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Drilling study on Cu, Mo, W and Ti by using SBS pulse compressed steep leading edge hundred picoseconds laser
Optik 127 (2016) 11156–11160
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Drilling study on Cu, Mo, W and Ti by using SBS pulse compressed steep leading edge hundred picoseconds laser Zhenxu Bai, Can Cui, Zhaohong Liu, Hang Yuan, Hongli Wang, Yulei Wang, Zhiwei Lu ∗ National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 13 July 2016 Accepted 13 September 2016 Keywords: Laser drilling SBS pulse compression Hundred picoseconds Micro structure
a b s t r a c t For the first time, studies on stimulated Brillouin scattering (SBS) pulse compression based hundred picoseconds laser processing of Cu, Mo, W and Ti have been experimentally investigated. Different from other laser output, the pulse width of the SBS pulse compressed laser is about 450 ps with a steep rising edge. The number of pulses to drill through the four kinds of metals with thicknesses 1 mm have been measured. The diameters, depths and morphologies of the drilled craters were observed with optical microscope and scanning electron microscope (SEM). Different micro structures on the four metals are obtained in our experiment. Our research provides a potential application of high energy hundred picoseconds lasers in the future. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction In the past decades, laser processing and manufacturing techniques have been widely used in high-tech industry, due to the characteristics of non-contact, adjustable parameter, high accuracy, high efficiency and good economic benefits [1–3]. In most application areas such as material micro-processing, optical parametric oscillator (OPO) pumping, laser induced plasma (LIP), laser shock peening (LSP), etc., high peak power lasers with narrow pulse width are required [4–7]. Especially in the field of material processing, with continuous improved hardness and melting point of the materials, people often need to carry out studies on drilling or damage threshold. Theoretical and experimental results have indicated that shorter pulse width and greater peak energy density can effectively reduce the heat-affected zone (HAZ) and avoid the formation of the recasting layer. When pulse durations smaller than the electron-phonon coupling time, a few picoseconds in metals generally, heat conduction will be decreased [8]. In addition, as a hot research direction currently, surface-enhanced Raman scattering (SERS) can be used to detect the chemical components in food, medicine, polluted water etc [9,10]. In most cases, the enhancement need to take place at a metal surface which has micro- and nano-scale roughness, while different microstructures of metal surface can also obtain different enhancement effects. Therefore, it has great application prospects to obtain the metal materials with different surface structures [11,12]. Although a signification amount of research has been conducted on laser drilling with ultra-short pulses from nanosecond to femtosecond, only a few of high energy processing has been reported. Meanwhile, due to the low stability and single pulse energy of picosecond and femtosecond lasers, they still far from adequate for industrial use. In practical application, people not only need to make the pulse width as narrow as possible, but also hope that the laser has high energy to improve the
∗ Corresponding author. E-mail address: zw [email protected] (Z. Lu). http://dx.doi.org/10.1016/j.ijleo.2016.09.061 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
Z. Bai et al. / Optik 127 (2016) 11156–11160
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Table 1 Thermal and physical characteristics of the selected metals.
Degrees of purity Density, V (g cm3 ) Specific Heat, Cl (J kg−1 K−1 ) Melting Temperature, Tm (K) Vaporization Temperature, Tv (K) Thermal conductivity, K (W m−1 K−1 ) Latent Heat of Fusion, Lm (J g−1 ) Latent Heat of Evaporation, Lv (J g−1 )
Cu
Mo
W
Ti
≥99.95% 8.96 385 1358 2841 401 205 4796
≥99.98% 10.28 251 2893 4885 138 290 6153
≥99.95% 19.35 130 3685 5934 160 192 4009
≥99.90% 4.507 520 1941 3560 21.9 365 8893
Fig. 1. Schematic illustration of the experimental setup.
