Periodic nanostructures on titanium dioxide film produced using femtosecond laser with wavelengths of 388 nm and 775 nm Togo Shinonaga,1,* Masahiro Tsukamoto,1 and Godai Miyaji2 1
2
Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka, 567-0047, Japan Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei-shi, Tokyo 184-8588, Japan * [email protected]
Abstract: Titanium dioxide (TiO2) film is an important biomaterial used to improve the biocompatibility of titanium (Ti). We have used a film coating method with an aerosol beam and femtosecond laser irradiation to form periodic structures on biomaterials for control of the cell spreading. The control of cell spreading on biomaterials is important for the development of advanced biomaterials. In this study, nanostructures with periods of 130 and 230 nm were formed on a film using a femtosecond laser with wavelengths of 388 and 775 nm, respectively. The nanostructure period on the film was 30% of the laser wavelengths. Periods produced with wavelengths of 388 and 775 nm were calculated using a surface plasmon polariton (SPP) model and the experimental results for both wavelengths were in the range of the calculated periods, which suggests that the mechanism for the formation of the periodic nanostructures on the film with a femtosecond laser was due to the excitation of SPPs. ©2014 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (140.7090) Ultrafast lasers; (310.6628) Subwavelength structures, nanostructures; (160.1435) Biomaterials.
References and links 1. 2.
T. Hanawa, “Biofunctionalization of titanium for dental implant,” Jpn. Dent. Sci. Rev. 46(2), 93–101 (2010). L. H. Li, Y. M. Kong, H. W. Kim, Y. W. Kim, H. E. Kim, S. J. Heo, and J. Y. Koak, “Improved biological performance of Ti implants due to surface modification by micro-arc oxidation,” Biomaterials 25(14), 2867– 2875 (2004). 3. X. Liu, P. K. Chub, and C. Ding, “Surface modification of titanium, titanium alloy sand related materials for biomedical applications,” Mater. Sci. Eng. Rep. 47(3–4), 49–121 (2004). 4. Y. Tsutsumi, M. Niinomi, M. Nakai, H. Tsutsumi, H. Doi, N. Nomura, and T. Hanawa, “Micro-arc oxidation treatment to improve the hard-tissue compatibility of Ti-29Nb-13Ta-4.6Zr alloy,” Appl. Surf. Sci. 262, 34–38 (2012). 5. J. Akedo, M. Ichiki, K. Kikuchi, and R. Maeda, “Jet molding system for realization of three-dimensional microstructures,” Sens. Actuators A Phys. 69(1), 106–112 (1998). 6. M. Tsukamoto, T. Fujihara, N. Abe, S. Miyake, M. Katto, T. Nakayama, and J. Akedo, “Hydroxyapatite coating on titanium plate with an ultrafine particle beam,” Jpn. J. Appl. Phys. 42(2A), L120–L122 (2003). 7. J. Lu, M. P. Rao, N. C. MacDonald, D. Khang, and T. J. Webster, “Improved endothelial cell adhesion and proliferation on patterned titanium surfaces with rationally designed, micrometer to nanometer features,” Acta Biomater. 4(1), 192–201 (2008). 8. A. Matsugaki, G. Aramoto, and T. Nakano, “The alignment of MC3T3-E1 osteoblasts on steps of slip traces introduced by dislocation motion,” Biomaterials 33(30), 7327–7335 (2012). 9. S. Fujita, M. Ohshima, and H. Iwata, “Time-lapse observation of cell alignment on nanogrooved patterns,” J. R. Soc. Interface 6(Suppl 3), S269–S277 (2009). 10. K. Matsuzaka, X. F. Walboomers, J. E. de Ruijter, and J. A. Jansen, “The effect of poly-L-lactic acid with parallel surface micro groove on osteoblast-like cells in vitro,” Biomaterials 20(14), 1293–1301 (1999). 11. M. Tsukamoto, K. Asuka, H. Nakano, M. Hashida, M. Katto, N. Abe, and M. Fujita, “Periodic microstructures produced by femtosecond laser irradiation on titanium plate,” Vacuum 80(11–12), 1346–1350 (2006). 12. M. Tsukamoto, T. Kayahara, H. Nakano, M. Hashida, M. Katto, M. Fujita, M. Tanaka, and N. Abe, “Microstructures formation on titanium plate by femtosecond laser ablation,” J. Phys. Conf. Ser. 59, 666–669 (2007).
