Open Access
Add to BibSonomy
Abstract
With ultrashort pulse durations and ultrahigh peak intensities, ultrafast lasers can create different types of micro/nano-structures to functionalize the processed surface with new properties. However, the applications of this method on freeform surfaces are still limited by the short length of a laser focusing spot and complex control of the 3D moving trajectory in the fabrication process. In this paper, we overcome this problem by shaping the on-axis intensity along the propagation axis using the spatial light modulator. By designing the phase mask, we increased the length of the stable-intensity zone (intensity fluctuation < 10%) by more than 3 times compared to that of an unshaped Bessel beam. The energy deposition was also optimized to be less than 2% fluctuation based on simulations. Using this method, we fabricated micro/nano structures on 3D surfaces at different fluences and demonstrated various properties including colorization, anti-reflection, and hydrophobicity in large height range. We demonstrated the applications of the proposed method in creating hydrophobicity on complex freeform syringe tip surfaces. This improved the minimum manipulatable volume of a liquid droplet to 2 times smaller compared with untreated syringe, thus greatly extending its performance for micro-droplet manipulation. This method offers an alternative approach for reliable and affordable freeform curved-surface processing.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With ultrashort pulse durations and ultrahigh peak intensities, ultrafast lasers have been used for laser surface processing to create different types of micro/nano-structures such as laser induced periodical surfaces structures (LIPSS) and spikes [1,2], etc. These structures will provide novel functionalities such as anti-reflection [3], surface structural color [4–6], and super-hydrophobicity/hydrophilicity [7–11]. Because of the simplicity of its procedure and high flexibility, ultrafast laser has been intensively studied to obtain these special functionalities on material surface in recent years [12–14]. In practical applications, many surfaces are not flat, so the capability for freeform curved-surface processing is crucial for real-world applications.
Several methods have been reported to extend the surface processing capability to 3D. The one most frequently used is moving the laser focal point or the sample according to the 3D profile of the workpiece. For example, galvo-mirrors are often combined with f-theta lenses to control the laser focal point for surfaces marking/cleaning. When the height difference of the samples is larger than the Rayleigh length, another translation stage should be synchronized with the beam movement to avoid defocusing on the sample surface [15]. In this case, the surface of the workpiece should be modeled or measured to calculate the moving trajectories. This requires advanced modeling tools and/or precise measuring machines, thereby imposing higher requirement on 3D fabrication. Laser shaping has also been used to tailor the 3D intensity distribution according to the surface’s morphologies. For example, Tian et al. used diffractive optical elements to shape the beam in three dimensions, so that curved surfaces can be processed at the same time [16]. Similarly, Chen et al. proposed using a 3D micro-lens array to shape the focusing spot so that curved surfaces can be processed [17]. These methods can largely increase the efficiency of curved surfaces fabrication by a single irradiation without movement of samples. However, the shape of the samples should still be measured before modifications to design the 3D light shaping accordingly, and the height range is limited because of the shaping capability, so that the application range is still limited.
To avoid the noted drawbacks, one of the applicable methods is using a laser beam with a long and stable intensity distribution along propagation axis. In this way, the height range of modifications can be greatly increased, and 3D surfaces can be fabricated with a 2D translation stage with no need to control the height change [18]. Non-diffraction Bessel beam has a long and stable focusing length, therefore suitable for such applications [19,20]. However, as far as we know, there are limited research of using Bessel beam in other types of functional surfaces fabrications, such as colorization, anti-reflections, and wettability. All of these applications require higher homogeneity of energy deposition in the laser processing, which is difficult for the special intensity distribution of Bessel beam. In addition, the length of Bessel beam is still limited for 3D surfaces with large height difference. These drawbacks remain for improving Bessel beam modifications of 3D surfaces.
In this paper, we investigate the functional surfaces fabrications by shaping the on-axis intensity along the propagation axis. Long and stable Bessel-like beam are obtained for 3D surfaces functions including diffraction-induced colorization, anti-reflections, and wettability. We simulate the energy deposition process to optimize the homogeneity during the ablation process. In addition, by using the algorithm we recently proposed to increase the length of Bessel-like beam [21], we also improve the capability of larger height difference for 3D surfaces. The effectiveness of the proposed method is verified by surfaces modifications experiments. We also demonstrate the fabrication of syringe tips, which have 3D complex hydrophobic surfaces, to show the applications prospect in micro-fluidic devices fabrications.
