Influence of laser-induced air breakdown on femtosecond laser ablation of aluminum Hang Zhang,1 Fangteng Zhang,1 Xi Du,1 Guoping Dong,1 and Jianrong Qiu1, 2* 1
State Key Laboratory of Luminescent Materials and Devices, and Institute of Optical Communication Materials South China University of Technology, Guangzhou 510640, China 2 The China-Germany Research Center for Photonic Materials and Devices, Guangzhou 510640, China * [email protected]
Abstract: We investigated the influence of laser-induced air breakdown on the femtosecond laser ablation of aluminum target using time-resolved pump-probe shadowgraphic imaging method. The early-stage plasma expanding dynamics and subsequent expanding behaviors of shockwaves and material ejection plume were analyzed through shadowgraphs recorded at different time delays. The dominated mechanisms were clarified at different stages during femtosecond laser pulses ablating aluminum, which provide very valuable information for ultrashort laser ablation of metals. ©2015 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (140.3440) Laser-induced breakdown; (320.7100) Ultrafast measurements; (160.3900) Metals.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
A. S. Mahmood, M. Sivakumar, K. Venkatakrishnan, and B. Tan, “Enhancement in optical absorption of silicon fibrous nanostructure produced using femtosecond laser ablation,” Appl. Phys. Lett. 95(3), 034107 (2009). T. Chen, J. Si, X. Hou, S. Kanehira, K. Miura, and K. Hirao, “Luminescence of black silicon fabricated by high-repetition rate femtosecond laser pulses,” J. Appl. Phys. 110(7), 073106 (2011). M. Yamaji, H. Kawashima, J. Suzuki, and S. Tanaka, “Three dimensional micromachining inside a transparent material by single pulse femtosecond laser through a hologram,” Appl. Phys. Lett. 93(4), 041116 (2008). S. Amoruso, G. Ausanio, R. Bruzzese, M. Vitiello, and X. Wang, “Femtosecond laser pulse irradiation of solid targets as a general route to nanoparticle formation in a vacuum,” Phys. Rev. B 71(3), 033406 (2005). B. Liu, Z. Hu, Y. Che, Y. Chen, and X. Pan, “Nanoparticle generation in ultrafast pulsed laser ablation of nickel,” Appl. Phys. Lett. 90(4), 044103 (2007). V. Margetic, A. Pakulev, A. Stockhaus, M. Bolshov, K. Niemax, and R. Hergenröder, “A comparison of nanosecond and femtosecond laser-induced plasma spectroscopy of brass samples,” Acta, Part B 55(11), 1771–1785 (2000). F. C. De Lucia, Jr., J. L. Gottfried, and A. W. Miziolek, “Evaluation of femtosecond laser-induced breakdown spectroscopy for explosive residue detection,” Opt. Express 17(2), 419–425 (2009). S. M. Angel, D. N. Stratis, K. L. Eland, T. Lai, M. A. Berg, and D. M. Gold, “LIBS using dual- and ultra-short laser pulses,” Fresenius J. Anal. Chem. 369(3-4), 320–327 (2001). W. Tan, H. Liu, J. Si, and X. Hou, “Control of the gated spectra with narrow bandwidth from a supercontinuum using ultrafast optical Kerr gate of bismuth glass,” Appl. Phys. Lett. 93(5), 051109 (2008). H. Liu, W. Tan, J. Si, X. Hou, and X. Hou, “Acquisition of gated spectra from a supercontinuum using ultrafast optical Kerr gate of lead phthalocyanine-doped hybrid glasses,” Opt. Express 16(17), 13486–13491 (2008). X. Wang and X. Xu, “Thermoelastic wave in metal induced by ultrafast laser pulses,” J. Therm. Stresses 25(5), 457–473 (2002). M. Sakakura and M. Terazima, “Oscillation of the refractive index at the focal region of a femtosecond laser pulse inside a glass,” Opt. Lett. 29(13), 1548–1550 (2004). M. Sakakura, M. Terazima, Y. Shimotsuma, K. Miura, and K. Hirao, “Observation of pressure wave generated by focusing a femtosecond laser pulse inside a glass,” Opt. Express 15(9), 5674–5686 (2007). H. Hu, X. Wang, and H. Zhai, “Neutrals ejection in intense femtosecond laser ablation,” Opt. Lett. 36(2), 124–126 (2011). C. A. Zuhlke, J. Bruce 3rd, T. P. Anderson, D. R. Alexander, and C. G. Parigger, “A fundamental understanding of the dependence of the laser-induced breakdown spectroscopy (LIBS) signal strength on the complex focusing dynamics of femtosecond laser pulses on either side of the focus,” Appl. Spectrosc. 68(9), 1021–1029 (2014). M. Mlejnek, M. Kolesik, J. V. Moloney, and E. M. Wright, “Optically turbulent femtosecond light guide in air,” Phys. Rev. Lett. 83(15), 2938–2941 (1999). P. Lorazo, L. J. Lewis, and M. Meunier, “Short-pulse laser ablation of solids: from phase explosion to
#228268 - $15.00 USD © 2015 OSA
Received 20 Nov 2014; revised 28 Dec 2014; accepted 29 Dec 2014; published 20 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.001370 | OPTICS EXPRESS 1370
fragmentation,” Phys. Rev. Lett. 91(22), 225502 (2003). 18. W. G. Roeterdink, L. B. F. Juurlink, O. P. H. Vaughan, J. Dura Diez, M. Bonn, and A. W. Kleyn, “Coulomb explosion in femtosecond laser ablation of Si(111),” Appl. Phys. Lett. 82(23), 4190–4192 (2003). 19. Y. T. Li, J. Zhang, H. Teng, K. Li, X. Y. Peng, Z. Jin, X. Lu, Z. Y. Zheng, and Q. Z. Yu, “Blast waves produced by interactions of femtosecond laser pulses with water,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 67(5), 056403 (2003). 20. X. Zeng, X. L. Mao, R. Greif, and R. E. Russo, “Experimental investigation of ablation efficiency and plasma expansion during femtosecond and nanosecond laser ablation of silicon,” Appl. Phys., A Mater. Sci. Process. 80(2), 237–241 (2005). 21. X. Zeng, X. Mao, S. S. Mao, S.- B. Wen, R. Greif, and R. E. Russo, “Laser-induced shockwave propagation from ablation in a cavity,” Appl. Phys. Lett. 88(6), 061502 (2006). 