Fig. 2. Characteristics of the SBS pulse width compressed laser beam (a) Temporal pulse profiles; (b) 3D intensity distribution of the far-field.
processing efficiency. However, nearly all previous high energy processing studies have focused on the nanosecond pulse that no related reports on the processing with high energy hundred picoseconds or narrower pulse width lasers [13–16]. Compared already well developed and commercially available high energy nanosecond laser, high energy hundred picoseconds laser system is still not widely available. In this research work, we experimentally investigated the pulse drilling of four metallic materials including Cu, Mo, W and Ti by using the self-developed hundred picoseconds laser system at 1064 nm wavelength. Those metals are widely used in the field of aerospace engineering, manufacturing, semiconductor and life science. The thermal and physical characteristics of the materials used in our experiment are given in Table 1. The purities of the four metals are all above 99.9%. With the same incident pulse energy, we compared the drilling results of the four materials by changing the number of pulses from 1 to 500. Although experiments on the ablation of metals with picosecond laser pulses have been reported, to the best of our knowledge, it is the first time that stimulated Brillouin scattering (SBS) pulse compressed steep leading edge hundred picoseconds laser is used in material drilling study. Because hundred picoseconds laser based on SBS compression can obtain pulse output up to joule level with high compact structure, our work can offer a new approach to realize high energy picosecond pulse laser processing; also provide some guidance for the potential industrial applications of high energy hundred picoseconds laser in the future. 2. Experimental setup and procedure Fig. 1 shows the schematic of the experimental setup. The hundred picoseconds laser is self-demonstrated based on SBS pulse compression [17–19]. Single-longitudinal-mode (SLM) with pulse with of 10 ns and single pulse energy 10 mJ is used as a seed source, which is based on a Fabry-Perot etalon mode selection Q-switched Nd:YAG oscillator. The optical isolator, consisting a polarizer P1 , a 1/2 wave-plate and a Faraday rotator, is used to prevent the backward Stokes light returning to the oscillator and ensure the stability of the oscillator. In the stage of SBS pulse compressor, FC-43 is chosen as the SBS medium in the SBS cell for its relative high SBS reflectivity and high pulse compression efficiency [20,21]. 1.0 at.% doped Nd:YAG rod with diameter of 8 mm and a length of 145 mm is applied as gain medium. After reflecting through M1 , SLM
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Fig. 3. The relationship between the number of drilling pulses and (a) diameter of the hole, (b) depth of the hole.
Fig. 4. Optical microscope 2D and SEM images of the hole entrances at 15 mJ and 500 pulses of (a–c) Cu, (d–f) Mo, (g–i) W, and (j–l) Ti.
seed is focused on the SBS Cell, producing the backward scattering Stokes light. Through a 1/4 wave-plate, backward Stokes light is reflected from the polarizer P2 with polarization angle rotated from p- to s-polarized. Maximum output pulse energy at 1064 nm was 200 mJ with temporal pulse width of 450 ps. The pulse to pulse stability measured was less than 3%. Pulses energy was adjusted continuously with a combination of a 1/2 wave-plate and a polarizer P3 . The pulse generated from the SBS pulse compressed laser has steep leading edge in the time domain, as shown in Fig. 2(a). The output beam traversed through the convex lens L2 with f = +200 mm. Focused spot size was calculated as the following equation [22]: 2ω0 =
4f · M2 d
(1)
Z. Bai et al. / Optik 127 (2016) 11156–11160
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Fig. 5. Optical microscope 3D images of the holes at 15 mJ and the number of pulses 500 of (a) Cu, (b) Mo, (c) W, and (d) Ti.
