2382
OPTICS LETTERS / Vol. 39, No. 8 / April 15, 2014
Ablation area quasiperiodic oscillations in semiconductors with femtosecond laser double-pulse delay Xin Li,1 Cong Li,1 Lan Jiang,1,* Xuesong Shi,1 Ning Zhang,1 and Yongfeng Lu2 1
NanoManufacturing Fundamental Research Joint Laboratory of National Science Foundation of China, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China 2 Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0511, USA *Corresponding author: [email protected] Received December 20, 2013; revised March 4, 2014; accepted March 16, 2014; posted March 17, 2014 (Doc. ID 203449); published April 9, 2014 A surprising repeatable phenomenon regarding semiconductor ablation area changes has been discovered. Irradiated by femtosecond double pulses, the ablation area quasiperiodically oscillates as the pulse delay increases from 0 to 1 ps at a material-dependent fluence range. In contrast, the ablation area monotonically decreases as the pulse delay increases beyond 1 ps or if the total fluence increases close to or beyond the single-shot threshold. Similar unexpected patterns of area quasiperiodic oscillations with the double-pulse delay are observed in various semiconductors, including Ge, Si, GaAs, and ZnO. The comparison study shows the same phenomenon in Au-plated ZnO. Yet, its oscillation periods are shorter and more stable than those in bulk ZnO, which implies that the localized carrier density is the key factor in oscillation periods. © 2014 Optical Society of America OCIS codes: (140.3330) Laser damage; (320.7130) Ultrafast processes in condensed matter, including semiconductors; (320.2250) Femtosecond phenomena; (320.5540) Pulse shaping. http://dx.doi.org/10.1364/OL.39.002382
A femtosecond (fs) laser has unique advantages in nonthermal processing due to its ultrashort pulse durations and ultrahigh power density, making it an ideal method for improving the accuracy and quality in micro/ nanofabrication. During the last several decades, fs-laserinduced ablation has drawn extensive theoretical and experimental research attention, not only for its scientific significance but also for its numerous industrial applications [1–5]. For example, Oktem et al. presented a new method by which the unprecedented uniform metaloxide nanostructures can be fabricated by a fs laser at high speed and low cost and on nonplanar or flexible surfaces [5]. Femtosecond double pulses consist of two subpulses with a uniform intensity distribution and pulse delays ranging from several femtoseconds to several hundred picoseconds [6–13]. Femtosecond double pulses may make it possible to adjust the laser–material interaction process, as compared with the conventional fs pulse. With pulse delays in picoseconds, a monotonic decrease of ablation areas was observed as the pulse delay increased [6–8]. Surprisingly, some abrupt changes have also been obtained in experiments using fs double pulses with pulse delays in femtoseconds [9–11]. For example, Deng et al. found that a systematic dip of the second pulse optical breakdown threshold in silica is observed for pulse delays from 200 to 300 fs [10]. Liebig et al. reported that laser-induced damage in semimetal bismuth is enhanced or suppressed by laser-induced coherent optical phonon oscillations for temporally shaped fs pulses [11]. In this Letter, ablation areas are measured in germanium (Ge), silicon (Si), gallium arsenide (GaAs), zinc oxide (ZnO), and gold (Au)-plated ZnO for fs double-pulse delays ranging from 0 to 3 ps. A quasiperiodic oscillation decrease phenomenon is observed with the increase in pulse delays within several hundred femtoseconds. 0146-9592/14/082382-04$15.00/0
The oscillation periods of Au-plated ZnO are shorter and more stable than those of ZnO, which indicates that the localized carrier density has an effect on the oscillation period of ablation areas. A regenerative Ti:sapphire laser (Spectra-Physics, Inc.) is used to generate the linearly polarized fs laser beam with a central wavelength of 800 nm, pulse duration of 50 fs, and repetition of 20 Hz. A 4f -configurationbased pulse shaper (Biophotonic Solutions Inc., MIIPSBOX 640) is employed to shape a fs pulse into double fs pulses, where the pulse delays can range from 0 to 5.0 ps (at a resolution of 0.1 fs). The laser beam is incident normal to the sample surface by a microscope objective (Olympus, ×5∕0.1) in ambient air, corresponding to a spot diameter of about 8 μm. In the ablation experiments, several semiconductor crystals, including Ge, Si, GaAs, and ZnO, are employed. The corresponding bandgaps are 0.67, 1.12, 1.42, and 3.37 eV; the crystal orientations are h111i, h111i, h111i, and h0001i, respectively. For all of the semiconductors, the 10 mm × 10 mm × 1 mm samples are optically polished to a surface roughness of 0.5 nm. 20 shots (fs double pulses per shot) are employed at the given total fluences (F) for accurate measurable results. After irradiation, the ablation structures are observed by scanning electron microscopy (SEM) (FEI Quanta 200 FEG). The fs double-pulse ablation in ZnO has been carefully studied. The SEM images of ablation craters at F 1.34 J∕cm2 are shown in Figs. 1(a)–1(i), and the measurements of the ablation areas in ZnO as a function of fs double-pulse delays (Δt) are shown in Fig. 1(j). At Δt 0 fs, the ablation area is about 67.0 μm2 . At Δt 100 fs, the ablation area decreases to 26.5 μm2 , and high-spatial-frequency laser-induced periodic surface structures (LIPSSs), with average spatial periods of 240 20 nm, are observed over the whole ablation crater. At Δt 200 fs, no ablation is observed. Surprisingly, at © 2014 Optical Society of America
April 15, 2014 / Vol. 39, No. 8 / OPTICS LETTERS
Fig. 1. (a)–(i) SEM images of ablation crater morphologies in ZnO are shown with (j) measurement results at the total fluence of 1.34 J∕cm2 . Pulse delays (Δt): (a) 0 fs, (b) 100 fs, (c) 200 fs, (d) 300 fs, (e) 500 fs, (f) 700 fs, (g) 800 fs, (h) 900 fs, and (i) 1000 fs. Each site is irradiated by 20 shots (femtosecond double pulses per shot).
