1. Introduction

Unique properties of nanomaterials have been extensively studied in the past and various nanostructures have found numerous applications in optics [1], optoelectronics [2], enhanced x-ray emission [3], chemical catalysis [4], and optical biosensing [5]. For this reason, the development of new techniques for producing nanostructures is important for nanoscience and nanotechnology. Recently, femtosecond lasers have been demonstrated to be a promising tool for both producing nanostructures by deposition from plume of ablated material [6,7] and direct surface nanomachining of solids [8–11]. For direct surface nanostructing, Nolte et al. [8] have proposed a technique based on the use of a femtosecond laser in combination with a scanning near-field microscope. Koch et al. [9] have reported an ablation-free technique suitable for femtosecond laser nanotexturing of metal thin films only. In contrast to the above-mentioned specific techniques for direct surface nanostructuring, we show in this paper that direct surface nanostructuring (not from the ablated plume deposition) is in fact a natural consequence of femtosecond laser ablation under certain experimental conditions. This type of surface nanostructures can be used in a number of technological applications, for example, manipulation of optical properties of solids [12], catalysts, dental implants, etc. However, a systematic study of the physical mechanism of the surface nanostructuring as well as the optimal conditions for a controllable nanostructuring using femtosecond laser ablation technique are lacking.

In this paper, we perform a detailed study of the morphology of surface nanomodifications produced on bulk metals using femtosecond laser ablation technique. The effects of laser fluence and number of applied pulses on the generated surface nanostructures are studied with a scanning electron microscope (SEM). We show, for the first time, that nanostructures are a natural consequence following femtosecond laser ablation of metals. We determine a set of optimal laser irradiation conditions for the surface nanostructuring and propose a mechanism for the formation of nanostructures.

2. Experimental setup

In our experiment, we use an amplified Ti:sapphire laser system that consists of a mode-locked oscillator and a two-stage amplifier including a regenerative amplifier and a two-pass power amplifier. The laser system produces 65-fs pulses with energy around 1 mJ/pulse at a 1-kHz repetition rate with a central wavelength λ = 800 nm. The experimental setup follows that described in our previous works [12,13]. Briefly, to produce ablation, the laser beam is focused normally onto a bulk sample mounted vertically. To measure the incident pulse energy, a certain fraction of the incident light is split off by a beam splitter and measured with a pyroelectric joulemeter. The uncertainty of this measurement is estimated to be 5%. The number of laser shots, N, applied to the sample is controlled using an electromechanical shutter. All experiments are performed in air under atmospheric pressure. The morphology of femtosecond laser-induced surface modifications is studied using a SEM. The samples studied are mechanically polished copper, gold, and platinum. The range of laser fluence used in the ablation is between 0.084 and 9.6 J/cm². The number of applied pulses is varied from 1 to 5 × 10⁴ shots. The ablation threshold is determined as the minimum fluence to generate a surface damage seen under the SEM.