22. N. Zhang, X. Zhu, J. Yang, X. Wang, and M. Wang, “Time-resolved shadowgraphs of material ejection in intense femtosecond laser ablation of aluminum,” Phys. Rev. Lett. 99(16), 167602 (2007). 23. H. Hu, X. Wang, H. Zhai, N. Zhang, and P. Wang, “Generation of multiple stress waves in silica glass in high fluence femtosecond laser ablation,” Appl. Phys. Lett. 97(6), 061117 (2010). 24. S. S. Mao, X. Mao, R. Greif, and R. E. Russo, “Initiation of an early-stage plasma during picoseconds laser ablation of solids,” Appl. Phys. Lett. 77(16), 2464–2466 (2000). 25. N. M. Bulgakova and A. V. Bulgakov, “Comment on ‘Time-resolved shadowgraphs of material ejection in intense femtosecond laser ablation of aluminum’,” Phys. Rev. Lett. 101(9), 099701, author reply 099702 (2008). 26. J. König, S. Nolte, and A. Tünnermann, “Plasma evolution during metal ablation with ultrashort laser pulses,” Opt. Express 13(26), 10597–10607 (2005). 27. A. E. Martirosyan, C. Altucci, A. Bruno, C. de Lisio, A. Porzio, and S. Solimeno, “Time evolution of plasma afterglow produced by femtosecond laser pulses,” J. Appl. Phys. 96(10), 5450–5455 (2004). 28. V. V. Bukin, S. V. Garnov, A. A. Malyutin, and V. V. Strelkov, “Femtosecond laser optical gas breakdown microplasma: the ionization and postionisation dynamics,” Quantum Electron. 37(10), 961–966 (2007). 29. X. L. Liu, X. Lu, X. Liu, T. T. Xi, F. Liu, J. L. Ma, and J. Zhang, “Tightly focused femtosecond laser pulse in air: from filamentation to breakdown,” Opt. Express 18(25), 26007–26017 (2010). 30. W. Hu, Y. C. Shin, and G. King, “Early-stage plasma dynamics with air ionization during ultrafast laser ablation of metal,” Phys. Plasmas 18(9), 093302 (2011). 31. N. Zhang, Z. Wu, K. Xu, and X. Zhu, “Characteristics of micro air plasma produced by double femtosecond laser pulses,” Opt. Express 20(3), 2528–2538 (2012). 32. L. I. Sedov, Similarity and Dimensional Methods in Mechanics (CRC Press, 1993).
1. Introduction The clearly understanding of femtosecond laser pulses ablating dynamics of various materials has been very important due to the fast developments of laser physics on micro- or nano-machining [1–3], nano-material generation [4, 5], and laser-induced breakdown spectroscopy [6–8], etc. When intense femtosecond pulses with laser fluence reach to several J/cm2 interact with materials, various phenomena can be observed, such as supercontinuum generation [9, 10], thermoelastic waves expanding [11–13], material ejections [14, 15]. The physical mechanisms, for instance the laser-induced air breakdown [16], phase explosion [17], and Coulomb explosion [18], used for interpreting these phenomena during femtosecond laser ablating are dependent on the laser parameters and the target properties. Actually, in the case of intense femtosecond laser ablation of materials, not only one mechanism dominates the ablation processes. Time-resolved pump-probe shadowgraphic method with nanosecond up to femtosecond time resolution has been employed for obtaining the details of ablation dynamics in pulsed laser ablation experiments [19–23]. In the experiments of pulsed laser ablating metals, when the free electrons absorb the energy of incident laser pulse at a femtosecond time scale, they will be heated initially, and then emit from the target surface by photoelectric and thermionic effects. The emitted electrons seed the formation of a plasma plume, which is consisted of the emitted electrons and subsequent material ejection of atomic and ionic mass [24]. A shockwave is induced by the rapid expansion of plasma plume into the ambient gas, which would last tens to hundreds of nanoseconds after the laser pulse. The well-known behavior of shockwave helps us understand the pulsed laser ablation process a lot. However, the original mechanisms of the expanding shockwaves have been discussed between laser-induced air breakdown [25, 26] and phase explosion [17, 22]. It is instructive to ascertain the expanding
#228268 - $15.00 USD © 2015 OSA
Received 20 Nov 2014; revised 28 Dec 2014; accepted 29 Dec 2014; published 20 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.001370 | OPTICS EXPRESS 1371
characteristics of these two types of shockwaves, which may clarify the forming mechanisms of the shockwaves. In this study, we investigated the processes of femtosecond laser ablating aluminum by time-resolved shadowgraphic method. Two types of shockwaves, induced by laser-induced air breakdown and phase explosion respectively, were observed at different time delays. The influence of laser-induced air breakdown on propagation of phase explosion induced shockwave and material ejection was analyzed, which clarified the temporal evolution processes of femtosecond laser pulse ablation of aluminum. 2. Experiments In the experiments, a femtosecond two-color pump-probe apparatus is employed for recording the time-resolved shadowgraphs of femtosecond laser ablating aluminum, as described elsewhere [22, 23]. A commercial Ti: sapphire regenerative amplifier system, which emitted pulses with central wavelength of 800 nm, pulses duration of 100 fs, and repetition rate of 1 kHz, is used as the laser source. The output beam is split to two relatively delayed parts via a beam splitter. The pump beam, which passes a time-delay device, is focused onto the aluminum sample by a 10 × microscopic objective (NA = 0.25). The probe beam is frequency doubled by a BBO crystal with 1 mm thickness and Type I phase matching. A digital delay generator (DG535) is used to trigger a 12-bit high-resolution (1280 × 1024 pixels) gated intensified CCD camera (PCO DICAM-Pro),allowing the time-resolved shadowgraphic system operated at single-shot mode. On our purpose of observation of laser-induced air breakdown, the focus of the pump beam is adjusted about 40 μm above the aluminum target surface. The pump pulse has 18 μJ energy with a measured focal spot diameter of 10.6 μm and, therefore, the power density is about 2.0 × 1014 W/cm2 at the focal point. The aluminum target is moved to a fresh spot after each laser pulse is irradiated and one shadowgraph is recorded. 