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Abstract
The formation of laser-induced periodic surface structures (LIPSS) on Ti thin films, their phase and stoichiometric evolution, as a function of well below ablation threshold laser fluence, and the number of pulses, is investigated. The experiments were carried out in ambient air by using a femtosecond laser at a wavelength of 1030 nm with a pulse duration of 270 fs, operating at a repetition rate of 18.6 kHz. On the one hand, the formation of LSFL composed by an either single phase (C-Ti2O3 or r-TiO2-x) or a two-phase mixture of titanium oxide (C-Ti2O3 +r-TiO2-x) is reported. The titanium oxide phases were obtained by micro-Raman spectroscopy. The orientation of the LSFL, either parallel or perpendicular with respect to the polarization of the incident beam, correlate well with the electronic nature of the titanium oxide phases. On the other hand, the results also show the formation of HSFL, this under low cumulative fluence, with periods of 398 and 460 nm with a two-phase mixture of titanium oxide in the form ofa-Ti2O3+ C-Ti2O3and C-Ti2O3+ r-TiO2-x, respectively. To the best of our knowledge this is the first time that single phase C-Ti2O3 or r-TiO2-x LIPSS are reported.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The ultrashort-pulse laser material processing is a powerful technique for precise nano and microstructuring that allow us to modify the material properties for various applications. Particularly, the formation of Laser Induced Periodic Surface Structures (LIPSS) [1] is a potentially strong method to fabricate micro/nano-structured surfaces, which are of high interest for biomedical applications [2], texturing of metals [3], solar cells [4], plasmonics [5], etc. The nanostructures can be generated in a single step process and allow us to modify the optical, mechanical and chemical surface properties. However, due to the high sensitivity of the LIPSS formation process to the type of substrate material, environment and laser parameters, accomplishing fine control over the structure formation is a difficult task. Different theoretical models have been proposed to explain the LIPSS formation, usually, a model based on the so called efficacy factor proposed by Sipe et al. is used to interpret the origin of the LIPSS formation [6]. Titanium is a very relevant metal that has been vastly use in industry due to its high corrosion and temperature resistance, light weight and biocompatibility. In this context, the formation of LIPSS on Ti has been widely studied. By exploring different laser processing parameters, such as wavelength, polarization, fluence, number of pulses, etc., different types of LIPSS have been observed in this material. LIPSS with periods close to the laser wavelength, i. e. low spatial frequency LIPSS (LSFL) [7],LIPSS with a period much smaller than the laser wavelength, i. e. high spatial frequency LIPSS (HSFL) [8] and LIPSS formed due to metal oxidation, called thermochemical LIPSS (TLIPSS) [9, 10] where, in the majority of the cases, the main composition is titanium dioxide (TiO2).The formation of LSFL is attributed to interference of the incident laser light and the surface scattered waves or surface plasmons, as a result the intensity maxima of the periodic pattern generate material ablation. In metals, LSFL have an orientation perpendicular to the polarization of the incident beam with periods approximately equal to the laser wavelength. TLIPSS are generated in a similar way, however, in this case the metal is not removed or ablated but chemically modified due to oxidation [11]. Their orientation is parallel to the polarization of the incident beam and their period depends on both the wavelength of the incident radiation and the metal film thickness [10, 12]. On the other hand, HSFL are induced by ultrashort laser pulses and several theories have been proposed to explain their formation, one of such considers second and third harmonic generation (SHG, THG) as the mechanisms responsible for the formation of this type of LIPSS [13]. In the particularly case of Ti different reports suggest that the oxidation process induced by the laser irradiation, like in our case the formation on TixOy, enhances the nonlinear response and lead the formation of HSFL [8,14]. It is well-known, nanostructured TiO2 has broad potential applications due to their low toxicity, good biocompatibility and intrinsic properties. It has been used in different fields of technology such as energy, medicine and sensors. In this respect, recently, various nanostructured non-stoichiometric titanium oxides, TiO2-x, have attracted significant attention for applications such as catalysis [15], batteries, optoelectronic devices and development and design of gas sensors [16]. In [16] a TiO2-x/TiO2- based hetero-structure was generated from a thin metallic titanium layer by hydrothermal oxidation. This hetero-structure was used as a sensing layer and it was demonstrated that it can be applied to design sensors with distinct selectivity and sensitivity. On the other hand, Ti2O3 is a non-magnetic semiconductor and it shows a progressive transition to metallic state around 450K [17]. This metal oxide thermochromic material is particularly interesting due to its capacity to adopt different color states at different temperatures and can return to their initial color countless times in response to temperature fluctuations. This effect is useful for adaptive solar control applications [18]. Framed in this context, the present study reports on the generation of HSFL and LSFL on Ti thin films, which composition varies from initial native oxide, Ti2O3, to TiO2-x, or a mix of phases, by using femtosecond laser pulses with fluence well below the ablation threshold, and normal incidence irradiation in air. The gradual change in the chemical composition and morphology of the nanostructured surface as a function of the number of pulses and per pulse laser fluence were analyzed with micro-Raman spectroscopy and scanning electron microscopy (SEM), respectively. Since only a few works have focused on the study of the chemical transformation influence on the Ti LIPSS formation [5,8,19], in this paper the obtained titanium oxide LIPSS are discussed in view of both their morphology and stoichiometry.
