One-dimensional quasiperiodic structures whose period is much smaller than the wavelength of exciting radiation have been obtained on a titanium surface under the multipulse action of linearly polarized femtosecond laser radiation with various surface energy densitiesye

Emel’yanov, V. I.

Golosov, E. V.

Ionin, A. A.

Kolobov, Yu. R.

Kudryashov, S. I.

DSpace at Belgorod State University

Femtosecond Laser Writing of Subwave One-Dimensional Quasiperiodic Nanostructures on a Titanium SurfaceE. V. Golosov", V. I. Emel’yanov*, A. A. Ionin% Yu. R. Kolobov", S. I. Kudryashov' *,A. E. Ligachev'7, Yu. N. Novoselov% L. V. Seleznevc, and D. V. Sinitsynca Belgorod State University, Belgorod, 308015 Russia b Moscow State University, Moscow, 119992 Russia c Lebedev Physical Institute, Russian Academy o f  Sciences, Leninskii pr. 53, Moscow, 119991 Russia d Prokhorov General Physics Institute, Russian Academy o f  Sciences, ul. Vavilova 38, Moscow, 117942 Russia* e-mail: sikudr@sci.Iebedev.ruOne-dimensional quasiperiodic structures whose period is m uch smaller than the wavelength of exciting radi­ation have been obtained on a titanium  surface under the multipulse action of linearly polarized femtosecond laser radiation with various surface energy densities. As the radiation energy density increases, the one­dimensional surface nanorelief oriented perpendicularly to the radiation polarization evolves from quasipe­riodic ablation nanogrooves to regular lattices with subwave periods (100—400 nm ). In contrast to the preced­ing works for various metals, the period of lattices for titanium  decreases with increasing energy density. The formation of the indicated surface nanostructures is explained by the interference of the electric fields of inci­dent laser radiation and a surface electromagnetic wave excited by this radiation, because the length of the surface electromagnetic wave for titanium  with significant interband absorption decreases with an increase in the electron excitation of the material.1. It was recently found that the multipulse actionof femtosecond laser radiation of the visible and near- infrared ranges on the surface of solids makes it possi­ble to reproducibly obtain subwave (period A =  70—900 nm  < /., where /. is the wavelength of exciting laser radiation) one-dimensional quasiperiodic nanostruc­tures (nanolattices) [1—8]. Such nanolattices on the surfaces of various materials are interesting both because nanoscale periodicity leads to unusual physi­cal or physicochemical properties of a surface contin­uously varying with A [4] and as sources of nanoparti­cles [1,3,  4—6]. The necessity of the variation of the nanolattice period in a wide range by varying the parameters of laser radiation and choosing materials for writing initiates a number of investigations of theeffect of the laser polarization (vector e), the wave­length /. and duration of a pulse, the surface energydensity F, and the num ber of acting pulses N  on the A value [1—8]. In particular, it was found that the wave vector of nanolattices q is always collinear to e and the lattice period A increases linearly with /. for femtosec­ond and short picosecond laser pulses in a limited interval covering the visible and near-infrared ranges[6, 7]. At first glance, variations in the parameters /(within several orders of magnitude) [8] and TV (within several orders of magnitude) [7] provide wider possi­bilities of varying A. However, the preceding worksshowed that A remains almost unchanged [3, 4] or monotonically increases with the surface energy den­sity [5, 8], whereas the duration of the irradiation of the surface usually makes it possible only to develop a nanostructure that initially appears on this surface [2, 6, 7]. The indicated relation between A and /  hinders the formation of developed one-dimensional nanolat­tices with minimum possible periods interesting for m odern nanotechnologies, because the nanotransport of m atter decreases sharply at lower energy densities. In view of this circumstance, it is necessary to seek new regimes and materials for the femtosecond laser writing of subwave one-dimensional