3. Experimental results and discussion In the previous works [16, 27, 28], it was shown that the power density of air breakdown threshold was 1013 ~1014 W/cm2 for a 100 fs and 800 nm femtosecond laser pulse. The intensity of the pump laser pulse increased very fast due to the tight focusing condition in our experiments. Bright sparking spot was observed near the geometric focus, and blast sound could be heard. Further, X. Lu et al. observed that the femtosecond laser induced filamentation in air broke into small-scale filaments at the input power density reached to 1014 W/cm2 [29]. Therefore, femtosecond induced air breakdown occurred at the focal spot region in our experimental scheme. Figure 1 shows the shadowgraphs of early-stage plasma evolution recorded at the time delay up to 160 ps. When the intense femtosecond laser pulse propagating through the air, a plasma channel was formed due to the laser-induce air breakdown. The electrons and atoms in the ionized plasma reached the target surface and were reflected backward. Simultaneously, the free electrons on the target surface absorbed the laser energy and emitted from the target surface by photoelectric and thermionic effects on a femtosecond to tens of picosecond time scale. It has been demonstrated that [30], the air breakdown threshold would be lowered 2 to 3 orders of magnitude near the target surface due to the seeding of reflected electrons from the plasma channel and the emitted free electrons from the target surface. Therefore, a shockwave (S1) was formed by the expansion of ionized electrons on the aluminum surface. On the top of S1, a contact front between S1 and the plasma channel was observed. Because the densities and temperatures were very high in plasma channel at small time delays, the contact front lasted about ten picoseconds due to plasma shielding [31]. When the time delay increased from 20 to 100 ps, the laser-induced plasma decayed gradually, exhibiting the length decreased longitudinally while the radius increased transversely. The contact front of S1 broke through the weakened plasma shielding and propagated in the plasma channel. The front of S1 became distortion on the longitudinal direction due to the faster expansion speed on this direction,
#228268 - $15.00 USD © 2015 OSA
Received 20 Nov 2014; revised 28 Dec 2014; accepted 29 Dec 2014; published 20 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.001370 | OPTICS EXPRESS 1372
which was caused by the lower refractive index in the plasma channel. A transparent bulge on the aluminum surface was observed at the time delay of 160 ps, which was considered the starting of phase explosion.
Fig. 1. Shadowgraphs of shockwave (S1) induced by laser-induced air breakdown at time delays from 20 to 160 ps. The fringes paralleled to the target surface were due to the edge diffraction of probe laser beam. The ellipse of dotted line denoted the focus position.
Fig. 2. Shadowgraphs of a second shockwave (S2) and material ejection induced by phase explosion at time delays from 200 ps to 4 ns.
When the femtosecond laser pulse hits the aluminum surface, the free electrons absorbed the laser energy, and then transferred the energy to the metal lattice. Because the target was overheated in the absorbed region, an extremely high pressure was created, which released through adiabatic expansion. The outward expansion induced another shockwave (S2) propagating outward the target surface, as shown in Fig. 2. The shockwave S2 expanded fast at the time delay from 160 ps to 800 ps, and then caught up with S1, exhibiting that the fronts of S1 and S2 overlapped at 800 ps time delay. After 800 ps, the shockwave S2 continued to expand to ambient environment. Similar to S1, the front of S2 on longitudinal direction expanded faster than that on other directions. The inward expansion of phase explosion compressed the inner part of the target, where the thermal energy transferred to mechanical energy occurred. The material ejection could be observed due to the recoil effect induced by the thermal expansion at larger time delays. The material ejection started at 300 ps, when an opaque bulge on the target surface was observed. The opaque bulge suffered the restrain from surface tension until to 1 ns time delay, and then started to erupt outward. The material ejection
#228268 - $15.00 USD © 2015 OSA
Received 20 Nov 2014; revised 28 Dec 2014; accepted 29 Dec 2014; published 20 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.001370 | OPTICS EXPRESS 1373
with erupted direction normal to the target surface could be observed at time delays longer than 2 ns.
Fig. 3. Radial and longitudinal expansion of shockwave (a) S1and (b) S2 as a function of time delay, respectively. (c) The dependence of radial and longitudinal expansion of shockwave S2 on time delay, respectively.