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14696
13. K. Okamuro, M. Hashida, Y. Miyasaka, Y. Ikuta, S. Tokita, and S. Sakabe, “Laser fluence dependence of periodic grating structures formed on metal surfaces under femtosecond laser pulse irradiation,” Phys. Rev. B 82(16), 165417 (2010). 14. S. Sakabe, M. Hashida, S. Tokita, S. Namba, and K. Okamuro, “Mechanism for self-formation of periodic grating structures on a metal surface by a femtosecond laser pulse,” Phys. Rev. B 79(3), 033409 (2009). 15. S. K. Das, D. Dufft, A. Rosenfeld, J. Bonse, M. Bock, and R. Grunwald, “Femtosecond laser induced quasiperiodic nanostructures on TiO2 surfaces,” J. Appl. Phys. 105(8), 084912 (2009). 16. N. Yasumaru, K. Miyazaki, and J. Kiuchi, “Femtosecond-laser-induced nanostructure formed on hard thin films of TiN and DLC,” Appl. Phys. A Mater.Sci. Proc. 76, 983–985 (2003). 17. D. Dufft, A. Rosenfeld, S. K. Das, R. Grunwald, and J. Bonse, “Femtosecond laser-induced periodic surface structures revisited: a comparative study on ZnO,” J. Appl. Phys. 105(3), 034908 (2009). 18. G. Miyaji and K. Miyazaki, “Origin of periodicity in nanostructuring on thin film surfaces ablated with femtosecond laser pulses,” Opt. Express 16(20), 16265–16271 (2008). 19. G. Miyaji, K. Miyazaki, K. Zhang, T. Yoshifuji, and J. Fujita, “Mechanism of femtosecond-laser-induced periodic nanostructure formation on crystalline silicon surface immersed in water,” Opt. Express 20(14), 14848– 14856 (2012). 20. K. Miyazaki and G. Miyaji, “Nanograting formation through surface plasmon fields induced by femtosecond laser pulses,” J. Appl. Phys. 114(15), 153108 (2013). 21. T. Shinonaga, M. Tsukamoto, A. Nagai, K. Yamashita, T. Hanawa, N. Matsushita, G. Xie, and N. Abe, “Cell spreading on titanium dioxide film formed and modified with aerosol beam and femtosecond laser,” Appl. Surf. Sci. 288, 649–653 (2014). 22. J. Reif, F. Costache, M. Henyk, and S. V. Pandelov, “Ripples revisited: non-classical morphology at the bottom of femtosecond laser ablation craters in transparent dielectrics,” Appl. Surf. Sci. 197–198, 891–895 (2002). 23. R. Buividas, L. Rosa, R. Sliupas, T. Kudrius, G. Slekys, V. Datsyuk, and S. Juodkazis, “Mechanism of fine ripple formation on surfaces of (semi) transparent materials via a half-wavelength cavity feedback,” Nanotechnology 22(5), 055304 (2011). 24. C. Wang, H. Huo, M. Johnson, M. Shen, and E. Mazur, “The thresholds of surface nano-/micro-morphology modifications with femtosecond laser pulse irradiations,” Nanotechnology 21(7), 075304 (2010). 25. Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultrashort light pulses,” Phys. Rev. Lett. 91(24), 247405 (2003). 26. F. A. Umran, Y. Liao, M. M. Elias, K. Sugioka, R. Stoian, G. Cheng, and Y. Cheng, “Formation of nanogratings in a transparent material with tunable ionization property by femtosecond laser irradiation,” Opt. Express 21(13), 15259–15267 (2013). 27. M. Tsukamoto, N. Abe, Y. Soga, M. Yoshida, H. Nakano, M. Fujita, and J. Akedo, “Control of electrical resistance of TiO2films by short-pulse laser irradiation,” Appl. Phys. A Mater. Sci. Proc. 93, 193–196 (2008). 28. M. Tsukamoto, T. Shinonaga, M. Takahashi, M. Fujita, and N. Abe, “Photoconductive properties of titanium dioxide film modified by femtosecond laser irradiation,” Appl. Phys. A Mater. Sci. Proc. 110, 679–682 (2013). 29. K. Sokolowski-Tinten and D. von der Linde, “Generation of dense electron-hole plasmas in silicon,” Phys. Rev. B 61(4), 2643–2650 (2000). 30. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings (Springer, 1988). 31. E. D. Palik, Handbook of Optical Constants of Solids (Academic, 1997). 32. B. Enright and D. Fitzmaurice, “Spectroscopic determination of electron and hole effective masses in a nanocrystalline semiconductor film,” J. Phys. Chem. 100(3), 1027–1035 (1996).
1. Introduction Titanium (Ti) is an attractive biomaterial because of its excellent chemical resistance and high strength. However, Ti has problems for long term applications and biofunction [1]; therefore, its biocompatibility must be improved. It has recently been suggested that coating a titanium dioxide (TiO2) film on Ti plates may improve the biocompatibility of Ti [2–4]. We have developed a method for coating a TiO2 film a on Ti plate using an aerosol beam composed of submicron-sized functional ceramic particles and helium gas [5,6]. The thickness of the film can be controlled to approximately several micrometers. Controlling the cell spreading on biomaterials is another useful method to improve the biocompatibility of Ti plates [7]. Increasing endothelial cell functions [7], anisotropic morphogenesis of bone tissue [8] and control of differentiation [9] have been achieved for biomaterials by the control of cell spreading. Thus, the control of cell spreading on biomaterials is important for the development of advanced biomaterials. The formation of periodic nanostructures on biomaterials is a useful method for the control of cell spreading [7,9,10]. Femtosecond lasers can be used to form periodic nanostructures on metals [11–14], semiconductors [15–24] and inside transparent materials [25,26]. Periodic nanostructures are self-organized on the laser focal spot. The nanostructure period is dependent on the
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14697