2. Material and methods
2.1 fs-laser irradiation
Titanium thin films (400 nm thickness) were fabricated by the Ar ion DC-sputtering technique and the films were grown onto glass substrates. A commercial Ytterbium-doped photonic crystal fiber laser amplifier system (Satsuma Amplitude HP2), with a central wavelength of 1030 nm and a pulse width of 270 fs, was used at a repetition rate of 18.6 kHz to irradiate the Ti samples. The experimental setup is shown in Fig. 1. The pulse energy is conveniently adjusted by using a half-wave plate and a linear polarizer (LP). A beam splitter (BS) located on the back-reflected light path B (blue), and a CCD camera were used to measure the laser beam intensity profile and the spatial distribution of the fluence. The beam was focused onto the sample by a spherical lens (PL, 150 mm focal length) obtaining an elliptical laser spot with a minor axis of 90 µm, oriented along the x direction (2ω0x, 1/e2), and major axis of 100 µm (2ω0y, 1/e2) along the y direction. A motorized x-y-z linear translation stage (Sigma Koki Co. SG-SP20-85) was used to position the sample. The number of delivered pulses was precisely controlled by using a digital delay/pulse generator (SRS DG645) synchronized to the laser. All the experiments were performed at normal incidence in air.
To generate LIPSS below the ablation threshold, it was necessary to determine the per pulse ablation threshold fluence. For such purpose, the phenomenological model described in [20] was used. According with this model, it is possible to determine the ablation threshold for a single pulse by knowing the accumulation damage generated for N pulses through the expression: Fth(N) = Fth(1) Nξ-1. Where Fth(1) and Fth(N) are the ablation threshold for a single pulse and for N pulses, respectively, the incubation factor for the accumulative phenomenon is represented with ξ. In this way, the values found for our titanium thin film are: Fth(1) = 397 ±13 mJ/cm2and ξ=0.897 ± 0.003. Once the ablation threshold has been determined, the experimental parameters for LIPSS formation were set. The irradiations were carried on using different values of per pulse fluence, from 10% to 25% the Fth(1) value. The laser irradiation conditions used in this study are summarized in Table 1. All the experiments were performed in air at normal incidence.
Table 1. Experimental parameters considered for the experiments.
For the different values of fluence considered in this work (Table 1), both the LIPSS morphology and their phase/chemical evolution were analyzed as a function of the delivered number of pulses. In general, LSFL and HSFL formation was observed across different zones of the irradiated areas. These structures show different orientations with respect to the polarization of the incident pulses, and their periodicity varies between 953 nm ($\frac{\mathrm{\lambda }}{{1.08}}$) and 398 nm ($\frac{\mathrm{\lambda }}{{2.59}}$).
3. Results and discussion
3.1 Characterization of the laser processed Ti thin film by OM, SEM, and micro-Raman spectroscopy
The morphological characteristics of the non-irradiated Ti thin film were investigated by SEM and AFM in terms of RMS roughness. Figure 2(a) shows a SEM micrograph reveling a compact morphology with a smooth grain background. Figure 2(b) shows a 2D-AFM image where it can be seen that the grains are uniformly distributed on the surface. The RMS roughness is 15 nm.
In order to facilitate the morphological and phase/chemical composition analysis of the irradiated areas, the examination was carried out by defining zones Z1 and Z2, where the parameters D1X and D2X represent their respective diameters along the x direction as described in Fig. 3.
Fig. 3. Schematic of the zones defined for the analysis of the morphology and phase/chemical composition features on the laser irradiated area. 2ω0x (90μm) and 2ω0y(100μm) represent the laser spot minor axis, along the x direction, and major axis, along the y direction. The modified surface is divided in two zones, the center (Z1, green) and the periphery (Z2, orange) with diameters D1X and D2X, respectively, along the x direction with values smaller than 2ω0x. D1X and D2Xvalues vary depending on the irradiation parameters.
The central area (Z1) and the periphery (Z2) of the modified surface were analyzed through optical microscopy, SEM and micro-Raman spectroscopy. Additionally, the two-dimensional fast Fourier transform (2D-FFT) was applied on the SEM micrographs to estimate the period (Λ) and the orientation of the LIPSS. For the first set of experiments, the Ti thin film was irradiated with laser pulses of F=42.6mJ/cm2. The evolution of LIPSS formation as a function of the number of pulses (N) was investigated. Figure 4 shows optical micrographs of the surface after being irradiated with N = 100, 1500 and 10000. A circular mark corresponding to the modification of the surface texture and coloration can be clearly distinguished.