Figure 3(a) shows the radial and longitudinal expansion of shockwave S1 as a function of time delay, respectively. The shockwave S1 expanded fast on both the radial and longitudinal directions at the early-stage time delays, and then slowed down. On the longitudinal direction, the average expansion velocity of shockwave S1 was about 1.1 × 105 m/s at the first 160 ps, and then slowed down to about 2 × 104 m/s at the time delay from 160 to 800 ps. The average velocities were obtained by measuring the slope between the expansion distance and the measured time interval. The very fast expansion velocity on the longitudinal direction was due to the low refractive index in the plasma channel, which was induced by laser-induced air breakdown. When the time delay increased to hundreds of picoseconds, the plasma channel decayed gradually. The increasing of refractive index in plasma channel slowed down the expansion velocity of shockwave S1 on longitudinal direction. On the radial direction, due to the larger refractive index in ambient air condition, the expansion velocity was about 6 × 104 m/s at the first 150 ps, which was smaller than that on longitudinal direction. However, the shockwave front of S1 maintained in the time delay from 200 to 800 ps. Because the energy absorbed from the femtosecond laser pulse to the air plasma was small, the ionized electrons at the front of shockwave S1 lost the kinetic energy through colliding with ambient air molecular quickly, which caused the balance of pressure and temperature inside and outside the S1 shockwave front. On the other hand, with the expansion of the plasma channel, the air pressure inside the channel decreases. The expanding force from the airflow in S1 gathered on the longitudinal direction, which weakened the expanding velocity on radial direction. Therefore, the plateau between 200 and 800 ps on radial direction was observed in Fig. 3(a). The top front of S1 on longitudinal direction was breakthrough to the plasma channel at 800 ps time delay, simultaneously, the shockwave front of S2 caught up with that of S1 on radial direction. Therefore, we could conclude that the shockwave S1 decayed absolutely to the ambient air at this time delay.
#228268 - $15.00 USD © 2015 OSA
Received 20 Nov 2014; revised 28 Dec 2014; accepted 29 Dec 2014; published 20 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.001370 | OPTICS EXPRESS 1374
Figure 3(b) shows the expansion distance of S2 as a function of time delay on radial and longitudinal directions, respectively. In the first time delay from 160 to 800 ps, the average expansion velocities of S2 front on radial and longitudinal direction were 4.6 × 104 m/s and 5.0 × 104 m/s, which were larger than those on the later time delays, respectively. The larger velocities were not only caused by the initial phase explosion, but also the pre-propagation of S1, which diluted the ambient air density and decreased the refractive index outside S2. After the time delay of 800 ps, the average expansion velocities of S2 were 4.8 × 103 m/s and 1.0 × 104 m/s on radial and longitudinal directions, respectively. The velocity on longitudinal direction was larger than that on radial direction. This was because the refractive index was smaller in the plasma channel, which decayed at several nanosecond time scales. The expansion distances of the shockwave S2 on radial and longitudinal directions after 800 ps time delay were analyzed using Sedov-Taylor scaling [32]: E R = λ ( )1/ (2 + β ) t 2/ (2 + β )
ρ
(1)
where λ is a constant approximately equal to 1, E is the energy release responsible for the observed shock, ρ is the mass density of the undisturbed air, and t is the time delay. The parameter β represents the dimensionality of propagation (for spherical propagation β = 3, for cylindrical propagation β = 2 and for planar propagation β = 1). By fitting the expansion distance of S2 as the function of time delays on radial and longitudinal directions, the slopes were obtained respectively, as shown in Fig. 3(c). On the radial direction, the Slope(R) = 0.38 (equal to β~3) denoted that the front of S2 propagating as a spherical wave. On the longitudinal direction, the Slope(L) = 0.52 (equal to β~2), denoted that the front of S2 propagating as a cylindrical wave, which fitted the limitation of two-dimensional expansion in a plasma channel condition.
Fig. 4. Shadowgraphs of femtosecond laser pulse ablating aluminum target recorded at (a) 7 ns and (b) 15 ns time delay, respectively.
For the longer time delays, the propagation characteristics of shockwave S2 are shown in Fig. 4. The competition between the longitudinal and radial expansion velocities determined the overall shockwave profiles. The bulge on the top of shockwave S2 on longitudinal direction denoted the faster expansion velocity compared with that on radial direction. If we reduced the pump power of femtosecond laser pulse to a value that the plasma channel could not be formed, the femtosecond laser pulse would ablate the aluminum surface directly. Though the air breakdown on the aluminum surface might happen, both the shockwaves induced by expansion
#228268 - $15.00 USD © 2015 OSA
Received 20 Nov 2014; revised 28 Dec 2014; accepted 29 Dec 2014; published 20 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.001370 | OPTICS EXPRESS 1375
of ionized electrons and phase explosion would expand in hemispherical manner, such as reported in previous works [22, 30]. The material ejection plume started from 2 ns time delay underwent a transition from a superheated metal liquid to a mixture of vapor and liquid in several nanoseconds. Therefore, the opaque region was observed near the target surface at early time delays due to the great scattering and absorption of the probe beam. The vapor plume expanded larger and became more and more transparent from 2 ns to 15 ns time delays, indicating a cooling process of ejected material. Similar to the profile of S2, the profile of the material ejection plume was also determined by the competition between the vertical and horizontal expansion velocities. The average expanding velocity of the ejection plume were about 2.1 × 103 m/s on vertical direction, which was larger than that on horizontal direction (about 1.2 × 103 m/s). Due to the influence of laser-induced air breakdown, the ejection plume also expanded faster on vertical direction, which resulted in a cylindrical shape of the material ejection plume. This was different from a hemi-spherical shape reported in previous experiments [22, 24]. 4. Conclusion In summary, we investigated the temporal dynamics of shockwaves induced by laser-induced air breakdown and phase explosion, respectively. The early-stage shockwave generated by expansion of ionized electrons on the aluminum surface only lasted about 800 ps due to the depleting of kinetic energy. The velocities of shockwave induced by phase explosion were estimated at different time delays, showing the different expansion dynamics on different front directions. Due to the influence of laser-induced air breakdown, the shockwave propagated as a cylindrical wave on the longitudinal direction. Further, the material ejection plume expanded in a cylindrical shape, which was not a hemispherical shape as reported in previous researches. Acknowledgments This work was financially supported by the China Postdoctoral Science Foundation (Grant No. 2014M550435), Fundamental Research Funds for the Central Universities (Grant No. 2014ZB0027), Guangdong Natural Science Foundation (Grant No. S2011030001349), the Natural Science Foundation of China (Grant Nos. 11404114, 51132004), and the National Basic Research Program of China (973 program) (Grant No. 2011CB808102).
