- Email: [email protected]
Investigations of morphological features of picosecond dual-wavelength laser ablation of stainless steel
Optics & Laser Technology 58 (2014) 94–99
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Investigations of morphological features of picosecond dual-wavelength laser ablation of stainless steel Wanqin Zhao, Wenjun Wang n, Xuesong Mei, Gedong Jiang, Bin Liu State Key Laboratory for Manufacturing Systems Engineering, Xi'an Jiaotong University, Xi'an 710049, China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 June 2013 Received in revised form 24 September 2013 Accepted 4 November 2013 Available online 26 November 2013
Investigations on the morphological features of holes and grooves ablated on the surface of stainless steel using the picosecond dual-wavelength laser system with different powers combinations are presented based on the scarce researches on morphology of dual-wavelength laser ablation. The experimental results show the profiles of holes ablated by the visible beam appear V-shaped while those for the nearinfrared have large openings and display U-shaped, which are independent of the ablation mechanism of ultrafast laser. For the dual-wavelength beam (a combination of visible beam and near-infrared), the holes resemble sunflower-like structures and have smoother ring patterns on the bottom. In general, the holes ablated by the dual-wavelength beam appear to have much flatter bottoms, linearly sloped sidewalls and spinodal structures between the bottoms of the holes and the side-walls. Furthermore, through judiciously combining the powers of the dual-wavelength beam, high-quality grooves could be obtained with a flat worm-like structure at the bottom surface and less resolidified melt ejection edges. This study provides insight into optimizing ultrafast laser micromachining in order to obtain desired morphology. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Morphology Picosecond laser Dual-wavelength beam
1. Introduction In the last few decades, many studies have been conducted on characteristics of ultrafast lasers micromachining. Some studies have focused on laser parameters, such as fluence, wavelength, pulse duration, repetition rate and pulse number [1–4], while others have been concerned with external laser processing methods, such as changing the processing environment (from or to an ambient liquid, gas or vacuum) [5–9]. All these studies have been aimed at improving the efficiency and morphology of laser micromachining. In recent years, many researchers have adopted combined strategies, such as using dual-laser systems, dualwavelength systems or generating pulse trains, etc., and obtained better ablation efficiency. For double-laser systems, integrated femtosecond (fs) and nanosecond (ns) dual-beam laser systems have been found to be more efficient at removing multiple materials than mono-laser systems [10–12]; in addition, another study reviewed the vacuum ultraviolet (VUV)–ultraviolet (UV) dual-laser excitation process and found that it could perform high-efficiency ablation of different materials attributing to the absorption of the UV laser by excited-states formed by the VUV laser irradiation [13]. For double-wavelength systems, one study
n
Corresponding author. Tel./fax: þ 86 29 82663870. E-mail address: [email protected] (W. Wang).
0030-3992/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2013.11.004
showed that ablation efficiency could be enhanced by using prepulses (fs) of 260 nm with linear absorption followed by main pulses (fs) of 780 nm with three-photon absorption during the selected removal of insulating layers from substrate materials [14]; furthermore, micromachining with dual-wavelength system has been found to improve the aspect ratios of holes and ablation yields [15,16]. With respect to the pulse train technique, one study showed that material removal rates could be increased significantly by using a train of carefully timed pairs of nanosecond laser pulses [17]. However, for the ablated morphology also highly relevant to numerous practical applications, only a few researches have been reported as a small portion of the total papers. For example, one study did report that two lasers could be combined to create holes with sharp edges, flat side-walls and smooth bottoms [13], a few other studies showed that double-wavelength laser could ablate holes with more regular profiles and surface morphologies that appeared to be smoother [15,16]. Based on this information, dualablation strategies may be able to improve the morphological features of ablated structures, further, it is very necessary for the comprehensive studies on the dual-combination ablation morphology attributing to the scarcity of systematical and regular researches. To contribute to this knowledge gap, the morphological features of dual-wavelength beam (a combination of visible beam, 532 nm and near-infrared, 1064 nm) ablation holes and grooves on
W. Zhao et al. / Optics & Laser Technology 58 (2014) 94–99
the surface of stainless steel are presented. In this study, a series of experiments of mono- and dual-wavelength beams ablation were performed. First, the morphological features of holes ablated by two mono-wavelengths, namely visible beam and near-infrared, were compared. Then, the morphology of hole for the dualwavelength beam was analyzed. Subsequently, the ablation of grooves was carried out for the dual-wavelength, and the highquality grooves were obtained by collocating double wavelengths beams powers judiciously.