where ω0 is the radius of beam waist, is the wavelength, f is the focal length of the focusing lens, d is the diameter of laser beam and M2 is the beam quality factor. Using the values of = 1064 nm, f = 200 mm, d = 7 mm and M2 = 2.4, the focused waist diameter 2ω0 was calculated to be ∼100 m. The three-dimensional far-field laser intensity distribution profiles is given in Fig. 2(b). Samples were fixed on the two-dimensional displacement platform with precision up to ∼10 m. A shutter is adopted to control the number of pulses worked on the target. Laser indicator is used to indicate the position of the laser on the material. In our experiment, the samples were exposed to the laser light at 1064 nm and pulse width of 450 ps. As too high energy density will exceed the breakdown threshold of air, the single pulse energy we adopted in the experiment is 15 mJ. 3. Results and discussion We drilled a series of holes with different number of pulses (10, 20, 50, 100, 200, 300, 400 and 500 respectively). The hole diameter and depth of the four samples as a function of the number of drilling pulses are shown in Fig. 3. The error bars in the diagram are the error range of the measurement. With increase of the number of pulses, both diameter and depth grow rapidly in the first 100 pulses; however, the grow speed slow down after 100 pulses. The reason for this phenomenon is that the oxide layer and sputtering formed gradually on the surface of the metals during ablating, making the pulses cannot interact sufficiently with the metals. In addition, we can observe from the results that the diameter of a hole is almost inversely proportional to its depth. For example, W and Mo have the largest diameter but with the minimum depth. Although the depth of Ti is deeper than Cu when the number of pulses is 400 and 500, it is also consistent with the results within the range of measurement error. Fig. 4 presents the optical microscope and scanning electron microscope (SEM) photographs of the hole drilled by the laser light at 1064 nm wavelength, pulse energy 15 mJ and pulse width 450 ps with the number of pulses 500. The external contours of the holes of Cu, Mo, W, and Ti are shown in Fig. 4(a), (d), (g) and (j) with diameters 196 m, 298 m, 365 m and 233 m respectively. We measured the 3D surface features of the holes with the Carl Zeiss microscope system (Smart Zoom 5). As Fig. 5 shows, the depth of the four holes are 47.2 m, 35 m, 29.9 m and 49.8 m respectively. We can observe that all the circumference of the holes is very smooth with higher roundness, which shows that the SBS compressed laser has high quality in far-field. This is consistent with the results we observed with CCD, as shown in Fig. 2(b). At the same time, a certain amount of sputtering can be seen on surface of the hole, accompanied by a certain of HAZ around. The HAZ of the four materials is in the range of 50–100 m in width, which is caused by the relative high pulse energy [23,24]. Fig. 4(b), (e), (h) and (k) are low magnification (∼20 m) SEM pictures of the four materials with both center and edge microstructures of the holes. Compared with the center, the microstructures at the edge has obvious sputtering accumulations; moreover, with the increase of the number of pulses, both the sputtering layer and HAZ increase showing a circular gradient around the drilled hole. Fig. 4(c), (f), (i) and (l) present the microstructures of the holes at center. The microstructure of Mo and W is
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very similar with irregular small spherical particles; Cu with relatively uniform distributed linear structures; while, Ti with large size slightly convex flat structures. 4. Conclusion Hundred picoseconds laser single-pulse drilling of Cu, Mo, W and Ti was first experimentally studied for potential micromachining applications. By using the self-developed SBS pulse compressed hundred picoseconds laser system, the influence of different pulse number on the ablated diameter and depth of the four metals is studied. The measurement was carried out by means of 3D optical microscope and SEM. In the case of the same energy injection, maximum depth 49.8 m in Ti plate was obtained. From the experiment data, we observed that the diameter is near inversely proportional to the depth of the hole in the four kinds of metal materials. The microstructure of the ablated place was also observed, where Cu and Ti exhibit a regular linear and flat structure respectively. Our study is very useful to reveal the ablation and micro-structures of metals with SBS pulse compressed hundred picoseconds laser. At the same time, it can also provide some guidance for the high energy hundred picoseconds LSP, material processing and preparation of SERS substrate. In addition, taking into account of the air breakdown threshold, we did not use all the energy for processing in our current study. At present, joule-level hundred picoseconds SBS pulse width compressed laser output with high beam quality and high stability can obtained in our group, which