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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 24 — Nov. 26, 2007
  • pp: 15741–15746
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Self-organized tungsten nanospikes grown on subwavelength ripples induced by femtosecond laser pulses

Quan-Zhong Zhao, Stefan Malzer, and Li-Jun Wang  »View Author Affiliations


Optics Express, Vol. 15, Issue 24, pp. 15741-15746 (2007)
http://dx.doi.org/10.1364/OE.15.015741


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Abstract

We report on the formation of nanoscale tungsten spikes generated on subwavelength periodic ripples which built up by single beam 800 nm femtosecond laser pulses. The nanospikes have a diameter ranging from 10 to 100 nm and are up to 250 nm in length. The nanospikes orientate from the ridges toward the valleys of the ripple structures independent of the polarization of the light. The heterogeneous nucleation of the liquid phase at the irradiated surface and the inhomogeneous surface roughness are considered as the mechanism of this nanospike formation.

© 2007 Optical Society of America

1. Introduction

Fabrication of nanostructures by laser technology has attracted great attention since nanotechnology revealed its great potential. Nanostructures, from zero- to three-dimensional structures such as quantum dots, nanowires, and nanogratings, etc., have been achieved in metal, dielectrics, semiconductors, and polymers by using laser technology [1

1. W. Shi, Z. Chen, N. Liu, H. Lu, Y. Zhou, D. Cui, and G. Yang, “Nonlinear optical properties of self-organized complex oxide Ce:BaTiO3 quantum dots grown by pulsed laser deposition,” Appl. Phys. Lett. 75, 1547–1549 (1999). [CrossRef]

10

10. F. Korte, J. Koch, and B. N. Chichkov, “Formation of microbumps and nanojets on gold targets by femtosecond laser pulses,” Appl. Phys. A 79, 879–881 (2004). [CrossRef]

]. Among them, femtosecond laser fabrication of nanostructures has especially become the focus of intensive research due to the ultra-high intensity and ultra-fast properties of femtosecond lasers [3

3. Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultrashort light pulses,” Phys. Rev. Lett. 91, 247405 (2003). [CrossRef] [PubMed]

10

10. F. Korte, J. Koch, and B. N. Chichkov, “Formation of microbumps and nanojets on gold targets by femtosecond laser pulses,” Appl. Phys. A 79, 879–881 (2004). [CrossRef]

]. Several approaches have been used to fabricate nanoscale structures by femtosecond lasers, including: (a) taking advantage of the well-defined modification threshold of materials to make only the central part of the beam able to modify the material, the diffraction limit can be overcome by choosing the peak laser fluence slightly above the threshold value [4

4. S. Kawata, H. B. Sun, T. Tanaka, and K. Takada, “Finer features for functional microdevices,” Nature 412, 697–698 (2001). [CrossRef] [PubMed]

, 5

5. F. Korte, J. Serbin, J. Koch, A. Egbert, C. Fallnich, A. Ostendorf, and B.N. Chichkov, “Towards nanostructuring with femtosecond laser pulses,” Appl. Phys. A 77, 229–235 (2003).

], (b) using femtosecond pulses in combination with a scanning near-field optical microscope to directly ablate materials to achieve nanostructuring [6

6. S. Nolte, B.N. Chichkov, H. Welling, Y. Shani, K. Lieberman, and H. Terkel, “Nanostructuring with spatially localized femtosecond laser pulses,” Opt. Lett. 24, 914–916 (1999). [CrossRef]

], (c) applying single beam femtosecond laser pulses interference with excited plasma waves to induce nanoscale periodic structures [3

3. Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultrashort light pulses,” Phys. Rev. Lett. 91, 247405 (2003). [CrossRef] [PubMed]

], (d) using femtosecond pulses to induce chemical reactions for the production of nanocomposites [7

7. J. Qiu, X. Jiang, C. Zhu, M. Shirai, J. Si, N. Jiang, and K. Hirao, “Manipulation of gold nanoparticles inside transparent materials,” Angew. Chem. Int. Ed. 43, 2230–2234 (2004). [CrossRef]

], (e) using femtosecond laser direct ablation to induce nanoparticles or nanowires [8

8. A. V. Kabashin and M. Meunier, “Synthesis of colloidal nanoparticles during femtosecond laser ablation of gold in water,” J Appl. Phys. 94, 7941–7943 (2003). [CrossRef]

, 9

9. T. Q. Jia, H. X. Chen, M. Huang, X. J. Wu, F. L. Zhao, M. Baba, M. Suzuki, H. Kuroda, J. R. Qiu, R. X. Li, and Z. Z. Xu, “ZnSe nanowires grown on the crystal surface by femtosecond laser ablation in air,” Appl. Phys. Lett. 89, 101116 (2006). [CrossRef]

].