Fig. 4. Optical micrographs of the surface of the Ti thin film right after being irradiated at a per pulse fluence of 42.6 mJ/cm2 for (a)100, (b)1500 and (c)10000 pulses. The arrows indicate the polarization direction of the incident laser beam.
As it can be observed, the main modification of the surface occurred at the center of the irradiated region, which is expected because of the Gaussian intensity beam profile. As the number of pulses increases different effects take place: the modified area grows, the metal-oxidation process takes place and different structures arise. The coloration of the surface could be the mixed result of light diffraction due to texturing and the formation of multi-phase titanium oxide with different stoichiometry [21]. The femtosecond laser-induced surface structures can be seen in more detail through the SEM micrographs showed in Fig. 5.
Fig. 5. SEM micrographs of Ti thin film after being irradiated with (a)100, (b)1500 and (c)10000delivered laser pulses. By zooming in the selected green and orange regions of the SEM images it shows the formation of small fractures in (d), well defined LIPSS at the center(e) and also at the periphery (f)of the irradiated spot. The inserts are the corresponding 2D-FFT images. (g) Gaussian fluence profile. The diameter of the modified region, labeled for zones Z1 and Z2, along the x axis is indicated for different values of N.
The SEM micrographs (a)-(c), show the modified area produced by a beam with vertical polarization. Two zones are analyzed, center (Z1) and periphery (Z2), as it was described in Fig. 3. The diameter of the modified surface at Z1 (D1X) and Z2 (D2X), along the x axis (horizontal), was obtained from the SEM micrographs showing that its value grows as N increases. For N = 100, small cracks appear at the center of the irradiation, which seem to be aligned perpendicularly to the polarization of the incident laser beam, this can be verified by the 2D-FFT in Fig. 5(d). Also notice some nanoparticles (∼100 nm) decorating the irradiated spot and the presence of nanoholes or nanocavities (tiny black dots in Fig. 5(d)). For N = 1500, HSFL format Z1as it is seen in Fig. 5(e). The ripples have perpendicular orientation to the incident polarization, as it is shown in the 2D-FFT image (inset, Fig. 5(e)). These structures have a period Λ=460 nm. Again, a good number of nanoparticles populate across the irradiated spot. When the number of pulses increases up to N = 10000 the ripples have been lost in Z1 (Fig. 5(c)); however, a second type of ripples appears at the periphery(Z2) of the laser modified area, as shown in Fig. 5(f). The corresponding 2D-FFT analysis of this second structure indicates the formation of LSFL (Λ=953 nm), with an orientation parallel to the polarization of the incident beam. As it has been recently demonstrated in different works, it is possible to induce the formation of thermochemical LIPSS (TLIPSS) in Ti under femtosecond laser irradiation [22]. These structures are characterized by having a parallel orientation with respect to the processing light polarization and are formed due to laser-induced metal oxidation. Our group published one of the first reports on the formation of this type of LIPSS under nanosecond pulsed laser irradiation on Ti films [23]. Aiming proper understanding of the influence of the metal oxidation on the 90-degree flip of the structure, the phase and chemical composition of the irradiated area was analyzed using micro-Raman spectroscopy. The Raman bands, as reported in the literature, for the amorphous Ti2O3(a-Ti2O3), crystalline Ti2O3(C-Ti2O3) and sub-stoichiometry rutile TiO2-x (r-TiO2-x) are listed in Table 2.
Table 2. Raman peak positions reported for laser irradiated titanium thin films.
Figure 6 shows the Raman spectra of the as-deposited Ti film when irradiated at laser fluence of 42.6 mJ/cm2 and for the delivery of 100, 1500 and 10000 pulses. Figures 6(a), 6(b) correspond to zones 1 and 2 of the irradiated spot, respectively. For 100 laser pulses, the Raman spectrum recorded at 5 mW is constituted by two wide bands located at 280 and 500 cm-1. According to the works reported by Perez del Pino et al. [21] for Ti films irradiated with ns pulses; Landis el al. for irradiation of Ti plates with fs pulses [24]; and Kirner et al. for a titanium cylinder irradiated with fs pulses [19], these two bands can be assigned to the amorphous-Ti2O3 phase. For 1500 laser pulses (Z1and Z2), the Raman spectrum present peaks at 245, 332, 432, 553, 600 cm-1 (Z1) and 249, 320, 436, 538, 604 cm-1 (Z2).Considering the works reported by Perez del Pino et al. [21] for Ti films irradiated with ns pulses, Kirner et al. for a titanium cylinder irradiated with fs pulses, Villaroel et al, for sputtered thin films [25], Escobar et al. for TiO2 thin films grown by laser ablation [27], Parker et al. TiO2 nanopowders [26], this set of Raman peaks can be attributed to the C-Ti2O3 and r-TiO2-x phases. When analyzing the Raman spectrum for 10000 pulses, a clear evolution of the material can be observed. In this case the Raman spectrum has peaks at 245, 354, 436, 564, 604cm-1 (Z1) and 250, 328, 436, 546, 604 cm-1 (Z2). In Z1 the Raman spectrum is dominated intensity wise by the bands corresponding to the rutile TiO2-x phase. While in Z2, the Raman spectrum is dominated by the bands corresponding to the Ti2O3 phase. One can see that the synthesized titanium oxide evolves with the number of delivered laser pulses, it starts form an amorphous Ti2O3phase, native oxide, as the number of pulses increases a mixture of the C-Ti2O3 and rutile TiO2-x crystalline phases are obtained, with a dominant rutile TiO2-xphase at Z1 and a dominant C-Ti2O3 phase at Z2.