Δt 300 fs, the ablation area sharply increases to 32.5 μm2 ; and low spatial frequency LIPSSs (periods of 680 25 nm) are observed at the central region of the ablation crater, while the high-spatial-frequency LIPSSs (periods of 235 15 nm) are observed at the edge of the ablation crater. During 300 fs < Δt < 800 fs, a similar oscillation of the ablation areas is found in ZnO for fs double-pulse ablation. At Δt 1000 fs, no ablation is observed again. On the whole, as the pulse delay increases, the peaks of ablation areas are observed at pulse delays of 0, 350, and 800 fs, while the valleys are observed at 200, 500, and 1000 fs [Fig. 1(j)]. The oscillation period and the oscillation amplitude are adopted for oscillation phenomenon characterization, where the oscillation period is defined as the pulse delay difference between the adjacent peaks and the oscillation amplitude is defined as half of the ablation area difference between the valley and the average of peaks in a certain oscillation period. In this case, the oscillation periods are 350 and 450 fs, respectively. Hence, ablation areas exhibit a quasiperiodic oscillation, where the oscillation periods increase with the increase in the pulse delays. In addition, the oscillation amplitudes are 24.8 and 11.3 μm2 , which indicates a decrease in the oscillation amplitudes [Fig. 1(j)].
2383
Generally, a quasiperiodic oscillation decrease of the ablation areas is observed in ZnO during fs double-pulse irradiation, with the pulse delay ranging from 0 to 1.0 ps. The quasiperiodic oscillation decrease of the ablation areas in ZnO can only be observed at a particular fluence range (Fosc) of 1.20–2.25 J∕cm2 (below a single-shot threshold of 2.50 J∕cm2 ) using 20 shots per site (Fig. 2). Two different variation laws of ablation areas are observed for two distinct fluence ranges, where the largest oscillation amplitudes appear at about 1.40 J∕cm2 . In a lower fluence range (1.20–1.40 J∕cm2 ), only the quasiperiodic oscillation decrease of the ablation areas is observed. As the total fluences increase from 1.20, 1.25, and 1.34 J∕cm2 to 1.40 J∕cm2 , the first oscillation periods remain at 350 fs and the oscillation amplitudes increase from 22.0, 24.8, and 26.7 μm2 to 28.7 μm2 . That is, as the total laser fluences increase, oscillation periods remain almost unchanged, while oscillation amplitudes increase. Also, the second oscillation periods remain at 450 fs for total fluences of 1.25, 1.34, and 1.40 J∕cm2 , which indicates the oscillation periods increase with the increase in pulse delays. In a higher fluence range (1.40–2.25 J∕cm2 ), the quasiperiodic oscillation decrease of the ablation areas is observed for pulse delays less than 1.0 ps, while a monotonic decrease occurs for longer pulse delays. As the total laser fluences increase, the oscillation periods decrease slightly and the oscillation amplitudes decrease gradually. As the total fluence increases to 2.25 J∕cm2 , the oscillation amplitude is reduced to 11.4 μm2 . Similarly, the oscillation periods increase with the increase in pulse delays. In addition, with the oscillation of ablation areas, the average spatial periods of LIPSSs also have an obvious oscillatory behavior with the increase in pulse delays. For example, at F 1.50 J∕cm2 , the average spatial periods of LIPSSs at the central region of the ablation crater are approximately 250 20 nm, 670 35, 260 25, 660 30, and 240 15 nm at pulse delays of 200, 400, 600, and 800 fs and 1.0 ps, respectively. Furthermore, the quasiperiodic oscillation decrease phenomenon of the ablation areas is also observed in other semiconductors, including Ge, Si, and GaAs, for fs
Fig. 2. Ablation area as a function of pulse delay in ZnO at different total laser fluences. Each site is irradiated by 20 shots (femtosecond double pulses per shot).