It is well-known that ripple-like periodic structures can be generated by a single laser beam on the surface of semiconductor [11

11. M. Birnbaum, “Semiconductor Surface Damage Produced by Ruby Lasers,” J. Appl. Phys. 36, 3688–3689 (1965). [CrossRef]

], metal [12

12. R. M. Osgood and D. J. Ehrlich Jr., “Optically induced microstructures in laser-photodeposited metal films,” Opt. Lett. 7, 385–387 (1982). [CrossRef] [PubMed]

], and dielectric [13

13. F. Keilmann and Y. H. Bai, “Periodic surface structures frozen into CO2 laser-melted quartz,” Appl. Phys. A 29, 9–18 (1982). [CrossRef]

] materials due to the interference of incident pulses with light scattered from defects at the surface [14

14. G. Zhou, P. M. Fauchet, and A. E. Siegman, “Growth of spontaneous periodic surface structures on solids during laser illumination,” Phys. Rev. B 26, 5366–5381 (1982). [CrossRef]

17

17. J. F. Young, J. S. Preston, H. M. van Driel, and J. E. Sipe, “Laser-induced periodic surface structure. II. Experiments on Ge, Si, Al, and brass,” Phys. Rev. B 27, 1155–1172 (1983). [CrossRef]

]. Very recently, we found that ripples with spatial period from subwavelength to half-wavelength can be observed by using linearly and circularly polarized femtosecond light with variation of laser fluence, number of pulses, and incident angle [18

18. Q. Z. Zhao, S. Malzer, and L. J. Wang, “Formation of subwavelength periodic structures on tungsten induced by ultrashort laser pulses,” Opt. Lett. 32, 1932–1934 (2007). [CrossRef] [PubMed]

], which is in contradiction to the conventional observations [11

11. M. Birnbaum, “Semiconductor Surface Damage Produced by Ruby Lasers,” J. Appl. Phys. 36, 3688–3689 (1965). [CrossRef]

17

17. J. F. Young, J. S. Preston, H. M. van Driel, and J. E. Sipe, “Laser-induced periodic surface structure. II. Experiments on Ge, Si, Al, and brass,” Phys. Rev. B 27, 1155–1172 (1983). [CrossRef]

].

In this paper, we report on the formation of nanoscale tungsten spikes grown on subwavelength periodic ripples induced by single beam, 800 nm femtosecond laser pulses. The orientation of the nanospikes is from the ridges towards the valleys of the ripple structures independent from the polarization-type of the light (linear or circular). The nanospikes have a diameter ranging from 10 to 100 nm and are up to 250 nm in length. Our observation provides a new approach to the fabrication of nanostructures which is different from the approaches (a) to (e) in the above classification.

2. Experimental

A commercial multi-pass amplified Ti:Sapphire mode-locked laser system (Femtopower Compact PRO, Femtolasers Produktions GmbH) with 800 nm wavelength, 33 fs pulse duration, ~1 mJ average energy, and 1 kHz repetition rate was used as the irradiation source. After passing through a variable neutral density filter and a mechanical shutter, the beam was guided into a microscope and was focused normally on the sample surface by a 10× objective (NA=0.30). The sample could be moved with the help of a computer controlled 3D-XYZ stage. The number of laser pulses applied to the sample was controlled by using a frequency control shutter system (FPC, Femtolasers Produktions GmbH). The laser polarization was adjusted by inserting half-wave or quarter-wave plates in the beam path. All experiments were performed in air under atmospheric pressure. The morphology of femtosecond laser-induced surface modifications is inspected by using an optical microscope and an ultra-high resolution field emission scanning electron microscope (FE-SEM, Hitachi S-4800). Tungsten foils with thickness of 50 µm were used in this study. Before and after laser irradiation, the samples are cleaned by acetone to remove ambient dust and most of the plume deposited on the sample surface.

