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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 15 — Jul. 21, 2008
  • pp: 11259–11265
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Ultra-broadband enhanced absorption of metal surfaces structured by femtosecond laser pulses

Yang Yang, Jianjun Yang, Chunyong Liang, and Hongshui Wang  »View Author Affiliations


Optics Express, Vol. 16, Issue 15, pp. 11259-11265 (2008)
http://dx.doi.org/10.1364/OE.16.011259


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Abstract

We investigate the enhanced absorption properties in a wavelength range of 0.2-25 μm for NiTi alloy targets structured by femtosecond laser pulses in air. Three different types of surface structures are produced with varying laser fluences. Measured reflectances through integrating sphere technique show that their couplings of incident electromagnetic irradiations are improved greatly over the broadband wavelength range. In particular, for coral-like micro-structures on the metal surfaces, approximate 90% absorption can be achieved from ultraviolet to mid-infrared region. Cut-off wavelengths of the enhanced absorption for the varied dimensional surface structures are determined experimentally. Chemical analysis by X-ray photoelectron spectroscopy indicates that blackness of metal surfaces is not attributed to the change in elemental composition. The physics of such remarkable absorption for the structured metal surfaces are discussed as well.

© 2008 Optical Society of America

1. Introduction

Since nanostructured metals show very complex and unique optical properties, their interaction with incident electromagnetic radiations can generate several interesting and curious phenomena. For examples, Teperik et al. found theoretically that nearly total resonant absorption of light could be achieved on a nanoporous surface of metal [1

1. T. V. Teperik, V. V. Popov, and F. J. García de Abajo, “Total Resonant Absorption of Light by Plasmons on the Nanoporous Surface of a Metal,” Physics of the Solid State 47, 172–175 (2005). [CrossRef]

]. Murnane et al. demonstrated experimentally the higher conversion efficiency of X-rays from ultrashort laser pulses through using gold gratings [2

2. M. M. Murnane, H. C. Kapteyn, S. P. Gordon, J. Bokor, and R. W. Falcone, “Efficient coupling of high intensity subpicosecond laser pulses into solids,” Appl. Phys. Lett. 62, 1068–1070 (1993). [CrossRef]

]. Coyle et al. pointed out that the enhanced absorption could be tuned by the nanostructure dimensions [3

3. S. Coyle, M. C. Netti, J. J. Baumberg, M. A. Ghanem, P. R. Birkin, P. N. Bartlett, and D. M. Whittaker, “Confined Plasmons in Metallic Nanocavities,” Phys. Rev. Lett. 87, 176801 (2001). [CrossRef] [PubMed]

]. However, how to fabricate metal nanostructures with varied dimensions in a convenient way becomes a big challenge for the research and development today.

Fortunately, with rapid progresses of laser technology in recent years, femtosecond lasers have been established to be an excellent and universal tool for micro- and nano-structuring solid materials by direct ablative writing. Mazur et al. recently discovered that femtocecond laser-induced micro-spikes on silicon could be employed to improve its opto-electron conversion efficiency [4–6

4. T. H. Her, R. J. Finlay, C. Wu, S. Deliwala, and E. Mazur, “Microstructuring of silicon with femtosecond laser pulses,” Appl. Phys. Lett. 73, 1673–1675 (1998). [CrossRef]

]. Guo et al. conducted the pioneering work in modification of reflectance of metal surfaces through femtosecond laser nanostructuring under the stationary or scanning focus, which indicated the controllable optical properties from UV to terahertz [7–9

7. A. Y. Vorobyev and C. Guo, “Enhanced absorptance of gold following multipulse femtosecond laser ablation,” Phys. Rev. B 72, 195422 (2005). [CrossRef]

]. More recently, Paivasaari et al. proposed a four-beam interferometric ablation method to create a cylindrical hole-array on steel surfaces [10

10. K. Paivasaari, J. J. Kaakkunen, M. Kuittinen, and T. Jaaskelainen, “Enhanced optical absorptance of metals using interferometric femtosecond ablation,” Opt. Express 15, 13838–13843 (2007). [CrossRef] [PubMed]

]. Their measurements of the sample absorption were shown in a band of 200-800 nm, where a gradual increase of reflectance to more than 30% could be observed in the red light. However, the situation of absorption changes in the longer wavelengths was not surveyed. Moreover, some extra optical diffraction elements should be required in this experiment, and the contribution of surface chemicals to the enhanced absorption remains ambiguous as well.