2384
OPTICS LETTERS / Vol. 39, No. 8 / April 15, 2014
Fig. 3. Quasiperiodic oscillation decrease of the ablation areas with pulse delays in femtoseconds and the monotonic decrease in picosecond pulse delays in Ge, Si, GaAs, and ZnO, respectively. Each site is irradiated by 20 shots, and the total fluences are 0.55, 0.90, 1.20, and 1.50 J∕cm2 , respectively.
double-pulse ablation with pulse delays less than 1.0 ps (Fig. 3). Similar patterns for oscillation periods and oscillation amplitudes of ablation areas are observed at particular fluence ranges. The fluence ranges are closely related to the bandgaps of semiconductor materials. For Ge, Si, GaAs, and ZnO, the bandgaps are 0.67, 1.12, 1.42, and 3.37 eV, respectively. In our experiments, the fluence ranges fF osc g in Ge, Si, GaAs, and ZnO are 0.5–0.6, 0.8–1.2, 1.0–1.4, and 1.20–2.25 J∕cm2 . The widths of fluence ranges fF osc g increase approximately with the increase in the bandgaps: fF osc Geg < fF osc Sig ≈ fF osc GaAsg < fF osc ZnOg. In addition, the average laser fluences for the quasiperiodic oscillation decrease phenomenon are found to be around 0.7 F th (F th is the single-shot threshold for a certain semiconductor). The ablation area is determined by the area of the phase changes [1–4]. Phase changes depend on the interatomic potential that is changed by the atomic motion [11–17]. For a period of a few picoseconds, the oscillation of the atomic motion, mainly corresponding to the coherent optical phonon mode, has been experimentally observed for many materials [11–17] including semiconductors [14]. The oscillation of the atomic motion can be adjusted by changing the delay of the fs double pulse around the oscillation periods of the coherent optical phonon [11–13]. The amplification or cancellation of the atomic motion oscillation amplitude significantly depends on the relative phase of the coherent optical phonon between two successive subpulses, which has been confirmed in semimetal Bi by the UV and hard x-ray pump–probe experiments [11,12] and the first principles calculations [13]. Furthermore, the carrier density change is the key reason for the coherent optical phonon
and is observed in the aforementioned experiments. The atomic potential energy, strongly affected by the carrier density, significantly impacts the oscillation period of the coherent optical phonon [12,13]. In summary, the oscillation of the atomic motion caused by carrier density changes leads to the quasiperiodic variations in the ablation area, when the pulse delay is around the oscillation periods of the coherent optical phonon. Fundamentally speaking, the localized electron dynamics plays a key role in the formation of ablation morphologies. The formation mechanisms of LIPSSs in ablation craters, including the surface plasmon [18–22] and the secondharmonic generation [23], are all decided by fs laser– electron interactions. Hence, the carrier density is significantly related to the observed phenomenon. However, more detailed studies are still required to discover the underlying mechanisms. For comparison, the fs double-pulse ablation in Au-plated ZnO has also been carefully studied, where a 20 nm Au film is plated on bulk ZnO. The quasiperiodic oscillation decrease of the ablation areas is observed for pulse delays less than 1.0 ps at a fluence range of 1.15–1.80 J∕cm2 . Generally, as the pulse delay increases, the peaks of the ablation areas are observed at pulse delays of 0, 300, 600, and 900 fs, while the valleys are observed at 150, 500, and 800 fs (Fig. 4). That is, in Au-plated ZnO the oscillation periods stay at about 300 fs with the increase in the pulse delays, while in bulk ZnO, the oscillation periods increase with the increase in the pulse delays. Hence, the oscillation periods in Au-plated ZnO are shorter and more stable than those in bulk ZnO. And at the same total fluence, the oscillation amplitudes in Au-plated ZnO are much larger. Electrons in Au film are heated to a high temperature during irradiation, and the fast hot-electron diffusion by the Au film increases the interfacial carrier density and enhances the interfacial electron–phonon coupling [24,25]. The higher interfacial carrier density in the Au-plated ZnO can lead to a shorter oscillation period of ablation areas. In summary, a quasiperiodic oscillation of ablation areas is discovered in semiconductors, including Ge, Si, GaAs, and ZnO, as the pulse delay increases from 0 to 1 ps at a particular fluence range, while a monotonic decrease is observed for longer pulse delays. With the
Fig. 4. Ablation area as a function of pulse delay in Au-plated ZnO at different total fluences. Each site is irradiated by 20 shots (femtosecond double pulses per shot).