3. Results and discussion

Fig. 1. SEM images of subwavelength ripple structures induced by different polarization and their 2D Fourier transformation. These structures are generated with 100 shots irradiation under a fluence of 4.2 J/cm2 at (a) vertical, (b) horizontal, (c) left circular, and (d) right circular polarization, respectively. The arrows indicate the direction of polarization. The corresponding 2D Fourier transformations are shown in (e)–(h).

In order to investigate the induced ripple structures in more detail, the images in Figs. 1(a)–(d) were subjected to a two-dimensional Fourier transformation, as shown in Figs. 1(e)–(h). One can see that the central areas reflect large spatial frequencies, such as large-scale corruption and, since the Fourier images are symmetrical to their center, four points in each figure correspond to the frequencies of the ripples in the original SEM images. Note that the four points lie on a vertical line [Fig. 1(e)], a horizontal line [Fig. 1(f)], and diagonal lines [Figs. 1(g) and 1(h)] through the image centers, because the image intensities in the spatial domain change the most when traversing them vertically, horizontally, and diagonally, respectively. If the surface morphology is observed with higher magnification, some interesting nanostructures on the subwavelength ripples can be observed, as shown in Fig. 2. A great number of nanospikes with smooth sidewalls are observed that have formed on the subwavelength ripples induced by different polarized light beams. If careful observation is made of the apparent random growth direction, it is found that most of the nanospikes root on the ridge of the ripples and grow towards the valley of the ripples. Explicitly, the formation of the nanospikes is not dependent on the ripple formation, be it by linearly or circularly polarized light. Most of the nanospikes grow with caps like mushrooms and a few of them without caps, like needles. The nanospikes have a diameter ranging from 10 to 100 nm (at the neck of the mushroom-like spike) and are up to 250 nm in length.

Fig. 2. SEM images of nanospikes grown on ripples induced by (a, e) vertical, (b, f) horizontal, (c, g) left circular, and (d, h) right circular polarization. Each row has the same magnification.

When the SEM accelerating voltage is increased (15 kV instead of 10 kV used for Figs. 1 and 2) a double-layer structure can be observed in the nanospikes (Fig. 3). Here we only give cases of the structures induced by vertically and left circularly polarized light, as the structures induced by horizontally and right circularly polarized light are similar.

Fig. 3. SEM images of double-layer nanospikes induced under a fluence of 7 J/cm2 with 30 shots irradiation at (a, b) vertical and (c, d) left circular polarization, respectively, which are detected at a higher accelerating voltage of 15 kV.

Further investigation was made of the evolution of the formation of nanospikes by applying varied fluences and number of pulses. We applied 1 to 100 shots with fluences in the range from 3 to 12 J/cm2. The SEM analysis indicates that the formation of nanospikes depends on both the laser energy and the number of shots. With a single shot, no nanospikes were observed within the applied fluence range. With a minimum fluence of 6 J/cm2 and 10 shots, the formation of nanospikes starts with morphologies similar to those presented in Fig. 2. It is worth mentioning that some interesting surface structures were observed when a different number of shots with a fluence of 3 J/cm2 was applied, In Figure 4(a) the structural changes caused by 10 shots are shown. It can be observed that a new class of nanoscale ripples with periods ranging from 30 to 100 nm with orientation parallel to the direction of polarization. This type of nanoripples is similar to type c structures defined in [17

17. J. F. Young, J. S. Preston, H. M. van Driel, and J. E. Sipe, “Laser-induced periodic surface structure. II. Experiments on Ge, Si, Al, and brass,” Phys. Rev. B 27, 1155–1172 (1983). [CrossRef]

], i.e. periodic structures with a spacing of λ/cosθ and orientation parallel to the polarization [17

17. J. F. Young, J. S. Preston, H. M. van Driel, and J. E. Sipe, “Laser-induced periodic surface structure. II. Experiments on Ge, Si, Al, and brass,” Phys. Rev. B 27, 1155–1172 (1983). [CrossRef]