2. Experiments

In our experiments, a chirped pulse amplification of Ti:sapphire femtosecond laser system (HP-Spitfire, New Port Inc.) was adopted , which was able to deliver 50 fs, 2 mJ, 800 nm pulse trains at a 1 kHz repetition rate. A single-shot autocorrelator was used to monitor the time duration of laser pulses. After passing through a variable neutral–density filter, the linearly polarized laser beam was focused with a microscope objective (10×, N. A=0.25) down to the surface of the NiTi alloy plate at the normal incidence. The estimated radius of the beam focus was around 5 μm. The sample was mounted on a computer-controlled three-dimensional (x-y-z) translation stage. All experiments were carried out in ambient air within a Class 1000 clean room. Before and after the laser processing, the samples were cleaned by an ultrasonic cleaner.

Selection of titanium alloys as the sample in our experiment is due to their wide applications in space manufacturing, photo-electrochemical systems and so on, which makes the investigation of their surface properties with micro- or nano-structures become important and much desirable. The NiTi alloy plate in the experiments has a size of 10 × 10 × 2 mm3, whose surfaces were polished with SiC emery paper to remove the oxide layers. After being pasted on a microscope slide, it was mounted on the translation stages. The sample surface was adjusted above the focal plane, and the beam diameter on it was calculated about 25 μm. The electric field of the incident laser beam was parallel to the sample moving direction. By adopting a scan speed of 0.8 mm/s and a move step of 10 μm between the two consecutive lines, we produced three surface structures with each large area of 10 × 10 mm2 on different metal targets by varying laser energies. In this case, about 30 pulses partially overlapped each other per micrometer during one scan process when the size of beam spot was considered.

The morphologies of the structured metal surfaces were examined by a scanning electron microscopy (SEM, HITACHI S4800) and an atomic force microscopy (AFM, Veeco Inc.). Their reflectance situations from 200 nm to 2 μm were obtained by using an UV-VIS-NIR spectrophotometer (V-570, JASCO Corp.). To explore their reflections in the mid-infrared region, we employed a FT-IR spectrometer (TensorR27, Bruker Optik GmbH) with a wavelength range of 2.5–25 μm. Measurements in the above two wavelength regions are based on the integrating sphere technique, which allows us to compare the reflectance spectra of the polished surface with specular and diffuse reflections of the structured surface. Changes in the chemical and electronic properties of the femtosecond laser structured areas were analyzed by X-ray photoelectron spectroscopy (Model PHI5300, Physical Electronics) equipped with an aluminum anode (13 kV, 1486.6 eV) and a quartz monochromator with take-off angle of 45°.

3. Results and discussions

Figure 1(a) shows a SEM image of the metal surface structured by femtosecond lasers with the fluence of 80 J/cm2. As can be seen, the coral-like surface structures consist of micro-cavities with random orientations. The cavities have diameters of 2–35 μm and the hole depth of about several microns. As shown by its zoom-in picture in Fig. 1(b), some protrusions are found on the cavity edges, which can be considered as conglomeration of nanoparticles expelled from the ablation craters. Both the random micro-cavities and protrusions greatly increase the area of the metal surface. The high degree of roughness inside the cavity wall and on the protrusion surface makes these structures as a porous absorber, which takes on a jet black color. Yet, the development of such surface structures during our scribing processes is still unclear. The measured integrating reflectance over the wavelength range of 0.2–2 μm for such structures is shown by a red curve in Fig. 2(a), where the integrated reflectance of a polished substrate sample without any laser treatments (see a black curve) are also added for the comparison. From this figure, one can see that coral-like structures induced by femtosecond laser pulses have only about 10% reflectance, which is evenly distributed over the UV-infrared spectrum region. It is 80% decrease than that of the substrate sample. Although its reflectance in the ultraviolet region can be a little higher with 30%–15%, it is still much less than that of the metal surface without laser treatments.