wavelength of the femtosecond laser. The directions of the grooves of periodic nanostructures lie perpendicular to the laser electric polarization vector. We have already reported that periodic nanostructures perpendicular to the laser electric polarization vector were formed on TiO2 films with a femtosecond laser using the fundamental wavelength of 775 nm [21]. The period of these nanostructures was approximately 230 nm, which is shorter than the laser wavelength. Cell testing revealed that cells spread along the grooves of the periodic (230 nm) nanostructures, whereas cell spreading did not have a definite direction on a film without periodic nanostructures [21]. The effect of the nanostructure period on cell spreading has not yet been investigated and it is expected that the period could be varied according to the laser wavelength. The mechanism for the formation of periodic nanostructures on the film should be investigated. Various mechanisms have been proposed for the formation of periodic nanostructures, such as second-harmonic generation [15,17], self-organization due to nonequilibrium surface [22], formation of nanoplasma [23] and a refractive index change during femtosecond laser irradiation [24]. For semiconductors, the period of the nanostructures is much shorter than the laser wavelength. However, there is no mechanism applicable to the formation of periodic nanostructures comprised of different materials. Miyaji et al. recently reported that the formation of periodic nanostructures with a femtosecond laser can be attributed to the excitation of surface plasmon polaritons (SPPs) in the glassy carbon layer formed on a diamond-like carbon (DLC) surface [18]. Calculations using the SPP model reproduced the observed nanosize periodicity. The SPP model can also be applied for periodic nanostructures formed on Si [19] and GaN [20]. In our previous study, we suggested that a multi-photon process may occur on a TiO2 film (TiO2 band gap of 3.2 eV) during femtosecond laser irradiation with a wavelength of 775 nm (photon energy of 1.6 eV) [27,28], as shown in Fig. 1(a). Thus, a high electron density region could be formed in the film, as shown in Fig. 1(b). SPPs may be excited at the interface between the high electron density region and the film, as shown in Fig. 1(b). SPPs may also be excited by femtosecond laser irradiation with a wavelength of 388 nm (photon energy is 3.2 eV) because electrons could be excited by a one photon process, as shown in Fig. 1(a). Hence, periodic nanostructures could also be formed on a film using a femtosecond laser with a wavelength of 388 nm. However, calculation of the periods with different laser wavelengths using the SPP model has not yet been investigated. In this study, periodic nanostructures were produced on a TiO2 film by irradiation with a femtosecond laser at wavelengths of 388 and 775 nm. The irradiated area was then observed using scanning electron microscopy (SEM) and the periodicity of the nanostructures was examined. The period was calculated for the different wavelengths using the SPP model to assess the validity of the SPP model for the formation of periodic nanostructures on TiO2 films. 2. Experimental procedure The TiO2 film was produced on a pure Ti plate by aerosol beam irradiation. An aerosol beam was formed by mixing TiO2 particles and helium gas, as shown in Fig. 2. The diameters of the TiO2 particles were between 100 and 1000 nm. The particles were accelerated by the helium
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14698
Fig. 1. (a) Multi and one photon processes in a TiO2 film with a femtosecond laser at wavelengths of 775 and 388 nm, respectively. (b) Formation of high electron density region on the film surface and excitation of SPPs at the interface between the high electron density region and the film.
Fig. 2. (a) Schematic illustration of aerosol beam irradiation to produce the TiO2 film coating on a Ti plate.
gas flow to velocities of several hundred meters per second. Impact of the particles with the substrate resulted in film deposition on the substrate. The thickness of the resultant film was approximately 5 µm. Schematic diagrams of femtosecond laser irradiation and the scanning direction are shown in Figs. 3(a) and 3(b), respectively. A commercial femtosecond Ti:sapphire laser system was employed and is based on the chirped pulse amplification technique. The wavelength, pulse duration, repetition rate and beam diameter for the femtosecond laser were 775 nm, 150 fs, 1 kHz and about 5 mm, respectively. The laser wavelength of 388 nm was obtained with a harmonic generator. The laser beam was focused on the film surface using a lens with a focal length of 100 mm. The Gaussian laser beam had a diameter of 60 μm (at the 1/e2 intensity points) on the film surface. The femtosecond laser focal spot was scanned over the film
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Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14699
surface using an XY stage to form periodic nanostructures, as shown in Fig. 3(b). The scanning speed was fixed at 0.1 mm/s. Laser fluences with wavelengths of 388 and 775 nm
Fig. 3. Schematic diagrams of (a) the experimental setup for femtosecond laser irradiation of the film surface, and (b) scanning direction of the femtosecond laser focal spot.
were 0.25 and 0.35 J/cm2, respectively. The periodicities of the nanostructures formed on the film were examined using SEM. The periodicity was calculated according to the SPP model. 3. Results and Discussion Figure 4(a) shows an SEM image of the bare film surface (no laser irradiation). SEM images of the film surface after scanning of the femtosecond laser spot at wavelengths of 388 and 775 nm are shown in Figs. 4(b) and 4(c), respectively. Figures 4(b) and 4(c) show laser irradiated area in the center region. No periodic nanostructures were observed on the non-irradiated film surface, whereas periodic nanostructures lying perpendicular to the laser electric polarization vector E, were formed on the area irradiated with the laser at a wavelength of 388 nm (Fig. 4(d)). Periodic nanostructures were also formed on the area irradiated at a wavelength of 775 nm (Fig. 4(c)).
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14700
Fig. 4. SEM images of (a) a bare film surface, and films with periodic nanostructures formed by femtosecond laser irradiation at wavelengths of (b) 388 and (c) 775 nm. The double-headed arrow indicates the direction of the laser electric polarization vector.
Fig. 5. Period of the nanostructures as a function of the femtosecond laser wavelength.