provides the possibility for the potential applications requiring high energy, high peak power and high efficiency. Acknowledgements The work is supported by National Natural Science Foundation of China (grant Nos. 61378007, 61138005 and 61378016). The authors are grateful to Dr. Peng Gu of Institute of Laser Engineering, Beijing University of Technology for sample analysis; and Opton Inc. for providing the Carl Zeiss microscope. References [1] B.N. Chichkov, C. Momma, S. Nolte, F. Alvensleben, A. Tünnermann, Femtosecond, picosecond and nanosecond laser ablation of solids, Appl. Phys. A 63 (1996) 109–115. [2] Y. Long, Q. Liu, Z. Zhong, L. Xiong, T. Shi, Experimental study on the processes of laser-enhanced electrochemical micromachining stainless steel, Optik 126 (2015) 1826–1829. [3] J. Jiang, M. Chen, Z. Bai, C. Yang, G. Li, Influence of polarization on the hole formation with picosecond laser, Opt. Rev. 20 (2013) 496–499. [4] A. Tyazhev, G. Marchev, V. Badikov, A. Esteban-Martin, D. Badikov, V. Panyutin, G. Shevyrdyaeva, S. Sheina, A. Fintisova, V. Petrov, High-power HgGa2 S4 optical parametric oscillator pumped at 1064 nm and operating at 100 Hz, Laser Photonics Rev. 7 (2013) L21–L24. [5] J. Han, Y. Li, W. Fan, C. He, P. Wang, G. Feng, C. Guo, The ablation of Ta2 O5 film by pulsed nanosecond Gaussian laser beams, Optik 126 (2015) 2327–2330. [6] A. Gorodetsky, A.D. Koulouklidis, M. Massaouti, S. Tzortzakis, Physics of the conical broadband terahertz emission from two-color laser-induced plasma filaments, Phys. Rev. A 89 (2014) 033838. [7] X. Zhang, C. Wang, Z. Ye, Y. Qi, C. Liu, Z. Xiang, J. Ge, Numerical analysis for single-frequency nanosecond optical parametric oscillators and optical parametric amplifiers, Optik 126 (2015) 1128–1132. [8] A. Ancona, S. Döring, C. Jauregui, F. Röser, J. Limpert, S. Nolte, A. Tünnermann, Femtosecond and picosecond laser drilling of metals at high repetition rates and average powers, Opt. Lett. 34 (2009) 3304–3306. [9] R. Zhang, Y. Zhang, Z.C. Dong, S. Jiang, C. Zhang, L.G. Chen, L. Zhang, Y. Liao, J. Aizpurua, Y. Luo, J.L. Yang, J.G. Hou, Chemical mapping of a single molecule by plasmon-enhanced Raman scattering, Nature 498 (2013) 82–86. [10] X. Lin, W.L.J. Hasi, S.Q.G.W. Han, X.T. Lou, D.Y. Lin, Z.W. Lu, Fabrication of transparent SERS platform via interface self-assembly of gold nanorods and gel trapping technique for on-site real time detection, Phys. Chem. Chem. Phys. 17 (2015) 31324–31331. [11] D. Liu, C. Li, F. Zhou, T. Zhang, H. Zhang, X. Li, G. Duan, W. Cai, Y. Li, Rapid synthesis of monodisperse Au nanospheres through a laser irradiation-induced shape conversion, self-assembly and their electromagnetic coupling SERS enhancemen, Sci. Rep. 5 (2015) 7686. [12] B.B. Xu, Z.C. Ma, L. Wang, R. Zhang, L.G. Niu, Z. Yang, Y.L. Zhang, W.H. Zheng, B. Zhao, Y. Xu, Q.D. Chen, H. Xia, H.B. Sun, Localized flexible integration of high-efficiency surface enhanced Raman scattering (SERS) monitors into microfluidic channels, Lab Chip 19 (2011) 3347–3351. [13] H.K. Tönshoff, C. Momma, A. Ostendorf, S. Nolte, G. Kamlage, Microdrilling of metals with ultrashort laser pulses, J. Laser Appl. 12 (2000) 23–27. [14] J. Cheng, W. Perrie, M. Sharp, S.P. Edwardson, N.G. Semaltianos, G. Dearden, K.G. Watkins, Single-pulse drilling study on Au, Al and Ti alloy by using a picosecond laser, Appl. Phys. A 95 (2009) 739–746. [15] A. Spiro, M. Lowe, G. Pasanik, Drilling rate of five metals with picosecond laser pulses at 355, 532, and 1064 nm, Appl. Phys. A 107 (2012) 801–808. [16] Y.-C. Hung, Y.-J. Chang, C.-L. Kuo, J.-C. Hsu, C.-C. Ho, Comparison between laser and stamping without die (SWD) for micro tapered hole forming, Appl. Sci. 6 (2016) 77. [17] Z. Bai, Y. Wang, Z. Lu, H. Yuan, Z. Zheng, S. Li, Y. Chen, Z. Liu, C. Cui, H. Wang, R. Liu, High compact, high quality single longitudinal mode hundred picoseconds laser based on stimulated brillouin scattering pulse compression, Appl. Sci. 6 (2016) 29. [18] Z. Bai, Y. Wang, Z. Lu, Y. Chen, S. Li, H. Yuan, Z. Liu, T. Tan, C. Cui, H. Wang, A single-longitudinal-mode Nd:Ce:YAG Q-switched laser based on three-plan resonant reflector, J. Russ. Laser Res. 37 (2016) 382–388. [19] Z. Bai, Y. Wang, Z. Lu, H. Yuan, L. Jiang, T. Tan, Z. Liu, H. Wang, C. Cui, W. Hasi, Efficient KDP frequency doubling SBS pulse compressed 532 nm hundred picosecond laser, Optik 127 (2016) 9201–9205. [20] Z.X. Zheng, W.L.J. Hasi, H. Zhao, S.X. Cheng, X.Y. Wang, D.Y. Lin, W.M. He, Z.W. Lü, Compression