April 15, 2014 / Vol. 39, No. 8 / OPTICS LETTERS
increase in the pulse delay, the oscillation periods increase and oscillation amplitudes decrease. With the increase in the total fluence, oscillation periods slightly decrease, and oscillation amplitudes increase first and then decrease. The enhancement or cancellation of atomic motion oscillation is related to the observed phenomenon, as when the pulse delay is around the oscillation periods of the coherent optical phonon. The carrier density plays a key role in the coherent optical phonon and the corresponding phenomenon, especially in the oscillation periods. The aforementioned phenomenon is also observed in Au-plated ZnO with shorter and more stable oscillation periods with an increase in pulse delays. This research is supported by the National Basic Research Program of China (973 Program) (Grant No. 2011CB013000) and the National Natural Science Foundation of China (NSFC) (Grant Nos. 91323301 and 51105037). References 1. A. Rousse, C. Rischel, S. Fourmaux, I. Uschmann, S. Sebban, G. Grillon, P. Balcou, E. Förster, J. P. Geindre, P. Audebert, J. C. Gauthier, and D. Hulin, Nature 410, 65 (2001). 2. T. Q. Jia, H. X. Chen, M. Huang, F. L. Zhao, J. R. Qiu, R. X. Li, Z. Z. Xu, X. K. He, J. Zhang, and H. Kuroda, Phys. Rev. B 72, 125429 (2005). 3. J. Wang and C. Guo, J. Appl. Phys. 102, 053522 (2007). 4. N. M. Bulgakova, R. Stoian, A. Rosenfeld, I. V. Hertel, and E. E. B. Campbell, Phys. Rev. B 69, 054102 (2004). 5. B. Oktem, I. Pavlov, S. Ilday, H. Kalaycıoğlu, A. Rybak, S. Yavas, M. Erdoğan, and F. Ö. Ilday, Nat. Photonics 7, 897 (2013). 6. S. Höhma, A. Rosenfeld, J. Krüger, and J. Bonse, Appl. Surf. Sci. 278, 7 (2013). 7. M. Barberoglou, D. Gray, E. Magoulakis, C. Fotakis, P. A. Loukakos, and E. Stratakis, Opt. Express 21, 18501 (2013). 8. J. Bonse, J. Krüger, S. Höhm, and A. Rosenfeld, J. Laser Appl. 24, 042006 (2012). 9. N. Zhang, X. Li, L. Jiang, X. Shi, C. Li, and Y. Lu, Opt. Lett. 38, 3558 (2013).
2385
10. Y. P. Deng, X. H. Xie, H. Xiong, Y. X. Leng, C. F. Cheng, H. H. Lu, R. X. Li, and Z. Z. Xu, Opt. Express 13, 3096 (2005). 11. C. M. Liebig, Y. Wang, and X. Xu, Opt. Express 18, 20498 (2010). 12. P. Beaud, S. L. Johnson, A. Streun, R. Abela, D. Abramsohn, D. Grolimund, F. Krasniqi, T. Schmidt, V. Schlott, and G. Ingold, Phys. Rev. Lett. 99, 174801 (2007). 13. É. D. Murray, D. M. Fritz, J. K. Wahlstrand, S. Fahy, and D. A. Reis, Phys. Rev. B 72, 060301(R) (2005). 14. K. Sokolowski-Tinten, C. Blome, J. Blums, A. Cavalleri, C. Dietrich, A. Tarasevitch, I. Uschmann, E. Förster, M. Kammler, M. Horn-von-Hoegen, and D. von der Linde, Nature 422, 287 (2003). 15. D. M. Fritz, D. A. Reis, B. Adams, R. A. Akre, J. Arthur, C. Blome, P. H. Bucksbaum, A. L. Cavalieri, S. Engemann, S. Fahy, R. W. Falcone, P. H. Fuoss, K. J. Gaffney, M. J. George, J. Hajdu, M. P. Hertlein, P. B. Hillyard, M. Horn-von Hoegen, M. Kammler, J. Kaspar, R. Kienberger, P. Krejcik, S. H. Lee, A. M. Lindenberg, B. McFarland, D. Meyer, T. Montagne, É. D. Murray, A. J. Nelson, M. Nicoul, R. Pahl, J. Rudati, H. Schlarb, D. P. Siddons, K. Sokolowski-Tinten, T. Tschentscher, D. von der Linde, and J. B. Hastings, Science 315, 633 (2007). 16. H. Dachraoui and W. Husinsky, Phys. Rev. Lett. 97, 107601 (2006). 17. X. Zhao and Y. C. Shin, J. Phys. D 46, 335501 (2013). 18. K. Furusawa, K. Takahashi, H. Kumagai, K. Midorikawa, and M. Obara, Appl. Phys. A 69, S359 (1999). 19. G. D. Tsibidis, M. Barberoglou, P. A. Loukakos, E. Stratakis, and C. Fotakis, Phys. Rev. B 86, 115316 (2012). 20. T. J.-Y. Derrien, J. Krüger, T. E. Itina, S. Höhm, A. Rosenfeld, and J. Bonse, Opt. Express 21, 29643 (2013). 21. M. Barberoglou, G. D. Tsibidis, D. Gray, E. Magoulakis, C. Fotakis, E. Stratakis, and P. A. Loukakos, Appl. Phys. A 113, 273 (2013). 22. M. Huang, F. Zhao, Y. Cheng, N. Xu, and Z. Xu, ACS Nano 3, 4062 (2009). 23. D. Dufft, A. Rosenfeld, S. K. Das, R. Grunwald, and J. Bonse, J. Appl. Phys. 105, 034908 (2009). 24. Y. Wang, X. Ruan, and A. K. Roy, Phys. Rev. B 85, 205311 (2012). 25. J. B. Lee and S. H. Lee, Mater. Trans. 52, 1492 (2011).