2. Experiment The stainless steel (SS304) samples as one of the most important industrial materials were used in this study. The samples were of rectangular shape with dimensions of 30 mm 20 mm 1.5 mm (length width thickness). The composition of samples was monitored by EDX (energy dispersive X-ray detector) and the elemental analysis of the SS304 surface is given in Table 1. The laser utilized for irradiation was a neodymium– vanadate (Nd:VAN; Austria laser delivering pulses with 10 ps duration, a power of 2 W and a repetition rate of 1 kHz. Furthermore, it was able to emit radiation at fundamental (1064 nm) and second harmonic. A 150 optical lens was used to focus the beam on the sample. Stainless steel samples with 1.5 mm thickness were placed on a three-dimensional (3D) motorized translation stage to allow for high-precision (50 nm) positional control. An accurate imaging system using a charge-coupled device (CCD) camera allowed for high-resolution imaging of the ablation zone on the target. Fig. 1 is a schematic of the picosecond dual-wavelength laser micromachining system used in this study. First, the delay time between the two wavelengths was about 0.5 ns, whereby the visible beam was emitted first, followed by the near-infrared. Furthermore, the index of refraction is higher for the visible beam than the near-infrared [15], so the focal plane of the shortwavelength beam was closer than that of the long-wavelength beam, the distance via experimentation between the two foci was approximately 1.7 mm. The 1/e2 focused spot diameters (2ω0) at each focal plane for the two wavelengths were simultaneously calculated using the following equation: 2ω0 ¼
4λf M 2 πd
ð1Þ
Table 1 Elemental analysis of SS304 surface using energy dispersive X-ray detector (EDX). Spectrum (wt%) C O
Si
Cr
Mn
Fe
Ni
8.28
0.51
16.72
1.11
62.81
7.80
2.77
95
where ω0 is the radius of beam waist, λ is the wavelength, f is the focal length of the lens, d is the diameter of the incident beam and M2 is the beam quality factor [18,19]. Consequently, the focused waist diameter of a Gaussian beam depends on the beam quality and wavelength in the focal plane, which were approximately 64.1 mm and 70.2 mm for the visible beam and near-infrared, respectively. In order to minimize the influence of spots sizes coming from the different wavelengths, likewise, reduce the difference between the two spot diameters, during irradiation the processing surface was set in focal plane of the near-infrared, such that the visible beam produced divergent beam ablation (the divergent distance was about 1.7 mm), resulting in some amplification of the visible beam spot. Irradiation was carried out under ambient conditions. Various techniques were used to analyze the morphological features of the samples after irradiation, such as scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM).
3. Results and discussion Morphological features of holes and grooves were primarily observed after ablation with the dual-wavelength beam, made up of the visible beam (532 nm) combined with the near-infrared (1064 nm). The results of the induced morphology and geometrical features are presented in the following sections. 3.1. Morphological features of holes ablated by mono-wavelength laser system A series of experiments was conducted for holes ablation with 10 ps pulse duration, a repetition rate of 1 kHz and the same processing surface. Each beam (the visible beam and the nearinfrared) was individually applied at various powers, and then the dual-wavelength beam (combining the visible beam and nearinfrared) was applied at different powers combinations. Fig. 2 shows the SEM images and cross-section profiles of holes on stainless steel surfaces ablated by different wavelengths, demonstrating their dramatically different morphologies. In Fig. 2(A1), which shows a hole ablated by the visible beam with a power of 8 mW, it can be clearly seen that the bottom of the hole was filled with coarse particles and varyingly sized pinholes, most of which had diameters less than 2 mm, at the same time, one larger pinhole with a diameter of about 7 mm appeared in the center of the bottom of the hole. Significant differences were observed for holes ablated by the near-infrared with the same power. As shown in Fig. 2(B1), the bottom of hole consisted of regular ripples and higher-period furrows that were vertical to the ripples. For the visible beam, the side-walls of the holes were mainly composed of symmetrical ravines along the direction of hole depth, otherwise, there was only a small amount of melt ejection, such as debris and droplets, around the hole surface,
Fig. 1. Schematic of the picosecond dual-wavelength laser micromachining system.
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W. Zhao et al. / Optics & Laser Technology 58 (2014) 94–99
Fig. 2. SEM images and cross-section profiles (datum fitting of LSCM) of holes on stainless steel ablated with 10 ps pulse duration, a repetition rate of 1 kHz and 2000 pulses by visible beam (A, 8 mW) and near-infrared (B, 8 mW).
as shown in Fig. 2(A2). Side-walls of holes ablated by the nearinfrared, consisted of finer and more regular ripples, while some shallower ravines appeared on side-walls near hole surfaces, as shown in Fig. 2(B2). It is possible that these morphological features are associated with ablation mechanisms. For ultrafast lasers, vaporization and phase explosion are the primary ablation mechanisms, and these mechanisms correspond to the two separate ablation regimes: “gentle” ablation and “strong” ablation [20,21]. Fig. 2(B1 and B2) shows little evidence of material melting for the hole, thus vaporization was assumed to be the mechanism responsible for removing material and creating the hole when the near-infrared was used. Unlike holes ablated by the near-infrared, those ablated by the visible beam showed obvious melt ejection, which suggests that phase explosion was the primary mechanism for hole creation in this case. Meanwhile, the variously sized pinholes were discovered in the hole bottom, just as shown in Fig. 2 (A1 and A2), resulting from self-focusing characteristic of air, this is filament [22,23]. In our study, when the other parameters were held constant, the visible beam ablation is more intense than near-infrared due to the energy accumulation significantly from the higher absorptivity and the lower damage threshold of material for the shorter wavelength [24], and the threshold values calculated were 0.05 J/cm2 and 0.20 J/cm2 for visible beam and near-infrared, respectively, in reasonable agreement with other reports. For the holes ablated by visible beam, like the one shown in Fig. 2(A3), the bottom of the hole was rough containing several local peaks and valleys, and the side-walls tended to slope linearly. Overall, the cross-section profile of the hole was V-shaped. For hole ablated by the near-infrared, the main geometric features included a flat hole bottom, curved side-walls, and a large hole opening at the surface, the cross-section profile was U-shaped, as shown in Fig. 2(B3). By comparing Fig. 2(A3) and Fig. 2(B3), it can be seen that the different wavelengths created strikingly different profiles even when all other parameters were held constant. In order to thoroughly study the relationship between ablation mechanism and morphology of hole, another set of experiments was performed for the two mono-wavelengths. Fig. 3 shows the SEM images and cross-section profiles of holes on stainless steel ablated by the visible beam with a lower power (2 mW) and the near-infrared with a higher power (40 mW). First, Fig. 3(A1), which shows only a small amount of material melting around the hole surface, suggests that ablation for the visible beam with the lower power of 2 mW employed the “gentle” ablation mechanism. Meanwhile, in Fig. 3(B1), which shows a hole ablated by nearinfrared with higher power of 40 mW, the bottom of the hole
Fig. 3. SEM images and cross-section profiles (datum fitting of LSCM) of holes on stainless steel ablated with 10 ps pulse duration, a repetition rate of 1 kHz and 2000 pulses by visible beam (A, 2 mW) and near-infrared (B, 40 mW).