characteristics of two new SBS mediums to generate 100-ps pulse for shock ignition, Appl. Phys. B 116 (2014) 659–663. [21] W. Hasi, H. Zhao, D. Lin, W. He, Z. Lu, Characteristics of perfluorinated amine media for stimulated Brillouin scattering in hundreds of picoseconds pulse compression at 532 nm, Chin. Opt. Lett. 13 (2015) 061901. [22] W.M. Steen, K. Watkins, Laser Material Processing, 4th ed., Springer, London, 2003. [23] J.P. Duan, M. Chen, Z.X. Bai, G. Li, Research on the metal ablation thresholds of picosecond laser, in: 3rd International Conference on Digital Manufacturing and Automation, Guangxi, China, 1–2 August, 2012, pp. 547–550. [24] R.L. Harzic, N. Huot, E. Audouard, C. Jonin, P. Laporte, S. Valette, A. Fraczkiewicz, R. Fortunier, Comparison of heat-affected zones due to nanosecond and femtosecond laser pulses using transmission electronic microscopy, Appl. Phys. Lett. 80 (2002) 3886–3888.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Drilling study on Cu, Mo, W and Ti by using SBS pulse compressed steep leading edge hundred picoseconds laser Zhenxu Bai, Can Cui, Zhaohong Liu, Hang Yuan, Hongli Wang, Yulei Wang, Zhiwei Lu ∗ National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 13 July 2016 Accepted 13 September 2016 Keywords: Laser drilling SBS pulse compression Hundred picoseconds Micro structure
a b s t r a c t For the first time, studies on stimulated Brillouin scattering (SBS) pulse compression based hundred picoseconds laser processing of Cu, Mo, W and Ti have been experimentally investigated. Different from other laser output, the pulse width of the SBS pulse compressed laser is about 450 ps with a steep rising edge. The number of pulses to drill through the four kinds of metals with thicknesses 1 mm have been measured. The diameters, depths and morphologies of the drilled craters were observed with optical microscope and scanning electron microscope (SEM). Different micro structures on the four metals are obtained in our experiment. Our research provides a potential application of high energy hundred picoseconds lasers in the future. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction In the past decades, laser processing and manufacturing techniques have been widely used in high-tech industry, due to the characteristics of non-contact, adjustable parameter, high accuracy, high efficiency and good economic benefits [1–3]. In most application areas such as material micro-processing, optical parametric oscillator (OPO) pumping, laser induced plasma (LIP), laser shock peening (LSP), etc., high peak power lasers with narrow pulse width are required [4–7]. Especially in the field of material processing, with continuous improved hardness and melting point of the materials, people often need to carry out studies on drilling or damage threshold. Theoretical and experimental results have indicated that shorter pulse width and greater peak energy density can effectively reduce the heat-affected zone (HAZ) and avoid the formation of the recasting layer. When pulse durations smaller than the electron-phonon coupling time, a few picoseconds in metals generally, heat conduction will be decreased [8]. In addition, as a hot research direction currently, surface-enhanced Raman scattering (SERS) can be used to detect the chemical components in food, medicine, polluted water etc [9,10]. In most cases, the enhancement need to take place at a metal surface which has micro- and nano-scale roughness, while different microstructures of metal surface can also obtain different enhancement effects. Therefore, it has great application prospects to obtain the metal materials with different surface structures [11,12]. Although a signification amount of research has been conducted on laser drilling with ultra-short pulses from nanosecond to femtosecond, only a few of high energy processing has been reported. Meanwhile, due to the low stability and single pulse energy of picosecond and femtosecond lasers, they still far from adequate for industrial use. In practical application, people not only need to make the pulse width as narrow as possible, but also hope that the laser has high energy to improve the
∗ Corresponding author. E-mail address: zw [email protected] (Z. Lu). http://dx.doi.org/10.1016/j.ijleo.2016.09.061 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
Z. Bai et al. / Optik 127 (2016) 11156–11160
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Table 1 Thermal and physical characteristics of the selected metals.