#228268 - $15.00 USD © 2015 OSA
Received 20 Nov 2014; revised 28 Dec 2014; accepted 29 Dec 2014; published 20 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.001370 | OPTICS EXPRESS 1376
State Key Laboratory of Luminescent Materials and Devices, and Institute of Optical Communication Materials South China University of Technology, Guangzhou 510640, China 2 The China-Germany Research Center for Photonic Materials and Devices, Guangzhou 510640, China * [email protected]
Abstract: We investigated the influence of laser-induced air breakdown on the femtosecond laser ablation of aluminum target using time-resolved pump-probe shadowgraphic imaging method. The early-stage plasma expanding dynamics and subsequent expanding behaviors of shockwaves and material ejection plume were analyzed through shadowgraphs recorded at different time delays. The dominated mechanisms were clarified at different stages during femtosecond laser pulses ablating aluminum, which provide very valuable information for ultrashort laser ablation of metals. ©2015 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (140.3440) Laser-induced breakdown; (320.7100) Ultrafast measurements; (160.3900) Metals.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
A. S. Mahmood, M. Sivakumar, K. Venkatakrishnan, and B. Tan, “Enhancement in optical absorption of silicon fibrous nanostructure produced using femtosecond laser ablation,” Appl. Phys. Lett. 95(3), 034107 (2009). T. Chen, J. Si, X. Hou, S. Kanehira, K. Miura, and K. Hirao, “Luminescence of black silicon fabricated by high-repetition rate femtosecond laser pulses,” J. Appl. Phys. 110(7), 073106 (2011). M. Yamaji, H. Kawashima, J. Suzuki, and S. Tanaka, “Three dimensional micromachining inside a transparent material by single pulse femtosecond laser through a hologram,” Appl. Phys. Lett. 93(4), 041116 (2008). S. Amoruso, G. Ausanio, R. Bruzzese, M. Vitiello, and X. Wang, “Femtosecond laser pulse irradiation of solid targets as a general route to nanoparticle formation in a vacuum,” Phys. Rev. B 71(3), 033406 (2005). B. Liu, Z. Hu, Y. Che, Y. Chen, and X. Pan, “Nanoparticle generation in ultrafast pulsed laser ablation of nickel,” Appl. Phys. Lett. 90(4), 044103 (2007). V. Margetic, A. Pakulev, A. Stockhaus, M. Bolshov, K. Niemax, and R. Hergenröder, “A comparison of nanosecond and femtosecond laser-induced plasma spectroscopy of brass samples,” Acta, Part B 55(11), 1771–1785 (2000). F. C. De Lucia, Jr., J. L. Gottfried, and A. W. Miziolek, “Evaluation of femtosecond laser-induced breakdown spectroscopy for explosive residue detection,” Opt. Express 17(2), 419–425 (2009). S. M. Angel, D. N. Stratis, K. L. Eland, T. Lai, M. A. Berg, and D. M. Gold, “LIBS using dual- and ultra-short laser pulses,” Fresenius J. Anal. Chem. 369(3-4), 320–327 (2001). W. Tan, H. Liu, J. Si, and X. Hou, “Control of the gated spectra with narrow bandwidth from a supercontinuum using ultrafast optical Kerr gate of bismuth glass,” Appl. Phys. Lett. 93(5), 051109 (2008). H. Liu, W. Tan, J. Si, X. Hou, and X. Hou, “Acquisition of gated spectra from a supercontinuum using ultrafast optical Kerr gate of lead phthalocyanine-doped hybrid glasses,” Opt. Express 16(17), 13486–13491 (2008). X. Wang and X. Xu, “Thermoelastic wave in metal induced by ultrafast laser pulses,” J. Therm. Stresses 25(5), 457–473 (2002). M. Sakakura and M. Terazima, “Oscillation of the refractive index at the focal region of a femtosecond laser pulse inside a glass,” Opt. Lett. 29(13), 1548–1550 (2004). M. Sakakura, M. Terazima, Y. Shimotsuma, K. Miura, and K. Hirao, “Observation of pressure wave generated by focusing a femtosecond laser pulse inside a glass,” Opt. Express 15(9), 5674–5686 (2007). H. Hu, X. Wang, and H. Zhai, “Neutrals ejection in intense femtosecond laser ablation,” Opt. Lett. 36(2), 124–126 (2011). C. A. Zuhlke, J. Bruce 3rd, T. P. Anderson, D. R. Alexander, and C. G. Parigger, “A fundamental understanding of the dependence of the laser-induced breakdown spectroscopy (LIBS) signal strength on the complex focusing dynamics of femtosecond laser pulses on either side of the focus,” Appl. Spectrosc. 68(9), 1021–1029 (2014). M. Mlejnek, M. Kolesik, J. V. Moloney, and E. M. Wright, “Optically turbulent femtosecond light guide in air,” Phys. Rev. Lett. 83(15), 2938–2941 (1999). P. Lorazo, L. J. Lewis, and M. Meunier, “Short-pulse laser ablation of solids: from phase explosion to
#228268 - $15.00 USD © 2015 OSA
Received 20 Nov 2014; revised 28 Dec 2014; accepted 29 Dec 2014; published 20 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.001370 | OPTICS EXPRESS 1370
fragmentation,” Phys. Rev. Lett. 91(22), 225502 (2003). 18. W. G. Roeterdink, L. B. F. Juurlink, O. P. H. Vaughan, J. Dura Diez, M. Bonn, and A. W. Kleyn, “Coulomb explosion in femtosecond laser ablation of Si(111),” Appl. Phys. Lett. 82(23), 4190–4192 (2003). 19. Y. T. Li, J. Zhang, H. Teng, K. Li, X. Y. Peng, Z. Jin, X. Lu, Z. Y. Zheng, and Q. Z. Yu, “Blast waves produced by interactions of femtosecond laser pulses with water,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 67(5), 056403 (2003). 20. X. Zeng, X. L. Mao, R. Greif, and R. E. Russo, “Experimental investigation of ablation efficiency and plasma expansion during femtosecond and nanosecond laser ablation of silicon,” Appl. Phys., A Mater. Sci. Process. 80(2), 237–241 (2005). 21. X. Zeng, X. Mao, S. S. Mao, S.- B. Wen, R. Greif, and R. E. Russo, “Laser-induced shockwave propagation from ablation in a cavity,” Appl. Phys. Lett. 88(6), 061502 (2006). 22. N. Zhang, X. Zhu, J. Yang, X. Wang, and M. Wang, “Time-resolved shadowgraphs of material ejection in intense femtosecond laser ablation of aluminum,” Phys. Rev. Lett. 99(16), 167602 (2007). 23. H. Hu, X. Wang, H. Zhai, N. Zhang, and P. Wang, “Generation of multiple stress waves in silica glass in high fluence femtosecond laser ablation,” Appl. Phys. Lett. 97(6), 061117 (2010). 24. S. S. Mao, X. Mao, R. Greif, and R. E. Russo, “Initiation of an early-stage plasma during picoseconds laser ablation of solids,” Appl. Phys. Lett. 77(16), 2464–2466 (2000). 25. N. M. Bulgakova and A. V. Bulgakov, “Comment on ‘Time-resolved shadowgraphs of material ejection in intense femtosecond laser ablation of aluminum’,” Phys. Rev. Lett. 101(9), 099701, author reply 099702 (2008). 