The nanograting inscription on the surface of a fused silica substrate is systematically investigated on a shot-to-shot basis. Three different evolutionary processes are observed with pulse fluence ranging from slightly below to well above the single shot ablation threshold. This dependence is explained by the interplay between local intensity distribution and incubation effect.
© 2012 Optical Society of America
During the past two decades, many experiments have been conducted to study the mechanism of nanograting formation with different writing parameters in diverse materials [1–8]. With the increase in the number of shots, nanograting period was found to decrease not only inside a fused silica substrate [3, 6, 8], but also on the surface of various materials [4, 7]. Most recently, Zhang et al.  has investigated multi-pulse irradiation on the surface of As2S3, and observed the creation of new nanogrooves and their elongation in the direction perpendicular to the electric field. The laser modified area was shown to increase with the number of shots per site and the nanograting period was independent of the pulse energy. Despite that a great many of experiments have been performed, a systematic study of nanograting inscription on a shot-to-shot basis is still absent. In this paper, we study the shot-to-shot evolution of nanograting inscription on the surface of fused silica in the static case. Three different evolutionary processes are observed depending on the pulse fluence’s conditions: slightly below, slightly above and ‘well’ above the single shot ablation threshold. This dependence provides further evidences of a newly proposed nanograting formation model  which consists of nanoplasmonic  and incubation effects [11,12]. Based on that model, detailed interpretations concerning the differences of the evolutionary processes are given. For the sake of completeness and clarity, a few pictures used in one of our previous papers  are included and also discussed.
A cw-pumped Titanium:sapphire regenerative amplifier (Coherent RegA 9000) was used in the experiment. The laser beam was pre-chirped negatively in an external compressor to compensate for the dispersion of optical components and a microscope objective crossed by the beam before reaching the target. The focal spot diameter was measured to be 2.4 μm with the technique proposed by Liu . The Fourier transform-limited pulse duration on the target was 80 fs. Single shot mode was used in the experiment. The number of pulses was controlled manually and the delay between two consecutive shots was about 2 seconds. A fast-response photodiode was used to make sure that the designed number of shots was applied by monitoring the transmitted part of a beam splitter (90/10). The pulse energy was varied from 80 to 120 nJ by a variable metallic ND filter. For a given pulse energy, a series of ablation spots was made with a continuously increased number of shots per site on the surface. The distance between each ablation spot was 10 μm. Following pulse exposure, the untreated sample (no cleaning, no coating, and no etching) was characterized in a scanning electron microscope (SEM, FEI Quanta 3D FEG).
3. Results and discussion
Figure 1 shows the shot-to-shot evolution of nanograting formation at 80 nJ, the pulse fluence (3.5 J/cm2) is smaller than the single shot ablation threshold (3.89 J/cm2) measured by Liang et al. . Therefore, the nanocraters observed at the first few shots are the consequence of laser ablation at the randomly distributed defect sites which are much easier to be ionized and thus ablated. The elongation of these nanocraters in the direction perpendicular to the electric field is observed after 4 shots, while the width of the nanocraters in the direction of the electric field looks nearly the same. With the increase in the number of shots, more nanocraters are induced due to the incubation effect [11,12] which progressively lowers the ablation threshold. As a consequence of that, more nanogrooves are formed through the elongation and merging of the nanocraters as shown in 5 to 20 shots. When a large number of shots are used (laser operated at 10 kHz), only straight nanogrooves are observed (bottom of Fig. 1). The number of the nanogrooves and the size of the interaction zone are roughly the same from 60 up to 800 shots indicating that the incubation effect gets saturated  between 20 and 60 shots.
As the pulse energy increases to 90 nJ (corresponding to pulse fluence of 3.98 J/cm2 which is slightly above the single shot ablation threshold), the merging of a few apparently randomly distributed small craters occurs at the first two shots. There is no sign of laser-induced surface ripples and the central zone seems to be smoothened by melting (Figs. 2(a) and 2(b)). The periodic surface ripples (nanogrooves) start to appear after a few consecutive shots and are orientated in the direction perpendicular to the electric field. The side and inner nanogrooves are found to be created in pairs which are symmetrical about the central one and evenly separated with respect to the ones formed earlier if just simply comparing the results of 4, 6, 9 and 13 shots (see arrows); while the width of the nanogrooves seems to decrease due to the redeposition of the ablated material. The evolution of the nanogrooves at a large number of shots is also presented (bottom of Fig. 2). The number of the nanogrooves and the size of interaction zone from 16 up to 800 shots are nearly the same. This indicates the saturation of the incubation effect.