Degrees of purity Density, V (g cm3 ) Specific Heat, Cl (J kg−1 K−1 ) Melting Temperature, Tm (K) Vaporization Temperature, Tv (K) Thermal conductivity, K (W m−1 K−1 ) Latent Heat of Fusion, Lm (J g−1 ) Latent Heat of Evaporation, Lv (J g−1 )
Cu
Mo
W
Ti
≥99.95% 8.96 385 1358 2841 401 205 4796
≥99.98% 10.28 251 2893 4885 138 290 6153
≥99.95% 19.35 130 3685 5934 160 192 4009
≥99.90% 4.507 520 1941 3560 21.9 365 8893
Fig. 1. Schematic illustration of the experimental setup.
Fig. 2. Characteristics of the SBS pulse width compressed laser beam (a) Temporal pulse profiles; (b) 3D intensity distribution of the far-field.
processing efficiency. However, nearly all previous high energy processing studies have focused on the nanosecond pulse that no related reports on the processing with high energy hundred picoseconds or narrower pulse width lasers [13–16]. Compared already well developed and commercially available high energy nanosecond laser, high energy hundred picoseconds laser system is still not widely available. In this research work, we experimentally investigated the pulse drilling of four metallic materials including Cu, Mo, W and Ti by using the self-developed hundred picoseconds laser system at 1064 nm wavelength. Those metals are widely used in the field of aerospace engineering, manufacturing, semiconductor and life science. The thermal and physical characteristics of the materials used in our experiment are given in Table 1. The purities of the four metals are all above 99.9%. With the same incident pulse energy, we compared the drilling results of the four materials by changing the number of pulses from 1 to 500. Although experiments on the ablation of metals with picosecond laser pulses have been reported, to the best of our knowledge, it is the first time that stimulated Brillouin scattering (SBS) pulse compressed steep leading edge hundred picoseconds laser is used in material drilling study. Because hundred picoseconds laser based on SBS compression can obtain pulse output up to joule level with high compact structure, our work can offer a new approach to realize high energy picosecond pulse laser processing; also provide some guidance for the potential industrial applications of high energy hundred picoseconds laser in the future. 2. Experimental setup and procedure Fig. 1 shows the schematic of the experimental setup. The hundred picoseconds laser is self-demonstrated based on SBS pulse compression [17–19]. Single-longitudinal-mode (SLM) with pulse with of 10 ns and single pulse energy 10 mJ is used as a seed source, which is based on a Fabry-Perot etalon mode selection Q-switched Nd:YAG oscillator. The optical isolator, consisting a polarizer P1 , a 1/2 wave-plate and a Faraday rotator, is used to prevent the backward Stokes light returning to the oscillator and ensure the stability of the oscillator. In the stage of SBS pulse compressor, FC-43 is chosen as the SBS medium in the SBS cell for its relative high SBS reflectivity and high pulse compression efficiency [20,21]. 1.0 at.% doped Nd:YAG rod with diameter of 8 mm and a length of 145 mm is applied as gain medium. After reflecting through M1 , SLM
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Fig. 3. The relationship between the number of drilling pulses and (a) diameter of the hole, (b) depth of the hole.
Fig. 4. Optical microscope 2D and SEM images of the hole entrances at 15 mJ and 500 pulses of (a–c) Cu, (d–f) Mo, (g–i) W, and (j–l) Ti.
seed is focused on the SBS Cell, producing the backward scattering Stokes light. Through a 1/4 wave-plate, backward Stokes light is reflected from the polarizer P2 with polarization angle rotated from p- to s-polarized. Maximum output pulse energy at 1064 nm was 200 mJ with temporal pulse width of 450 ps. The pulse to pulse stability measured was less than 3%. Pulses energy was adjusted continuously with a combination of a 1/2 wave-plate and a polarizer P3 . The pulse generated from the SBS pulse compressed laser has steep leading edge in the time domain, as shown in Fig. 2(a). The output beam traversed through the convex lens L2 with f = +200 mm. Focused spot size was calculated as the following equation [22]: 2ω0 =
4f · M2 d
(1)
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Fig. 5. Optical microscope 3D images of the holes at 15 mJ and the number of pulses 500 of (a) Cu, (b) Mo, (c) W, and (d) Ti.