2. Laser shaping and optimization of fluence
The principal pulse-shaping setup is shown in Fig. 1(a). The laser system is a Ti:sapphire laser, emitting 35 fs (FWHM), 800 nm center wavelength laser pulses at 1 kHz. The maximum laser power can be tuned up to 3 W. The incident laser pulses are Gaussian beams with flat phase. Spatial light modulators (SLM) were used to change the spatial phase distribution of the incident laser beam. According to previous research, the on-axis intensity of a propagating laser field is highly related to the spatial phase [19,20]. Therefore, by properly designing the phase, the intensity on the propagation axis can be controlled. After the SLM, a telescope consisting of a thin lens and a microscope objective was used to increase the beam fluence. Thus, a high-fluence long Bessel-like beam can be obtained beyond the microscope objective. By tuning the fluence, different types of surface structures can be obtained on free-form surfaces, which can be used for colorization (Fig. 1(b)), anti-reflection (Fig. 1(c)), or hydrophobicity (Fig. 1(d)).
Fig. 1. (a) Schematic of Bessel beam generation and surface fabrication. SLM represents spatial light modulator; MO represents Micro-Objective lens. (b) Laser-induced colorization structure fabricated on curved surfaces with curvature around 8 cm-1; (c) Anti-reflection patterns fabricated on a pillar with curvature of 0.25 cm-1; (d)Droplet on a hydrophobic curved surface with a curvature of 1.8 cm-1.
The shaping method used in this research is based on a new strategy that we proposed recently [21] to control the length of the Bessel-like beam. The main idea is to change the lateral shape of the axicon from straight (Fig. 2(a)) to curved one (Fig. 2(b)). By this method, the Bessel zone can be extended drastically compared to unshaped one. Comparison of the theoretical on-axis intensity distribution of Bessel beam and shaped Bessel-like beam is shown in Fig. 2(c). For the Bessel beam, the corresponding base angle (defined as the slant angle between cone surface and the base plane) is 2°, while the Bessel-like beam has the same base angle with a positive curvature of 8×102 mm. The horizontal axis represents the distance between the axicon tip and the CCD, while the vertical axis represents the relative intensity. The insets show pseudo-color maps of the lateral intensity at several different distances: 13 cm, 15 cm, 17 cm, and 21 cm. These show that the intensity remains relatively stable within this range. Compared with unshaped Bessel beam, the stable zone with fluctuation smaller than 10% is around 3 times longer. This improvement increases the stable intensity zone for 3D surfaces with larger height difference.
Fig. 2. The phase design and corresponding axicon shape of Bessel beam (a) and Bessel-like beam (b) used in the research. (c) Measurement of the intensity in the propagation direction; the shaped Bessel-like beam.
The cross section of Bessel beam is different with that of Gaussian beam, so that the fabrication parameters might be different. One of the important parameters is the distance between different pulses, as shown in Fig. 3(a). The distances between nearby pulses usually have a large influence on the energy deposition homogeneity. By decreasing the distances between pulses, the homogeneity of energy deposition can be improved, whereas the efficiency will be reduced. Therefore, the distance between pulses should be chosen properly.
Fig. 3. (a) The scanning route of the samples. The inset shows two adjacent Bessel pulses with a distance of d. (b) The evolution of standard deviation of the fluence with the increasing of distance ratio, which is defined as the ratio distance between adjacent pulses and the diameter of the main lobe of the Bessel beam; (c) The accumulated fluence map of four different distance ratios of 1.2, 1, 0.8, 0.6, respectively. The scale bar shows the length of the size of the main Bessel lobe.