appeared to contain coarse particles and the hole surface showed obvious melt ejection, these two key features indicate that the near-infrared with this power of 40 mW used the “strong” ablation mechanism. Comparing Figs. 2 and 3, regardless of whether the ablation mechanism was “gentle” or “strong”, the cross-section profiles of the holes were still V-shaped for the visible beam and U-shaped for the near-infrared. Therefore, the cross-section profile of hole is independent of the ablation mechanisms. In addition, the defined ravines observed on the side-walls of hole ablated by visible beam made it a flower petal-like appearance, whereas the ravines in hole ablated by near-infrared were shallow and only appeared near the surface. These phenomena suggest that there is no close relationship between ravine morphology and ablation mechanism. While the previous results show that the cross-section profiles of holes ablated by each mono-wavelength beam are independent of ablation mechanism, meanwhile, the profiles have nothing to do with the processing surfaces coming from the experiments where the processing surface for visible beam and near-infrared were set at the convergent, focal and divergent planes. The experimental results show that the cross-section profiles of holes ablated by each beam maintained their respective characteristic shapes. In general, for most of the targets, the holes profiles did not correspond to the spatial intensity distribution of the laser, only at the lowest power, did holes profiles coincide with the laser intensity distribution for parts of pulse durations and wavelengths [25,26]. It is inferred that wavelength or wavelength-dependent
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Fig. 4. SEM images and cross-section profile (datum fitting of LSCM) of hole on stainless steel ablated with 10 ps pulse duration, a repetition rate of 1 kHz and 2000 pulses by the dual-wavelength beam with the power of 8 mW for visible beam combining the power of 8m W for near-infrared.
Fig. 5. SEM (1) and 3D LSCM (2) images of groove on stainless steel ablated with 10 ps, a repetition rate of 1 kHz and the scan speed 0.05 mm/s by visible beam with the power of 8 mW.
parameters, such as the ablation threshold, absorptivity and heat conduction, may be responsible for creating the observed profiles, and further exploration would be developed in future. Overall, the obvious differences displayed in the cross-section profiles of holes ablated by different wavelengths provide additional ways customize surface morphologies for various practical applications.
3.2. Morphological features of holes ablated by dual-wavelength laser system Fig. 4 shows the SEM images and cross-section profile of hole on stainless steel ablated by the dual-wavelength beam which consisted of the visible beam (8 mW) combined with the nearinfrared (8 mW). In Fig. 4(A1), it can be noticed that the bottom of the hole is much smoother, patterned with ring-like structures and containing more fine particles. The interaction of the beams during ablation may be responsible for creating the observed structures at the bottom of the hole: the delayed near-infrared plays a re-role in pre-visible beam, in other words, the ripples play a re-role in particles, so the particles are scattered, thereby spreading to the surroundings, which results in a smoother bottom with redistributed ring-like morphology and smaller particles. The side-wall, as shown in Fig. 4(A2), was composed of fine ripples in the interior of the hole, while sunflower-like structures with defined ravines near to the surface which were probably associated to the influence of the very large opening on the hole surface for near-infrared to the ravines obtained mainly by visible beam. In addition, some melt ejection was observed around the hole surface as a result of energy accumulation. The cross-section profile of the hole ablated by the dual-wavelength beam appeared to have its own characteristics, including a much flatter bottom, linearly sloping side-walls, and double spinodal structures between the bottom of the hole and side-walls, likely related to the extension of the U-shaped profile
(associated with the near-infrared) to the V-shaped (associated with the visible beam), as shown in Fig. 4(A3). 3.3. Morphological features of grooves By studying the holes ablated by the visible beam and nearinfrared, both alone and combined, we found that the dualwavelength beam created significant morphological and geometric changes compared with that created by mono-wavelength beams. To further investigate the morphological characteristics associated with dual-wavelength laser ablation, a series of experiments was performed for ablation of grooves on stainless steel with 10 ps pulse duration, a repetition rate of 1 kHz and the scan speed of 0.05 mm/s. Fig. 5 shows SEM and 3D LSCM images of the groove ablated on stainless steel by visible beam with a power of 8 mW. From this figure, it can be seen that the groove edges were flat and linear but the bottom of the groove contained irregular and coarse particles whose surfaces were covered with nano-sized stripes. For this reason, we expected that the bottom of the groove would be smooth and regular in the premise of guaranteeing the edges of groove were unbroken. In other words, we expected the nearinfrared would assist the main visible beam in making the bottom of the groove much flatter. When the near-infrared with a lower power (2 mW) was used as an assistant beam, particles in the bottom of the groove had been diminished to form the same sized sub-particles with relatively small gaps, as shown in Fig. 6(A1). The dual-wavelength beam, in which the assistant beam had a power of 2 mW and the main beam had a power of 8 mW, smoothed the bottom of the groove, but not considerably. Then the power of 8 mW was given to the assistant beam and the results are shown in Fig. 6(B). Under these conditions, the appearance of the groove bottom changed dramatically to a finer worm-like structure with smaller gaps
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W. Zhao et al. / Optics & Laser Technology 58 (2014) 94–99
Fig. 6. SEM and 3D LSCM images of grooves on stainless steel ablated with 10 ps, a repetition rate of 1 kHz and the scan speed 0.05 mm/s by dual-wavelength beam, (A) with the power of 8 mW (main visible beam) and 2 mW (assistant near-infrared), (B) with the power of 8 mW (main visible beam) and 8 mW (assistant near-infrared).