3. Results and discussion

A SEM picture of the copper sample surface prior to laser irradiation is shown in Fig. 1(a). For a reference, the ablation threshold for a copper sample is determined to be F_abl = 0.084 J/cm² following a total of N = 100 shots. The morphology of the irradiated surface is studied following ablation with laser fluence of F = 0.084, 0.16, 0.35, 1.52, 3.7, and 9.6 J/cm² and the number of applied pulses in the range of 1 – 5 × 10⁴. A number of representative surface structures produced on the copper sample are shown in Figs. 1–3. An analysis of the SEM data obtained in our experiments shows that the morphology of femtosecond laser-induced surface nanostructures depends both on laser fluence and the number of applied pulses. The effect of the total shots on nanostructuring at F = 0.35 and 1.52 J/cm² is shown in Figs. 1 and 2, respectively. Figure 1(b) shows that ^{nanostuctures begin to occur on some random localized sites after one shot at F = 0.35 J/cm2}. A few larger-size structural features are also observed in the central part of the ablated area, as seen in Fig. 1(b). We believe that these larger structures are associated with surface defects and/or laser beam intensity inhomogeneities. Figure 1 (c) shows a nanoscale surface structure produced by two-shot ablation. The structure composes of both larger nanocavities and nanoprotrusions with spherical tips of a diameter up to about 75 nm. Therefore, the one additional shot transforms the sparsely distributed nanoscale features in Fig. 1(b) to cellular-like structures in Fig. 1(c). The surface morphology after ablation with 1000 pulses is shown in Fig. 1(d). One can see that the mean size of nanoprotrusions becomes larger while at the same time some nanocavities develop into microcavities. The evolution of the surface structures following ablation at F = 1.52 J/cm² and various N is shown in Fig. 2(a–d). At this middle fluence, pure nanostructures are only generated by ablation with one or two laser shots [Fig. 2(a) and 2(b)]. As shown in Fig. 2(c), ten-shot ablation produces both random nano- and micro-structures. With further increasing N, the proportion of nanostructures decreases and this can be seen in Fig. 2(d), where microscale structures become dominant. At the highest fluence used in our experiment, nanostructures are not present over most of the irradiated area and a dominant morphological feature is microroughness. However, nanostructuring can still be observed on the periphery of the ablated spot, where the Gaussian beam intensity is low enough for nanostructural formation. An example of these surface structural modifications are shown in Fig. 3 for two-shot ablation at F = 9.6 J/cm².

 

Fig. 1. SEM images of nanoscale structures in the center of the irradiated spot on copper following ablation at F = 0.35 J/cm². (a) Sample surface before irradiation. [Note, this figure does not show the same spot on the sample as in (b)], (b) surface after one-shot ablation featuring random fine nanostructures in form of nanoprotrusions, nanocavities, and nanorims, (c) after two-shot ablation, (d) after 1000-shot ablation.

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Fig. 2. SEM images of the central part of the irradiated spot on copper following ablation at F = 1.52 J/cm² (a) surface after 1 shot featuring only random nanostructures in the form of nanoprotrusions and nanocavities, (b) surface after 2 shot featuring only random nanostructures in the form of spherical nanoprotrusions and nanocavities, (c) surface after 10 shots featuring both nano- and micro-structures, (d) surface after 1000 shots showing microstructures being dominating.

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Fig. 3. SEM images of copper following two-shot ablation at F = 9.6 J/cm². Only microstructures are present in the central area. However, nanostructures are observed on the periphery of the ablated spot. The insertion shows micro-structural details in the central area.

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The effect of laser fluence on surface structuring can be seen from analyzing the surface modifications produced at various F and fixed N, see, for example Fig. 1(c) (F = 0.35 J/cm², N = 2), Fig. 2(b) (F = 1.52 J/cm², N = 2), and Fig. 3 (F = 9.6 J/cm², N = 2). These images show that ablation with high laser fluence does not actually induce nanostructures and therefore there exist optimal laser ablation conditions for surface nanostructuring. In order to determine the optimal conditions for nanostructuring, we performed an SEM study of laser-induced surface modifications following ablation with a large variety of F and N, and the obtained data are summarized in Fig. 4. One can see that the most favorable conditions for pure nanostructuring are ablation at low and medium values of laser fluence (F < 1.5 J/cm²). This ablation regime has been referred to as “gentle” ablation in studying dielectric materials in Ref. [10]. Figure 4 also shows the range of laser irradiation parameters where femtosecond laser ablation produces different combinations of surface nano-, micro-, and macro-structures.

 

Fig. 4. Different types of surface structures produced on copper at various combinations of laser fluence and number of shots.

