The formation of periodic structures on stainless steel under linearly polarized multi-burst picosecond laser pulses irradiation was experimentally investigated. The resulting structures were characterized by scanning electron microscopy (SEM) analysis. This analysis of images revealed four distinctive (quasi-) periodic structures depending on the laser irradiation parameters, i.e., LSFLs, HSFLs, micro-grooves and nano-holes. It is demonstrated that the multi-burst picosecond pulses technique is capable of fabricating periodic structures with different scales and shapes.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Laser induced periodic surface structures (LIPSS), usually composed of (quasi-) periodic patterns, are a universal phenomenon that occurs on all classes of solid materials upon irradiation near their ablation threshold . Formation of LIPSS on material has wide potential applications, such as control of colorization , generation of hydrophobic/-philic surfaces [2, 3], control of cell and bacterial film growth  and enhancement of tribological properties . The periodic surface structures of the material are of critical importance for tailoring of the surface interaction properties in the aforementioned applications. In general, the emerging periodic structures after the irradiation with linearly polarized ultrafast laser are classified into two distinct types, low spatial frequency LIPSS (LSFL, λ > Λ > λ∕2) and high spatial frequency LIPSS (HSFL, Λ < λ∕2) . It is generally accepted that the formation mechanisms of LSFLs can be attributed to surface plasmon polaritons (SSPs) and interference [6, 7]. The origin of HSFLs, however, is not yet completely understood and the debate still continuous. Possible explanations comprise self-organization , second harmonic generation (SHG) , twining , and cavitation instability  etc. In most cases the LIPSS are generated with low repetition rate of 100 kHz or below, corresponding to pulse intervals longer than 10 μs, which is considered long enough to permit effective heat conduction .
However, surface processing at repetition rates within the MHz range could induce inter-pulses heat accumulation, resulting in diverse surface morphologies compared to LSFLs and HSFLs. Micro-grooves, craters, micro-holes and conical micro-spikes have been fabricated using ultrafast laser with very high repetition rate . The formation of these new (quasi-) periodic structures could be ascribed to material movement due to Marangoni flow, driven by a local thermal gradient . Additionally, recent advancements in laser burst technology have found many applications in the fabrication of metals [15, 16]. Using laser burst technology with pulse-to-pulse separation times on the ns-scale or less enables the development of remarkable improved surface qualities due to interactions of subsequent pulses within the burst with the already hot surface. The combination of multi-burst subpulses and low repetition rate was expected to have a positive effect for creating new periodic structures due to synergetic impact of the heat accumulation and SPPs. Guay et al.  showed that use of burst picosecond pulses could create a new large laser induced periodic surface structures (LLIPSS) that were 10 times the laser wavelength and parallel to the laser polarization. Shi et al.  applied femtosecond subpulse trains to form a double grating structure on silicon, which was probably attributable to the grating-assisted enhanced energy deposition and subsequent thermal effects. However, there is still a lack of information related to the formation of different types of LIPSS using burst technology.
According to the previous works mentioned above, heat accumulation induced by tiny time interval between two successive laser pulses has a great effect on the surface morphology using ultrafast laser irradiation. Due to the significant potential, extending multi-burst pulse sequences to the formation of LIPSS may be a promising technique to create new periodic structures. Hence, this study presents the formation of periodic structures on stainless steel surface upon irradiation with multi-burst picosecond pulses, including single-, double-, triple-, and quadruple-pulse sequences. The evolution and corresponding mechanisms of the periodic structures with three different laser fluences are investigated in detail.
For the experimental formation of LIPSS, a commercial Nd:YVO4 picosecond laser (PX50, Edgewave, Germany) producing a pulse duration of 10 ps and a wavelength of 532 nm was used. The laser can be operated in single pulse mode as well as in burst mode. The time separation between subpulses within a burst is 20 ns, which is defined by the oscillator repetition of 50 MHz. The number of subpulses within each burst, which is commonly referred to as pulses per burst (PPB), can be turned from 1 (single pulse mode) to 10. The energy of the burst (sum of the energies of the subpulses) is equal to the energy of the pulse in single pulse mode, and the amplitude of each subpulse within a burst is decreasing (i.e., reducing in energy), as shown in Fig. 1(a). The laser beam was linearly polarized with a 3 mm diameter and had a Gaussian energy density distribution. The beam was brought to the focusing lens (f = 50 mm) through a beam delivery system. Through the focusing lens, the beam was focused to a spot with diameter (2w0) of ~16 μm, which was obtained using D2-method, as described in .