26. J. König, S. Nolte, and A. Tünnermann, “Plasma evolution during metal ablation with ultrashort laser pulses,” Opt. Express 13(26), 10597–10607 (2005). 27. A. E. Martirosyan, C. Altucci, A. Bruno, C. de Lisio, A. Porzio, and S. Solimeno, “Time evolution of plasma afterglow produced by femtosecond laser pulses,” J. Appl. Phys. 96(10), 5450–5455 (2004). 28. V. V. Bukin, S. V. Garnov, A. A. Malyutin, and V. V. Strelkov, “Femtosecond laser optical gas breakdown microplasma: the ionization and postionisation dynamics,” Quantum Electron. 37(10), 961–966 (2007). 29. X. L. Liu, X. Lu, X. Liu, T. T. Xi, F. Liu, J. L. Ma, and J. Zhang, “Tightly focused femtosecond laser pulse in air: from filamentation to breakdown,” Opt. Express 18(25), 26007–26017 (2010). 30. W. Hu, Y. C. Shin, and G. King, “Early-stage plasma dynamics with air ionization during ultrafast laser ablation of metal,” Phys. Plasmas 18(9), 093302 (2011). 31. N. Zhang, Z. Wu, K. Xu, and X. Zhu, “Characteristics of micro air plasma produced by double femtosecond laser pulses,” Opt. Express 20(3), 2528–2538 (2012). 32. L. I. Sedov, Similarity and Dimensional Methods in Mechanics (CRC Press, 1993).
1. Introduction The clearly understanding of femtosecond laser pulses ablating dynamics of various materials has been very important due to the fast developments of laser physics on micro- or nano-machining [1–3], nano-material generation [4, 5], and laser-induced breakdown spectroscopy [6–8], etc. When intense femtosecond pulses with laser fluence reach to several J/cm2 interact with materials, various phenomena can be observed, such as supercontinuum generation [9, 10], thermoelastic waves expanding [11–13], material ejections [14, 15]. The physical mechanisms, for instance the laser-induced air breakdown [16], phase explosion [17], and Coulomb explosion [18], used for interpreting these phenomena during femtosecond laser ablating are dependent on the laser parameters and the target properties. Actually, in the case of intense femtosecond laser ablation of materials, not only one mechanism dominates the ablation processes. Time-resolved pump-probe shadowgraphic method with nanosecond up to femtosecond time resolution has been employed for obtaining the details of ablation dynamics in pulsed laser ablation experiments [19–23]. In the experiments of pulsed laser ablating metals, when the free electrons absorb the energy of incident laser pulse at a femtosecond time scale, they will be heated initially, and then emit from the target surface by photoelectric and thermionic effects. The emitted electrons seed the formation of a plasma plume, which is consisted of the emitted electrons and subsequent material ejection of atomic and ionic mass [24]. A shockwave is induced by the rapid expansion of plasma plume into the ambient gas, which would last tens to hundreds of nanoseconds after the laser pulse. The well-known behavior of shockwave helps us understand the pulsed laser ablation process a lot. However, the original mechanisms of the expanding shockwaves have been discussed between laser-induced air breakdown [25, 26] and phase explosion [17, 22]. It is instructive to ascertain the expanding
#228268 - $15.00 USD © 2015 OSA
Received 20 Nov 2014; revised 28 Dec 2014; accepted 29 Dec 2014; published 20 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.001370 | OPTICS EXPRESS 1371
characteristics of these two types of shockwaves, which may clarify the forming mechanisms of the shockwaves. In this study, we investigated the processes of femtosecond laser ablating aluminum by time-resolved shadowgraphic method. Two types of shockwaves, induced by laser-induced air breakdown and phase explosion respectively, were observed at different time delays. The influence of laser-induced air breakdown on propagation of phase explosion induced shockwave and material ejection was analyzed, which clarified the temporal evolution processes of femtosecond laser pulse ablation of aluminum. 2. Experiments In the experiments, a femtosecond two-color pump-probe apparatus is employed for recording the time-resolved shadowgraphs of femtosecond laser ablating aluminum, as described elsewhere [22, 23]. A commercial Ti: sapphire regenerative amplifier system, which emitted pulses with central wavelength of 800 nm, pulses duration of 100 fs, and repetition rate of 1 kHz, is used as the laser source. The output beam is split to two relatively delayed parts via a beam splitter. The pump beam, which passes a time-delay device, is focused onto the aluminum sample by a 10 × microscopic objective (NA = 0.25). The probe beam is frequency doubled by a BBO crystal with 1 mm thickness and Type I phase matching. A digital delay generator (DG535) is used to trigger a 12-bit high-resolution (1280 × 1024 pixels) gated intensified CCD camera (PCO DICAM-Pro),allowing the time-resolved shadowgraphic system operated at single-shot mode. On our purpose of observation of laser-induced air breakdown, the focus of the pump beam is adjusted about 40 μm above the aluminum target surface. The pump pulse has 18 μJ energy with a measured focal spot diameter of 10.6 μm and, therefore, the power density is about 2.0 × 1014 W/cm2 at the focal point. The aluminum target is moved to a fresh spot after each laser pulse is irradiated and one shadowgraph is recorded. 