OPTICS LETTERS / Vol. 39, No. 8 / April 15, 2014
Ablation area quasiperiodic oscillations in semiconductors with femtosecond laser double-pulse delay Xin Li,1 Cong Li,1 Lan Jiang,1,* Xuesong Shi,1 Ning Zhang,1 and Yongfeng Lu2 1
NanoManufacturing Fundamental Research Joint Laboratory of National Science Foundation of China, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China 2 Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0511, USA *Corresponding author: [email protected] Received December 20, 2013; revised March 4, 2014; accepted March 16, 2014; posted March 17, 2014 (Doc. ID 203449); published April 9, 2014 A surprising repeatable phenomenon regarding semiconductor ablation area changes has been discovered. Irradiated by femtosecond double pulses, the ablation area quasiperiodically oscillates as the pulse delay increases from 0 to 1 ps at a material-dependent fluence range. In contrast, the ablation area monotonically decreases as the pulse delay increases beyond 1 ps or if the total fluence increases close to or beyond the single-shot threshold. Similar unexpected patterns of area quasiperiodic oscillations with the double-pulse delay are observed in various semiconductors, including Ge, Si, GaAs, and ZnO. The comparison study shows the same phenomenon in Au-plated ZnO. Yet, its oscillation periods are shorter and more stable than those in bulk ZnO, which implies that the localized carrier density is the key factor in oscillation periods. © 2014 Optical Society of America OCIS codes: (140.3330) Laser damage; (320.7130) Ultrafast processes in condensed matter, including semiconductors; (320.2250) Femtosecond phenomena; (320.5540) Pulse shaping. http://dx.doi.org/10.1364/OL.39.002382
A femtosecond (fs) laser has unique advantages in nonthermal processing due to its ultrashort pulse durations and ultrahigh power density, making it an ideal method for improving the accuracy and quality in micro/ nanofabrication. During the last several decades, fs-laserinduced ablation has drawn extensive theoretical and experimental research attention, not only for its scientific significance but also for its numerous industrial applications [1–5]. For example, Oktem et al. presented a new method by which the unprecedented uniform metaloxide nanostructures can be fabricated by a fs laser at high speed and low cost and on nonplanar or flexible surfaces [5]. Femtosecond double pulses consist of two subpulses with a uniform intensity distribution and pulse delays ranging from several femtoseconds to several hundred picoseconds [6–13]. Femtosecond double pulses may make it possible to adjust the laser–material interaction process, as compared with the conventional fs pulse. With pulse delays in picoseconds, a monotonic decrease of ablation areas was observed as the pulse delay increased [6–8]. Surprisingly, some abrupt changes have also been obtained in experiments using fs double pulses with pulse delays in femtoseconds [9–11]. For example, Deng et al. found that a systematic dip of the second pulse optical breakdown threshold in silica is observed for pulse delays from 200 to 300 fs [10]. Liebig et al. reported that laser-induced damage in semimetal bismuth is enhanced or suppressed by laser-induced coherent optical phonon oscillations for temporally shaped fs pulses [11]. In this Letter, ablation areas are measured in germanium (Ge), silicon (Si), gallium arsenide (GaAs), zinc oxide (ZnO), and gold (Au)-plated ZnO for fs double-pulse delays ranging from 0 to 3 ps. A quasiperiodic oscillation decrease phenomenon is observed with the increase in pulse delays within several hundred femtoseconds. 0146-9592/14/082382-04$15.00/0
The oscillation periods of Au-plated ZnO are shorter and more stable than those of ZnO, which indicates that the localized carrier density has an effect on the oscillation period of ablation areas. A regenerative Ti:sapphire laser (Spectra-Physics, Inc.) is used to generate the linearly polarized fs laser beam with a central wavelength of 800 nm, pulse duration of 50 fs, and repetition of 20 Hz. A 4f -configurationbased pulse shaper (Biophotonic Solutions Inc., MIIPSBOX 640) is employed to shape a fs pulse into double fs pulses, where the pulse delays can range from 0 to 5.0 ps (at a resolution of 0.1 fs). The laser beam is incident normal to the sample surface by a microscope objective (Olympus, ×5∕0.1) in ambient air, corresponding to a spot diameter of about 8 μm. In the ablation experiments, several semiconductor crystals, including Ge, Si, GaAs, and ZnO, are employed. The corresponding bandgaps are 0.67, 1.12, 1.42, and 3.37 eV; the crystal orientations are h111i, h111i, h111i, and h0001i, respectively. For all of the semiconductors, the 10 mm × 10 mm × 1 mm samples are optically polished to a surface roughness of 0.5 nm. 20 shots (fs double pulses per shot) are employed at the given total fluences (F) for accurate measurable results. After irradiation, the ablation structures are observed by scanning electron microscopy (SEM) (FEI Quanta 200 FEG). The fs double-pulse ablation in ZnO has been carefully studied. The SEM images of ablation craters at F 1.34 J∕cm2 are shown in Figs. 1(a)–1(i), and the measurements of the ablation areas in ZnO as a function of fs double-pulse delays (Δt) are shown in Fig. 1(j). At Δt 0 fs, the ablation area is about 67.0 μm2 . At Δt 100 fs, the ablation area decreases to 26.5 μm2 , and high-spatial-frequency laser-induced periodic surface structures (LIPSSs), with average spatial periods of 240 20 nm, are observed over the whole ablation crater. At Δt 200 fs, no ablation is observed. Surprisingly, at © 2014 Optical Society of America
April 15, 2014 / Vol. 39, No. 8 / OPTICS LETTERS
Fig. 1. (a)–(i) SEM images of ablation crater morphologies in ZnO are shown with (j) measurement results at the total fluence of 1.34 J∕cm2 . Pulse delays (Δt): (a) 0 fs, (b) 100 fs, (c) 200 fs, (d) 300 fs, (e) 500 fs, (f) 700 fs, (g) 800 fs, (h) 900 fs, and (i) 1000 fs. Each site is irradiated by 20 shots (femtosecond double pulses per shot).
