients work together, driving Ca2+ ions to migrate to the periphery of the laser-affected zone [Fig. 2(b)]. The higher pulse power not only leads to a larger diameter of the crystallization circle, but also gives rise to sharper temperature and pressure gradients. All of these result in a larger ring-shaped enrichment zone of Ca2+ ions [Figs. 2(c) and 2(d)] compared with the one in Fig. 2(b). When the pulse energy rises to a certain value (3.52 μJ) due to the accumulation of a large amount of heat, the temperature in the laser-affected zone cannot rapidly drop to the solidification point of glass after laser irradiation. This keeps the glass in a molten state for a short time after laser irradiation, and some Ca2+ ions may migrate back to the central area under the influence of the concentration gradient. Moreover, during the relatively lengthy cooling process, the temperature of the focal region may get into the temperature range of nucleation and crystallization. It is possible for CaF2 crystallization to take place in the central area a short time after laser irradiation. This may result in a central enrichment zone of Ca2+ ions in the laser-affected region, as presented in Fig. 2(d).

The redistribution of Yb3+ ions is generally similar with Ca2+ ions. When pulse energy is low (2 μJ), CaF2 crystallization takes place in the center of the irradiated region, incorporating some Yb3+ ions into the CaF2 crystal. Therefore, Yb3+ ions show relatively high concentration in the focal region [Fig. 2(e)]. As the pulse energy increases, Ca2+ ions migrate to the periphery of the laser-affected zone. The crystallization region of CaF2 changes from the central point to a circle around the focal point. Some Yb3+ ions are incorporated into the CaF2 crystal circle, forming a ring-shaped enrichment zone, which is presented as the outer ring in Fig. 2(f). Meanwhile, temperature and pressure gradients drive the Yb3+ ions to migrate outward, forming another ring-shaped enrichment zone, which is presented as the inner ring in Fig. 2(f). As the ratio of charge/ion radius for Yb3+ is larger than that of Ca2+, which may result in poorer mobility of Yb3+ than that of Ca2+ to pass through the glass network during irradiation, it can be inferred that Yb3+ ions have a lower migration speed away from the focal center compared with Ca2+ ions. Considering the difference in migration speed, it is reasonable that the diameter of the ring-shaped enrichment zone of Yb3+ ions in Fig. 2(f) is smaller than that of Ca2+ ions in Fig. 2(b). Comparing the distribution of Yb3+ ions in Figs. 2(f), 2(g), and 2(h), it is also observed that both the inner and outer rings become larger as the pulse energy increases. Two factors may lead to this phenomenon. Firstly, the higher pulse power leads to a larger diameter of the CaF2 crystallization circle at the periphery of the laser-affected zone. Yb3+ ions are incorporated into the CaF2 crystal and form a larger outer ring-shaped enrichment zone. Secondly, the higher pulse power also leads to sharper temperature and pressure gradients, which pull the Yb3+ ions outward to form a larger inner ring-shaped enrichment zone of Yb3+ ions. Besides the ring-shaped enrichment zones, the high concentration areas in the central part of Figs. 2(g) and 2(h) are remarkable. We pose a hypothesis to explain this phenomenon that occurs at high pulse energy. Firstly, some Yb3+ ions may diffuse back to the central area under the influence of the concentration gradient, since the intense accumulation of heat keeps the glass in a molten state for a short time after the high-power laser irradiation. Secondly, the temperature in the laser focal zone at high pulse energy may be suitable for the crystallization of YbF3. The nucleation and crystallization of YbF3 attract the surrounding Yb3+ ions, driving them to migrate inward. Thus, central enrichment zones of Yb3+ ions are formed in the laser-affected region. However, further investigations are needed to prove this hypothesis.

The distribution of Si4+ ions shows an opposite tendency with Ca2+ ions at different pulse energies. Since the network formers have a stronger binding energy with oxygen than the network modifiers, the diffusion coefficient of the network former Si4+ ions is generally much smaller than that of the modifier Ca2+ ions. Si4+ ions are apt to migrate slowly, filling the vacancies resulted from migration of other ions for change balance^[2]. Thus, when pulse energy is low (2 μJ), Ca2+ ions diffuse towards the focal center and show relatively high concentration in the focal region. Simultaneously, a few Si4+ ions migrate outward to fill the vacancies around the central enrichment zone of Ca2+ ions, causing a slight decrease of Si4+ ions at the focal point [Fig. 3(a)]. As the pulse energy increases, Ca2+ ions diffuse away from the center, leaving a lot of vacancies in the central area. Si4+ ions migrate inward, forming a central enrichment zone in the center of the laser-affected area [Figs. 3(b), 3(c), and 3(d)].

In order to investigate micro-property changes of the glass resulted from the element redistribution, the fluorescence intensities of Yb3+ at pulse energies of 2.72  and 3.12 μJ were measured. Confocal fluorescence spectra were acquired at different positions (a–l) of the laser-affected zone under 785 nm laser irradiation (Fig. 4). The infrared emission at 975 nm [Fig. 4(b)] in the glass can be attributed to the F25/2(Yb3+)→F27/2(Yb3+) transitions of Yb3+. Representative points in which the relative concentration of Yb3+ ions is extremely high or low were selected and presented clearly in Figs. 4(a) and 4(c). The fluorescence intensities of these points are illustrated in Figs. 4(d) and 4(e). Through these figures [Figs. 4(a), 4(c), 4(d), and 4(e)], it is easy to associate the change in fluorescence intensities with the element redistribution induced by laser irradiation. Although the micro-fluorescence intensity of Yb3+ is affected by many factors, it is clear that the controllable element distribution plays a significant role in changing the micro-fluorescence property of glass. It should be pointed out that not only the micro-fluorescence property can be changed by laser-induced element migration. This technique can be used to modify many other micro-property of materials, like refractive index. More applications will be developed based on laser-induced element migration, such as micro-shaped waveguides.

In the present work, we have reported on abnormal element redistribution in oxyfluoride glasses induced by a 250 kHz fs laser. The elemental distribution analyses demonstrate complex migration behaviors of Ca2+, Yb3+, and Si4+ ions at different pulse energies. We attribute these complex migration behaviors to the influences of temperature and pressure gradients, the attraction of CaF2 crystallization, and the incorporation of Yb3+ ions caused by crystallization. The change in fluorescence intensities caused by element redistribution is also investigated. This study opens a new way to understand the mechanism of fs-laser-induced element redistribution. It is proved that a single fs laser beam can realize diversiform micro-modification of elemental redistribution, which is crucial for improving the micro-property of materials. We also believe this result will help in resolving other irregular phenomena in the area of modification of materials with ultrafast lasers, such as abnormal refractive index changes in some glasses. These findings in the present work may be helpful for fabricating microstructures that may find promising applications in integrated optics, such as nonlinear optical waveguides.