where ω0 is the radius of beam waist, is the wavelength, f is the focal length of the focusing lens, d is the diameter of laser beam and M2 is the beam quality factor. Using the values of = 1064 nm, f = 200 mm, d = 7 mm and M2 = 2.4, the focused waist diameter 2ω0 was calculated to be ∼100 m. The three-dimensional far-field laser intensity distribution profiles is given in Fig. 2(b). Samples were fixed on the two-dimensional displacement platform with precision up to ∼10 m. A shutter is adopted to control the number of pulses worked on the target. Laser indicator is used to indicate the position of the laser on the material. In our experiment, the samples were exposed to the laser light at 1064 nm and pulse width of 450 ps. As too high energy density will exceed the breakdown threshold of air, the single pulse energy we adopted in the experiment is 15 mJ. 3. Results and discussion We drilled a series of holes with different number of pulses (10, 20, 50, 100, 200, 300, 400 and 500 respectively). The hole diameter and depth of the four samples as a function of the number of drilling pulses are shown in Fig. 3. The error bars in the diagram are the error range of the measurement. With increase of the number of pulses, both diameter and depth grow rapidly in the first 100 pulses; however, the grow speed slow down after 100 pulses. The reason for this phenomenon is that the oxide layer and sputtering formed gradually on the surface of the metals during ablating, making the pulses cannot interact sufficiently with the metals. In addition, we can observe from the results that the diameter of a hole is almost inversely proportional to its depth. For example, W and Mo have the largest diameter but with the minimum depth. Although the depth of Ti is deeper than Cu when the number of pulses is 400 and 500, it is also consistent with the results within the range of measurement error. Fig. 4 presents the optical microscope and scanning electron microscope (SEM) photographs of the hole drilled by the laser light at 1064 nm wavelength, pulse energy 15 mJ and pulse width 450 ps with the number of pulses 500. The external contours of the holes of Cu, Mo, W, and Ti are shown in Fig. 4(a), (d), (g) and (j) with diameters 196 m, 298 m, 365 m and 233 m respectively. We measured the 3D surface features of the holes with the Carl Zeiss microscope system (Smart Zoom 5). As Fig. 5 shows, the depth of the four holes are 47.2 m, 35 m, 29.9 m and 49.8 m respectively. We can observe that all the circumference of the holes is very smooth with higher roundness, which shows that the SBS compressed laser has high quality in far-field. This is consistent with the results we observed with CCD, as shown in Fig. 2(b). At the same time, a certain amount of sputtering can be seen on surface of the hole, accompanied by a certain of HAZ around. The HAZ of the four materials is in the range of 50–100 m in width, which is caused by the relative high pulse energy [23,24]. Fig. 4(b), (e), (h) and (k) are low magnification (∼20 m) SEM pictures of the four materials with both center and edge microstructures of the holes. Compared with the center, the microstructures at the edge has obvious sputtering accumulations; moreover, with the increase of the number of pulses, both the sputtering layer and HAZ increase showing a circular gradient around the drilled hole. Fig. 4(c), (f), (i) and (l) present the microstructures of the holes at center. The microstructure of Mo and W is
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very similar with irregular small spherical particles; Cu with relatively uniform distributed linear structures; while, Ti with large size slightly convex flat structures. 4. Conclusion Hundred picoseconds laser single-pulse drilling of Cu, Mo, W and Ti was first experimentally studied for potential micromachining applications. By using the self-developed SBS pulse compressed hundred picoseconds laser system, the influence of different pulse number on the ablated diameter and depth of the four metals is studied. The measurement was carried out by means of 3D optical microscope and SEM. In the case of the same energy injection, maximum depth 49.8 m in Ti plate was obtained. From the experiment data, we observed that the diameter is near inversely proportional to the depth of the hole in the four kinds of metal materials. The microstructure of the ablated place was also observed, where Cu and Ti exhibit a regular linear and flat structure respectively. Our study is very useful to reveal the ablation and micro-structures of metals with SBS pulse compressed hundred picoseconds laser. At the same time, it can also provide some guidance for the high energy hundred picoseconds LSP, material processing and preparation of SERS substrate. In addition, taking into account of the air breakdown threshold, we did not use all the energy for processing in our current study. At present, joule-level hundred picoseconds SBS pulse width compressed laser output with high beam quality and high stability can obtained in our group, which provides the possibility for the potential applications requiring high energy, high peak power and high efficiency. Acknowledgements The work is supported by National Natural Science Foundation of China (grant Nos. 61378007, 61138005 and 61378016). 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