The periodicity of the nanostructures shown in Figs. 4(b) and 4(c) were examined as a functional of the laser wavelengths and the results are shown in Fig. 5. The period of the nanostructures on the film was increased from 130 to 230 nm as the laser wavelength was increased from 388 and 775 nm. These periods with 130 and 230 nm were average values from SEM measurement. The standard deviation of the period with approximately 130 and 230 nm is about 10 and 10 nm, respectively. Thus, the nanostructure period on the film was 30% of the femtosecond laser wavelength. Multi- or one-photon processes may occur in the film by femtosecond laser irradiation at wavelengths of 775 and 388 nm, respectively (Fig. 1(a)). The incident femtosecond laser pulse produces a high electron density region in the film, as shown in Fig. 1(b). Using the Drude model for electrical conduction, the relative refractive index of the high electron density region in the film is calculated as:
ε high = ε TiO 2 −
ω pb 2 , ω0 2 + i ω0 τ
(1)
where εhigh is the relative dielectric constant of the high electron density region in the film, εTiO2 is the relative dielectric constant of TiO2, ωpb is the plasma frequency with the dielectric constant of a vacuum, ω0 is the incident light frequency in a vacuum, τ is the Drude damping time for free electrons. The plasma frequency ωpb is calculated using:
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Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14701
ω pb 2 =
Ne2 , ε 0m * m
(2)
where N is the electron density, ε0 is the dielectric constant of a vacuum, e is the electron charge, m is the electron mass, and m* is the optical effective mass for carriers [29]. When the dispersion relation is satisfied [30], the SPPs can be excited at the interface between the high electron density region and the film: 12
k SPP
ε ε = k0 high TiO 2 , ε high + ε TiO 2
(3)
where kspp is the plasmon wave number, k0 is the wavenumber of the incident light in a vacuum. The optical near-field in the interface is enhanced along the laser polarization direction, at the half SPP wavelength, due to the spatial standing wave of SPPs [18–20]. The nanostructure period may thus correspond to half of the SPP wavelength [18–20]. The SPP wavelength is calculated as:
λSPP =
2π , Re{k SPP }
(4)
where Re {kspp} is the real part of kspp. The nanostructure periodicity in the film, dTiO2, is calculated as dTiO2 = λSPP/2. The relative dielectric constant of TiO2 for a wavelength of 775 nm, εTiO2 (775 nm) = 7.83 + 0.45i [31] and the relative dielectric constant of TiO2 for a wavelength of 388 nm, εTiO2 (388 nm) = 12.18 [31], were used in the calculation of kspp. In addition, m* = 0.8 [32] and τ = 1 fs [29] were used for both wavelengths of 775 and 388 nm. Figure 6 shows the period dTiO2 = λSPP/2 of the nanostructures produced by femtosecond laser irradiation at 775 nm and the real part of the relative dielectric constant of the high electron density region Re{εhigh} calculated as a function of electron density. The condition of Re{εhigh} < 0 must be satisfied for the excitation of SPPs [30], which is indicated by the shaded region of N > 1.4 × 1022 cm–3 shown in Fig. 6. The calculated periods with the wavelength of 775 nm are in the range from 120 to 260 nm, as shown in Fig. 6. Figures 5 and 6 show that the experimental result of approximately 230 nm is in the range of the calculated period for a wavelength of 775 nm. For the wavelength of 338 nm, the period and the real part of relative dielectric constant of the high electron density region were also calculated as a function of electron density, and are shown in Fig. 7. The condition of Re{εhigh} < 0 must be satisfied for the excitation of SPPs, which is indicated by the shaded region of N > 7.6 × 1022 cm–3 shown in Fig. 7. The calculated periods for a wavelength of 388 nm are in the range from 40 to 170 nm, as shown in Fig. 7. The experimental result of approximately 130 nm is in the range of the calculated period for a wavelength of 388 nm, as shown in Figs. 5 and 7. These results suggest that the mechanism for the formation of periodic nanostructures on TiO2 film by irradiation with a femtosecond laser is due to the excitation of SPPs. The influence of the change in nanostructure periodicity on cell spreading will be reported elsewhere. Thickness of high electron density region might correspond to skin depth in the high electron density region [19]. The skin depths, zskin, in the high electron density region with the wavelength of 775 and 388 nm were calculated. The skin depth with the wavelength of 775 nm rapidly decreases to zskin < 55 nm as N increases to N > 1.4 × 1022 cm–3 for the condition of Re{εhigh} < 0. The skin depth with the wavelength of 388 nm also decreases to zskin < 25 nm as N increases to N > 7.4 × 1022 cm–3 for the condition of Re{εhigh} < 0. These results suggest
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14702
Fig. 6. Periods of nanostructures formed with the femtosecond laser at a wavelength of 775 nm, and the real part of the relative dielectric constant for the high electron density region εhigh, calculated as a function of electron density, N.
Fig. 7. Period of nanostructures formed with the femtosecond laser at a wavelength of 388 nm, and the real part of the relative dielectric constant for the high electron density region εhigh, calculated as a function of electron density, N.
that laser energy is reflected and absorbed in the high electron density region, and contributed to the excitation of the SPP for the condition of Re{εhigh} < 0. S. K. Das et al. reported that periodic nanostructures were formed on rutile type TiO2 single crystals [15]. They observed periods of 170 nm with the laser wavelength of 800 nm on TiO2 single crystals in which explained by interference relation d = λ /(2n sinθ), where d is period of the periodic nanostructures, λ is the wavelength of the incident radiation, n is the corresponding refractive index of the material, and θ is the half angle between two interfering partial beams. From interference relation d = λ /(2n sinθ), calculated periods were in the range from 140 to 160 nm (n = 2.78 or 2.52), which slightly shorter than that of experimental result of the period with 170 nm [15]. In present study, periodic nanostructures were formed on
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14703
anatase type TiO2 films. Period with the laser wavelength of 775 nm on the film was about 230 nm. From equation d = λ /(2n sinθ), calculated periods were in the range from 140 to 150 nm (n = 2.80 or 2.52), which shorter than that of experimental result of the period with 230 nm. These results suggest that another factor except for interference relation is necessary to clarify mechanism of the periodic nanostructures formation on TiO2 film. Excitation of SPP during the femtosecond laser irradiation is one of the possibilities for the mechanism of the periodic nanostructures formation on TiO2 film. 4. Summary Periodic nanostructures were formed on a TiO2 film surface using a femtosecond laser at wavelengths of 388 and 775 nm. The periods of the nanostructures formed with wavelengths of 388 and 775 nm were 130 and 230 nm, respectively, and were 30% of the laser wavelengths. The periodicity of nanostructures formed with wavelengths of 388 and 775 nm were calculated using the SPP model and the experimental results were in the range of the calculated periods. These results suggest that the mechanism for the formation of periodic nanostructures on TiO2 film by femtosecond laser irradiation is due to the excitation of SPPs.