nanolattices that would allow for the efficient creation of one-dim en- sional surface nanolattices of minimum and maxi­m um  sizes.In this work, subwave one-dimensional quasiperi­odic nanostructures written under the action of femto­second laser pulses with different surface energy den­sities on a polished surface of chemically pure tita­nium, which is one of the basic materials of the aerospace industry and implant medicine, are investi­gated using a scanning electron microscope with a magnification o f 100 000.2. The experiments were performed on an experi­mental setup including a Ti:sapphire laser (Avesta Proekt) with fundamental radiation pulses (centralGOLOSOV et al.Fig. 1. Scanning electron microscope images of nano­structures written on the dry titanium  surface for the energy densities F  =  (a) 17, (b) 25, (c) 33, and (d) 250 m J/cm 2. The arrows indicate the coinciding direc­tions of the polarization vector o f the laser field e, the wave vector o f the nanostructures q, and the target velocity v. The dashed circles show the features o f the nanorelief, which are ablation fragments, their flaky clusters, and nanoparticles.Fig. 2. N anolattice periods A on (squares) dry and (circles) humid titanium surfaces versus the surface energy density F.wavelength is 744 nm, the FWHM of the generation band is about 15 nm) with a duration of about 80 fs (FW HM) and an energy of up to 8 mJ [9]. The trans­verse spatial distribution of the laser field corre­sponded to the TEM 00 mode. Normally, incident laser radiation was focused on a spot with a diameter of 1.2 m m  (at a level of l /e 2) on the surface of a target (half-bar, a diameter of 8 mm) made from chemically pure, multiply annealed, mechanically polished (rms < 50 nm) VT1-0 titanium  with a m ean grain sizeof 0.25 |am (Center for Nanostructure Materials and Technologies, Belgorod State University). The target was placed on a three-dimensional motorized plate with a computer control. The energy of laser pulses was varied and controlled by, respectively, a reflecting polarization attenuator (Avesta Proekt) and a DET- 210 calibrated photodiode (Thorlab) illuminated by a weak laser glare through a rotary dielectric mirror. The writing of nanostructures was performed by scanning of the titanium  target surface with a speed of 20 |_im/s at low laser radiation energies (<0.5 mJ, peak power W  <4  GW) in order to avoid a noticeable degradation of the energy density distribution on the target surface associated with self-focusing in air (the critical self- focusing power is Wcr ~ 3 GW  [10]) and the accompa­nying effects of chromatic emission, filamentation, and scattering on a plasma [10, 11].3. In the process of the writing of nanostructures at low energy densities l  ~ 17 m J/cm 2 and the number of incident pulses N  ~ 500, pronounced quasiperiodic (mean period A ~ 0.4 |im) narrow (A < 0.1 |im) grooves with the wave vector q (|q | =  2n/A) collinear to the radiation polarization vector e and scanning direc­tion v are formed on the titanium  target surface (see Fig. la). The target surface beyond the grooves does not contain any traces of the removal of the material, but is contaminated owing to the reprecipitation of the fragments of ablation products from the grooves. At higher energy densities 1 ~ 25—350 m J/cm 2 and the same number A, pronounced nanolattices with q || e, v and subwave periods in the range of 0.2—0.4 |im are formed on the surface (see Figs. lb —Id), although the degradation of the grooves occurs with increasing / and they degrade to linear sets of nanopeaks at F  > 400 m J/cm 2. In particular, even at intermediate l v a l ­ues, the lattices contain flaky traces of the reprecipita­tion of the material (see Fig. lb). In the entire Grange, round (drop) fragments (nanoparticles) are seen on the lattice grooves. This likely indicates that ablation is supercritical thermal rather than subcritical (slabbing) [12]. Indeed, in this case, the condensation of the vapor drop products of ablation in their gas-dynamic scattering expansion from narrow nanovalleys of the surface relief can be expected (see Fig. 