], where λ is laser wavelength and θ is incident angle, while the period of nanoripples in our case is much less than the wavelength of the laser. We suggest that the inhomogeneous surface roughness is the origin of the nanoripples. When the laser irradiates the sample surface, surface plasmon waves are generated, whose density would not be uniform on the nanoscale area due to this inhomogeneous surface roughness. The nonuniform plasmon waves will interfere with incident light, causing a localized melting or ablation area with a much smaller size than the laser wavelength. Figure 4(b) shows some nanospheres with diameters ranging from 20 to 120 nm on the spot when the sample was irradiated with 40 shots at 3 J/cm2 pulse energy. Figure 4(c) shows that the nanospikes occur when the sample is irradiated with 100 shots. Compared to the nanospikes shown in Fig. 2, their dimension is smaller. For comparison, we as well show an image of sample surface before laser irradiation in Fig. 4(d).

Fig. 4. Nanoripples (a), nanospheres (b) and nanospikes (c) induced by femtosecond laser with number of shots 10 (a), 40 (b) and 100 (c) under fluence 3 J/cm2. (d) Surface without femtosecond laser irradiation.

Based on the evolution of the nanospike formation, several points can be concluded: 1) The nanospike formation has a threshold fluence for any number of shots no matter if ripples occur or not, 2) The dimension of nanospikes changes slightly with variation of fluence and number of shots, 3) The orientation of nanospikes is random without ripple formation. When ripples are formed, the nanospikes are mostly perpendicular to the ripples.

Studies of the ultrafast laser induced solid-liquid phase transitions in metals [19

19. J. Hohlfeld, S.-S. Wellershoff, J. Gudde, U. Conrad, V. Jahnke, and E. Matthias, “Electron and lattice dynamics following optical excitation of metals,” Chem. Phys. 251, 237–258, (2000). [CrossRef]

, 20

20. B. J. Siwick, J. R. Dwyer, R. E. Jordan, and R. J. D. Miller, “An atomic-level view of melting using femtosecond electron diffraction,” Science 302, 1382–1385, (2003). [CrossRef] [PubMed]

], performed by time-resolved pump-probe techniques, suggest that the melting of the surface layer can take from several to hundreds of picoseconds. At such a time scale, the heterogeneous nucleation of the liquid phase at the irradiated surface and propagation of the melting front can penetrate into the bulk of the crystal. On the other hand, the original surface roughness will increase the optical absorption, causing melting on localized nanoscale sites, which will subsequently cool and freeze to form nanospikes. With the increase of the number of pulses, the formation of nanospikes will further enhance laser absorption to excite surface plasmons that will interfere with the incident light. The interference patterns will be inscribed on the surface to form the subwavelength ripple structures. Without subwavelength ripple formation, the orientation of the nanospikes is random. With subwavelength ripple formation, the orientation of the nanospikes is from the ridge toward the valley of the ripple. This is because a periodic modulated intensity field is formed due to the incident light interfering with the excited plasmons. Consequently, the temperature at the surface heated by high intensity parts of the intensity field is higher than that of surface heated by low intensity parts of the intensity field, causing a large temperature gradient from the ridge to the valley of the ripple. Thus the orientation of nanospikes is mainly from the bridge to the valley of the ripple, following this temperature gradient. The exact physical mechanism of the formation of nanospikes is still under investigation.

4. Conclusions

In summary, we have observed the formation of tungsten nanospikes growing on periodic ripples induced by single beam 800 nm femtosecond laser pulses. The formation of nanospikes depends both on laser fluence and the number of shots. The heterogeneous nucleation of the liquid phase at the irradiated surface and the inhomogeneous surface roughness are considered as the mechanism of this nanospike formation. Other nanostructures, nanoripples and nanospheres were observed on surfaces without the ripple-like structure. With this method of femtosecond laser material processing a new way for structuring material surfaces at the nanometer scale is demonstrated, which may find applications in modifying field emission [21

21. H. Y. Yang, S. P. Lau, S. F. Yu, L. Huang, M. Tanemura, J. Tanaka, T Okita, and H. H. Hng, “Field emission from zinc oxide nanoneedles on plastic substrates,” Nanotechnology 16, 1300–1303 (2005). [CrossRef]

