A novel two-dimensional (2-D) periodic structure on the ablated hole wall is formed by using the method of single-beam femtosecond laser pulses irradiating a titanium target. This 2-D structure on the ablated surface, after consecutive irradiation of 10000 pulses, presents different spatial periods which characters a 1500 nm period along the hole wall in the vertical direction and a 400 nm period in the azimuthal direction of the hole. The reported experimental results can be well interpreted by a prophetic theoretical model of Bonch-Bruevich (A. M. Bonch-Bruevich et al., Opt. Eng. 31, 718 (1992)).
©2009 Optical Society of America
Periodic structures (also termed ripples or nanogratings) induced by single-beam femtosecond laser ablating solids, have attracted extensive attention [1–6], including their distinguishing features [7–9], formation mechanism [10–14] and potential applications [15–18]. In majority of the publications, the ripples frequently appeared on the materials surface or at the bottom of the shallow crater are one-dimension periodic structures [1–18]. Recent, one-dimensional ripple-like structures on the ablated hole wall are also observed on some different materials, such as copper  and some semiconductors in liquid environment . However, to the best of our knowledge, until now there has been no observation of the 2-D ripple-like periodic structure generated on the hole wall of some metal just by a single laser beam, and its formation mechanism has not been fully understood. In this paper, we have observed a 2-D ripple structure on the ablated hole wall of the titanium irradiated by a single beam 800nm femtosecond laser pulses in the air. This phenomenon is interpreted as the interference between the incident light field and the excited surface plasmons. And it was confirmed by the relevant elevation angle measurement of the 2-D structure on the hole wall with a confocal laser scanning microscope. Additional a fine cone structure with size of 400 nm was also observed on an ablated hole under different laser fluence.
In our experiment, an amplified Ti:sapphire laser system(Legend Elite-USP-HE, Coherent Corp.) operating at a central wavelength λ L = 800nm and 1 kHz repetition rate was used as the irradiation source. The pulse energy and pulse duration were kept constant at 3.5mJ and 35fs, respectively. The linear polarized laser beam was focused onto a vertically standing sample with a 40-cm-focal-length lens at normal incidence. The sample was mounted on a three-dimensional translation stage. The laser spot size on the sample was approximately 70 μm. The number of laser pulses, N, applied to the sample is selected by an electromechanical shutter. A half-wave plate and polarizer assembly was used to control the laser fluence, F. The laser fluence is determined by F = P/[f rep × π × (d/2)2], where P is the laser power, d is the diameter of the focused spot which is measured by a beam quality diagnostics (LaserCam-HR(RoHS), Coherent Corp.), and f rep is the repetition frequency of the femtosecond laser. After mechanical polished, a pure titanium bulk sample (99.999%) was used in this experiment. All experiments were performed in air.
The morphology of femtosecond laser-induced surface modifications and the character of the holes wall were inspected by using an ultrahigh resolution field emission scanning electron microscopy (SEM). To give a better description, the wall of the ablated hole is which covers the 2-D periodic structure is an important parameter to evaluate the interaction between the laser and hole wall. To obtain this angle, a confocal laser scanning microscope was used to reconstruct the three-dimensional profile of the ablated hole. A suitable section plane corresponding to the location of the SEM studied 2-D periodic structure was created to extract a profile curve and to calculate the relevant elevation angle of the hole wall. The relationship between the relevant elevation angle and the laser incident angle on the hole wall are shown in Fig. 1(b) .
3. Results and discussion
At the laser fluence of 3.54J/cm2, single laser pulse can ablate the material. As the number of pulses increases, more materials were ablated and ejected out of the surface. After the Ti surface irradiated with N = 10000 pulses, a main hole with depth of 100μm was formed in the laser irradiation zoom, as shown in Fig. 2(a) . At the bottom of this main hole, a small hole is formed as shown in the Fig. 3(b) . Figure 2(b) shows a pronounced 2-D ripple structure on the central zone of the area 1.These two inter-perpendicular ripples show different spatial frequencies and different orientations. The spatial period d 1 of the high-spatial-frequency structure (HSFS) is about 400nm, and the spatial period d 2 of the low-spatial-frequency structure (LSFS) is about 1500nm. The details of the relative orientation of these two ripple structures can refer to the Fig. 1 and Fig. 2(b). The LSFS is along the hole wall in the vertical direction, and the HSFS is in the azimuthal direction of the hole. For the 2-D ripple structure on the central zone of the area 1, the grating vector of its HSFS is parallel to the incident light polarization, and the grating vector of its LSFS is perpendicular to the incident light polarization. Different to the structure on the central zone of the area 1, there is just one-dimensional ripple structure appeared on the center zone of the area 2, which is shown in Fig. 2(c). In sharp contrast to the ripple structure, the nanocone structure was found on a small hole wall at bottom of main hole, which is shown in Fig. 2(d).