Fig. 1. Surface morphologies of the NiTi alloy plates irradiated by femtosecond lasers with three different laser fluences. (a) and (b) for 80 J/cm2, (c) and (d) for 2 J/cm2, (e) and (f) for 20 J/cm2. E is the direction of the laser polarization. S is the direction of the sample scan.

Another modification of metal surfaces by femtosecond laser pulses could be achieved with laser fluence of 20 J/cm2. As shown by SEM pictures in Fig. 1(e) and Fig. 1(f), the surface patterns generation under this circumstance is characterized by some complex cellular-like nanostructures, which can be recognized as a combination of cavities and gratings. In this case, however, the diameter of cavities is reduced to about 500 nm, and their spatial distributions become loose but regular. For the grating components with the orientation perpendicular to the laser polarization, although the period still remains about 630 nm, the length of the grating ridges is shortened greatly. Its integrating reflectance spectrum in the range of 0.2–2 μm is shown by an olive curve in Fig. 2(a). Apparently, its around 13% reflectance is located between those of the coral-like and grating-like structures.

In order to investigate their enhanced absorption behaviors in the mid-infrared region, we employed the Fourier transform spectrometer with a broad wavelength range of 2.5–25 μm equipped with an integrating sphere. The measured integrating reflectance spectra are illustrated in Fig. 2(b). Each curve is in fact the average result of multiple measurements (100 times). In this figure, the black curve represents the reflectance of the polished substrate sample without laser treatments, which shows a slow rising tendency from 50% to 70% with increasing wavelength. The blue and olive curves describe the reflectance spectra for the grating-like and cellular-like structures, respectively. In the case of grating-like structures, a big bump of reflectance at the wavelength of 6 μm can be found, which makes a puzzle to be explored. For the two curves, it is seen that the variation tendency of the reflectance with increasing wavelength follows a very unique pattern: an initial small and a much less change in response to the incident wavelength and then a faster increase with larger wavelengths. Such a phenomenon suggests that a cut-off wavelength for the enhanced absorption exist for a given type of surface structures, which appears to separate the two different change tendency of the absorption processes. For example, in Fig. 2(b), the cut-off wavelength of the enhanced absorption for grating-like surface structures was measured about 3 μm, while 9 μm could be found for the cellular-like surface structures. Another interesting thing is that when the metal surface was modified with coral-like structures, the cut-off wavelength for the enhanced absorption can be extended to more than 25 μm. And the reflectance before this critical value can be decreased to be about 15%. This means that the incident light energies between 2.5 μm and 25 μm can be almost greatly trapped by this kind of peculiar surface structures, which becomes similar to a typical blackbody. In fact, for the three different surface structures, the reflectance after their cut-off wavelengths will grow up dramatically to converge with that of the polished substrate sample at the larger wavelength. This result indicates that the enhanced absorption would become gradually malfunction to the larger wavelengths, which actually depends on the characteristic dimensions of the surface structures.

Fig. 2. Measured integrated reflectance spectra for different surface structures induced by femtosecond lasers on the NiTi alloy plates. (a) in the UV-infrared range of 0.2–2 μm; (b) in the mid-infrared range of 2.5–25 μm. Among them, blue curves are for grating-like structures, olive ones for cellular-like structures, red ones for coral-like structures, and black ones for the polished metallic substrates without any laser treatments. Cut-off wavelengths of the enhanced absorption for different surface structures are marked by arrows in (b).

