Using two beams of femtosecond laser pulses linearly polarized in different directions, we demonstrate a new phenomenon of eliminating the periodic subwavelength surface structures recorded on Fe-based metallic glass. It is found that such femtosecond laser erasing process can be efficiently controlled by varying the temporal delay between two laser beams while maintaining the amorphous properties of the sample surface. The underlying mechanisms are substantially attributed to the transient enhancement of the surface mobility of the sample by two laser-matter interactions. These investigations may be helpful in high precision manipulation of the material surface for rewritable applications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Recently, femtosecond laser-induced periodic surface structures (LIPSSs) have emerged as a novel and versatile technology for micro- or nano-processing of materials, including metals , semiconductors  and dielectrics , because it is self-assembled with flexibility, simplicity and a range of capabilities. The resultant nanostructured materials can be employed for a wide range of applications, such as structural color [4,5], biocompatibility , wetting properties [7,8], mechanical properties  and others , which undoubtedly arouses great interest in various disciplines. Extensive studies have been explored to understand the underlying mechanisms and control LIPSSs with different laser parameters [11–14]. Besides the commonly observed one-dimensional grating-like ripple patterns, the direct formation of two-dimensional subwavelength surface structure arrays has been also reported in our previous studies [15,16]. However, the removal of the laser-induced surface structures is just as important as the formation, because there is often a need to repair or rewrite the structures for flexible exploitation of the surface functionalization. In addition, it has been reported that the formation of volume nanograting structures inside fused silica can be rewritten by the newly incident femtosecond laser pulses, where the structure orientation was determined by the linear polarization direction of the subsequent rewriting laser beam [17,18].
In this paper, we introduce a new method for erasing the subwavelength LIPSSs on Fe-based metallic glass (Fe82Si11C7) under irradiation of two femtosecond laser beams with different linear polarizations. The experimental results reveal that the laser erasing effects closely depend on the time delay between the two pulses, and the recovered smooth surface can maintain the amorphous properties, which is confirmed as preferable for proper re-writing of LIPSSs. Finally, we provide some explanation for the experimental observations.
2. Experimental setup
Figure 1 shows a schematic diagram of the experimental setup. A commercial chirped-pulse-amplification Ti:sapphire laser system was employed to deliver 50-fs pulse trains centered at the wavelength of 800 nm with a repetition rate of 1 kHz. The 8 mm-diameter of output laser beam was linearly polarized in the horizontal direction. In order to have a good beam profile of the laser intensity , a 3.2 mm-diameter hard aperture was adopted in the optical path. Then each laser pulse was split into two parts by a 50:50 beam splitter BS1. The direction of the linear polarization of the laser beam was altered by a half-wave plate in one optical arm while keeping polarization unchanged in another arm, which results in different polarization directions for two laser beams. Moreover, a high-precision delay line was used to adjust the time delay between two laser beams [Note: a positive time delay Δt means that the laser pulse of the linear polarization E2 is temporally delayed with respect to that of E1]. After passing through a mirror BS2, the two laser beams were aligned into spatially overlapping and collinear propagation towards an objective lens (4 × , NA = 0.1). The adopted material is Fe-based metallic glass with a thickness of 40 μm and it was irradiated by the focal laser beams at normal incidence. During the experiments, the sample was scanned by a speed of 1 mm/s along the direction perpendicular to the laser irradiation. In order to avoid severe ablation damages, the sample surface was placed 500 μm before the focal plane. A diameter of the focused laser spot on the sample surface is calculated approximately 100 μm and the experimentally measured single pulse ablation threshold fluence for this material is 0.23 J/cm2. Our choice of Fe-based metallic glass is due to its superior physics and chemical properties, such as soft magnetic properties , excellent corrosion and wear resistance, bonding strength and super-high hardness . The surface morphology was characterized by scanning electron microscopy (SEM, Keyence, VE-9800) and no surface preparation including etching and chemical treatment was carried out for imaging with SEM. The crystalline properties of the sample was analyzed by an X-ray diffractometer (Rigaku SmartLab XRD) before and after the laser treatments.