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14704
2
Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka, 567-0047, Japan Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei-shi, Tokyo 184-8588, Japan * [email protected]
Abstract: Titanium dioxide (TiO2) film is an important biomaterial used to improve the biocompatibility of titanium (Ti). We have used a film coating method with an aerosol beam and femtosecond laser irradiation to form periodic structures on biomaterials for control of the cell spreading. The control of cell spreading on biomaterials is important for the development of advanced biomaterials. In this study, nanostructures with periods of 130 and 230 nm were formed on a film using a femtosecond laser with wavelengths of 388 and 775 nm, respectively. The nanostructure period on the film was 30% of the laser wavelengths. Periods produced with wavelengths of 388 and 775 nm were calculated using a surface plasmon polariton (SPP) model and the experimental results for both wavelengths were in the range of the calculated periods, which suggests that the mechanism for the formation of the periodic nanostructures on the film with a femtosecond laser was due to the excitation of SPPs. ©2014 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (140.7090) Ultrafast lasers; (310.6628) Subwavelength structures, nanostructures; (160.1435) Biomaterials.
References and links 1. 2.
T. Hanawa, “Biofunctionalization of titanium for dental implant,” Jpn. Dent. Sci. Rev. 46(2), 93–101 (2010). L. H. Li, Y. M. Kong, H. W. Kim, Y. W. Kim, H. E. Kim, S. J. Heo, and J. Y. Koak, “Improved biological performance of Ti implants due to surface modification by micro-arc oxidation,” Biomaterials 25(14), 2867– 2875 (2004). 3. X. Liu, P. K. Chub, and C. Ding, “Surface modification of titanium, titanium alloy sand related materials for biomedical applications,” Mater. Sci. Eng. Rep. 47(3–4), 49–121 (2004). 4. Y. Tsutsumi, M. Niinomi, M. Nakai, H. Tsutsumi, H. Doi, N. Nomura, and T. Hanawa, “Micro-arc oxidation treatment to improve the hard-tissue compatibility of Ti-29Nb-13Ta-4.6Zr alloy,” Appl. Surf. Sci. 262, 34–38 (2012). 5. J. Akedo, M. Ichiki, K. Kikuchi, and R. Maeda, “Jet molding system for realization of three-dimensional microstructures,” Sens. Actuators A Phys. 69(1), 106–112 (1998). 6. M. Tsukamoto, T. Fujihara, N. Abe, S. Miyake, M. Katto, T. Nakayama, and J. Akedo, “Hydroxyapatite coating on titanium plate with an ultrafine particle beam,” Jpn. J. Appl. Phys. 42(2A), L120–L122 (2003). 7. J. Lu, M. P. Rao, N. C. MacDonald, D. Khang, and T. J. Webster, “Improved endothelial cell adhesion and proliferation on patterned titanium surfaces with rationally designed, micrometer to nanometer features,” Acta Biomater. 4(1), 192–201 (2008). 8. A. Matsugaki, G. Aramoto, and T. Nakano, “The alignment of MC3T3-E1 osteoblasts on steps of slip traces introduced by dislocation motion,” Biomaterials 33(30), 7327–7335 (2012). 9. S. Fujita, M. Ohshima, and H. Iwata, “Time-lapse observation of cell alignment on nanogrooved patterns,” J. R. Soc. Interface 6(Suppl 3), S269–S277 (2009). 10. K. Matsuzaka, X. F. Walboomers, J. E. de Ruijter, and J. A. Jansen, “The effect of poly-L-lactic acid with parallel surface micro groove on osteoblast-like cells in vitro,” Biomaterials 20(14), 1293–1301 (1999). 11. M. Tsukamoto, K. Asuka, H. Nakano, M. Hashida, M. Katto, N. Abe, and M. Fujita, “Periodic microstructures produced by femtosecond laser irradiation on titanium plate,” Vacuum 80(11–12), 1346–1350 (2006). 12. M. Tsukamoto, T. Kayahara, H. Nakano, M. Hashida, M. Katto, M. Fujita, M. Tanaka, and N. Abe, “Microstructures formation on titanium plate by femtosecond laser ablation,” J. Phys. Conf. Ser. 59, 666–669 (2007).