Fig. 6. Raman spectra of the Ti film irradiated at 42.6 J/cm2 with different number of laser pulses. Raman spectra were taken at (a) zone Z1 and (b) zone Z2. 
To investigate the effect of the per pulse laser fluence on the LIPSS formation, the same analysis was carried on for areas irradiated with a fluence of 69.5 mJ/cm2. As the fluence increases, a fewer number of laser pulses are needed to induce the formation of superficial periodic structures. Figure 7 shows the optical micrographs of the irradiated surface for different number of pulses. As already seen for a lower laser fluence, the modification of the material surface includes coloring and texture changes. As expected, the modified area is larger in this case since the per pulse laser fluence was increased.
Fig. 7. Optical micrographs of the modified Ti surface irradiated at 69.5 mJ/cm2 with (a)30, (b)1500 and (c) 10000pulses. The arrows indicate the polarization direction of the incident laser beam.
Figure 8 shows the SEM images for the laser fluence F = 69.5 mJ/cm2. While the modified surface for the irradiation with per pulse fluence 42.6 mJ/cm2 has a diameter of 25 μm for 1500 pulses and 38μm for 10000 pulses (Figs. 5(b), 5(c), 5(g)); when the per pulse fluence is increased up to 69.5 mJ/cm2the modified area gets slightly elliptical, the diameter along the short axis x grows to 51 μm and the long axis to 60 μm for 1500 pulses, and 55 μm (short axis) and 65μm (long axis) for 10000 pulses (Figs. 8(b), 8(c), 8(i)).In this case, 30 pulses was enough to get well defined LIPSS (Figs. 8(a), 8(d)) with Λ=398 nm (HSFL) perpendicular to the incident beam polarization. For N = 1500 two regions can be clearly identified. In the center zone (Z1), according with the 2D-FFT, a combination of LIPSS with perpendicular orientations are formed. At the periphery zone (Z2) well formed LIPSS, with Λ=630 nm, are oriented perpendicularly to the incident laser beam polarization. When N = 10000, at Z1 a granular randomly oriented structure is formed, probably as a consequence of the destruction of the LIPSS. However, as in the case of the lowest per pulse fluence (Fig. 5(f)), the generation of LIPSS with Λ=984 (LSFL), and orientation parallel to the incident polarization, occurs at the periphery (Z2) (Fig. 8(h)).
Fig. 8. SEM micrographs of the Ti thin film after being irradiated with (a) 30, (b) 1500, and (c) 10000 delivered laser pulses. Zoom in of the LIPSS observed at Z1 (d,e,f) and Z2 (g,h). The inserts are the corresponding 2D-FFT images. (i) Gaussian fluence profile. The diameter of the modified region (Dx1,2), labeled for zones Z1 and Z2, along the x axis is indicated for different values of N.
Figure 9 shows the Raman spectra of the as-deposited Ti film irradiated at laser fluence of 69.5 mJ/cm2 and for the delivery of 30, 1500 and 10000 pulses. Figures 9(a) and 9(b) correspond to zones 1 and 2 in the irradiated, respectively. In this case, one can observe for 30 pulses that the Raman spectra are constituted by several bands located at 211, 242,300, 443 and 524 cm-1 (Z1 and Z2). The band at 242 cm-1 is the more intense, indicating that the irradiated material is constituted by a mixture of the amorphous and the C-Ti2O3 phases. For 1500 laser pulses, the Raman spectrum measured on Z1 presents only two bands located at 432 and 600 cm-1 that indicate the formation of the r-TiO2-x crystalline phase in Z1. Meanwhile the spectrum measured on Z2 has several bands at 208, 249, 428, 524, 597 cm-1, which are assigned to the C-Ti2O3 and the rutile TiO2-x crystalline phases (see Table 2). For 10000 pulses, the Raman spectrum has peaks at 432 and 600 cm-1 (Z1), indicating the presence of the pure r-TiO2-x crystalline phase. For Z2, the Raman spectrum has the band corresponding to ther-TiO2-x crystalline phase and, additionally, some features at 208, 253, 527 cm-1showing the presence of C-Ti2O3. In Z1, one can describe the laser-induced oxidation of titanium as a function of the increasing number of laser pulses as Ti→a-Ti2O3+C-Ti2O3→r-TiO2-x. In Z2, the transformation occurs in the following way Ti→a-Ti2O3+C-Ti2O3→C-Ti2O3+r-TiO2-x.