Fig. 7. SEM (1) and 3D LSCM (2) images of groove on stainless steel ablated with 10 ps, a repetition rate of 1 kHz and the scan speed 0.05 mm/s by dual-wavelength beam with the power of 8 mW (main visible beam) and 4 mW (assistant near-infrared).
leading to a level groove bottom. However, excessive power created lower-quality and irregular edges containing the same bulk structures just as shown in Fig. 4(A1 and A2). A high-quality groove, with a smooth bottom and sharp edges, was obtained by judiciously combining the dual-wavelength beam powers using visible beam with a power of 8 mW assisted by nearinfrared with a power of 4 mW, as shown in Fig. 7. The bottom of the groove was flat worm-like structure, the edges of groove were sharper and resolidified ejected material was less obvious than for the grooves shown in Fig. 6. Consequently, the combination of 8mW main beam and 4-mW assistant beam was optimal for this ablation system. The experimental results show that high-quality laser machining of the grooves could be achieved by optimizing the power ratio between the main beam and the assistant beam that made up the
dual-wavelength beam. In this case, the optimal power ratio of the main (visible beam) beam to the assistant (near-infrared) beam was equal to 2. This research indicates that when beam powers are combined appropriately and judiciously, the dual-wavelength beam can create high-quality ablated structures.
4. Conclusion Morphology is an important parameter for estimating the quality of ultrafast laser micromachining. In this study, the morphological features of holes and grooves ablated on the surface of stainless steel samples using both mono-wavelength and dual-wavelength picosecond laser systems with different powers combinations are presented. For ablated holes, side-walls are
W. Zhao et al. / Optics & Laser Technology 58 (2014) 94–99
composed of regular ripples and some ravines for both visible beam and near-infrared, diversely, the ravines only appear near the hole surface and are shallower for the latter; the cross-section profiles for the visible beam appear V-shaped while those for the near-infrared have large openings and appear U-shaped. Holes ablated by the dual-wavelength beam resemble sunflower-like structures and have smoother ring patterns on the bottom. In general, the holes ablated by the dual-wavelength beam appear to have much flatter bottoms, linearly sloped side-walls and spinodal structures between the bottom of the hole and the side-walls. Furthermore, through judiciously combining the powers of the dual-wavelength beam, high-quality grooves could be obtained with a flat worm-like structure at the bottom surface and less resolidified melt ejection edges. This study provides insight into optimizing ultrafast laser micromachining in order to obtain desired morphology. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant no. 91123024), Program for Changjiang Scholars and Innovative Research Team in University (Grant no. IRT1172) and the Fundamental Research Funds for the Central Universities (Grant no. xjj2012113). References [1] Spiro A, Lowe M, Pasmanik G. Appl Phys 2012;107:801–8. [2] Utéza O, Sanner N, Chimier B, Brocas A, Varkentina N, Sentis M, et al. Appl Phys 2011;105:131–41. [3] Raciukaitis G, Brikas M, Gecys P, Gedvilas M. Accumulation effects in laser ablation of metals with high-repetition-rate lasers. Proc SPIE 2008;7005:70052L-1. [4] Latif A, Khaleeq-ur-Rahman M, Rafique MS, Siraj K, Bhatti KA, Perveen A. Radiat Eff Defects Solids 2012;167:199–203.