To investigate the influence of surface chemicals on the reflectance of metal structures, we plotted the typical XPS spectra for Ti and Ni elements on the modified surface regions with different laser energies in Fig. 3. Two dominant peaks at 458 eV and 464.8 eV in Fig. 3(a) can be identified as the generation of compound of TiO2 oxides. While the doublet peaks at 851 eV and 855 eV in Fig. 3(b) can be assigned to be nickel in the metallic and compound state, respectively. Since the oxides of nickel and titanium usually have only relatively weak absorption in UV-visible-infrared range, the main contribution from the surface chemical changes to the enhanced absorption can be ruled out. This result suggests that strong absorption of the metal surfaces within such a broadband region should be attributed to the micro- and nano-structures on metal surfaces.

Fig. 3. Typical XPS spectra of Ti (a), and Ni (b) elements on the metal surfaces before and after treatments by femtosecond laser pulses.

For the nanoparticle-covered ripples in Fig. 1(c) and Fig. 1(d), the periodic metallic corrugations on the subwavelength scale can be identified to induce the external radiations into SPs, leading to the more incident energy coupling to the metals [14

14. M. B. Sobnack, W. C. Tan, N. P. Wanstall, T. W. Preist, and J. R. Sambles, “Stationary Surface Plasmons on a Zero-Order Metal Grating,” Phys. Rev. Lett. 80, 5667–5670 (1998). [CrossRef]

]. On the other hand, dense distribution of nanoparticles over the grating ridges is also very helpful to improve the light absorption [16

16. J. Cesario, R. Quidant, G. Badenes, and S. Enoch, “Electromagnetic coupling between a metal nanoparticle grating and a metallic surface,” Opt. Lett. 30, 3404–3406 (2005). [CrossRef]

]. As concerning the cellular-like structures on metal surfaces, a mixture of subwavelength gratings and micro-cavities makes its reflectance within the broad wavelength range locate between the above two situations. In addition, its cut-off wavelength for the enhanced absorption can also be found larger than that of grating-like surface structures, but smaller than that of coral-like structures. A deep survey of the relationships between the cut-off wavelength for the enhanced absorption and the morphology of the surface structures is under way.

Conclusions

In summary, we have demonstrated that efficient trapping external irradiations could be achieved within a broadband wavelength region from 200 nm to 25 μm through generating some surface structures on the metal surfaces by femtosecond lasers in air. Both morphologies of three distinct surface structures and their corresponding reflectance spectra have been examined. About 90% absorption over the entire wavelength range has been demonstrated for the coral-like surface structures. The enhanced absorption behaviors have been bounded by the cut-off wavelengths, which can be varied with different surface structure dimensions. The highly improved coupling efficiency between light and solid targets may be attributed to the excitation of confined plasmon modes on structured metal surfaces. We believe that this investigation provides new insights into ultrafast laser micro-processing, and has great potential applications such as eliminating stray light, constructing sensitive detectors, stealth technology, as well as increasing emissions from the structured devices.

Acknowledgments

The authors would like to thank Profs. Guoguang Mu and Xiaonong Zhu for their consistent supports to this research work, Hua-Kuang Liu for helpful discussions, Yong Yang and Dengfeng Kuang for AFM inspections. Kaiping Yuan from Bruker Optics China is greatly appreciated for his help during integrated reflectance measurements over mid-infrared region. This research is financially supported by National Natural Science Foundation of China under Grant No. 60637020, and the Fund of Doctoral Program of Higher Education under Grant No. 20070055066.

References and links

1.

T. V. Teperik, V. V. Popov, and F. J. García de Abajo, “Total Resonant Absorption of Light by Plasmons on the Nanoporous Surface of a Metal,” Physics of the Solid State 47, 172–175 (2005). [CrossRef]

2.

M. M. Murnane, H. C. Kapteyn, S. P. Gordon, J. Bokor, and R. W. Falcone, “Efficient coupling of high intensity subpicosecond laser pulses into solids,” Appl. Phys. Lett. 62, 1068–1070 (1993). [CrossRef]

3.

S. Coyle, M. C. Netti, J. J. Baumberg, M. A. Ghanem, P. R. Birkin, P. N. Bartlett, and D. M. Whittaker, “Confined Plasmons in Metallic Nanocavities,” Phys. Rev. Lett. 87, 176801 (2001). [CrossRef] [PubMed]

4.