Fig. 9. Raman spectra of the Ti film irradiated at 69.5 J/cm2 with different number of laser pulses. Raman spectra were taken at (a) zone Z1 and (b) zone Z2.
The highest per pulse laser fluence used in the present study was F=103 mJ/cm2. For this case, the maximum number of pulses was reduced in order to avoid the destruction of the LIPSS at Z1. As it can be expected at increasing laser fluence a shorter number of pulses are needed to create LIPSS and to generate ablation. In this particular case, 2000 pulses were the maximum number of pulses that we can afford before the starting of ablation. Figure 10 shows the optical micrographs of the surface irradiated with N = 10, 1500 and 2000 pulses. At this fluence, 10 pulses were enough to generate LIPSS at the center zone of the processed area. For 1500 and 2000 pulses we can see the coloring of the surface as similarly observed for the lower fluence values.
Fig. 10. Optical micrographs of the modified surface of the Ti thin film irradiated at 103 mJ/cm2 with (a) 10, (b) 1500 and (c) 2000 laser pulses. The arrows indicate the polarization direction of the incident laser beam.
Figure 11 shows the SEM micrographs of the modified surface. The dimensions for the modified surface are 59 μm for the short axis x and 66 μm for the long axis for N = 2000 (Fig. 11(i)). For N = 10 (Fig. 11(a)) well defined LIPSS are generated with Λ=671 nm whose orientation is perpendicular to the laser beam polarization (Figs. 11(a), 11(d)). For 1500 pulses (Figs. 11(b), 11(e), 11(g)) two different types of LIPSS are formed. At Z1 the ripples have an orientation parallel to the polarization of the laser beam with Λ=677 nm, while in the periphery, LIPSS with perpendicular orientation to the incident polarization are generated with Λ=762 nm. For N = 2000, the surface does not exhibit a significant difference (Figs. 11(c), 11(f), 11(h)) in comparison to the case for N = 1500, i.e. the LIPSS orientation is preserved parallel to incident polarization in Z1 and perpendicular to incident polarization in Z2. However, analyzing the 2D-FFT, the LIPSS formed at Z1 seem to be better defined, at N = 2000, with a periodicity of Λ=672. At Z2, the LIPSS show a larger periodicity of Λ=708 nm.
Fig. 11. SEM micrographs of the Ti thin film after being irradiated with (a)10, (b)1500 and (c)2000 laser pulses. Magnified micrographs of the LIPSS observed at Z1 (d,e,f) and Z2 (g,h). The insets are the corresponding2D-FFT images. (i) Gaussian fluence per pulse profile. The diameter of the modified region (Dx1,2), labeled for zones Z1 and Z2, along the x axis is indicated for different values of N.
Figure 12 shows the Raman spectra of the as-deposited Ti film irradiated at 103 mJ/cm2 laser fluence and 10, 1500 and 2000 pulses. Figures 12(a) and 12(b) correspond to zones 1 and 2 of the irradiated spot, respectively. For 10 laser pulses, the Raman bands are attributed to the C − Ti2O3 phase. Meanwhile for both 1500 and 2000 laser pulses, the Raman spectra are very similar, having features corresponding to the r-TiO2-x phase in Z1 and a mixture of the C − Ti2O3+r-TiO2-x phases in Z2. The band at 549 cm-1showing up at Z1corresponds to the glass substrate, indicating that under these experimental conditions the center of the laser irradiated zone has been fully oxidized all the way down to the substrate; notice that since TiO2-x is a transparent material, the laser beam from the Raman equipment reaches to the substrate then producing the 549 cm-1signal.
Fig. 12. Raman spectra of the Ti film irradiated at 103 J/cm2 with different number of laser pulses. The Raman spectra were taken at (a) zone Z1 and (b) zone Z2.
3.2. Discussion
Table 3 presents a comprehensive summary of our experimental findings; some interesting features can be pointed out.
- 1. In the regime of low number of pulses, small fractures (a-Ti2O3), HSFL (a-Ti2O3 + C-Ti2O3), and LSFL (C-Ti2O3) were generated at Z1 using different fluence values. It is relevant to note that in this regime the induced oxidation forms predominantly Ti2O3. Considering the semiconductor/metallic nature of Ti2O3[28] it is expected to obtain LSFL with orientation perpendicular to the polarization of the incident radiation [1] as it is clearly the case for LIPSS formed at the center region (Z1) with 10 laser pulses and laser fluence of 103 mJ/cm2.