However, a strange and interesting phenomenon shows up (Fig. 3) when the pulse energy increases up to 100 nJ (4.42 J/cm2). At the first shot, a rather big and smooth crater is created at the center of the beam with several smaller ones surrounded, and there is no evidence of laser-induced surface ripples. The characteristic of phase explosion is very obvious at the first few shots, and the molten material splashes around the crater. Some ridges within the crater somewhat parallel to the electric field are observed. After 4 shots, this ridge-type structure seems to disappear, and the nanogrooves start to form and evolve as in the case of 90 nJ. At 120 nJ (5.3 J/cm2), similar phenomena occur as shown in Fig. 4. The phase explosion becomes more severe as judged by the resolidification of the molten material. The ridge-type structure appears within the first 5 shots, and then gradually disappears with the nanogrooves evolving in the same way as in the case of 90 nJ except that now the nanogrooves are formed apparently at the bottom of a big crater.
Based on the experimental results, three apparently different evolutionary processes are observed:
Case 1. Slightly above threshold. In the case of 90 nJ, the pulse fluence (3.98 J/cm2) is slightly above the single shot ablation threshold. The detailed evolution of the nanogrooves for an ideal case has been discussed by a recently proposed nanograting formation model . In that model, an ideal crater is created at the first shot. With the increase in the number of shots, incubation takes place along the equatorial direction of the crater as well as at the two side maxima which results from the nanoplasmonic effect . The interplay between the local intensity distribution (i.e. nanoplasmonic effect) and incubation effect leads to the progressive creation of pairs of side and inner nanogrooves. In the real case, however, where random surface defects and impurities exist, near-threshold ablation would be induced preferably at these defect sites and thus gives rise to a random set of craters as experimentally observed (Figs. 2(a) and 2(b)). With the increase in the number of shots (e.g. 3 and 4 shots), the early created crater(s) will be elongated in the direction perpendicular to the electric field due to the local field enhancement [5,15] and self-seeding effect originating from the laser-induced defects ; meanwhile new ablation zones and thus nanogrooves are created at the positions of the side maxima where the incubation has taken place (see arrows in Figs. 2(c) and 2(d)).
Note that the evolution of the local intensity distribution is significantly dependent on the surface morphology induced by the previous shots due to the incubation effect. Therefore, the randomness of the defect sites would strongly influence the local intensity distribution from the very beginning when the pulse fluence is lower than or close to the single shot ablation threshold, resulting in “site-dependent” evolution of the nanogrooves. This means that the incubation effect actually acts as a positive feedback in control of the local intensity distribution for the next shot, which in turn controls the incubation areas. With the increase in the number of shots, the interplay between the local intensity distribution and incubation is taking place leading to the progressive creation of these ordered nanogrooves (Fig. 2).
Case 2. Slightly below threshold. In the case of 80 nJ, the pulse fluence (3.5 J/cm2) is lower than the single shot ablation threshold. The modifications of the local intensity and incubation effect become relatively weaker. As a consequence, the laser ablation only occurs at the randomly distributed defect sites resulting in the formation of many nanocraters. With the increase in the number of shots, these nanocraters are gradually elongated in the direction perpendicular to the electric field and some of them merge together during the elongation (Figs. 1(e)–1(t)). Meanwhile, many new nanocraters are also created with the contributions from both laser-induced and pre-existing defects within the interaction zone. All these nanocraters extend themselves along the direction perpendicular to the electric field while maintaining certain spacing between themselves in the orthogonal direction because of the decrease in the local intensity at the poles. For large number of shots, the local intensity distribution is progressively modified as the surface morphologies evolve. Stronger ablation zones (corresponding to certain grooves) could lead to weaker field around the poles resulting in the suppression of near-by weaker ablation zones. The weaker ablation zones are very likely to be buried by the re-deposition of the ablated material and/or smoothened out by melting. After the irradiation of sufficient number of shots, regular nanogrooves are created (bottom of Fig. 1). No sign of the production of side or inner nanogrooves in pairs is observed.