Δt 300 fs, the ablation area sharply increases to 32.5 μm2 ; and low spatial frequency LIPSSs (periods of 680 25 nm) are observed at the central region of the ablation crater, while the high-spatial-frequency LIPSSs (periods of 235 15 nm) are observed at the edge of the ablation crater. During 300 fs < Δt < 800 fs, a similar oscillation of the ablation areas is found in ZnO for fs double-pulse ablation. At Δt 1000 fs, no ablation is observed again. On the whole, as the pulse delay increases, the peaks of ablation areas are observed at pulse delays of 0, 350, and 800 fs, while the valleys are observed at 200, 500, and 1000 fs [Fig. 1(j)]. The oscillation period and the oscillation amplitude are adopted for oscillation phenomenon characterization, where the oscillation period is defined as the pulse delay difference between the adjacent peaks and the oscillation amplitude is defined as half of the ablation area difference between the valley and the average of peaks in a certain oscillation period. In this case, the oscillation periods are 350 and 450 fs, respectively. Hence, ablation areas exhibit a quasiperiodic oscillation, where the oscillation periods increase with the increase in the pulse delays. In addition, the oscillation amplitudes are 24.8 and 11.3 μm2 , which indicates a decrease in the oscillation amplitudes [Fig. 1(j)].
2383
Generally, a quasiperiodic oscillation decrease of the ablation areas is observed in ZnO during fs double-pulse irradiation, with the pulse delay ranging from 0 to 1.0 ps. The quasiperiodic oscillation decrease of the ablation areas in ZnO can only be observed at a particular fluence range (Fosc) of 1.20–2.25 J∕cm2 (below a single-shot threshold of 2.50 J∕cm2 ) using 20 shots per site (Fig. 2). Two different variation laws of ablation areas are observed for two distinct fluence ranges, where the largest oscillation amplitudes appear at about 1.40 J∕cm2 . In a lower fluence range (1.20–1.40 J∕cm2 ), only the quasiperiodic oscillation decrease of the ablation areas is observed. As the total fluences increase from 1.20, 1.25, and 1.34 J∕cm2 to 1.40 J∕cm2 , the first oscillation periods remain at 350 fs and the oscillation amplitudes increase from 22.0, 24.8, and 26.7 μm2 to 28.7 μm2 . That is, as the total laser fluences increase, oscillation periods remain almost unchanged, while oscillation amplitudes increase. Also, the second oscillation periods remain at 450 fs for total fluences of 1.25, 1.34, and 1.40 J∕cm2 , which indicates the oscillation periods increase with the increase in pulse delays. In a higher fluence range (1.40–2.25 J∕cm2 ), the quasiperiodic oscillation decrease of the ablation areas is observed for pulse delays less than 1.0 ps, while a monotonic decrease occurs for longer pulse delays. As the total laser fluences increase, the oscillation periods decrease slightly and the oscillation amplitudes decrease gradually. As the total fluence increases to 2.25 J∕cm2 , the oscillation amplitude is reduced to 11.4 μm2 . Similarly, the oscillation periods increase with the increase in pulse delays. In addition, with the oscillation of ablation areas, the average spatial periods of LIPSSs also have an obvious oscillatory behavior with the increase in pulse delays. For example, at F 1.50 J∕cm2 , the average spatial periods of LIPSSs at the central region of the ablation crater are approximately 250 20 nm, 670 35, 260 25, 660 30, and 240 15 nm at pulse delays of 200, 400, 600, and 800 fs and 1.0 ps, respectively. Furthermore, the quasiperiodic oscillation decrease phenomenon of the ablation areas is also observed in other semiconductors, including Ge, Si, and GaAs, for fs
Fig. 2. Ablation area as a function of pulse delay in ZnO at different total laser fluences. Each site is irradiated by 20 shots (femtosecond double pulses per shot).