3. Results and discussions
In all the experiments, the energy fluence of each laser pulse was given to be 0.02 J/cm2, which corresponds to a single pulse energy of 1.75 μJ. As shown in Fig. 2(a), under irradiation of the single beam femtosecond laser pulses, the one-dimensional grating-like periodic ripples structures can be induced on the material surface, with orientation perpendicular to the laser polarization direction E, which is very similar to previous observations [22,23]. The measured spatial periodicity of the ripple structure is approximately 644 nm, less than the incident laser wavelength of 800 nm. When two beams of femtosecond laser pulses with linear polarization in different directions (at an intersection angle of φ = 60°) were employed to simultaneously strike the surface (at Δt = 0 ps), the periodic ripple structures are still obtained, as shown in Fig. 2(b). Compared with that of the single beam irradiation, the available ripple orientation is seen to become slantwise, neither perpendicular nor parallel to any of the two laser polarizations, and the ripple period is slightly increased to about 665 nm. Remarkably, in this case the ripple surface structures begin to have extraordinarily uniform distribution with a clean and clear appearance, in sharp contrast to the previous reports [8,13]. In addition, the ripple-covered surface region induced by two laser beams is larger than that of the single beam irradiation due to the increased overall energy fluence. Actually, such well-organized periodic ripple structures can be maintained until the time delay of two femtosecond laser beams is enlarged to Δt = 10 ps. With continuously increasing the time delays, the formation of the ripple surface structures became gradually deteriorated. Figure 2(c) displays a result induced by two femtosecond laser beams at the time delay of Δt = 15 ps. Beyond our expectations, it is found for the first time that no periodic ripple structures appear within a narrow region of the laser-exposed surface. More interestingly, as the time delay between two laser beams is increased to Δt = 30 ps, the periodic ripple structures can be largely suppressed within most of the laser-exposed area, only appearing on both lateral edges. In other words, under irradiation of two laser beams with different linear polarization directions, the suppressed formation of the ripple structures on the metallic glass surface seems to be more drastic within the appropriate range of time delays between two laser beams.
Inspired by the above phenomenon, we tried to explore the possibilities of erasing subwavelength periodic ripple structures on Fe-based metallic glass surface via the following steps. First, a spatially periodic distribution of the subwavelength ripple structures was developed on the sample surface by the single beam femtosecond laser pulses that is linearly polarized in the horizontal direction, as shown in Fig. 2(a). Then two beams of femtosecond laser pulses with variable time delays were employed to re-irradiate the ripple-covered surface region. Figure 3(a) shows the obtained result for two laser pulses with the time delay of Δt = 8 ps during the second step, where the formation of the periodic ripple structures is clearly strengthened rather than suppressed [In fact, this phenomenon is consistently observed for time delays less than 8ps]. When the time delay of two laser pulses in the second step is increased to Δt = 30 ps, we can find that the central area of the previously existing structures on the sample surface is erased completely, as shown in Fig. 3(b), only leaving a small fraction of ripple traces on both lateral edges. However, when the time delay of two second-step laser pulses is further increased to Δt = 50 ps, the effect of laser erasing on the existing surface structures worsens, as shown in Fig. 3(c).
Figure 3(d) shows a typical result of the laser erasure when the time delay of two second-step laser pulses is given by Δt = 20 ps, where the most of the initially formed ripple structures are completely erased on the sample surface. This behavior is just like that of an eraser used to rub off writing from paper. In order to identify the physical properties of the laser erased surface region, we adopted other single beam femtosecond laser pulses linearly polarized along the horizontal (similar to the first step) for re-irradiation, and it was found that the uniform periodic ripple structures can be well organized on the sample surface, as shown in Fig. 3(e).
In the experiments, the erasing of the existing periodic ripple surface structures was also carried out by varying the intersection angle between two laser polarization directions, and the obtained results are similar to the situation of φ = 60°. In order to quantitatively evaluate the laser erasing effect, we here define a parameter γ, a ratio of the ripple-erased area to the originally structured surface area. Figure 4 shows the measured dependences of the ripple-erased ratio γ on the time delay between two laser pulses. Clearly, for two laser polarization directions with different intersection angles, the time-delay dependent ripple-erased ratios present very similar variation trends, i.e. the ripple-erased ratios gradually increase from zero at the time delay of Δt = 0 ps to the maximum of γ≈75% around Δt = 30 ps, then they dramatically fall down to zero again at the large time delay of Δt = 60 ps. This indicates that the laser erasing performance can be controlled more efficiently by adjusting the time delay. In order to enable direct visualization of the microstructure removing process, we have measured the erased surface areas (Table 1) for different time delays of Δt during the second step when the intersection angle of φ = 60° was taken as an example. Under such circumstances, the largest area of the erased surface was 395.90 μm2 at the time delay of Δt = 30 ps.