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[5] Ith M, Pratisto H, Altermatt HJ, Frenz M, Weber HP. Appl Phys B 1994;59:621–9. [6] Shazia B, Hamza V, Khaliq M. Appl Phys A 2013;110:389–95. [7] Mahdieh MH, Nikbakht M, Moghadam ZE, Sobhani M. Appl Surf Sci 2010;256:1778–83. [8] Wynne AE, Stuart BC. Appl Phys A 2003;76:373–8. [9] Petrović SM, Peruško D, Salatić B, Bogdanović-Radovi ĆI, Panjan P, Gakovi ĆB, et al. Opt Laser Technol 2013;54:22–9. [10] Cheng HL, Zheng HR, Lan J, Wu JT, Ping HW, Chi WC, et al. Enhancement of ablation efficiency by a femto/nano-second dual-beam micromachining system. Proc SPIE 2010;7585:75850I-1. [11] Cheng HL, Zheng HR, Lan J, Wu JT, Ping HW, Chi WC, et al. Opt Lett 2010;35:2490–2. [12] Theberge F, Chin SK. Appl Phys A 2005;80:1505–10. [13] Sugioka K, Midorikawa K. VUV-UV multiwavelength excitation process for high-quality ablation of fused silica. Proc SPIE 2013;8777:877704-1. [14] Masanao K, Susumu T, Tetusmi S, Hitoshi S. Dual wavelength femtosecond laser materials processing. Lasers and electro-optics. Baltimore, MD: CLEO; 2007. [15] Tan B, Venkatkrishnan K, Sivakumar NR, Gan GK. Opt Laser Technol 2003;35:199–202. [16] Sandra Z, Johann Z, Georg AR. Appl Surf Sci 2007;253:7692–5. [17] Forsman AC, Banks PS, Perry MD, Campbell EM, Dodell AL, Armas MS. J Appl Phys 2005;98:033302–7. [18] Cheng J, Perrie W, Sharp M, Edwardson SP, Semaltianos NG, Dearden G, et al. Appl Phys A 2009;95:739–46. [19] Otani T, Herbst I, Heglin M, Govorkov SV, Wiessner AO. Appl Phys A 2004;79:1335–9. [20] Mannion PT, Magee J, Coyne E, O'Connor GM, Glynn TJ. Appl Surf Sci 2004;233:275–87. [21] Wenjun W, Xuesong M, Gedong J. Int J Adv Manuf Technol 2009;41:504–12. [22] Wang XC, Lim GC, Zheng HY, Ng FL, Liu W, Chua SJ. Appl Surf Sci 2004;228:221–6. [23] Semaltianos NG, Perrie W, French P, Sharp M, Dearden G, Logothetidis S, et al. Appl Phys A 2009;94:999–1009. [24] Sun YW, Lu QS, Fang ZX, Chen YZ, Li CF, Guan JL, et al. The effect of laser irradiation. Beijing: National Defence Industry Press; 2002. [25] Semerok A, Salle B, Lacour JL, Wagner JF, Petite G, Gobert O, et al. Femtosecond, picosecond and nanosecond laser microablation: laser plasma and crater investigation. Proc SPIE 2001;4424:574–9. [26] Salle B, Chaleard C, Detalle V, Lacour JL, Mauchien P, Nouvellon C, et al. Appl Surf Sci 1999;138:302–5.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Investigations of morphological features of picosecond dual-wavelength laser ablation of stainless steel Wanqin Zhao, Wenjun Wang n, Xuesong Mei, Gedong Jiang, Bin Liu State Key Laboratory for Manufacturing Systems Engineering, Xi'an Jiaotong University, Xi'an 710049, China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 June 2013 Received in revised form 24 September 2013 Accepted 4 November 2013 Available online 26 November 2013
Investigations on the morphological features of holes and grooves ablated on the surface of stainless steel using the picosecond dual-wavelength laser system with different powers combinations are presented based on the scarce researches on morphology of dual-wavelength laser ablation. The experimental results show the profiles of holes ablated by the visible beam appear V-shaped while those for the nearinfrared have large openings and display U-shaped, which are independent of the ablation mechanism of ultrafast laser. For the dual-wavelength beam (a combination of visible beam and near-infrared), the holes resemble sunflower-like structures and have smoother ring patterns on the bottom. In general, the holes ablated by the dual-wavelength beam appear to have much flatter bottoms, linearly sloped sidewalls and spinodal structures between the bottoms of the holes and the side-walls. Furthermore, through judiciously combining the powers of the dual-wavelength beam, high-quality grooves could be obtained with a flat worm-like structure at the bottom surface and less resolidified melt ejection edges. This study provides insight into optimizing ultrafast laser micromachining in order to obtain desired morphology. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Morphology Picosecond laser Dual-wavelength beam
1. Introduction In the last few decades, many studies have been conducted on characteristics of ultrafast lasers micromachining. Some studies have focused on laser parameters, such as fluence, wavelength, pulse duration, repetition rate and pulse number [1–4], while others have been concerned with external laser processing methods, such as changing the processing environment (from or to an ambient liquid, gas or vacuum) [5–9]. All these studies have been aimed at improving the efficiency and morphology of laser micromachining. In recent years, many researchers have adopted combined strategies, such as using dual-laser systems, dualwavelength systems or generating pulse trains, etc., and obtained better ablation efficiency. For double-laser systems, integrated femtosecond (fs) and nanosecond (ns) dual-beam laser systems have been found to be more efficient at removing multiple materials than mono-laser systems [10–12]; in addition, another study reviewed the vacuum ultraviolet (VUV)–ultraviolet (UV) dual-laser excitation process and found that it could perform high-efficiency ablation of different materials attributing to the absorption of the UV laser by excited-states formed by the VUV laser irradiation [13]. For double-wavelength systems, one study
n
Corresponding author. Tel./fax: þ 86 29 82663870. E-mail address: [email protected] (W. Wang).
0030-3992/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2013.11.004
showed that ablation efficiency could be enhanced by using prepulses (fs) of 260 nm with linear absorption followed by main pulses (fs) of 780 nm with three-photon absorption during the selected removal of insulating layers from substrate materials [14]; furthermore, micromachining with dual-wavelength system has been found to improve the aspect ratios of holes and ablation yields [15,16]. With respect to the pulse train technique, one study showed that material removal rates could be increased significantly by using a train of carefully timed pairs of nanosecond laser pulses [17]. However, for the ablated morphology also highly relevant to numerous practical applications, only a few researches have been reported as a small portion of the total papers. For example, one study did report that two lasers could be combined to create holes with sharp edges, flat side-walls and smooth bottoms [13], a few other studies showed that double-wavelength laser could ablate holes with more regular profiles and surface morphologies that appeared to be smoother [15,16]. Based on this information, dualablation strategies may be able to improve the morphological features of ablated structures, further, it is very necessary for the comprehensive studies on the dual-combination ablation morphology attributing to the scarcity of systematical and regular researches. To contribute to this knowledge gap, the morphological features of dual-wavelength beam (a combination of visible beam, 532 nm and near-infrared, 1064 nm) ablation holes and grooves on
W. Zhao et al. / Optics & Laser Technology 58 (2014) 94–99
the surface of stainless steel are presented. In this study, a series of experiments of mono- and dual-wavelength beams ablation were performed. First, the morphological features of holes ablated by two mono-wavelengths, namely visible beam and near-infrared, were compared. Then, the morphology of hole for the dualwavelength beam was analyzed. Subsequently, the ablation of grooves was carried out for the dual-wavelength, and the highquality grooves were obtained by collocating double wavelengths beams powers judiciously.