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14696
13. K. Okamuro, M. Hashida, Y. Miyasaka, Y. Ikuta, S. Tokita, and S. Sakabe, “Laser fluence dependence of periodic grating structures formed on metal surfaces under femtosecond laser pulse irradiation,” Phys. Rev. B 82(16), 165417 (2010). 14. S. Sakabe, M. Hashida, S. Tokita, S. Namba, and K. Okamuro, “Mechanism for self-formation of periodic grating structures on a metal surface by a femtosecond laser pulse,” Phys. Rev. B 79(3), 033409 (2009). 15. S. K. Das, D. Dufft, A. Rosenfeld, J. Bonse, M. Bock, and R. Grunwald, “Femtosecond laser induced quasiperiodic nanostructures on TiO2 surfaces,” J. Appl. Phys. 105(8), 084912 (2009). 16. N. Yasumaru, K. Miyazaki, and J. Kiuchi, “Femtosecond-laser-induced nanostructure formed on hard thin films of TiN and DLC,” Appl. Phys. A Mater.Sci. Proc. 76, 983–985 (2003). 17. D. Dufft, A. Rosenfeld, S. K. Das, R. Grunwald, and J. Bonse, “Femtosecond laser-induced periodic surface structures revisited: a comparative study on ZnO,” J. Appl. Phys. 105(3), 034908 (2009). 18. G. Miyaji and K. Miyazaki, “Origin of periodicity in nanostructuring on thin film surfaces ablated with femtosecond laser pulses,” Opt. Express 16(20), 16265–16271 (2008). 19. G. Miyaji, K. Miyazaki, K. Zhang, T. Yoshifuji, and J. Fujita, “Mechanism of femtosecond-laser-induced periodic nanostructure formation on crystalline silicon surface immersed in water,” Opt. Express 20(14), 14848– 14856 (2012). 20. K. Miyazaki and G. Miyaji, “Nanograting formation through surface plasmon fields induced by femtosecond laser pulses,” J. Appl. Phys. 114(15), 153108 (2013). 21. T. Shinonaga, M. Tsukamoto, A. Nagai, K. Yamashita, T. Hanawa, N. Matsushita, G. Xie, and N. Abe, “Cell spreading on titanium dioxide film formed and modified with aerosol beam and femtosecond laser,” Appl. Surf. Sci. 288, 649–653 (2014). 22. J. Reif, F. Costache, M. Henyk, and S. V. Pandelov, “Ripples revisited: non-classical morphology at the bottom of femtosecond laser ablation craters in transparent dielectrics,” Appl. Surf. Sci. 197–198, 891–895 (2002). 23. R. Buividas, L. Rosa, R. Sliupas, T. Kudrius, G. Slekys, V. Datsyuk, and S. Juodkazis, “Mechanism of fine ripple formation on surfaces of (semi) transparent materials via a half-wavelength cavity feedback,” Nanotechnology 22(5), 055304 (2011). 24. C. Wang, H. Huo, M. Johnson, M. Shen, and E. Mazur, “The thresholds of surface nano-/micro-morphology modifications with femtosecond laser pulse irradiations,” Nanotechnology 21(7), 075304 (2010). 25. Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultrashort light pulses,” Phys. Rev. Lett. 91(24), 247405 (2003). 26. F. A. Umran, Y. Liao, M. M. Elias, K. Sugioka, R. Stoian, G. Cheng, and Y. Cheng, “Formation of nanogratings in a transparent material with tunable ionization property by femtosecond laser irradiation,” Opt. Express 21(13), 15259–15267 (2013). 27. M. Tsukamoto, N. Abe, Y. Soga, M. Yoshida, H. Nakano, M. Fujita, and J. Akedo, “Control of electrical resistance of TiO2films by short-pulse laser irradiation,” Appl. Phys. A Mater. Sci. Proc. 93, 193–196 (2008). 28. M. Tsukamoto, T. Shinonaga, M. Takahashi, M. Fujita, and N. Abe, “Photoconductive properties of titanium dioxide film modified by femtosecond laser irradiation,” Appl. Phys. A Mater. Sci. Proc. 110, 679–682 (2013). 29. K. Sokolowski-Tinten and D. von der Linde, “Generation of dense electron-hole plasmas in silicon,” Phys. Rev. B 61(4), 2643–2650 (2000). 30. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings (Springer, 1988). 31. E. D. Palik, Handbook of Optical Constants of Solids (Academic, 1997). 32. B. Enright and D. Fitzmaurice, “Spectroscopic determination of electron and hole effective masses in a nanocrystalline semiconductor film,” J. Phys. Chem. 100(3), 1027–1035 (1996).
1. Introduction Titanium (Ti) is an attractive biomaterial because of its excellent chemical resistance and high strength. However, Ti has problems for long term applications and biofunction [1]; therefore, its biocompatibility must be improved. It has recently been suggested that coating a titanium dioxide (TiO2) film on Ti plates may improve the biocompatibility of Ti [2–4]. We have developed a method for coating a TiO2 film a on Ti plate using an aerosol beam composed of submicron-sized functional ceramic particles and helium gas [5,6]. The thickness of the film can be controlled to approximately several micrometers. Controlling the cell spreading on biomaterials is another useful method to improve the biocompatibility of Ti plates [7]. Increasing endothelial cell functions [7], anisotropic morphogenesis of bone tissue [8] and control of differentiation [9] have been achieved for biomaterials by the control of cell spreading. Thus, the control of cell spreading on biomaterials is important for the development of advanced biomaterials. The formation of periodic nanostructures on biomaterials is a useful method for the control of cell spreading [7,9,10]. Femtosecond lasers can be used to form periodic nanostructures on metals [11–14], semiconductors [15–24] and inside transparent materials [25,26]. Periodic nanostructures are self-organized on the laser focal spot. The nanostructure period is dependent on the
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14697