3. Experimental results and discussion In the previous works [16, 27, 28], it was shown that the power density of air breakdown threshold was 1013 ~1014 W/cm2 for a 100 fs and 800 nm femtosecond laser pulse. The intensity of the pump laser pulse increased very fast due to the tight focusing condition in our experiments. Bright sparking spot was observed near the geometric focus, and blast sound could be heard. Further, X. Lu et al. observed that the femtosecond laser induced filamentation in air broke into small-scale filaments at the input power density reached to 1014 W/cm2 [29]. Therefore, femtosecond induced air breakdown occurred at the focal spot region in our experimental scheme. Figure 1 shows the shadowgraphs of early-stage plasma evolution recorded at the time delay up to 160 ps. When the intense femtosecond laser pulse propagating through the air, a plasma channel was formed due to the laser-induce air breakdown. The electrons and atoms in the ionized plasma reached the target surface and were reflected backward. Simultaneously, the free electrons on the target surface absorbed the laser energy and emitted from the target surface by photoelectric and thermionic effects on a femtosecond to tens of picosecond time scale. It has been demonstrated that [30], the air breakdown threshold would be lowered 2 to 3 orders of magnitude near the target surface due to the seeding of reflected electrons from the plasma channel and the emitted free electrons from the target surface. Therefore, a shockwave (S1) was formed by the expansion of ionized electrons on the aluminum surface. On the top of S1, a contact front between S1 and the plasma channel was observed. Because the densities and temperatures were very high in plasma channel at small time delays, the contact front lasted about ten picoseconds due to plasma shielding [31]. When the time delay increased from 20 to 100 ps, the laser-induced plasma decayed gradually, exhibiting the length decreased longitudinally while the radius increased transversely. The contact front of S1 broke through the weakened plasma shielding and propagated in the plasma channel. The front of S1 became distortion on the longitudinal direction due to the faster expansion speed on this direction,
#228268 - $15.00 USD © 2015 OSA
Received 20 Nov 2014; revised 28 Dec 2014; accepted 29 Dec 2014; published 20 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.001370 | OPTICS EXPRESS 1372
which was caused by the lower refractive index in the plasma channel. A transparent bulge on the aluminum surface was observed at the time delay of 160 ps, which was considered the starting of phase explosion.
Fig. 1. Shadowgraphs of shockwave (S1) induced by laser-induced air breakdown at time delays from 20 to 160 ps. The fringes paralleled to the target surface were due to the edge diffraction of probe laser beam. The ellipse of dotted line denoted the focus position.
Fig. 2. Shadowgraphs of a second shockwave (S2) and material ejection induced by phase explosion at time delays from 200 ps to 4 ns.
When the femtosecond laser pulse hits the aluminum surface, the free electrons absorbed the laser energy, and then transferred the energy to the metal lattice. Because the target was overheated in the absorbed region, an extremely high pressure was created, which released through adiabatic expansion. The outward expansion induced another shockwave (S2) propagating outward the target surface, as shown in Fig. 2. The shockwave S2 expanded fast at the time delay from 160 ps to 800 ps, and then caught up with S1, exhibiting that the fronts of S1 and S2 overlapped at 800 ps time delay. After 800 ps, the shockwave S2 continued to expand to ambient environment. Similar to S1, the front of S2 on longitudinal direction expanded faster than that on other directions. The inward expansion of phase explosion compressed the inner part of the target, where the thermal energy transferred to mechanical energy occurred. The material ejection could be observed due to the recoil effect induced by the thermal expansion at larger time delays. The material ejection started at 300 ps, when an opaque bulge on the target surface was observed. The opaque bulge suffered the restrain from surface tension until to 1 ns time delay, and then started to erupt outward. The material ejection
#228268 - $15.00 USD © 2015 OSA
Received 20 Nov 2014; revised 28 Dec 2014; accepted 29 Dec 2014; published 20 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.001370 | OPTICS EXPRESS 1373
with erupted direction normal to the target surface could be observed at time delays longer than 2 ns.
Fig. 3. Radial and longitudinal expansion of shockwave (a) S1and (b) S2 as a function of time delay, respectively. (c) The dependence of radial and longitudinal expansion of shockwave S2 on time delay, respectively.
Figure 3(a) shows the radial and longitudinal expansion of shockwave S1 as a function of time delay, respectively. The shockwave S1 expanded fast on both the radial and longitudinal directions at the early-stage time delays, and then slowed down. On the longitudinal direction, the average expansion velocity of shockwave S1 was about 1.1 × 105 m/s at the first 160 ps, and then slowed down to about 2 × 104 m/s at the time delay from 160 to 800 ps. The average velocities were obtained by measuring the slope between the expansion distance and the measured time interval. The very fast expansion velocity on the longitudinal direction was due to the low refractive index in the plasma channel, which was induced by laser-induced air breakdown. When the time delay increased to hundreds of picoseconds, the plasma channel decayed gradually. The increasing of refractive index in plasma channel slowed down the expansion velocity of shockwave S1 on longitudinal direction. On the radial direction, due to the larger refractive index in ambient air condition, the expansion velocity was about 6 × 104 m/s at the first 150 ps, which was smaller than that on longitudinal direction. However, the shockwave front of S1 maintained in the time delay from 200 to 800 ps. Because the energy absorbed from the femtosecond laser pulse to the air plasma was small, the ionized electrons at the front of shockwave S1 lost the kinetic energy through colliding with ambient air molecular quickly, which caused the balance of pressure and temperature inside and outside the S1 shockwave front. On the other hand, with the expansion of the plasma channel, the air pressure inside the channel decreases. The expanding force from the airflow in S1 gathered on the longitudinal direction, which weakened the expanding velocity on radial direction. Therefore, the plateau between 200 and 800 ps on radial direction was observed in Fig. 3(a). The top front of S1 on longitudinal direction was breakthrough to the plasma channel at 800 ps time delay, simultaneously, the shockwave front of S2 caught up with that of S1 on radial direction. Therefore, we could conclude that the shockwave S1 decayed absolutely to the ambient air at this time delay.