2384
OPTICS LETTERS / Vol. 39, No. 8 / April 15, 2014
Fig. 3. Quasiperiodic oscillation decrease of the ablation areas with pulse delays in femtoseconds and the monotonic decrease in picosecond pulse delays in Ge, Si, GaAs, and ZnO, respectively. Each site is irradiated by 20 shots, and the total fluences are 0.55, 0.90, 1.20, and 1.50 J∕cm2 , respectively.
double-pulse ablation with pulse delays less than 1.0 ps (Fig. 3). Similar patterns for oscillation periods and oscillation amplitudes of ablation areas are observed at particular fluence ranges. The fluence ranges are closely related to the bandgaps of semiconductor materials. For Ge, Si, GaAs, and ZnO, the bandgaps are 0.67, 1.12, 1.42, and 3.37 eV, respectively. In our experiments, the fluence ranges fF osc g in Ge, Si, GaAs, and ZnO are 0.5–0.6, 0.8–1.2, 1.0–1.4, and 1.20–2.25 J∕cm2 . The widths of fluence ranges fF osc g increase approximately with the increase in the bandgaps: fF osc Geg < fF osc Sig ≈ fF osc GaAsg < fF osc ZnOg. In addition, the average laser fluences for the quasiperiodic oscillation decrease phenomenon are found to be around 0.7 F th (F th is the single-shot threshold for a certain semiconductor). The ablation area is determined by the area of the phase changes [1–4]. Phase changes depend on the interatomic potential that is changed by the atomic motion [11–17]. For a period of a few picoseconds, the oscillation of the atomic motion, mainly corresponding to the coherent optical phonon mode, has been experimentally observed for many materials [11–17] including semiconductors [14]. The oscillation of the atomic motion can be adjusted by changing the delay of the fs double pulse around the oscillation periods of the coherent optical phonon [11–13]. The amplification or cancellation of the atomic motion oscillation amplitude significantly depends on the relative phase of the coherent optical phonon between two successive subpulses, which has been confirmed in semimetal Bi by the UV and hard x-ray pump–probe experiments [11,12] and the first principles calculations [13]. Furthermore, the carrier density change is the key reason for the coherent optical phonon
and is observed in the aforementioned experiments. The atomic potential energy, strongly affected by the carrier density, significantly impacts the oscillation period of the coherent optical phonon [12,13]. In summary, the oscillation of the atomic motion caused by carrier density changes leads to the quasiperiodic variations in the ablation area, when the pulse delay is around the oscillation periods of the coherent optical phonon. Fundamentally speaking, the localized electron dynamics plays a key role in the formation of ablation morphologies. The formation mechanisms of LIPSSs in ablation craters, including the surface plasmon [18–22] and the secondharmonic generation [23], are all decided by fs laser– electron interactions. Hence, the carrier density is significantly related to the observed phenomenon. However, more detailed studies are still required to discover the underlying mechanisms. For comparison, the fs double-pulse ablation in Au-plated ZnO has also been carefully studied, where a 20 nm Au film is plated on bulk ZnO. The quasiperiodic oscillation decrease of the ablation areas is observed for pulse delays less than 1.0 ps at a fluence range of 1.15–1.80 J∕cm2 . Generally, as the pulse delay increases, the peaks of the ablation areas are observed at pulse delays of 0, 300, 600, and 900 fs, while the valleys are observed at 150, 500, and 800 fs (Fig. 4). That is, in Au-plated ZnO the oscillation periods stay at about 300 fs with the increase in the pulse delays, while in bulk ZnO, the oscillation periods increase with the increase in the pulse delays. Hence, the oscillation periods in Au-plated ZnO are shorter and more stable than those in bulk ZnO. And at the same total fluence, the oscillation amplitudes in Au-plated ZnO are much larger. Electrons in Au film are heated to a high temperature during irradiation, and the fast hot-electron diffusion by the Au film increases the interfacial carrier density and enhances the interfacial electron–phonon coupling [24,25]. The higher interfacial carrier density in the Au-plated ZnO can lead to a shorter oscillation period of ablation areas. In summary, a quasiperiodic oscillation of ablation areas is discovered in semiconductors, including Ge, Si, GaAs, and ZnO, as the pulse delay increases from 0 to 1 ps at a particular fluence range, while a monotonic decrease is observed for longer pulse delays. With the
Fig. 4. Ablation area as a function of pulse delay in Au-plated ZnO at different total fluences. Each site is irradiated by 20 shots (femtosecond double pulses per shot).