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To determine the mechanism of nanostructuring, we further performed a careful SEM study on the origin of nanoscale modifications. A representative example of nascent nanostructures following ablation with F = 0.35 J/cm² and N =1 is shown in Fig. 5(b), where the characteristic types of initial nanostructures are labeled. For comparison, Fig. 5(a) shows an undamaged area of the sample using the same scale as in Fig 5(a). It is seen in Fig. 5(b) that surface structuring is initiated on random highly-localized nanoscale sites. The typical structures include circular nanopores with a diameter in the range of 40–100 nm, randomly-oriented nanoprotrusions with a diameter in the range of 20–70 nm and a length of 20–80 nm, nanocavities of arbitrary form, and nanorims around nanocavities. Therefore, under these femtosecond laser processing conditions, nanoscale features down to a size of 20 nm are produced. We can see from Fig. 5(b) that a nanopore or nanocavity is always immediately accompanied by a nanorim or nanoprotrusion, indicating a nanoscale material relocation to an adjacent site. This one-to-one nanoscale dips and protrusions occur randomly over the laser spot, suggesting an initial non-uniform laser energy deposition. Following are some possible factors responsible for the spatial variation of the absorbed laser energy: (1) the spatial inhomogeneity of the incident beam, (2) the enhancement of absorption by surface defects, (3) interference of the incident laser light with the excited surface electromagnetic waves due to structural defects. When the incident laser fluence is close to the laser ablation threshold, the spatial variations in deposited laser energy can produce a melt at localized nanoscale sites within the irradiated spot. Once the localized nanoscale melts have been formed, a high radial temperature gradient in a nanomelt can induce a radial surface tension gradient that expels the liquid to the periphery of the nanomelt [9]. This will lead to the formation of nanocavities, nanoprotrusions, and nanorims due to fast freezing of the expelled liquid on the boundary with the solid state material (see Fig. 5(b)). This mechanism is also used to explain the formation of nanobumps on a thin metal film [9]. These initially induced surface random nanostructures can enhance the absorption of laser light [12] and facilitate the further growth of surface nanoroughness due to the increased spatial nonuniform energy absorption. When laser fluence is sufficiently high to produce ablation, the atoms ejected from the nanomelts produce a recoil pressure that squirts liquid metal outside of the nanomelt. For multi-pulse ablation, the repeating vaporization and re-deposition of nanoparticles back onto the surface should also affect the surface nanostructuring but the detailed mechanism requires further investigation. SEM morphology study at high fluence (F > 5 J/cm²: strong ablation) shows that melt occurs over a large area of the ablated spot (see Fig. 3) and the flow dynamics in this large melt pool predominantly results in microstructuring. We have also performed studies on nanostructuring of gold and platinum, and nanostructures are observed on these metals as well indicating that nanostructuring is a natural consequence following femtosecond laser ablation of metals. In general, the size and the shape of nano-features on the three metals are similar. We have also studied the ambient gas pressure effect on nanostructuring by taking SEM images of platinum following single-pulse ablation in 1-atm air and in a vacuum at a base pressure of 8 × 10^-3 Torr. Although we have observed a greater amount of re-deposited nanoparticles in air than in vacuum, the morphology of nanostructures is still quite similar under different air pressures. Our study was performed with samples mounted vertically. It should be noted that the amount of re-deposited ablated particles back onto the sample surface may be different when the sample is positioned vertically versus horizontally, but further studies are required in this aspect of nanostructuring using fs laser pulses.

 

Fig. 5. (a) A typical image of sample surface before irradiation and (b) nascent nanostructures induced on copper by ablation at F = 0.35 J/cm² and N = 1. Note, Fig. 5(a) does not show exactly the same spot on the sample as in Fig. 5(b).

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4. Conclusions

In summary, we study the origin and formation of random surface nanostructures produced on metals using femtosecond laser ablation. We find that surface nanostructuring first occurs randomly on highly-localized nanoscale sites. Based on the morphology study with SEM, we believe that these nanostructures are formed due to the flow dynamics of a nanoscale melt that relocates material from the center of the melted site to the peripheral area resulting in a nanocavity, nanorim, or nanoprotrusions. A systematic study under a large variety of experimental conditions shows that the optimal conditions for pure nanostructuring are through femtosecond laser ablation at low and medium fluence with a certain number of pulses.