#228268 - $15.00 USD © 2015 OSA
Received 20 Nov 2014; revised 28 Dec 2014; accepted 29 Dec 2014; published 20 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.001370 | OPTICS EXPRESS 1374
Figure 3(b) shows the expansion distance of S2 as a function of time delay on radial and longitudinal directions, respectively. In the first time delay from 160 to 800 ps, the average expansion velocities of S2 front on radial and longitudinal direction were 4.6 × 104 m/s and 5.0 × 104 m/s, which were larger than those on the later time delays, respectively. The larger velocities were not only caused by the initial phase explosion, but also the pre-propagation of S1, which diluted the ambient air density and decreased the refractive index outside S2. After the time delay of 800 ps, the average expansion velocities of S2 were 4.8 × 103 m/s and 1.0 × 104 m/s on radial and longitudinal directions, respectively. The velocity on longitudinal direction was larger than that on radial direction. This was because the refractive index was smaller in the plasma channel, which decayed at several nanosecond time scales. The expansion distances of the shockwave S2 on radial and longitudinal directions after 800 ps time delay were analyzed using Sedov-Taylor scaling [32]: E R = λ ( )1/ (2 + β ) t 2/ (2 + β )
ρ
(1)
where λ is a constant approximately equal to 1, E is the energy release responsible for the observed shock, ρ is the mass density of the undisturbed air, and t is the time delay. The parameter β represents the dimensionality of propagation (for spherical propagation β = 3, for cylindrical propagation β = 2 and for planar propagation β = 1). By fitting the expansion distance of S2 as the function of time delays on radial and longitudinal directions, the slopes were obtained respectively, as shown in Fig. 3(c). On the radial direction, the Slope(R) = 0.38 (equal to β~3) denoted that the front of S2 propagating as a spherical wave. On the longitudinal direction, the Slope(L) = 0.52 (equal to β~2), denoted that the front of S2 propagating as a cylindrical wave, which fitted the limitation of two-dimensional expansion in a plasma channel condition.
Fig. 4. Shadowgraphs of femtosecond laser pulse ablating aluminum target recorded at (a) 7 ns and (b) 15 ns time delay, respectively.
For the longer time delays, the propagation characteristics of shockwave S2 are shown in Fig. 4. The competition between the longitudinal and radial expansion velocities determined the overall shockwave profiles. The bulge on the top of shockwave S2 on longitudinal direction denoted the faster expansion velocity compared with that on radial direction. If we reduced the pump power of femtosecond laser pulse to a value that the plasma channel could not be formed, the femtosecond laser pulse would ablate the aluminum surface directly. Though the air breakdown on the aluminum surface might happen, both the shockwaves induced by expansion
#228268 - $15.00 USD © 2015 OSA
Received 20 Nov 2014; revised 28 Dec 2014; accepted 29 Dec 2014; published 20 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.001370 | OPTICS EXPRESS 1375
of ionized electrons and phase explosion would expand in hemispherical manner, such as reported in previous works [22, 30]. The material ejection plume started from 2 ns time delay underwent a transition from a superheated metal liquid to a mixture of vapor and liquid in several nanoseconds. Therefore, the opaque region was observed near the target surface at early time delays due to the great scattering and absorption of the probe beam. The vapor plume expanded larger and became more and more transparent from 2 ns to 15 ns time delays, indicating a cooling process of ejected material. Similar to the profile of S2, the profile of the material ejection plume was also determined by the competition between the vertical and horizontal expansion velocities. The average expanding velocity of the ejection plume were about 2.1 × 103 m/s on vertical direction, which was larger than that on horizontal direction (about 1.2 × 103 m/s). Due to the influence of laser-induced air breakdown, the ejection plume also expanded faster on vertical direction, which resulted in a cylindrical shape of the material ejection plume. This was different from a hemi-spherical shape reported in previous experiments [22, 24]. 4. Conclusion In summary, we investigated the temporal dynamics of shockwaves induced by laser-induced air breakdown and phase explosion, respectively. The early-stage shockwave generated by expansion of ionized electrons on the aluminum surface only lasted about 800 ps due to the depleting of kinetic energy. The velocities of shockwave induced by phase explosion were estimated at different time delays, showing the different expansion dynamics on different front directions. Due to the influence of laser-induced air breakdown, the shockwave propagated as a cylindrical wave on the longitudinal direction. Further, the material ejection plume expanded in a cylindrical shape, which was not a hemispherical shape as reported in previous researches. Acknowledgments This work was financially supported by the China Postdoctoral Science Foundation (Grant No. 2014M550435), Fundamental Research Funds for the Central Universities (Grant No. 2014ZB0027), Guangdong Natural Science Foundation (Grant No. S2011030001349), the Natural Science Foundation of China (Grant Nos. 11404114, 51132004), and the National Basic Research Program of China (973 program) (Grant No. 2011CB808102).
#228268 - $15.00 USD © 2015 OSA
Received 20 Nov 2014; revised 28 Dec 2014; accepted 29 Dec 2014; published 20 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.001370 | OPTICS EXPRESS 1376