wavelength of the femtosecond laser. The directions of the grooves of periodic nanostructures lie perpendicular to the laser electric polarization vector. We have already reported that periodic nanostructures perpendicular to the laser electric polarization vector were formed on TiO2 films with a femtosecond laser using the fundamental wavelength of 775 nm [21]. The period of these nanostructures was approximately 230 nm, which is shorter than the laser wavelength. Cell testing revealed that cells spread along the grooves of the periodic (230 nm) nanostructures, whereas cell spreading did not have a definite direction on a film without periodic nanostructures [21]. The effect of the nanostructure period on cell spreading has not yet been investigated and it is expected that the period could be varied according to the laser wavelength. The mechanism for the formation of periodic nanostructures on the film should be investigated. Various mechanisms have been proposed for the formation of periodic nanostructures, such as second-harmonic generation [15,17], self-organization due to nonequilibrium surface [22], formation of nanoplasma [23] and a refractive index change during femtosecond laser irradiation [24]. For semiconductors, the period of the nanostructures is much shorter than the laser wavelength. However, there is no mechanism applicable to the formation of periodic nanostructures comprised of different materials. Miyaji et al. recently reported that the formation of periodic nanostructures with a femtosecond laser can be attributed to the excitation of surface plasmon polaritons (SPPs) in the glassy carbon layer formed on a diamond-like carbon (DLC) surface [18]. Calculations using the SPP model reproduced the observed nanosize periodicity. The SPP model can also be applied for periodic nanostructures formed on Si [19] and GaN [20]. In our previous study, we suggested that a multi-photon process may occur on a TiO2 film (TiO2 band gap of 3.2 eV) during femtosecond laser irradiation with a wavelength of 775 nm (photon energy of 1.6 eV) [27,28], as shown in Fig. 1(a). Thus, a high electron density region could be formed in the film, as shown in Fig. 1(b). SPPs may be excited at the interface between the high electron density region and the film, as shown in Fig. 1(b). SPPs may also be excited by femtosecond laser irradiation with a wavelength of 388 nm (photon energy is 3.2 eV) because electrons could be excited by a one photon process, as shown in Fig. 1(a). Hence, periodic nanostructures could also be formed on a film using a femtosecond laser with a wavelength of 388 nm. However, calculation of the periods with different laser wavelengths using the SPP model has not yet been investigated. In this study, periodic nanostructures were produced on a TiO2 film by irradiation with a femtosecond laser at wavelengths of 388 and 775 nm. The irradiated area was then observed using scanning electron microscopy (SEM) and the periodicity of the nanostructures was examined. The period was calculated for the different wavelengths using the SPP model to assess the validity of the SPP model for the formation of periodic nanostructures on TiO2 films. 2. Experimental procedure The TiO2 film was produced on a pure Ti plate by aerosol beam irradiation. An aerosol beam was formed by mixing TiO2 particles and helium gas, as shown in Fig. 2. The diameters of the TiO2 particles were between 100 and 1000 nm. The particles were accelerated by the helium
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14698
Fig. 1. (a) Multi and one photon processes in a TiO2 film with a femtosecond laser at wavelengths of 775 and 388 nm, respectively. (b) Formation of high electron density region on the film surface and excitation of SPPs at the interface between the high electron density region and the film.
Fig. 2. (a) Schematic illustration of aerosol beam irradiation to produce the TiO2 film coating on a Ti plate.
gas flow to velocities of several hundred meters per second. Impact of the particles with the substrate resulted in film deposition on the substrate. The thickness of the resultant film was approximately 5 µm. Schematic diagrams of femtosecond laser irradiation and the scanning direction are shown in Figs. 3(a) and 3(b), respectively. A commercial femtosecond Ti:sapphire laser system was employed and is based on the chirped pulse amplification technique. The wavelength, pulse duration, repetition rate and beam diameter for the femtosecond laser were 775 nm, 150 fs, 1 kHz and about 5 mm, respectively. The laser wavelength of 388 nm was obtained with a harmonic generator. The laser beam was focused on the film surface using a lens with a focal length of 100 mm. The Gaussian laser beam had a diameter of 60 μm (at the 1/e2 intensity points) on the film surface. The femtosecond laser focal spot was scanned over the film
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14699
surface using an XY stage to form periodic nanostructures, as shown in Fig. 3(b). The scanning speed was fixed at 0.1 mm/s. Laser fluences with wavelengths of 388 and 775 nm
Fig. 3. Schematic diagrams of (a) the experimental setup for femtosecond laser irradiation of the film surface, and (b) scanning direction of the femtosecond laser focal spot.
were 0.25 and 0.35 J/cm2, respectively. The periodicities of the nanostructures formed on the film were examined using SEM. The periodicity was calculated according to the SPP model. 3. Results and Discussion Figure 4(a) shows an SEM image of the bare film surface (no laser irradiation). SEM images of the film surface after scanning of the femtosecond laser spot at wavelengths of 388 and 775 nm are shown in Figs. 4(b) and 4(c), respectively. Figures 4(b) and 4(c) show laser irradiated area in the center region. No periodic nanostructures were observed on the non-irradiated film surface, whereas periodic nanostructures lying perpendicular to the laser electric polarization vector E, were formed on the area irradiated with the laser at a wavelength of 388 nm (Fig. 4(d)). Periodic nanostructures were also formed on the area irradiated at a wavelength of 775 nm (Fig. 4(c)).
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14700
Fig. 4. SEM images of (a) a bare film surface, and films with periodic nanostructures formed by femtosecond laser irradiation at wavelengths of (b) 388 and (c) 775 nm. The double-headed arrow indicates the direction of the laser electric polarization vector.
Fig. 5. Period of the nanostructures as a function of the femtosecond laser wavelength.