1). However, the monotonically decreasing dependence A(F) (see Fig. 2) is of particular interest; it certainly contradicts the results of preceding investigations for other metals [3, 8]. It is worth noting that the twofold decrease in the period of the nanostructures written on the tita­nium  surface by means of infrared femtosecond laser pulses was previously m entioned in [13], but was referred to two-dimensional recondensation peak structures and, in the latter case, was attributed (in the surface electromagnetic wave model) to the doubling of the period of dissipative surface nanostructures.In our case, the pronounced orientation of the obtained surface nanostructures perpendicular to the laser radiation polarization, the quasiperiodicity of theFEMTOSECOND LASER WRITING OF SUBWAVEha  (eV)Fig. 3. Spectral dependence of the (solid line) real, gj, and (dashed line) imaginary, g2, parts o f the relative perm ittiv­ity for unexcited titanium  (according to  [15]). The arrow marks the energy quantum  of a fem tosecond laser pulse (X a 744 nm).initial nanogrooves (see Fig. la), and the high-energy character of local ablation in nanogrooves in the absence of visible ablation of the material surface itself clearly indicate the local enhancement of the electric field of femtosecond laser radiation on the surface, which is most probable in the case of the excitation of the surface electromagnetic wave [1 ,3 , 5—7, 13, 14]. W hen the surface electromagnetic wave interfering with the light wave incident on the surface with the wavelength /. is generated, the roughness component with the wave vector q || e and period [14]A  = X I S il e l - 1± sin( ( 1)Fig. 4. Scanning electron microscope image o f the nano­lattice with the period A « 100 nm  w ritten on the humid titanium  surface at i 7« 300 m J/cm 2.face excited by femtosecond laser pulses, including the contributions from free and quasibound electrons; this component has the form6, = 61 ,inter "2 2 WpTg, 2 2 ' 1 +  00 T„(2)increases. Here, is the absolute value of the real part of the relative permittivity of the material at the wavelength of laser radiation and 0inc is the angle of incidence of radiation on the surface (in this work,0 m c * O ° ) .However, it is worth noting that the expected sub- wave periods of the modulation of the electromagnetic field on the titanium  surface in the range of 0.2— 0.4 |.im under femtosecond laser irradiation corre­spond to the values o, ~ —(1.1 — 1.5), which are stronglydifferent from the value ej ~ —3.5 for the unexcited material at /. ~ 744 nm  [15] (see Fig. 3) with the real, n =  2.65, and imaginary, k  =  3.24, parts of the refrac­tive index. Such an inconsistency was previously observed in [3 ,7], where it was explained by the inap­plicability of tabulated optical constants for the flat surface to the rough (nanostructured) surface and the electron excitation of the material, respectively. Simi­lar to [7], we assume that the nanostructuring of tita­nium  can be accompanied by a change in the com po­nent o | of the relative permittivity of the titanium  sur-This change can occur due to, first, a decrease in the relaxation time of the electron m om entum  x,oc 1/7^ as a result of the heating of the electron subsystem to temperature 1161 (in this case, the plasma frequencycop can increase due to the thermal ionization of the d  bands [17]) and, second, a characteristic decrease in the term  e , inter determined by transitions between nar­row d  bands with a high density of states near the Fermi level under the excitation by photons with ener­gies higher than the interband transition energy maxi­mum; this decrease is caused by the thermal spreading of the electron distribution near the Fermi level [18]. However, a detailed simulation of a change in the rel­ative permittivity of titanium  during its femtosecond laser excitation is complicated owing to the complex structure of its band spectrum [17]; for this reason, direct experimental investigations of the ultrafast dynamics of its optical constants are expedient.To illustrate the surface character of the local enhancement of the electromagnetic