The periodic structure on the ablated hole wall is a special case of the periodic surface structure induced by laser at an obliquely incident angle. As pointed out earlier, the formation of periodic ripple-like structure on the metal surface is due to the interference between the incident laser wave and the excited surface plasmons [21,22], and this interference causes a periodic intensity variation in the surface and gives rise to the formation of the periodic surface structure. In the case of p-polarized laser light and at a range of incident angles greater than 50°, the inter-perpendicular ripples have been observed. For a smooth planar metal surface, the periodic structure with grating vector parallel to the incident light polarization and the type-c periodic structure with grating vector perpendicular to the incident light polarization have been simultaneously obtained in the irradiating area . In most case, the type-c structure is only observed in the peripheral area of the beam spot and the other ripple structure is in the central area . Thus, it is hard to observe the crossed ripples. However, the ablated hole wall is curved surface, and at different area on the hole wall the laser polarization is different. It makes the exciting of the plasmons become more complex. A large numbers of surface plasmons may be excited and propagate along the curved surface. For 2-D ripple structure observed in this experiment, the laser induced two inter-perpendicular ripples is due to two plasmons strongly excited by p-polarized laser with peak efficacy factor .
For p-polarized laser light, the spatial period d 1 and d 2 of the inter-perpendicular ripples can be written as 23].Owing to the presence of ripple with grating vector parallel to the incident light polarization is 400nm which is shorter than the laser wavelength and the η for solid Ti is about 1, d 1 should choose λ/(η + sinθ) to evaluate. From the data d 1 = 400nm, d 2 = 1500nm and λ = 800nm, we deduced that η ≈1.0711, θ ≈68°. When the laser normally irradiates a metal surface, for the ablated hole wall, the incident angle is equal to the elevation angle of the hole wall, as shown in the Fig. 1(b). For the location of the SEM which is shown in Fig. 2(b), the local elevation angle is about 68° as shown in Fig. 3. The incident angle estimated by this experimental method is in good agreement with those obtained by using the theoretical model . Meanwhile, η is calculated to be 1.004 at λ = 800nm for Ti  and it is also in reasonable agreement with the results from solving the two equations above . In fact, it's reasonable that a little change of the value of the effective refractive index due to intense laser pulse heating and increasing of surface roughness on the metal surface . Thus, the experimental results are well explained by the analytical model .
It is worth noting that 2-D ripple structure cannot cover all of the hole wall. When a linear polarized laser irradiated the material surface, the laser polarization characteristic is different
in different areas of the hole wall. Two-dimensional ripple structure can be form on the wall where the laser polarization has a p-polarized component. And one-dimensional ripple was observed at the location where laser is s-polarized light. It means that different polarized character of the laser on the hole wall will cause different ripple characters. Besides the laser polarization, the laser fluence is also an important parameter to determine the feature of the structure on the hole wall. For example, when the laser fluence is 0.354J/cm2 and N = 100, periodic ripple structure is well formed on the metal surface as shown in Figs. 4(a) and 4(b). However when N = 10000, instead of the ripple structure, there is full of the cone structure covered on the ablated hole, which is show in Fig. 4(c) and 4(d). Different to the micrometer cones on the metal surface [25,26], the size of this cone structure is about 400nm. The reason of the structure change in our experiment is still uncertain.
In summary, we presented a 2-D ripple-like periodic structure on the ablated hole wall of a titanium, which was induced by single-beam femtosecond laser pulses with linear-polarized electric field. This 2-D structure on the ablated hole wall can be seen as a surface structure induced by femtosecond laser at a large angle of incidence. The measured spatial periods of the 2-D structure are successfully interpreted by the model of the interference between the incident light wave and the excited surface plasmons. In addition to the ripple structure, the nanocone structure on the ablated hole wall are also generated by linear polarized femtosecond laser. These different surface structures on the hole wall and how to control them would be potentially of interest to a variety scenarios including laser lithography, laser drilling holes for vias in electronics, microfluidics and applications requiring certain adhesion properties and wettability of the surface.
The authors would like to acknowledge Prof. Yuan Ji for assistance with SEM images and Dr. Yinzhou Yan for assistance with confocal scanning laser microscope measurement. This research is financially supported by Beijing Municipal Education Commission of Scientific Research under Grant No. 1010005466903.
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