The periodicity of the nanostructures shown in Figs. 4(b) and 4(c) were examined as a functional of the laser wavelengths and the results are shown in Fig. 5. The period of the nanostructures on the film was increased from 130 to 230 nm as the laser wavelength was increased from 388 and 775 nm. These periods with 130 and 230 nm were average values from SEM measurement. The standard deviation of the period with approximately 130 and 230 nm is about 10 and 10 nm, respectively. Thus, the nanostructure period on the film was 30% of the femtosecond laser wavelength. Multi- or one-photon processes may occur in the film by femtosecond laser irradiation at wavelengths of 775 and 388 nm, respectively (Fig. 1(a)). The incident femtosecond laser pulse produces a high electron density region in the film, as shown in Fig. 1(b). Using the Drude model for electrical conduction, the relative refractive index of the high electron density region in the film is calculated as:
ε high = ε TiO 2 −
ω pb 2 , ω0 2 + i ω0 τ
(1)
where εhigh is the relative dielectric constant of the high electron density region in the film, εTiO2 is the relative dielectric constant of TiO2, ωpb is the plasma frequency with the dielectric constant of a vacuum, ω0 is the incident light frequency in a vacuum, τ is the Drude damping time for free electrons. The plasma frequency ωpb is calculated using:
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14701
ω pb 2 =
Ne2 , ε 0m * m
(2)
where N is the electron density, ε0 is the dielectric constant of a vacuum, e is the electron charge, m is the electron mass, and m* is the optical effective mass for carriers [29]. When the dispersion relation is satisfied [30], the SPPs can be excited at the interface between the high electron density region and the film: 12
k SPP
ε ε = k0 high TiO 2 , ε high + ε TiO 2
(3)
where kspp is the plasmon wave number, k0 is the wavenumber of the incident light in a vacuum. The optical near-field in the interface is enhanced along the laser polarization direction, at the half SPP wavelength, due to the spatial standing wave of SPPs [18–20]. The nanostructure period may thus correspond to half of the SPP wavelength [18–20]. The SPP wavelength is calculated as:
λSPP =
2π , Re{k SPP }
(4)
where Re {kspp} is the real part of kspp. The nanostructure periodicity in the film, dTiO2, is calculated as dTiO2 = λSPP/2. The relative dielectric constant of TiO2 for a wavelength of 775 nm, εTiO2 (775 nm) = 7.83 + 0.45i [31] and the relative dielectric constant of TiO2 for a wavelength of 388 nm, εTiO2 (388 nm) = 12.18 [31], were used in the calculation of kspp. In addition, m* = 0.8 [32] and τ = 1 fs [29] were used for both wavelengths of 775 and 388 nm. Figure 6 shows the period dTiO2 = λSPP/2 of the nanostructures produced by femtosecond laser irradiation at 775 nm and the real part of the relative dielectric constant of the high electron density region Re{εhigh} calculated as a function of electron density. The condition of Re{εhigh} < 0 must be satisfied for the excitation of SPPs [30], which is indicated by the shaded region of N > 1.4 × 1022 cm–3 shown in Fig. 6. The calculated periods with the wavelength of 775 nm are in the range from 120 to 260 nm, as shown in Fig. 6. Figures 5 and 6 show that the experimental result of approximately 230 nm is in the range of the calculated period for a wavelength of 775 nm. For the wavelength of 338 nm, the period and the real part of relative dielectric constant of the high electron density region were also calculated as a function of electron density, and are shown in Fig. 7. The condition of Re{εhigh} < 0 must be satisfied for the excitation of SPPs, which is indicated by the shaded region of N > 7.6 × 1022 cm–3 shown in Fig. 7. The calculated periods for a wavelength of 388 nm are in the range from 40 to 170 nm, as shown in Fig. 7. The experimental result of approximately 130 nm is in the range of the calculated period for a wavelength of 388 nm, as shown in Figs. 5 and 7. These results suggest that the mechanism for the formation of periodic nanostructures on TiO2 film by irradiation with a femtosecond laser is due to the excitation of SPPs. The influence of the change in nanostructure periodicity on cell spreading will be reported elsewhere. Thickness of high electron density region might correspond to skin depth in the high electron density region [19]. The skin depths, zskin, in the high electron density region with the wavelength of 775 and 388 nm were calculated. The skin depth with the wavelength of 775 nm rapidly decreases to zskin < 55 nm as N increases to N > 1.4 × 1022 cm–3 for the condition of Re{εhigh} < 0. The skin depth with the wavelength of 388 nm also decreases to zskin < 25 nm as N increases to N > 7.4 × 1022 cm–3 for the condition of Re{εhigh} < 0. These results suggest
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14702
Fig. 6. Periods of nanostructures formed with the femtosecond laser at a wavelength of 775 nm, and the real part of the relative dielectric constant for the high electron density region εhigh, calculated as a function of electron density, N.
Fig. 7. Period of nanostructures formed with the femtosecond laser at a wavelength of 388 nm, and the real part of the relative dielectric constant for the high electron density region εhigh, calculated as a function of electron density, N.
that laser energy is reflected and absorbed in the high electron density region, and contributed to the excitation of the SPP for the condition of Re{εhigh} < 0. S. K. Das et al. reported that periodic nanostructures were formed on rutile type TiO2 single crystals [15]. They observed periods of 170 nm with the laser wavelength of 800 nm on TiO2 single crystals in which explained by interference relation d = λ /(2n sinθ), where d is period of the periodic nanostructures, λ is the wavelength of the incident radiation, n is the corresponding refractive index of the material, and θ is the half angle between two interfering partial beams. From interference relation d = λ /(2n sinθ), calculated periods were in the range from 140 to 160 nm (n = 2.78 or 2.52), which slightly shorter than that of experimental result of the period with 170 nm [15]. In present study, periodic nanostructures were formed on
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14703
anatase type TiO2 films. Period with the laser wavelength of 775 nm on the film was about 230 nm. From equation d = λ /(2n sinθ), calculated periods were in the range from 140 to 150 nm (n = 2.80 or 2.52), which shorter than that of experimental result of the period with 230 nm. These results suggest that another factor except for interference relation is necessary to clarify mechanism of the periodic nanostructures formation on TiO2 film. Excitation of SPP during the femtosecond laser irradiation is one of the possibilities for the mechanism of the periodic nanostructures formation on TiO2 film. 4. Summary Periodic nanostructures were formed on a TiO2 film surface using a femtosecond laser at wavelengths of 388 and 775 nm. The periods of the nanostructures formed with wavelengths of 388 and 775 nm were 130 and 230 nm, respectively, and were 30% of the laser wavelengths. The periodicity of nanostructures formed with wavelengths of 388 and 775 nm were calculated using the SPP model and the experimental results were in the range of the calculated periods. These results suggest that the mechanism for the formation of periodic nanostructures on TiO2 film by femtosecond laser irradiation is due to the excitation of SPPs.
#210723 - $15.00 USD (C) 2014 OSA
Received 22 Apr 2014; revised 29 May 2014; accepted 29 May 2014; published 6 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014696 | OPTICS EXPRESS 14704