- 2. The case for N = 1500 allow us following the evolution of the surface structures as a function of the fluence value. For 42.6 mJ/cm2, at Z1, HSFL (C-Ti2O3 + r-TiO2-x) with orientation perpendicular to the incident polarization are formed. When the fluence value increases to 69.5 mJ/cm2the HSFL disappear, and instead a granular structure that seems to be a mix of vertically and horizontally oriented LIPSS (see insert in Fig. 8 (e)) is observed. Finally, for the highest fluence value, LSFL (r-TiO2-x) are formed. In contrast to C-Ti2O3, the r-TiO2-xphase possess a dielectric nature, which explains well why in this case the orientation of the LIPSS is parallel to the polarization of the incident beam [1]. For the middle and highest fluence values at the periphery (Z2), LSFL (C-Ti2O3 + r-TiO2-x) with orientation perpendicular to the polarization of the incident light are formed. Both cases show very similar morphological characteristics, but it is the periodicity which increases with fluence. The orientation of the structures could suggest that the semiconductor/metallic nature of the C-Ti2O3phase is dominant at that the surface. This might be also interpreted as an indication of the two-phase ratio, being the C-Ti2O3phase more abundant.
- 3. It is interesting to observe how the orientation of the LSFL, generated at the center of the irradiated spot (Z1), with a fluence value of 103 mJ/cm2, evolves with the number of pulses. For this particular case, the number of pulses has no strong influence on the LIPSS periodicity but on their orientation. This seems to correlate with both the crystalline phase and stoichiometry progress from the single phase C-Ti2O3, in the low number of pulses regime, to the single phase r-TiO2-x, in the high number of pulses regime.
- 4. In the regime of high number of pulses, for fluence values of 42.6 and 69.5mJ/cm2, a granular structure is formed at the center zone (Z1). The Raman spectra shows peaks that match with C-Ti2O3 + r-TiO2-x and r-TiO2-x, respectively. At the periphery zone (Z2), LSFL with parallel orientation with respect to the polarization of the incident beam are generated in both cases with similar chemical composition (C-Ti2O3 + r-TiO2-x) but slightly different periodicity. The orientation of these LSFL suggest that in this case the dielectric nature of the r-TiO2-x phase is dominant at Z2. For the103 mJ/cm2 fluence, a maximum of 2000 pulses were considered. Formation of LSFL at both zones Z1 and Z2was obtained. However, the LIPSS orientation is either parallel at Z1 or perpendicular at Z2, with respect to the incident beam polarization. Consistently, the LIPSS oriented parallel to the incident polarization are composed by the singler-TiO2-x dielectric phase, while the mixture of the C-Ti2O3 + r-TiO2-x phases, with a dominant contribution of the semiconductor/metallic C-Ti2O3 phase drives the perpendicularly oriented LIPSS at zone Z2.
Table 3. Summary of morphological and chemical features of the laser processed regions at the center Z1 (green) and periphery Z2 (orange).
Table 4. Morphological characteristics and chemical composition of the LIPSS obtained in this work.
In the experiments HSFL, with periods of around half and one-third of the laser wavelength, were generated on the surface of a Ti thin film (Z1) when the sample, with a nativea-Ti2O3 layer, was irradiated with 42.6 mJ/cm2 (N=1500) and 69.5 mJ/cm2 (N=30). The laser induced the formation of r-TiO2-x for the first case, and a phase change from a-Ti2O3 to c-Ti2O3 in the second case. Apart from the laser induced oxidation, the presence of Ti oxides allowed the occurrence of nonlinear optical effects, such as SHG or THG, during the irradiation of Ti allowing the formation of HSFL [8]. As the number of pulses increases the HSFL are destroyed, and a granular structure emerge instead. The formation of the titanium oxides under laser irradiation is rather a complex process where several competing mechanisms take place. There is a fast electronic excitation triggered by every single femtosecond pulse, which produces hot electrons that transfer thermal energy to the lattice in a picosecond time scale. If the pulse delivery repetition rate is high enough an accumulative heat process is established, and high temperatures can be achieved in relatively short periods of time. The heating process is fluence dependent, therefore a heating profile (nearly Gaussian) is created following the laser intensity profile. This establishes a spatial temperature distribution which drives, together with oxygen availability, the spatially resolved synthesis of the different titanium oxides we observe across the obtained LIPSS. The number of pulses play a fine-tuning parameter which determines the very type(s) of titanium oxide(s) that is/are formed at each one of the identified zones (Z1 and Z2) with distinct structural, stochiometric and morphological features.