], improving directivity of thermal light sources [22

22. M. Laroche, C. Arnold, F. Marquier, R. Garminati, J. J. Greffet, S. Collin, N. Bardou, and J. L. Pelouard, “Highly directional radiation generated by a tungsten thermal source,” Opt. Lett. 30, 2623–2625 (2005). [CrossRef] [PubMed]

], and enhancing surface catalytic activities [23

23. Y. Song, W. A. Steen, D. Peña, Y. B. Jiang, C. J. Medforth, Q. Huo, J. L. Pincus, Y. Qiu, D. Y. Sasaki, J. E. Miller, and J. A. Shelnutt, “Foamlike nanostructures created from dendritic platinum sheets on liposomes,” Chem. Mater. 18, 2335–2346 (2006). [CrossRef]

].

Acknowledgments

Q. Z. Zhao acknowledges fellowship support from the Alexander von Humboldt Foundation.

References and links

1.

W. Shi, Z. Chen, N. Liu, H. Lu, Y. Zhou, D. Cui, and G. Yang, “Nonlinear optical properties of self-organized complex oxide Ce:BaTiO3 quantum dots grown by pulsed laser deposition,” Appl. Phys. Lett. 75, 1547–1549 (1999). [CrossRef]

2.

A. M. Morales and C. M. Lieber, “A laser ablation method for the synthesis of crystalline semiconductor nanowires,” Science 279, 208–211 (1998). [CrossRef] [PubMed]

3.

Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultrashort light pulses,” Phys. Rev. Lett. 91, 247405 (2003). [CrossRef] [PubMed]

4.

S. Kawata, H. B. Sun, T. Tanaka, and K. Takada, “Finer features for functional microdevices,” Nature 412, 697–698 (2001). [CrossRef] [PubMed]

5.

F. Korte, J. Serbin, J. Koch, A. Egbert, C. Fallnich, A. Ostendorf, and B.N. Chichkov, “Towards nanostructuring with femtosecond laser pulses,” Appl. Phys. A 77, 229–235 (2003).

6.

S. Nolte, B.N. Chichkov, H. Welling, Y. Shani, K. Lieberman, and H. Terkel, “Nanostructuring with spatially localized femtosecond laser pulses,” Opt. Lett. 24, 914–916 (1999). [CrossRef]

7.

J. Qiu, X. Jiang, C. Zhu, M. Shirai, J. Si, N. Jiang, and K. Hirao, “Manipulation of gold nanoparticles inside transparent materials,” Angew. Chem. Int. Ed. 43, 2230–2234 (2004). [CrossRef]

8.

A. V. Kabashin and M. Meunier, “Synthesis of colloidal nanoparticles during femtosecond laser ablation of gold in water,” J Appl. Phys. 94, 7941–7943 (2003). [CrossRef]

9.

T. Q. Jia, H. X. Chen, M. Huang, X. J. Wu, F. L. Zhao, M. Baba, M. Suzuki, H. Kuroda, J. R. Qiu, R. X. Li, and Z. Z. Xu, “ZnSe nanowires grown on the crystal surface by femtosecond laser ablation in air,” Appl. Phys. Lett. 89, 101116 (2006). [CrossRef]

10.

F. Korte, J. Koch, and B. N. Chichkov, “Formation of microbumps and nanojets on gold targets by femtosecond laser pulses,” Appl. Phys. A 79, 879–881 (2004). [CrossRef]

11.

M. Birnbaum, “Semiconductor Surface Damage Produced by Ruby Lasers,” J. Appl. Phys. 36, 3688–3689 (1965). [CrossRef]

12.

R. M. Osgood and D. J. Ehrlich Jr., “Optically induced microstructures in laser-photodeposited metal films,” Opt. Lett. 7, 385–387 (1982). [CrossRef] [PubMed]

13.

F. Keilmann and Y. H. Bai, “Periodic surface structures frozen into CO2 laser-melted quartz,” Appl. Phys. A 29, 9–18 (1982). [CrossRef]

14.

