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Abstract
In this work, microscale holes and intermediate self-organized structures were obtained on the surface of a thin 50-nm silver film in air and under a water layer using single tightly focused femtosecond laser pulses with variable energy. We study the differences in the relief of microholes using scanning electron microscopy. The comparison of the silver film ablation thresholds under irradiation in air and water is carried out. A non-monotonic change in the size of craters in a water medium at peak powers more than 1.5MW was found, which corresponds to the critical filamentation power of laser pulses in water.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Various kinds of nano- and microscale relief on the surface of thin metal films, formed during their direct femtosecond laser ablation, is the subject of numerous studies [1–15]. Ordered arrays of such structures (nano- and microholes, nanorings, nanocrowns, microcones, self-organized nanogratings [1–6]) have unique optical, nonlinear optical and spectral properties, and are now actively used in biosensorics, nanoplasmonics [7–10]. At the same time, the possibility to control the phase, amplitude, and polarization of light via subwavelength structures induced by laser radiation is actively used in holography [11,12].
Among the laser methods for the surface modification of thin films, the technique of direct laser recording by single ultrashort (femtosecond) tightly focused laser pulses is widely used [13–15]. This method is a universal tool for structuring various targets with the possibility of using it in combination with non-laser methods of exposure. However, the physical processes and dynamics of the matter redistribution during the formation of nano- and microrelief on thin metal films processed in air are not yet fully understood. Solidified nano- and microstructures [1–10], which are then studied by scanning (SEM) or transmission (TEM) electron microscopy, are indirect signs of the processes occurring during laser ablation of the surface.
To date, a systematic study of the dependence of the diameter and depth of micro- and nanoholes on focusing conditions, energy, wavelength and duration of laser pulses for thin films in air has been carried out [1–6,16–18]. On the other hand, much less attention is devoted to such studies in a liquid medium, mainly for the bulk targets [19,20]. Despite some enlightening, though rather narrow research on thin films [21], the correspondence between the topology of the surface relief and the mechanisms of its generation in air and water under the impact of tightly focused ultrashort laser pulses has not yet been experimentally established.
In this work, we report for the first time the results of a comparative analysis of the microholes formation in a thin silver film in air and under a layer of distilled water under the impact of single femtosecond (300 fs) sharply focused (NA = 0.25 and NA = 0.65) laser (515 nm) pulses of variable energy, thus broadly extending our previous studies and providing accurate, well-justified conclusions on the basic effects of wet fs-laser ablation of thin metallic films.
2. Experimental setup and materials
In our studies, the irradiation of the surface of a silver film with 50 ± 5 nm thickness in air and under a small (∼ 1 mm) layer of distilled water was carried out on a bench for precision laser structuring [16]. We used femtosecond second harmonic pulses of an ytterbium fiber laser: the second harmonic wavelength is 515 nm, the pulse duration is τ ≈ 300 fs, and the maximum energy in the pulse is 4 µJ (TEM00) in a single pulse mode.
Laser radiation was focused onto the surface of the samples through the objectives of an optical microscope (Levenhuk Bioview 630) with NA = 0.25 and 0.65 into a spot with radius R1/e ≈ 2.2 ± 0.1 µm and R1/e ≈ 1.6 ± 0.1 µm (on a dry surface), respectively (Fig. 1).
Fig. 1. Experimental setup scheme. RA – reflective attenuator; BS – beamsplitter; RM – reflective mirror; AC – autocorrelator; PM – power meter; OB – microscope objective for laser beam focusing; M – optical microscope; CCD – digital CCD camera; MS – motorized stage.
The sample was fixed on a three-coordinate motorized step translation platform with a minimum displacement step of 150 nm. A silver film used as a target was deposited onto the surface of a glass substrate (N-BK7) in an argon atmosphere by magnetron sputtering (SC7620, Quorum Technologies) of a foil obtained from a 99.99% silver ingot. The morphology of single craters on the target surface irradiated at different energies (0.024–1.32 μJ) of laser pulses was visualized using a JEOL7001F SEM.
3. Experimental results and discussion
Let us consider the morphology of craters obtained by ablation of a silver film by single laser pulses focused onto the surface by a microscope objective with a numerical aperture NA = 0.25. Impact of a single pulse with energy E = 0.024 μJ in air leads to the formation of a microcone with a submicron hole at the top as can be seen on Fig. 2(a). An increase in pulse energy to 0.1 μJ leads to the formation of a round crater with thin walls, along the edges of which regions of film peeling from the glass substrate are noticeable [Fig. 2(b)]. As the E increases from 0.48 μJ to 1.32 μJ, the hole size increases significantly as shown on Figs. 2(c), 2(d). It should be noted that there are no traces of melting along the periphery of the crater, and there is no melt ridge, which indicates a “cold” mechanism of silver separation from the substrate [22]. The swelling and slight rolling of the film at the edges of the holes, especially at the laser pulse energy close to the maximum, is apparently associated with the propagation of the shock wave [22,23].
Fig. 2. SEM images of microholes (at an angle of 30°) on the surface of a silver 50 nm film under single-pulse exposure with NA = 0.25 objective in: (a)-(d) air and (e)–(f) water medium.