April 15, 2014 / Vol. 39, No. 8 / OPTICS LETTERS
increase in the pulse delay, the oscillation periods increase and oscillation amplitudes decrease. With the increase in the total fluence, oscillation periods slightly decrease, and oscillation amplitudes increase first and then decrease. The enhancement or cancellation of atomic motion oscillation is related to the observed phenomenon, as when the pulse delay is around the oscillation periods of the coherent optical phonon. The carrier density plays a key role in the coherent optical phonon and the corresponding phenomenon, especially in the oscillation periods. The aforementioned phenomenon is also observed in Au-plated ZnO with shorter and more stable oscillation periods with an increase in pulse delays. This research is supported by the National Basic Research Program of China (973 Program) (Grant No. 2011CB013000) and the National Natural Science Foundation of China (NSFC) (Grant Nos. 91323301 and 51105037). References 1. A. Rousse, C. Rischel, S. Fourmaux, I. Uschmann, S. Sebban, G. Grillon, P. Balcou, E. Förster, J. P. Geindre, P. Audebert, J. C. Gauthier, and D. Hulin, Nature 410, 65 (2001). 2. T. Q. Jia, H. X. Chen, M. Huang, F. L. Zhao, J. R. Qiu, R. X. Li, Z. Z. Xu, X. K. He, J. Zhang, and H. Kuroda, Phys. Rev. B 72, 125429 (2005). 3. J. Wang and C. Guo, J. Appl. Phys. 102, 053522 (2007). 4. N. M. Bulgakova, R. Stoian, A. Rosenfeld, I. V. Hertel, and E. E. B. Campbell, Phys. Rev. B 69, 054102 (2004). 5. B. Oktem, I. Pavlov, S. Ilday, H. Kalaycıoğlu, A. Rybak, S. Yavas, M. Erdoğan, and F. Ö. Ilday, Nat. Photonics 7, 897 (2013). 6. S. Höhma, A. Rosenfeld, J. Krüger, and J. Bonse, Appl. Surf. Sci. 278, 7 (2013). 7. M. Barberoglou, D. Gray, E. Magoulakis, C. Fotakis, P. A. Loukakos, and E. Stratakis, Opt. Express 21, 18501 (2013). 8. J. Bonse, J. Krüger, S. Höhm, and A. Rosenfeld, J. Laser Appl. 24, 042006 (2012). 9. N. Zhang, X. Li, L. Jiang, X. Shi, C. Li, and Y. Lu, Opt. Lett. 38, 3558 (2013).
2385
10. Y. P. Deng, X. H. Xie, H. Xiong, Y. X. Leng, C. F. Cheng, H. H. Lu, R. X. Li, and Z. Z. Xu, Opt. Express 13, 3096 (2005). 11. C. M. Liebig, Y. Wang, and X. Xu, Opt. Express 18, 20498 (2010). 12. P. Beaud, S. L. Johnson, A. Streun, R. Abela, D. Abramsohn, D. Grolimund, F. Krasniqi, T. Schmidt, V. Schlott, and G. Ingold, Phys. Rev. Lett. 99, 174801 (2007). 13. É. D. Murray, D. M. Fritz, J. K. Wahlstrand, S. Fahy, and D. A. Reis, Phys. Rev. B 72, 060301(R) (2005). 14. K. Sokolowski-Tinten, C. Blome, J. Blums, A. Cavalleri, C. Dietrich, A. Tarasevitch, I. Uschmann, E. Förster, M. Kammler, M. Horn-von-Hoegen, and D. von der Linde, Nature 422, 287 (2003). 15. D. M. Fritz, D. A. Reis, B. Adams, R. A. Akre, J. Arthur, C. Blome, P. H. Bucksbaum, A. L. Cavalieri, S. Engemann, S. Fahy, R. W. Falcone, P. H. Fuoss, K. J. Gaffney, M. J. George, J. Hajdu, M. P. Hertlein, P. B. Hillyard, M. Horn-von Hoegen, M. Kammler, J. Kaspar, R. Kienberger, P. Krejcik, S. H. Lee, A. M. Lindenberg, B. McFarland, D. Meyer, T. Montagne, É. D. Murray, A. J. Nelson, M. Nicoul, R. Pahl, J. Rudati, H. Schlarb, D. P. Siddons, K. Sokolowski-Tinten, T. Tschentscher, D. von der Linde, and J. B. Hastings, Science 315, 633 (2007). 16. H. Dachraoui and W. Husinsky, Phys. Rev. Lett. 97, 107601 (2006). 17. X. Zhao and Y. C. Shin, J. Phys. D 46, 335501 (2013). 18. K. Furusawa, K. Takahashi, H. Kumagai, K. Midorikawa, and M. Obara, Appl. Phys. A 69, S359 (1999). 19. G. D. Tsibidis, M. Barberoglou, P. A. Loukakos, E. Stratakis, and C. Fotakis, Phys. Rev. B 86, 115316 (2012). 20. T. J.-Y. Derrien, J. Krüger, T. E. Itina, S. Höhm, A. Rosenfeld, and J. Bonse, Opt. Express 21, 29643 (2013). 21. M. Barberoglou, G. D. Tsibidis, D. Gray, E. Magoulakis, C. Fotakis, E. Stratakis, and P. A. Loukakos, Appl. Phys. A 113, 273 (2013). 22. M. Huang, F. Zhao, Y. Cheng, N. Xu, and Z. Xu, ACS Nano 3, 4062 (2009). 23. D. Dufft, A. Rosenfeld, S. K. Das, R. Grunwald, and J. Bonse, J. Appl. Phys. 105, 034908 (2009). 24. Y. Wang, X. Ruan, and A. K. Roy, Phys. Rev. B 85, 205311 (2012). 25. J. B. Lee and S. H. Lee, Mater. Trans. 52, 1492 (2011).