G. Zhou, P. M. Fauchet, and A. E. Siegman, “Growth of spontaneous periodic surface structures on solids during laser illumination,” Phys. Rev. B 26, 5366–5381 (1982). [CrossRef]

15.

P. M. Fauchet and A. E. Siegman, “Surface ripples on silicon and gallium arsenide under picosecond laser illumination,” Appl. Phys. Lett. 40, 824–826 (1982). [CrossRef]

16.

H. M. Vandriel, J. E. Sipe, and J. F. Young, “Laser-induced coherent modulation of solid and liquid surfaces,” J. Lumin. 30, 446–471 (1985). [CrossRef]

17.

J. F. Young, J. S. Preston, H. M. van Driel, and J. E. Sipe, “Laser-induced periodic surface structure. II. Experiments on Ge, Si, Al, and brass,” Phys. Rev. B 27, 1155–1172 (1983). [CrossRef]

18.

Q. Z. Zhao, S. Malzer, and L. J. Wang, “Formation of subwavelength periodic structures on tungsten induced by ultrashort laser pulses,” Opt. Lett. 32, 1932–1934 (2007). [CrossRef] [PubMed]

19.

J. Hohlfeld, S.-S. Wellershoff, J. Gudde, U. Conrad, V. Jahnke, and E. Matthias, “Electron and lattice dynamics following optical excitation of metals,” Chem. Phys. 251, 237–258, (2000). [CrossRef]

20.

B. J. Siwick, J. R. Dwyer, R. E. Jordan, and R. J. D. Miller, “An atomic-level view of melting using femtosecond electron diffraction,” Science 302, 1382–1385, (2003). [CrossRef] [PubMed]

21.

H. Y. Yang, S. P. Lau, S. F. Yu, L. Huang, M. Tanemura, J. Tanaka, T Okita, and H. H. Hng, “Field emission from zinc oxide nanoneedles on plastic substrates,” Nanotechnology 16, 1300–1303 (2005). [CrossRef]

22.

M. Laroche, C. Arnold, F. Marquier, R. Garminati, J. J. Greffet, S. Collin, N. Bardou, and J. L. Pelouard, “Highly directional radiation generated by a tungsten thermal source,” Opt. Lett. 30, 2623–2625 (2005). [CrossRef] [PubMed]

23.

Y. Song, W. A. Steen, D. Peña, Y. B. Jiang, C. J. Medforth, Q. Huo, J. L. Pincus, Y. Qiu, D. Y. Sasaki, J. E. Miller, and J. A. Shelnutt, “Foamlike nanostructures created from dendritic platinum sheets on liposomes,” Chem. Mater. 18, 2335–2346 (2006). [CrossRef]

OCIS Codes
(140.3390) Lasers and laser optics : Laser materials processing
(160.3900) Materials : Metals
(220.4000) Optical design and fabrication : Microstructure fabrication

ToC Category:
Laser Micromachining

History
Original Manuscript: September 12, 2007
Revised Manuscript: November 6, 2007
Manuscript Accepted: November 6, 2007
Published: November 12, 2007

Citation
Quan-Zhong Zhao, Stefan Malzer, and Li-Jun Wang, "Self-organized tungsten nanospikes grown on subwavelength ripples induced by femtosecond laser pulses," Opt. Express 15, 15741-15746 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-24-15741