field in the fem­tosecond laser writing of subwave lattices on the tita­nium  surface, we carried out similar experiments on its nanostructuring at high energy densities I ~ 0.3—0.4 J/cm 2 and the num ber of incident pulses N  ~ 500 under a thin (~ 1 mm, to avoid self-focusing) layer ofGOLOSOV et al.doubly purified distilled water. In this case, nanolat­tices with a m uch smaller period of A ~ 0.1 |im  were observed (see Figs. 2, 4); this was preliminarily attrib­uted to an additional decrease in the effective n value for titanium  in the aqueous medium, which promotes the formation of small-scale nanolattices according to Eq. (1).The revealed unusual dependence of the period A of one-dimensional nanolattices for titanium  on the surface energy density I of femtosecond laser radia­tion implies that not only titanium, but also many transition metals and other metals with a complex electronic structure (lanthanides, actinides) can likely exhibit similar dependences determined by the rela­tion between the laser radiation energy quantum  and the interband transition maximum. As a result, a vari­ation of the wavelength, angle of incidence, and energy density of femtosecond laser radiation opens possibilities for the writing of one-dimensional nano­lattices with periods varying from the minimally possi­ble subwave (A /.) to a superwave (A > /.) on the sur­face of such metals.4 . To summarize, sets of oriented quasiperiodic nanostructures (one-dimensional nanolattices) whose subwave periods lying in the range of 0.1—0.4 |im decrease with increasing surface energy density rather than increase as for other metals have been written on dry and hum id titanium  surfaces by means of fem to­second laser pulses. Such an anomalous behavior is explained by the features of the electron spectrum of this metal (the existence of interband transitions), which are manifested in the writing of nanolattices by femtosecond laser pulses of the visible and near-infra­red ranges.We are grateful to G.A. Shafeev (Prokhorov G en­eral Physics Institute, Russian Academy of Sciences) for valuable advice and recommendations.REFERENCES1. S. V. Zabotnov, L. A. Golovan’, I. A. Ostapenko, et al., Pis’ma Zh. Eksp. Teor. Fiz. 83 , 76 (2006) [JETP Lett. 83 , 69 (2006)].2. R. Waaner, J. G ottm ann, A. Horn, and E. W. Kreutz, Appl. Surf. Sei. 252, 8576 (2006).3. A. Y Vorobyev, V. S. Makin, and C. Guo, J. Appl. Phys. 101, 034903 (2007).4. Y Yana, J. Yana, C. Liana, and H. Wane, Opt. Express 16, 11259 (2008).5. G. Miyaji and K. Miyazaki, Opt. Express 16, 16265 (2008).6. M. Huang, F. Zhao, Y Cheng, et al., Opt. Express 16, 19354 (2008).7. M. Huana, F. Zhao, Y C-hena, et al., Phys. Rev. B 79, 125436 (2009).8. S. Sakabe, M. Hashida, S. Tokita, et al., Phys. Rev. B 79, 033409 (2009).9. V D. Zvorykin, A. A. Ionin, S. I. Kudryashov, et al., Pis’ma Zh. Eksp. Teor. Fiz. 88, 10 (2008) [JETP Lett. 8 8 ,8  (2008)].10. A. Couairon and A. Mysyrowicz, Phys. Rep. 441, 47(2007).11. S. M. Klimentov, T. V. Kononenko, P. A. Pivovarov, et al., Q uantum  Electron. 32 , 433 (2002).12. S. I. Anisimov and B. S. Luk’yanchuk, Usp. Fiz. Nauk 172, 301 (2002) [Phys. Usp. 45 , 293 (2002)].13. V S. M akin, R. S. Makin, A. Ya. Vorob’ev, and Ch. Guo, Pis’ma Zh. Tekh. Fiz. 34, 55 (2008) [Tech. Phys. Lett. 34 , 387 (2008)].14. S. A. Akhmanov, V. I. Em el’yanov, N. I. Koroteev, and V N. Seminogov, Usp. Fiz. N auk 147, 675 (1985) [Sov. Phys. Usp. 28 , 1084(1985)].15. Handbook o f  Optical Constants o f Solids, Ed. by E. D. Palik (Academic, Orlando, 1998).16. F. Ladstädter, U. Hohenester, P. Puschnig, and C. Ambrosch-Draxl, Phys. Rev. B 70, 235125 (2004).17. Z. Lin and L. V. Zhigilei, Phys. Rev. B 77, 075133(2008).18. C-.-K. Sun, F. Vallee, L. H. Acioli, et al., Phys. Rev. B 50, 15337(1994).Translated by R. TyapaevJE T P LETTERS Vol. 90 No. 2 2009

Femtosecond laser writing of subwave one-dimensional quasiperiodic nanostructures on a titanium surface

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Femtosecond laser writing of subwave one-dimensional quasiperiodic nanostructures on a titanium surface

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