To monitor the change of the amorphous phase on the sample surface, we carried out X-ray diffraction (XRD) measurements before and after the laser treatments, using Cu Kα radiation of λ = 1.541 Å. The typical results are shown in Fig. 5, where three curves with different colors represent XRD patterns for the fresh, ripple-covered, and laser-erased surface regions, respectively. Except for the broad scattering peaks appearing around 40-50°, which is the feature of Fe-based metallic glass [24,25], no narrow crystalline peaks can be observed in the results. This indicates that the amorphous phase of the sample surface can be well maintained even with irradiation of femtosecond laser pulses, which is in contrast to the previous reports [24,26].
The experimental observations are discussed as follows: for the irradiation of single beam femtosecond laser pulses, the time interval between two neighboring pulses is sufficiently long (1 ms at 1 kHz repetition rate) to separate their dynamic laser-matter interaction processes. The formation of the periodic ripple structures on the sample surface is mainly attributed to the interference between incident laser pulse and its excited surface plasmon polaritons (SPPs) . Moreover, because the direction of the linear polarization of the laser pulses remains the same during the single beam irradiation, the spatial arrangement of the ripple orientation, which is always perpendicular to the laser polarization, will not change.
When two femtosecond laser pulses linearly polarized in different directions irradiate the material surface at the time delay of Δt = 0 ps, two SPP waves can be excited respectively along different linear polarization directions, the addition of which leads to a new vector and consequently causes the final formation of ripples with slantwise orientation. Within an initial time delay range of Δt ≤ 10 ps after irradiation of the foregoing laser pulse, the absorbed energy of the sample is mostly deposited in the electron system without substantially disturbing the ions, so that the unchanged surface mobility of the metallic glass has little influence on the formation of the periodic ripple structures.
However, for the time delays of Δt > 10 ps after irradiation of the foregoing laser pulse, the ion temperature is expected to gradually increase via the electron-ion coupling. According to the previous study, the surface mobility of metallic glass can become much larger at higher ion temperatures . Under such circumstances, if the delayed incident laser pulse arrives at the target, the significantly increased surface mobility can deteriorate the molding process of the periodic ripple structures on the sample. For a given time delay, the deterioration strength depends on the spatial intensity distribution of the femtosecond laser beam. With increasing time delay, the surface region with higher ion temperatures is extended by both the electron diffusion and electron-ion coupling [29,30], which inevitably leads to a larger area of erased ripple surface structures. It should be mentioned that the thermal equilibration time between electrons and ions is usually about 10 ps for metals , but it is longer for the metallic glass because of its weaker electron-phonon coupling factor [32,33]. This may be the reason why the observed maximum laser-erasing ratio appears at the time delay of about 30 ps. Due to the Gaussian cross-sectional intensity distribution of the laser pulse, the central part of the beam path usually has higher temperatures, leading to the higher surface mobility and the deterioration of the ripple molding process. In contrast, because of lower temperature and smaller surface mobility on the edge areas, the ripple formation induced by the second laser pulse is mostly like to take place with orientation perpendicular to its polarization direction. However, if the time delay continues to increase, the thermal diffusion process becomes predominant enough to shrink the region of high temperature ions, thus resulting in the smaller laser erasing area. As noted in our experiments, the adopted energy fluence of each laser pulse was only 0.02 J/cm2, and the employed scanning speed of 1 mm/s corresponds to only 100 pulses partially overlapped within one laser beam spot. Such experimental conditions have been confirmed insufficient to support the crystallization process of amorphous metallic alloys , which can explain the achieved XRD results shown in Fig. 5.
In summary, we have successfully employed two time-delayed femtosecond laser beams as a microscale eraser to eliminate the subwavelength ripple structures on Fe-based metallic glass surface. It has been proven that the temporal delay between the two laser pulses plays a significant role during the erasure process, whose appropriate values ranging from 10 ps to 50 ps can be used to control the ripple-erased ratio. XRD measurements have revealed that the amorphous phase of the sample surface can be maintained well despite laser treatments, and the laser erased regions have been identified as rewritable for recording surface microstructures. The analyses suggest that the transient correlations between the two laser-matter interaction dynamics are responsible for the experimental observations, resulting in the increased surface mobility that deteriorates the molding process of the subwavelength ripple structures. It is expected that our method will benefit the microscale control of material surfaces for re-functionalization, which may have potential applications in many fields such as template repair, erasable storage and encryption technology.
National Key R&D Program of China (2017YFB1104700); National Natural Science Foundation of China (11674178); Natural Science Foundation of Tianjin City (17JCZDJC37900).
We greatly appreciate Prof. Rashid A. Ganeev for the critical reading.
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