T. H. Her, R. J. Finlay, C. Wu, S. Deliwala, and E. Mazur, “Microstructuring of silicon with femtosecond laser pulses,” Appl. Phys. Lett. 73, 1673–1675 (1998). [CrossRef]

5.

Z. Huang, J. E. Carey, M. Liu, X. Guo, E. Mazur, and J. C. Campbell, “Microstructured silicon photodetector,” Appl. Phys. Lett. 89, 033506 (2006). [CrossRef]

6.

R. A. Myers, R. Farrell, A. M. Karger, J. E. Carey, and E. Mazur, “Enhancing near-infrared avalanche photodiode performance by femtosecond laser microstructuring,” Appl. Opt. 45, 8825–8831 (2006). [CrossRef] [PubMed]

7.

A. Y. Vorobyev and C. Guo, “Enhanced absorptance of gold following multipulse femtosecond laser ablation,” Phys. Rev. B 72, 195422 (2005). [CrossRef]

8.

A. Y. Vorobyev and C. Guo, “Colorizing metals with femtosecond laser pulses” Appl. Phys. Lett. 92, 41914 (2008). [CrossRef]

9.

A. Y. Vorobyev and C. Guo, “Effect of nanostructure-covered femtosecond laser-induced periodic surface structures on optical absorptance of metals,” Appl. Phys. A 86, 321–324 (2007). [CrossRef]

10.

K. Paivasaari, J. J. Kaakkunen, M. Kuittinen, and T. Jaaskelainen, “Enhanced optical absorptance of metals using interferometric femtosecond ablation,” Opt. Express 15, 13838–13843 (2007). [CrossRef] [PubMed]

11.

J. Yang, Y. Zhao, and X. Zhu, “Transition between nonthermal and thermal ablation of metallic targets under the strike of high-fluence ultrashort laser pulses,” Appl. Phys. Lett. 88, 094101 (2006). [CrossRef]

12.

V. Mikhailov, G. A. Wurtz, J. Elliott, P. Bayvel, and A.V. Zayats, “Dispersing Light with Surface Plasmon Polaritonic Crystals,” Phys. Rev. Lett. 99, 083901 (2007). [CrossRef] [PubMed]

13.

K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, and M. S. Feld, “ Single molecule detection using surface-enhanced Raman scattering (SERS),” Phys. Rev. Lett. 78, 1667–1670 (1997). [CrossRef]

14.

M. B. Sobnack, W. C. Tan, N. P. Wanstall, T. W. Preist, and J. R. Sambles, “Stationary Surface Plasmons on a Zero-Order Metal Grating,” Phys. Rev. Lett. 80, 5667–5670 (1998). [CrossRef]

15.

J. Sukmanowskia, J. R. Viguié, B. Nölting, and F. X. Royer, “Light absorption enhancement by nanoparticles,” J. Appl. Phys. 97, 104332 (2005). [CrossRef]

16.

J. Cesario, R. Quidant, G. Badenes, and S. Enoch, “Electromagnetic coupling between a metal nanoparticle grating and a metallic surface,” Opt. Lett. 30, 3404–3406 (2005). [CrossRef]

17.

W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature , 424, 824–830 (2003). [CrossRef] [PubMed]

OCIS Codes
(140.3390) Lasers and laser optics : Laser materials processing
(140.7090) Lasers and laser optics : Ultrafast lasers
(220.4000) Optical design and fabrication : Microstructure fabrication

ToC Category:
Laser Micromachining

History
Original Manuscript: April 3, 2008
Revised Manuscript: May 26, 2008
Manuscript Accepted: June 30, 2008
Published: July 11, 2008

Citation
Yang Yang, Jianjun Yang, Chunyong Liang, and Hongshui Wang, "Ultra-broadband enhanced absorption of metal surfaces structured by femtosecond laser pulses," Opt. Express 16, 11259-11265 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-15-11259