4. Conclusions
We have demonstrated the formation of LIPSS composed by a single phase or a two-phase mixture of titanium oxide, under fs laser irradiation with per pulse fluence well below the ablation threshold. The single phase titanium oxide (either C-Ti2O3 or r-TiO2-x) is consolidated at the center of the irradiated area under high cumulative laser fluence, i.e. for the highest per pulse laser fluence and a number N of delivered pulses in the range 10-2000. At the periphery of the irradiated area, where the laser fluence is naturally low due to the incident laser beam Gaussian profile, the two-phase mixture C-Ti2O3 + r-TiO2-x characterizes the surface structures. These two phases possibly arise from native amorphous titanium dioxide (a-Ti2O3) which, under the fs laser irradiation, evolves into C-Ti2O3 plus the r-TiO2-x phase as a result of the increasing number of delivered laser pulses. As pointed out in the discussion section of this work, these two titanium oxide phases possess very distinct electronic properties, theC-Ti2O3 phase is a semiconductor/metallic material, opposite to the dielectric nature of the r-TiO2-x phase. It is well known in the literature that the orientation of LIPSS is closely correlated to the semiconductor/metallic or the dielectric nature of the materials, in general, LIPSS form perpendicularly to the incident polarization for semiconducting/metallic surfaces and parallel to the incident polarization for dielectrics. The orientation of the LSFL reported in the present work correlate well with the electronic nature of the titanium oxide phases we found through micro-Raman spectroscopy. Although HSFL and LSFL were obtained in this work, HSFL formed only at the center of the irradiated spot under low cumulative fluence, a two-phase mixture including a-Ti2O3+ C-Ti2O3 and C-Ti2O3+ r-TiO2-x characterize these LIPSS. It is not clear at this point what drives the HSFL formation in this titanium oxides, however, as it has been suggested by some authors nonlinear optical phenomena such as harmonic generation could help the HSFL formation. Therefore, it would be interesting to investigate on the nonlinear optical properties of the hereby synthesized titanium oxides. We must notice this is the first-time single phase LIPSS of either C-Ti2O3 or r-TiO2-x are reported. Due to the semiconductor/metallic or dielectric nature of these titanium oxides our results would impact current technological application of LIPSS in areas such as sensors and smart optoelectronic surfaces.
Funding
Consejo Nacional de Ciencia y Tecnología (CB17-18-A1-S-21245, FONCICyT 246648).
Acknowledgments
Authors thank the program Catedras-CONACYT. Also, authors acknowledge M Eng. Fabián Alonso-Cordero for his technical support with the SEM characterization.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. J. Bonse, S. Höhm, S. V. Kirner, A. Rosenfeld, and J. Krüger, “Laser-induced periodic surface structures, scientific evergreen,” IEEE J. Select. Topics Quantum Electron. 23(3), 9000615 (2017). [CrossRef]
2. R. A. Barb, C. Hrelescu, L. Dong, J. Heitz, J. Siegel, P. Slepicka, V. Vosmanska, V. Svorcik, B. Magnus, R. Marksteiner, M. Schernthaner, and K. Groschner, “Laser-induced periodic surface structures on polymers for formation of gold nanowires and activation of human cells,” Appl. Phys. A 117(1), 295–300 (2014). [CrossRef]
3. G. Rotella, L. Orazi, M. Alfano, S. Candamano, and I. Gnilitskyi, “Innovative high-speed femtosecond laser nano-patterning for improved adhesive bonding of Ti6Al4 V titanium alloy,” CIRP Journal of Manufacturing Science and Technology 18, 101–106 (2017). [CrossRef]
4. J. Bonse, G. Mann, J. Kruger, M. Marcinkowski, and M. Eberstein, “Femtosecond laser-induced removal of silicon nitride layers from doped and textured silicon wafers used in photovoltaics,” Thin Solid Films. 542, 420–425 (2013). [CrossRef]
5. A. Y. Vorobyev and C. Guo, “Multifunctional surfaces produced by femtosecond laser pulses,” J. Appl. Phys. 117(3), 033103 (2015). [CrossRef]
6. J. E. Sipe, Jeff F. Young, J. S. Preston, and H. M. van Driel, “Laser-induced periodic surface structure,” Phys. Rev. B 27(2), 1141–1154 (1983). [CrossRef]
7. I. Gnilitskyi, T. JY. Derrien, and Y. Levy, “High-speed manufacturing of highly regular femtosecond laser-induced periodic surface structures: physical origin of regularity,” Sci. Rep. 7(1), 8485 (2017). [CrossRef]