AISI 304 stainless steel sample (25 mm × 25 mm × 0.5 mm) with a surface roughness approximately 10 nm was used in the experiments. The sample, kept in air at room temperature, was attached on a motorized x-y-z stage (PS-30, Borui, China). The stage with a movement precision of 500 nm, was controlled by a commercial controller (PMAC, Delta, USA) via a personal computer and used to precisely position the sample. The experimental setup is depicted in Fig. 1(b).
During experimentation, the sample surface was irradiated by series of laser pulses at F = 10 kHz repetition rate. LIPSS along a line were created by scanning the laser beam with a fixed speed (v = 5 or 10 mm/s) in y direction. The polarization direction of the incident laser radiation was parallel to the scanning direction by adjusting the half-wave plate. The laser was operated in burst mode with PPB varying from 1 to 4, corresponding to single-, double-, triple-, and quadruple-pulse sequences. The effective pulses per spot (PPS, calculated by 2w0/(v/F)) were 32 or 16 under single pulse mode, which were then increased about 2-4 times with the increasing PPB from 2 to 4. Three different laser fluences (0.25, 0.54, 1.44 J/cm2, calculated via 2E/(πw02)) were utilized by adjusting the energy of burst E from 0.25 μJ to 1.45 μJ, as measured by an external power meter (Maestro, Gentec –EO, Canada). Note that laser fluence in this paper represents the sum of the fluence of subpulses. Single-shot ablation threshold fluence was used as a reference point and was estimated using D2-method to be 0.20 J/cm2. The generated LIPSS were observed by a scanning electron microscope (Quanta FEG 250, FEI, USA) following a thorough ultrasonic cleaning by ethanol and distilled water. LIPSS periods were obtained by analyzing the SEM micrographs using a Gwyddion software.
3. Results and discussion
3.1 Evolution of surface morphology for 0.25 J/cm2
Figure 2 shows SEM images of the surface structures induced by ps laser pulses with different subpulse numbers at the laser fluence of 0.25 J/cm2. For under single-pulse sequence irradiation, a uniform distribution of nano-structures covered the laser irradiated region, as shown in Fig. 2(a). In order to observe the detailed characteristics of these nano-structures, two partial enlarged images are shown in Figs. 2(b) and 2(c). It is clearly seen that a good deal of regular ripples was generated in the center of laser irradiated region. These regular ripples had the orientation perpendicular to the laser polarization direction and the spatial periods of ~450 nm, which was slightly smaller than the laser wavelength. Hence, these ripples can be inferred as LSFLs due to the similar characteristics with the ripples in low-spatial-frequency . Unlike the central region, the morphology of ripples in the peripheral region had the period around 130 nm lower than half of the laser wavelength, so they can be thought as HSFLs . The difference of periods between the center and periphery of the irradiated region is inferred from nonuniform Gaussian energy distribution of the laser. High fluence region in the center leads to formation of LSFLs, which is resulting from interference effects of the incident laser beam with surface electromagnetic waves (SEW) ; low fluence region in the periphery leads to formation of HSFLs, which may be due to self-organization  or the initial periodic distribution of the electron plasma concentration .
For double-pulse sequences, similar nano-structures consisting of HSFLs and LSFLs appeared on the stainless steel surface, as shown in Fig. 2(d). However, the width of central LSFLs presented non-homogeneous. In this situation, energy of 0.25 μJ for a single pulse was distributed over two subpulses leading to the unstable transition region mixed up with LSFLs and HSFLs. In Figs. 2(g) and 2(j), obvious different nano-structures were observed on the sample surface after irradiation with triple-pulse and quadruple-pulse trains. The parallel ultrafine ripples with periods of ∼100 nm, i.e., HSFLs, were regularly distributed at the overall laser irradiated region. This may be because the energy of 0.25 μJ was split into three or four subpulses leading to the fluence of each pulse lower than that of the LSFL formation threshold, which causes the absence of LSFLs at the laser irradiated region .
3.2 Evolution of surface morphology for 0.54 J/cm2
Figure 3 presents SEM images of the surface structures induced by ps laser with PPB varying from 1 to 4 at the laser fluence of 0.54 J/cm2. At the subpulse number of 1, orderly ripples were fully generated in the laser irradiated region, as shown in Fig. 3(a). With the increasing laser fluence, the central portion of the beam exceeded the molten LSFL threshold of the material resulting in a blurring of the LSFLs with periods of about 475 nm, as is revealed from Fig. 3(b). Similarly, HSFLs (Λ~100 nm), were formed at the edge of laser irradiated region. Meanwhile, some particles with the size of ~90 nm appeared on the top of ripples, as shown in Figs. 3(b) and 3(c). The resolidification of the splashed material during the laser scanning in the high energy radiating region is responsible for the forming of particles .