Laser ablation of the silver film with single pulses under a small layer of water and similar energy and focusing parameters changes the morphology of microholes. At energies E = 0.024–0.1 μJ, a damaged microcone with a submicron hole is formed on the surface; the film is noticeably swollen from the center to the periphery [Figs. 2(e), 2(f)]. As the energy increases up to the maximum values of E = 1.32 μJ, the size of the holes increases, and the edges of the craters become thicker in comparison to ablation in air [Figs. 3(g), 3(h)]. In this case, traces of material melting appear – a smooth edge, nanojets along the perimeter, indicating the direction of expansion of the melt during the formation of the structure.
Fig. 3. SEM images of microholes (at an angle of 30°) on the surface of a silver 50 nm film under single-pulse exposure with NA = 0.65 objective in: (a)-(d) air and (e)–(f) water medium.
Now consider the case of tighter focusing using an objective with a numerical aperture NA = 0.65. Single laser pulses with energy E = 0.024 μJ in air form microcones on the film surface with a submicron hole [Fig. 3(a)], the film is swollen at the edges, crystallized nanojets are visible at the periphery, and the walls of the structures are thin. An increase in energy to 0.1–0.48 μJ [Figs. 3(b), 3(c)] leads to the formation of microholes; a crater is formed in the center on the glass substrate, which also increases in size as the energy rises to 1.32 μJ [Fig. 3(d)].
A distinctive feature of single-pulse laser ablation of a silver film when focusing with NA = 0.65 objective in water and E = 0.024 μJ is a significant and almost symmetric swelling of the film with the formation of a submicron-sized structure resembling a burst bubble in shape [Fig. 3(e]). An increase in the energy to 0.1 μJ leads to the appearance of a microhole with a thick smooth rim along the edges [Fig. 3(f)], while the dimensions of the structure are comparable with the holes obtained with similar parameters in the air. As the energy increases to E = 1.32 μJ, crystallized nanojets appear, indicating the melting of the film material and splashing of the melt [Figs. 3(g), 3(h)], but at the same time, there is no crater on the glass substrate in the center of the processing area. It is also worth noting that microholes size in the water medium exceed those created with similar parameters in air.
The analysis of the change in the size of craters was made using the dependence of the square of the radius on the logarithm of the laser pulse energy (R2 – lnE). Linear approximation of the dependences shown in Fig. 4, allows to determine the focus radius w (1/e2 level) and the threshold value Eth of the formation of structures at a given focusing. Thus, it is possible to estimate the value of the threshold energy density (fluence) of ablation Fth = 2Eth/πw2. The calculation results and approximations are shown in Table 1.
Fig. 4. Dependence of the square of the crater radius on the logarithm of the laser pulse energy during ablation in water and air and focusing with micro objectives NA = 0.25 and NA = 0.65.
Table 1. Threshold energy density and calculation results.
The R2 – lnE dependences (Fig. 4) show the nonmonotonic character of the change in the crater size during single-pulse laser ablation under a layer of distilled water, which is obviously associated with the influence of the medium. The values of Fthr1 and w1 at low energies of laser pulses are practically identical (except for Fth1 in case of tighter focusing NA = 0.65).
However, as the energy increases, the nonlinear nature of fs-laser propagation in water unexpectedly begins to manifest itself on a microscale of the high-NA focal waist, which is still debatable effect, despite some pioneer works (see, e.g., [24]). In particular, in our work the break of the slope of the R2 – lnE curve in water occurs at critical powers Pcr ≈ 1.6 MW and 1.5 MW for the 0.25-NA and 0.65-NA strongly focusing objectives, respectively. Comparing these values with the critical self-focusing powers in water Pcr ≈ 1–1.8 MW, measured previously for collimated or weakly-focused laser beams in [21,25–27], one can still consistently conclude that the slope break in Fig. 4 is associated with the filamentation of laser pulses in water. Another strong argument for the filamentation-mediated fluence distribution in the waist in water at tighter (NA = 0.65) focusing is the absence of the craters in glass underneath the silver film [Figs. 3(c)–3(d), 3(g)–3(h)], indicating the strongly reduced peak fluence on the film surface immersed in water. Moreover, comparing to our previous, preliminary similar study for silver films [21], which exhibited similar break for a wet silver film on a glass surface, but not on a silicon substrate, here the intriguing microscale filamentation effect at the high-NA focusing in water was demonstrated much more consistently, systematically and broadly in terms of different focusing high-NA objectives.
4. Conclusion
In this work, we performed for the first time a comparative study of the microholes morphology and other self-organized structures on the surface of a 50-nm silver film on a glass substrate during its femtosecond single-pulse laser ablation with different sharp focusing parameters (NA = 0.25 and 0.65). SEM imaging shows differences in surface topology depending on the processing environment. A nonmonotonic change in the size of the obtained structures was observed upon ablation in water medium, which can be associated with the unusual microscale filamentation of the tightly focused ultrashort laser pulses at critical self-focusing powers Pcr > 1.5 MW.
Funding
Russian Science Foundation (16-12-10165); Ministry of Science and Higher Education of the Russian Federation (0705-2020-0041).
Acknowledgments
This work was supported by the Russian Science Foundation in part of laser nanofabrication and by the Ministry of Science and Higher Education of the Russian Federation in part of electron microscopy analysis.
Disclosures
The authors declare no conflicts of interest.
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