2. Experiment The stainless steel (SS304) samples as one of the most important industrial materials were used in this study. The samples were of rectangular shape with dimensions of 30 mm 20 mm 1.5 mm (length width thickness). The composition of samples was monitored by EDX (energy dispersive X-ray detector) and the elemental analysis of the SS304 surface is given in Table 1. The laser utilized for irradiation was a neodymium– vanadate (Nd:VAN; Austria laser delivering pulses with 10 ps duration, a power of 2 W and a repetition rate of 1 kHz. Furthermore, it was able to emit radiation at fundamental (1064 nm) and second harmonic. A 150 optical lens was used to focus the beam on the sample. Stainless steel samples with 1.5 mm thickness were placed on a three-dimensional (3D) motorized translation stage to allow for high-precision (50 nm) positional control. An accurate imaging system using a charge-coupled device (CCD) camera allowed for high-resolution imaging of the ablation zone on the target. Fig. 1 is a schematic of the picosecond dual-wavelength laser micromachining system used in this study. First, the delay time between the two wavelengths was about 0.5 ns, whereby the visible beam was emitted first, followed by the near-infrared. Furthermore, the index of refraction is higher for the visible beam than the near-infrared [15], so the focal plane of the shortwavelength beam was closer than that of the long-wavelength beam, the distance via experimentation between the two foci was approximately 1.7 mm. The 1/e2 focused spot diameters (2ω0) at each focal plane for the two wavelengths were simultaneously calculated using the following equation: 2ω0 ¼
4λf M 2 πd
ð1Þ
Table 1 Elemental analysis of SS304 surface using energy dispersive X-ray detector (EDX). Spectrum (wt%) C O
Si
Cr
Mn
Fe
Ni
8.28
0.51
16.72
1.11
62.81
7.80
2.77
95
where ω0 is the radius of beam waist, λ is the wavelength, f is the focal length of the lens, d is the diameter of the incident beam and M2 is the beam quality factor [18,19]. Consequently, the focused waist diameter of a Gaussian beam depends on the beam quality and wavelength in the focal plane, which were approximately 64.1 mm and 70.2 mm for the visible beam and near-infrared, respectively. In order to minimize the influence of spots sizes coming from the different wavelengths, likewise, reduce the difference between the two spot diameters, during irradiation the processing surface was set in focal plane of the near-infrared, such that the visible beam produced divergent beam ablation (the divergent distance was about 1.7 mm), resulting in some amplification of the visible beam spot. Irradiation was carried out under ambient conditions. Various techniques were used to analyze the morphological features of the samples after irradiation, such as scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM).
3. Results and discussion Morphological features of holes and grooves were primarily observed after ablation with the dual-wavelength beam, made up of the visible beam (532 nm) combined with the near-infrared (1064 nm). The results of the induced morphology and geometrical features are presented in the following sections. 3.1. Morphological features of holes ablated by mono-wavelength laser system A series of experiments was conducted for holes ablation with 10 ps pulse duration, a repetition rate of 1 kHz and the same processing surface. Each beam (the visible beam and the nearinfrared) was individually applied at various powers, and then the dual-wavelength beam (combining the visible beam and nearinfrared) was applied at different powers combinations. Fig. 2 shows the SEM images and cross-section profiles of holes on stainless steel surfaces ablated by different wavelengths, demonstrating their dramatically different morphologies. In Fig. 2(A1), which shows a hole ablated by the visible beam with a power of 8 mW, it can be clearly seen that the bottom of the hole was filled with coarse particles and varyingly sized pinholes, most of which had diameters less than 2 mm, at the same time, one larger pinhole with a diameter of about 7 mm appeared in the center of the bottom of the hole. Significant differences were observed for holes ablated by the near-infrared with the same power. As shown in Fig. 2(B1), the bottom of hole consisted of regular ripples and higher-period furrows that were vertical to the ripples. For the visible beam, the side-walls of the holes were mainly composed of symmetrical ravines along the direction of hole depth, otherwise, there was only a small amount of melt ejection, such as debris and droplets, around the hole surface,
Fig. 1. Schematic of the picosecond dual-wavelength laser micromachining system.