Case 3. Well above threshold. However, when the pulse fluence is well above the single shot ablation threshold (e.g. 100 nJ (4.42 J/cm2) and 120 nJ (5.3 J/cm2)), material removal by phase explosion accompanied by thermal melting occurs at the first few shots (Figs. 3 and 4). The evolutionary process can again be explained by the interplay between the local intensity distribution and incubation effect . At the first shot, a big crater is created because of the larger incident pulse fluence. Following this pulse exposure, defects are induced mainly in a band along the equatorial direction of the crater due to the local field enhancement as shown in Fig. 5. The defect concentration has a distribution in the band. It is higher in the central band (where ‘a’, ‘e’ and ‘b’ locate) because the local field is higher. It is lower in the side bands passing through ‘c’ and ‘d’ because the local field decreases from the equator towards the pole. Higher local field/fluence means more induced defects; this would lower the ablation threshold. Therefore, the ablation threshold is lower in the central band than in the two side bands passing through ‘c’ and ‘d’. However, during the interaction between the laser and material in the real case, a certain amount of energy is absorbed and scattered by the slightly overdense plasma. The laser field is thus reduced significantly, especially in the central zone, beyond the penetration depth. The residual laser field at the bottom of the crater is thus weaker resulting in less defects as compared to the other areas in the band (e.g. ‘a’, ‘b’, ‘c’ and ‘d’). On the other hand, the local fields in the zones ‘c’ and ‘d’ is lower than those in ‘a’ and ‘b’ as mentioned above. Therefore, among these five locations, ‘a’ and ‘b’ which are symmetrical about the center have the highest local field, hence, lowest ablation threshold. During the irradiation of the next pulse, due to the symmetrical distribution of the Gaussian beam about the beam propagation axis, two new ablation zones are induced at the symmetrical positions in the equator (‘a’ and ‘b’). As a consequence, resolidification of the molten material in these positions results in ridge-type structure parallel to the electric field as shown in Fig. 3(b) and Fig. 4(b), respectively. This process is repeated until a whole layer of material is removed accompanied by the disappearance of the ridges as shown in Fig. 3(d) and Fig. 4(e), respectively. This results in a degraded surface with significant material removal and reduced pulse fluence on the surface of the deep crater. In such a case, with the increase in the number of shots, the process of evolution similar to ‘Case 1’ occurs except that the ablation is now on a degraded surface (Figs. 3(e)–3(l) and Figs. 4(f)–4(l)) resulting in bad quality gratings. This indicates that well-shaped nanogratings cannot be achieved with relatively high pulse fluence due to the severe phase explosion.
In summary, the physical evolution of the nanogrooves inscription was systematically investigated. Three different evolutionary processes were observed. At the pulse fluence smaller than the single shot ablation threshold, the laser ablation is initiated at the pre-existing defect sites. With the increase in the number of shots, nanogrooves evolve through the elongation (in the direction perpendicular to the electric field due to the local field enhancement (i.e. nanoplasmonic effect) and self-seeding effect (i.e. incubation effect)) and merging of the randomly induced nanocraters. When the pulse fluence is slightly above the single shot ablation threshold, the side and inner nanogrooves are produced in pairs whose locations and evolution are governed by the interplay of the nanoplasmonic and incubation effects. With pulse fluence well above the threshold, a layer of molten material is first removed by the first few shots resulting in the reduction of pulse fluence, the nanogrooves are then created at the bottom of a big crater in the same way as ‘Case 1’. Our experimental results suggest that the nanograting cannot be induced by a single shot and the pulse fluence used for well-shaped nanograting inscription should not be ‘well’ above the single shot ablation threshold. Moreover, we confirm the validity of the recently proposed model that the interplay between the nanoplasmonic and incubation effects is the key to the nanograting inscription.
This work is supported by the Natural Sciences and Engineering Research Council of Canada, Canada Research Chair, Canada Foundation for Innovation and the Canadian Institute for Photonic Innovations. We thank S. Gagnon, M. Martin and D. Gingras for the technical support and Dr. Q. Q. Wang for the help in the experiment.
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