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References

  1. T. V. Teperik, V. V. Popov, F. J. García de Abajo, "Total Resonant Absorption of Light by Plasmons on the Nanoporous Surface of a Metal," Phys. Solid State 47, 172-175 (2005). [CrossRef]
  2. M. M. Murnane, H. C. Kapteyn, S. P. Gordon, J. Bokor, R. W. Falcone, "Efficient coupling of high- intensity subpicosecond laser pulses into solids," Appl. Phys. Lett. 62, 1068-1070 (1993). [CrossRef]
  3. S. Coyle, M. C. Netti, J. J. Baumberg, M. A. Ghanem, P. R. Birkin, P. N. Bartlett, and D. M. Whittaker, "Confined Plasmons in Metallic Nanocavities," Phys. Rev. Lett. 87, 176801 (2001). [CrossRef] [PubMed]
  4. T. H. Her, R. J. Finlay, C. Wu, S. Deliwala, and E. Mazur, "Microstructuring of silicon with femtosecond laser pulses," Appl. Phys. Lett. 73, 1673-1675 (1998). [CrossRef]
  5. Z. Huang, J. E. Carey, M. Liu X. Guo, E. Mazur, and J. C. Campbell, "Microstructured silicon photodetector," Appl. Phys. Lett. 89, 033506 (2006). [CrossRef]
  6. R. A. Myers, R. Farrell, A. M. Karger, J. E. Carey, and E. Mazur, "Enhancing near-infrared avalanche photodiode performance by femtosecond laser microstructuring," Appl. Opt. 45, 8825-8831 (2006). [CrossRef] [PubMed]
  7. A. Y. Vorobyev and C. Guo, "Enhanced absorptance of gold following multipulse femtosecond laser ablation," Phys. Rev. B 72, 195422 (2005). [CrossRef]
  8. A. Y. Vorobyev and C. Guo, "Colorizing metals with femtosecond laser pulses" Appl. Phys. Lett. 92, 41914 (2008). [CrossRef]
  9. A. Y. Vorobyev and C. Guo, "Effect of nanostructure-covered femtosecond laser-induced periodic surface structures on optical absorptance of metals," Appl. Phys. A 86, 321-324 (2007). [CrossRef]
  10. K. Paivasaari, J. J. Kaakkunen, M. Kuittinen, and T. Jaaskelainen, "Enhanced optical absorptance of metals using interferometric femtosecond ablation," Opt. Express 15, 13838-13843 (2007). [CrossRef] [PubMed]
  11. J. Yang, Y. Zhao, and X. Zhu, "Transition between nonthermal and thermal ablation of metallic targets under the strike of high-fluence ultrashort laser pulses," Appl. Phys. Lett. 88, 094101 (2006). [CrossRef]
  12. V. Mikhailov, G. A. Wurtz, J. Elliott, P. Bayvel, and A. V. Zayats, "Dispersing Light with Surface Plasmon Polaritonic Crystals," Phys. Rev. Lett. 99, 083901 (2007). [CrossRef] [PubMed]
  13. K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, and M. S. Feld, " Single molecule detection using surface-enhanced Raman scattering (SERS)," Phys. Rev. Lett. 78, 1667-1670 (1997). [CrossRef]
  14. M. B. Sobnack, W. C. Tan, N. P. Wanstall, T. W. Preist, and J. R. Sambles, "Stationary Surface Plasmons on a Zero-Order Metal Grating," Phys. Rev. Lett. 80, 5667-5670 (1998). [CrossRef]
  15. J. Sukmanowskia, J. R. Viguié, B. Nölting, and F. X. Royer, "Light absorption enhancement by nanoparticles," J. Appl. Phys. 97, 104332 (2005). [CrossRef]
  16. J. Cesario, R. Quidant, G. Badenes, and S. Enoch, "Electromagnetic coupling between a metal nanoparticle grating and a metallic surface," Opt. Lett. 30, 3404-3406 (2005). [CrossRef]
  17. W. L. Barnes, A. Dereux, and T. W. Ebbesen, "Surface plasmon subwavelength optics," Nature  424, 824-830 (2003). [CrossRef] [PubMed]

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