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Fig. 2. SEM images and cross-section profiles (datum fitting of LSCM) of holes on stainless steel ablated with 10 ps pulse duration, a repetition rate of 1 kHz and 2000 pulses by visible beam (A, 8 mW) and near-infrared (B, 8 mW).
as shown in Fig. 2(A2). Side-walls of holes ablated by the nearinfrared, consisted of finer and more regular ripples, while some shallower ravines appeared on side-walls near hole surfaces, as shown in Fig. 2(B2). It is possible that these morphological features are associated with ablation mechanisms. For ultrafast lasers, vaporization and phase explosion are the primary ablation mechanisms, and these mechanisms correspond to the two separate ablation regimes: “gentle” ablation and “strong” ablation [20,21]. Fig. 2(B1 and B2) shows little evidence of material melting for the hole, thus vaporization was assumed to be the mechanism responsible for removing material and creating the hole when the near-infrared was used. Unlike holes ablated by the near-infrared, those ablated by the visible beam showed obvious melt ejection, which suggests that phase explosion was the primary mechanism for hole creation in this case. Meanwhile, the variously sized pinholes were discovered in the hole bottom, just as shown in Fig. 2 (A1 and A2), resulting from self-focusing characteristic of air, this is filament [22,23]. In our study, when the other parameters were held constant, the visible beam ablation is more intense than near-infrared due to the energy accumulation significantly from the higher absorptivity and the lower damage threshold of material for the shorter wavelength [24], and the threshold values calculated were 0.05 J/cm2 and 0.20 J/cm2 for visible beam and near-infrared, respectively, in reasonable agreement with other reports. For the holes ablated by visible beam, like the one shown in Fig. 2(A3), the bottom of the hole was rough containing several local peaks and valleys, and the side-walls tended to slope linearly. Overall, the cross-section profile of the hole was V-shaped. For hole ablated by the near-infrared, the main geometric features included a flat hole bottom, curved side-walls, and a large hole opening at the surface, the cross-section profile was U-shaped, as shown in Fig. 2(B3). By comparing Fig. 2(A3) and Fig. 2(B3), it can be seen that the different wavelengths created strikingly different profiles even when all other parameters were held constant. In order to thoroughly study the relationship between ablation mechanism and morphology of hole, another set of experiments was performed for the two mono-wavelengths. Fig. 3 shows the SEM images and cross-section profiles of holes on stainless steel ablated by the visible beam with a lower power (2 mW) and the near-infrared with a higher power (40 mW). First, Fig. 3(A1), which shows only a small amount of material melting around the hole surface, suggests that ablation for the visible beam with the lower power of 2 mW employed the “gentle” ablation mechanism. Meanwhile, in Fig. 3(B1), which shows a hole ablated by nearinfrared with higher power of 40 mW, the bottom of the hole
Fig. 3. SEM images and cross-section profiles (datum fitting of LSCM) of holes on stainless steel ablated with 10 ps pulse duration, a repetition rate of 1 kHz and 2000 pulses by visible beam (A, 2 mW) and near-infrared (B, 40 mW).
appeared to contain coarse particles and the hole surface showed obvious melt ejection, these two key features indicate that the near-infrared with this power of 40 mW used the “strong” ablation mechanism. Comparing Figs. 2 and 3, regardless of whether the ablation mechanism was “gentle” or “strong”, the cross-section profiles of the holes were still V-shaped for the visible beam and U-shaped for the near-infrared. Therefore, the cross-section profile of hole is independent of the ablation mechanisms. In addition, the defined ravines observed on the side-walls of hole ablated by visible beam made it a flower petal-like appearance, whereas the ravines in hole ablated by near-infrared were shallow and only appeared near the surface. These phenomena suggest that there is no close relationship between ravine morphology and ablation mechanism. While the previous results show that the cross-section profiles of holes ablated by each mono-wavelength beam are independent of ablation mechanism, meanwhile, the profiles have nothing to do with the processing surfaces coming from the experiments where the processing surface for visible beam and near-infrared were set at the convergent, focal and divergent planes. The experimental results show that the cross-section profiles of holes ablated by each beam maintained their respective characteristic shapes. In general, for most of the targets, the holes profiles did not correspond to the spatial intensity distribution of the laser, only at the lowest power, did holes profiles coincide with the laser intensity distribution for parts of pulse durations and wavelengths [25,26]. It is inferred that wavelength or wavelength-dependent
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Fig. 4. SEM images and cross-section profile (datum fitting of LSCM) of hole on stainless steel ablated with 10 ps pulse duration, a repetition rate of 1 kHz and 2000 pulses by the dual-wavelength beam with the power of 8 mW for visible beam combining the power of 8m W for near-infrared.
Fig. 5. SEM (1) and 3D LSCM (2) images of groove on stainless steel ablated with 10 ps, a repetition rate of 1 kHz and the scan speed 0.05 mm/s by visible beam with the power of 8 mW.
parameters, such as the ablation threshold, absorptivity and heat conduction, may be responsible for creating the observed profiles, and further exploration would be developed in future. Overall, the obvious differences displayed in the cross-section profiles of holes ablated by different wavelengths provide additional ways customize surface morphologies for various practical applications.