8. X. F. Li, C. Y. Zhang, H. Li, Q. F. Dai, S. Lan, and S. L. Tie, “Formation of 100-nm periodic structures on a titanium surface by exploiting the oxidation and third harmonic generation induced by femtosecond laser pulses,” Opt. Express 22(23), 28086–28099 (2014). [CrossRef]
9. A. V. Dostovalov, V. P. Korolkov, V. S. Terentyev, K. A. Okotrub, F. N. Dultsev, and S. A. Babin, “Study of the formation of thermochemical laser-induced periodic surface structures on Cr, Ti, Ni and NiCr films under femtosecond irradiation,” Quantum Electron. 47(7), 631–637 (2017). [CrossRef]
10. A. V. Dostovalov, V. P. Korolkov, and S. A. Babin, “Formation of thermochemical laser-induced periodic surface structures on Ti films by a femtosecond IR Gaussian beam: regimes, limiting factors, and optical properties,” Appl. Phys. B 123(1), 30 (2017). [CrossRef]
11. B. Öktem, I. Pavlov, S. Ilday, H. Kalaycıoğlu, A. Rybak, S. Yavaş, M. Erdoğan, and F. Ömer Ilday, “Nonlinear laser lithography for indefinitely large-area nanostructuring with femtosecond pulses,” Nat. Photonics 7(11), 897–901 (2013). [CrossRef]
12. A. V. Dostovalov, V. P. Korolkov, K. A. Okotrub, K. A. Bronnikov, and S. A. Babin, “Oxide composition and period variation of thermochemical LIPSS on chromium films with different thickness,” Opt. Express 26(6), 7712–7723 (2018). [CrossRef]
13. A. Borowiec and H. K. Haugen, “Subwavelength ripple formation on the surfaces of compound semiconductors irradiated with femtosecond laser pulses,” Appl. Phys. Lett. 82(25), 4462–4464 (2003). [CrossRef]
14. S. K. Das, C. Schwanke, A. Pfuch, W. Seeber, M. Bock, G. Steinmeyer, T. Elsaesser, and R. Grunwald, “Highly efficient THG in TiO2 nanolayers for third-order pulse characterization,” Opt. Express 19(18), 16985–16995 (2011). [CrossRef]
15. X. Chen, L. Liu, and F. Huang, “Black titanium dioxide (TiO2) nanomaterials,” Chem. Soc. Rev. 44(7), 1861–1885 (2015). [CrossRef]
16. S. Ramanavicius, A. Tereshchenko, R. Karpicz, V. Ratautaite, U. Bubniene, A. Maneikis, A. Jagminas, and A. Ramanavicius, “TiO2-x/TiO2-structure based ‘self-heated’ sensor for the determination of some reducing gases,” Sensors 20(1), 74 (2020). [CrossRef]
17. Y. Guo, S. J. Clark, and J. Robertson, “Electronic and magnetic properties of Ti2O3, Cr2O3, and Fe2O3 calculated by the screened exchange hybrid density functional,” J. Phys.: Condens. Matter 24(32), 325504 (2012). [CrossRef]
18. M. Ferrara and M. Bengisu, Materials that Change Color: Smart Materials, Intelligent Design, PoliMI Springer Briefs (2018), pp. 5–38.
19. S. V. Kirner, T. Wirth, H. J. Sturm, and J. Bonse, “Nanometer-resolved chemical analyses of femtosecond laser-induced periodic surface structures on titanium,” J. Appl. Phys. 122(10), 104901 (2017). [CrossRef]
20. Y. Jee, M.F. Becker, and R. M. Walser, “Laser-induced damage on single-crystal metal surfaces,” J. Opt. Soc. Am. B 5(3), 648 (1988). [CrossRef]
21. A. Pérez del Pino, P. Serra and J. L. Morenza, “Oxidation of titanium through Nd:YAG laser irradiation,” Appl. Surf. Sci. 197-198, 887–890 (2002). [CrossRef]
22. A. V. Dostovalov, V. P. Korolkov, and S. Babin, “Simultaneous formation of ablative and thermochemical laser-induced periodic surface structures on Ti film at femtosecond irradiation,” Laser Phys. Lett. 12(3), 036101 (2015). [CrossRef]
23. S. Camacho-López, R. Evans, L. Escobar-Alarcón, M.A. Camacho-López, and M.A. Camacho-López, “Polarization-dependent single-beam laser-induced grating-like effects on titanium films,” Appl. Surf. Sci. 255(5), 3028–3032 (2008). [CrossRef]
24. E. C. Landis, K. C. Phillips, E. Mazur, and C. M. Friend, “Formation of nanostructured TiO2 by femtosecond laser irradiation of titanium in O2,” J. Appl. Phys. 112(6), 063108 (2012). [CrossRef]
25. R. Villarroel, R. Espinoza-González, J. Lisoni, and Guillermo González-Moraga, “Influence of the oxygen consumption on the crystalline structure of titanium oxides thin films prepared by DC reactive magnetron sputtering,” Vacuum 154, 52–57 (2018). [CrossRef]
26. J. C. Parker and R. W. Siegel, “Calibration of the Raman spectrum to the oxygen stoichiometry of nanophase TiO2,” Appl. Phys. Lett. 57(9), 943–945 (1990). [CrossRef]
27. L. Escobar-Alarcón, E. Haro-Poniatowski, M.A. Camacho-López, M. Fernández-Guasti, J. Jímenez-Jarquín, and A. Sánchez-Pineda, “Structural characterization of TiO2 thin films obtained by pulsed laser deposition,” Appl. Surf. Sci. 137(1-4), 38–44 (1999). [CrossRef]
28. S. H. Shin, G. V. Chandrashekhar, R. E. Loehman, and J. M. Honig, “Thermoelectric effects in pure and V-doped Ti2O3 single crystals,” Phys. Rev. B 8(4), 1364–1372 (1973). [CrossRef]
denotes r-TiO2-x, 
denotes C-Ti2O3.