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References

  1. W. Shi, Z. Chen, N. Liu, H. Lu, Y. Zhou, D. Cui, and G. Yang, "Nonlinear optical properties of self-organized complex oxide Ce:BaTiO3 quantum dots grown by pulsed laser deposition," Appl. Phys. Lett. 75, 1547-1549 (1999). [CrossRef]
  2. A. M. Morales and C. M. Lieber, "A laser ablation method for the synthesis of crystalline semiconductor nanowires," Science 279, 208-211 (1998). [CrossRef] [PubMed]
  3. Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, "Self-organized nanogratings in glass irradiated by ultrashort light pulses," Phys. Rev. Lett. 91, 247405 (2003). [CrossRef] [PubMed]
  4. S. Kawata, H. B. Sun, T. Tanaka, and K. Takada, "Finer features for functional microdevices," Nature 412, 697-698 (2001). [CrossRef] [PubMed]
  5. F. Korte, J. Serbin, J. Koch, A. Egbert, C. Fallnich, A. Ostendorf, and B.N. Chichkov, "Towards nanostructuring with femtosecond laser pulses," Appl. Phys. A 77, 229-235 (2003).
  6. S. Nolte, B.N. Chichkov, H. Welling, Y. Shani, K. Lieberman, and H. Terkel, "Nanostructuring with spatially localized femtosecond laser pulses," Opt. Lett. 24, 914-916 (1999). [CrossRef]
  7. J. Qiu, X. Jiang, C. Zhu, M. Shirai, J. Si, N. Jiang, and K. Hirao, "Manipulation of gold nanoparticles inside transparent materials," Angew. Chem. Int. Ed. 43, 2230-2234 (2004). [CrossRef]
  8. A. V. Kabashin and M. Meunier, "Synthesis of colloidal nanoparticles during femtosecond laser ablation of gold in water," J Appl. Phys. 94, 7941-7943 (2003). [CrossRef]
  9. T. Q. Jia, H. X. Chen, M. Huang, X. J. Wu, F. L. Zhao, M. Baba, M. Suzuki, H. Kuroda, J. R. Qiu, R. X. Li,and Z. Z. Xu, "ZnSe nanowires grown on the crystal surface by femtosecond laser ablation in air," Appl. Phys. Lett. 89, 101116 (2006). [CrossRef]
  10. F. Korte, J. Koch, and B. N. Chichkov, "Formation of microbumps and nanojets on gold targets by femtosecond laser pulses," Appl. Phys. A 79, 879-881 (2004). [CrossRef]
  11. M. Birnbaum, "Semiconductor Surface Damage Produced by Ruby Lasers," J. Appl. Phys. 36, 3688-3689 (1965). [CrossRef]
  12. R. M. Osgood and D. J. Ehrlich, "Optically induced microstructures in laser-photodeposited metal films," Opt. Lett. 7, 385-387 (1982). [CrossRef] [PubMed]
  13. F. Keilmann and Y. H. Bai, "Periodic surface structures frozen into CO2 laser-melted quartz," Appl. Phys. A 29, 9-18 (1982). [CrossRef]
  14. G. Zhou, P. M. Fauchet, and A. E. Siegman, "Growth of spontaneous periodic surface structures on solids during laser illumination," Phys. Rev. B 26, 5366-5381 (1982). [CrossRef]
  15. P. M. Fauchet and A. E. Siegman, "Surface ripples on silicon and gallium arsenide under picosecond laser illumination," Appl. Phys. Lett. 40, 824-826 (1982). [CrossRef]
  16. H. M. Vandriel, J. E. Sipe, and J. F. Young, "Laser-induced coherent modulation of solid and liquid surfaces," J. Lumin. 30, 446-471 (1985). [CrossRef]
  17. J. F. Young, J. S. Preston, H. M. van Driel, and J. E. Sipe, "Laser-induced periodic surface structure. II. Experiments on Ge, Si, Al, and brass," Phys. Rev. B 27, 1155-1172 (1983). [CrossRef]
  18. Q. Z. Zhao, S. Malzer, and L. J. Wang, "Formation of subwavelength periodic structures on tungsten induced by ultrashort laser pulses," Opt. Lett. 32, 1932-1934 (2007). [CrossRef] [PubMed]
  19. J. Hohlfeld, S.-S. Wellershoff, J. Gudde, U. Conrad, V. Jahnke, and E. Matthias, "Electron and lattice dynamics following optical excitation of metals," Chem. Phys. 251, 237-258, (2000). [CrossRef]
  20. B. J. Siwick, J. R. Dwyer, R. E. Jordan, and R. J. D. Miller, "An atomic-level view of melting using femtosecond electron diffraction," Science 302, 1382-1385, (2003). [CrossRef] [PubMed]
  21. H. Y. Yang, S. P. Lau, S. F. Yu, L. Huang, M. Tanemura, J. Tanaka, T Okita, and H. H. Hng, "Field emission from zinc oxide nanoneedles on plastic substrates," Nanotechnology 16, 1300-1303 (2005). [CrossRef]
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