3.2. Morphological features of holes ablated by dual-wavelength laser system Fig. 4 shows the SEM images and cross-section profile of hole on stainless steel ablated by the dual-wavelength beam which consisted of the visible beam (8 mW) combined with the nearinfrared (8 mW). In Fig. 4(A1), it can be noticed that the bottom of the hole is much smoother, patterned with ring-like structures and containing more fine particles. The interaction of the beams during ablation may be responsible for creating the observed structures at the bottom of the hole: the delayed near-infrared plays a re-role in pre-visible beam, in other words, the ripples play a re-role in particles, so the particles are scattered, thereby spreading to the surroundings, which results in a smoother bottom with redistributed ring-like morphology and smaller particles. The side-wall, as shown in Fig. 4(A2), was composed of fine ripples in the interior of the hole, while sunflower-like structures with defined ravines near to the surface which were probably associated to the influence of the very large opening on the hole surface for near-infrared to the ravines obtained mainly by visible beam. In addition, some melt ejection was observed around the hole surface as a result of energy accumulation. The cross-section profile of the hole ablated by the dual-wavelength beam appeared to have its own characteristics, including a much flatter bottom, linearly sloping side-walls, and double spinodal structures between the bottom of the hole and side-walls, likely related to the extension of the U-shaped profile
(associated with the near-infrared) to the V-shaped (associated with the visible beam), as shown in Fig. 4(A3). 3.3. Morphological features of grooves By studying the holes ablated by the visible beam and nearinfrared, both alone and combined, we found that the dualwavelength beam created significant morphological and geometric changes compared with that created by mono-wavelength beams. To further investigate the morphological characteristics associated with dual-wavelength laser ablation, a series of experiments was performed for ablation of grooves on stainless steel with 10 ps pulse duration, a repetition rate of 1 kHz and the scan speed of 0.05 mm/s. Fig. 5 shows SEM and 3D LSCM images of the groove ablated on stainless steel by visible beam with a power of 8 mW. From this figure, it can be seen that the groove edges were flat and linear but the bottom of the groove contained irregular and coarse particles whose surfaces were covered with nano-sized stripes. For this reason, we expected that the bottom of the groove would be smooth and regular in the premise of guaranteeing the edges of groove were unbroken. In other words, we expected the nearinfrared would assist the main visible beam in making the bottom of the groove much flatter. When the near-infrared with a lower power (2 mW) was used as an assistant beam, particles in the bottom of the groove had been diminished to form the same sized sub-particles with relatively small gaps, as shown in Fig. 6(A1). The dual-wavelength beam, in which the assistant beam had a power of 2 mW and the main beam had a power of 8 mW, smoothed the bottom of the groove, but not considerably. Then the power of 8 mW was given to the assistant beam and the results are shown in Fig. 6(B). Under these conditions, the appearance of the groove bottom changed dramatically to a finer worm-like structure with smaller gaps
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Fig. 6. SEM and 3D LSCM images of grooves on stainless steel ablated with 10 ps, a repetition rate of 1 kHz and the scan speed 0.05 mm/s by dual-wavelength beam, (A) with the power of 8 mW (main visible beam) and 2 mW (assistant near-infrared), (B) with the power of 8 mW (main visible beam) and 8 mW (assistant near-infrared).
Fig. 7. SEM (1) and 3D LSCM (2) images of groove on stainless steel ablated with 10 ps, a repetition rate of 1 kHz and the scan speed 0.05 mm/s by dual-wavelength beam with the power of 8 mW (main visible beam) and 4 mW (assistant near-infrared).
leading to a level groove bottom. However, excessive power created lower-quality and irregular edges containing the same bulk structures just as shown in Fig. 4(A1 and A2). A high-quality groove, with a smooth bottom and sharp edges, was obtained by judiciously combining the dual-wavelength beam powers using visible beam with a power of 8 mW assisted by nearinfrared with a power of 4 mW, as shown in Fig. 7. The bottom of the groove was flat worm-like structure, the edges of groove were sharper and resolidified ejected material was less obvious than for the grooves shown in Fig. 6. Consequently, the combination of 8mW main beam and 4-mW assistant beam was optimal for this ablation system. The experimental results show that high-quality laser machining of the grooves could be achieved by optimizing the power ratio between the main beam and the assistant beam that made up the
dual-wavelength beam. In this case, the optimal power ratio of the main (visible beam) beam to the assistant (near-infrared) beam was equal to 2. This research indicates that when beam powers are combined appropriately and judiciously, the dual-wavelength beam can create high-quality ablated structures.
4. Conclusion Morphology is an important parameter for estimating the quality of ultrafast laser micromachining. In this study, the morphological features of holes and grooves ablated on the surface of stainless steel samples using both mono-wavelength and dual-wavelength picosecond laser systems with different powers combinations are presented. For ablated holes, side-walls are
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composed of regular ripples and some ravines for both visible beam and near-infrared, diversely, the ravines only appear near the hole surface and are shallower for the latter; the cross-section profiles for the visible beam appear V-shaped while those for the near-infrared have large openings and appear U-shaped. Holes ablated by the dual-wavelength beam resemble sunflower-like structures and have smoother ring patterns on the bottom. In general, the holes ablated by the dual-wavelength beam appear to have much flatter bottoms, linearly sloped side-walls and spinodal structures between the bottom of the hole and the side-walls. Furthermore, through judiciously combining the powers of the dual-wavelength beam, high-quality grooves could be obtained with a flat worm-like structure at the bottom surface and less resolidified melt ejection edges. This study provides insight into optimizing ultrafast laser micromachining in order to obtain desired morphology. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant no. 91123024), Program for Changjiang Scholars and Innovative Research Team in University (Grant no. IRT1172) and the Fundamental Research Funds for the Central Universities (Grant no. xjj2012113). References [1] Spiro A, Lowe M, Pasmanik G. Appl Phys 2012;107:801–8. [2] Utéza O, Sanner N, Chimier B, Brocas A, Varkentina N, Sentis M, et al. Appl Phys 2011;105:131–41. [3] Raciukaitis G, Brikas M, Gecys P, Gedvilas M. Accumulation effects in laser ablation of metals with high-repetition-rate lasers. Proc SPIE 2008;7005:70052L-1. [4] Latif A, Khaleeq-ur-Rahman M, Rafique MS, Siraj K, Bhatti KA, Perveen A. Radiat Eff Defects Solids 2012;167:199–203.
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