Interestingly, a new type of laser induced structures with a higher spatial periodicity was observed in Fig. 3(d) by irradiation with double-pulse trains. The produced structures, micro-grooves [Fig. 3(e)] exhibited the orientation parallel to the laser polarization direction and the spatial periods of ~1200 nm, which was more than twice the laser wavelength. Similar micro grooves with parallel orientation and larger periodicity were also observed in solids [13, 14].
In case of double-pulse trains, the interval of 20 ns between the subpulses leads to electrons with a higher temperature at the beginning of the second subpulse irradiation, resulting in shorter relation time, a stronger electron-photon coupling as well as the change of transport properties of the material . For specific values of laser fluence and pulses per spot, the transient temperature of the material could reach the melting point under the action of heat accumulation . Moreover, Marangoni shear generated convection dominates the mechanism leading to hydrothermal waves and eventually to micro-grooves . It is important to note that micro-grooves did not appear under laser irradiation with similar pulses per spot and pulse energy in single pulse mode, as shown in Figs. 2(a)–2(c). At the same time, SPPs excitation results in the formation of LSFLs, as shown in Fig. 3(e). In the superposed region of LSFLs and micro-grooves, another new structure (i.e., nano-holes) was generated, as shown in Fig. 3(e). HSFLs still remained at the edge of laser irradiated region due to low energy accumulation. With the increasing PPB to 3, the micro-grooves shown in Fig. 3(g) became non-homogeneous and blurring. Hence, the nano-holes had almost vanished in the central region, as demonstrated by Fig. 3(h). For quadruple-pulse trains, a nonuniform distribution of LSFLs was observed in the central region while HSFLs still dominated the edge [Figs. 3(j)–3(l)]. The further reduction of the fluence of each subpulse at PPB = 3 or 4 may be responsible for the non-homogeneous micro-grooves and LSFLs.
3.3 Evolution of surface morphology for 1.44 J/cm2
Figure 4 summarizes the evolution of surface structures after irradiation with different PPB at the laser fluence of 1.44 J/cm2. Figures 4(a)–4(c) depict the surface structure after irradiation using single pulse mode. As shown in Fig. 4(a), periodic LSFLs were already severely blurred because of surface melting, while HSFLs still appeared on the rim. Compared to Fig. 1(a) and Fig. 2(a), the overall width of the HSFLs further reduced with the increasing laser fluence. For double-pulse trains, LSFLs, micro-grooves and melting phenomenon arose simultaneously in the central region [Figs. 4(d)–4(f)]. As for triple-pulse trains, disorganized micro-grooves got more distinct [Fig. 4(g)] and more nano-holes were generated [Fig. 4(h)]. Similarly, after irradiation of quadruple-pulse trains two types of ripples, i.e., LSFLs and micro grooves were superimposed in the central of whole ablated area, resulting in a local uniform distribution of nano-holes with diameter of ~300 nm. With the increasing laser fluence, each successive subpulse at PPB = 3 or 4 further increases the surface temperature, resulting in that the transient temperature of sample could reach the melting point. As previously explained the generation of micro-grooves is due to the heat accumulation and induced hydrothermal waves . The formation of nano-holes can be attributed to an enhancement of light absorption in the superimposed region, leading to locally increased ablation . These nano-holes were much smaller than that induced by ps laser trains in the nonburst case .
In this study, the formation and evolution mechanisms of periodic structures on stainless steel induced by linearly polarized multi-burst picosecond laser pulses were investigated. The final surface morphology using multi-bust pulses at three different laser fluences showed four distinctive periodic structures, i.e., LSFLs, HSFLs, micro-grooves and nano-holes. The experimental results demonstrated that the multi-burst picosecond pulse train technique is capable of creating periodic structures with different scales and shapes.
National Natural Science Foundation of China (51705258); Natural Science Foundation of Jiangsu Province (BK20150685); the Fundamental Research Funds for the Central Universities (KYZ201659); Foundation for Distinguished Young Talents, College of Engineering, Nanjing Agricultural University (YQ201604).
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