In order to improve the energy deposition homogeneity, we simulated the accumulated energy of every pulses at the same position. Herein, we defined distance ratio as the ratio of distance between adjacent pulses and the diameter of the main lobe of the Bessel beam. The diameter is defined as the distance between the first minimums on two sides. The simulated energy deposition maps of different distance ratios are shown in Fig. 3(c). Figure 3(c1-c4) shows the energy deposition map of 1.2, 1, 0.8, 0.6, respectively. The intensity map of 0.6 shows the best homogeneity of these four cases. For distance ratio of 0.8, the intensity fluctuation is within 100% and 90%. For even larger distance ratio, the homogeneity becomes worse. To quantitively demonstrate the relationship between the distance ratio and the energy deposition homogeneity, we calculated the standard deviation of different distance ratio, as shown in Fig. 3(b). We found that as the distance ratio increase, the standard deviation doesn’t increase with exact linear relationship, but in a fluctuating way. It implies that the homogeneity is improved only if the distance ration decreases, which is different with that of Gaussian pulses. This difference is attributed to the sidelobes of Bessel beam. According to the simulation, there is a rapid decrease of the standard deviation when the distance ratio is increased from 0.7 to 0.6, this is because of the overlap between the main lobe and the side lobes. After the distance ratio is decreased to 0.6, further decrease doesn’t show high improvement of the standard deviation. Considering the decreasing of distance ratio may reduce the efficiency, it is reasonable to fabricate the structures by using a distance ratio of 0.6. Despite the high homogeneity in the simulation, the structures homogeneity will still be influenced by the explosive nature of laser ablation. The target here is to obtain the optimized fabrication parameter rather than obtain better homogeneity than the Gaussian beam.
3. Fabrication of functional surfaces and applications
We further investigated the morphologies of the fabricated structures at different fluences and corresponding performances. The tested material is stainless steel (304#, the composition is 18Cr-9Ni), with the surface polished to the mirror level (roughness <0.1μm). According to Fig. 2(c), the cross section at different height has similar profile except for their fluence, so it is safe to conclude that the difference of structures fabricated at different heights are mainly related to the fluence. Therefore, we intentionally choose a range of different fluence and measure the surface morphologies and performance of different functionalities at different fluences. After we build the relationship between the fluence and the morphologies, we can predict the performance of different functionalities based on the fluence measured at different heights, as measured in Fig. 2(c). According to the morphology type, two different fluence range are discussed: low-fluence range (ripple-like structures), and high-fluence range (bumps-like structures). For the low-fluence modifications, the modulated beam after the SLM is directly used for modification. The diameter of the center lobe (defined as the distance between the two minimums near the main lobe) is around 46 μm. For the high-fluence modifications, a telescope consisting of a plano-convex lens (focusing lens of 200 mm) and a microscope objective (5×, NA = 0.15, Olympus Inc.) is used (as shown in Fig. 1(a)). The distance ratio is chosen to be 0.1 to ensure the homogeneity. In this paper, to quantitively describe the pulse, the value of the peak fluence is calculated based on the pulse energy and pulse profile.
At lower fluence, ripple-like structures can be obtained on the surface, as shown in Figs. 4(a-c). With the pulse fluence increasing, the period of the ripple structures become smaller (Fig. 4(c)). In order to obtain its periodicity, two-dimensional fast Fourier transformation (2D-FFT) were used to analyze the frequency components, as shown in the inset images. The horizontal central line of the 2D-FFT image is shown in Fig. 4(d), clearly showing that the period gradually increases from 630 nm to 315 nm. A possible explanation might be the splitting of the bump of the laser-induced periodic surface structures (LIPSS) [22–24]..
Fig. 4. (a-c) LIPSS structures fabricated with low fluence ranges: 1.02 J/cm2, 1.53 J/cm2, and 2.15 J/cm2. The scale bar is 5 μm. The insets in the left corner are the FFT images of the SEM results. Scaler bar: 5 μm. (d) The intensity of the center line along the horizontal axis of the FFT-transformed image. The black, red, blue curve corresponds to Fig. (a), (b) and (c) respectively. (e) The relative diffraction efficiency of surfaces fabricated by different fluences. (f) Diffraction-induced-color obtained at different heights. The unit in the image is the meter. The scale bar is 5 mm.
These LIPSS structures can be used for diffraction-induced color. Due to the periodicity, the LIPSS structures can act as a grating and show different colors when illuminated by white light [25]. The diffraction-induced color is determined by the period of the structure. We measured the diffraction beam of a single-color laser beam at a particular angle to indicate the stability of the fabricated surface structures. The incident laser pulse is a 532 nm continuous laser with an orthogonal incident angle. A camera is fixed at the angle near 58° (corresponding to the 1st order diffraction of the gratings in Fig. 4(a)) in respect to the incident laser pulse to receive the diffracted beam. The camera sensor has been tested to have a linear response to this wavelength, so it was used to quantify the relative intensity. Figure 4(e) shows the relative diffractive intensity of different fluence. The diffraction efficiency increases to the maximum near the fluence of 1.2 J/cm2, then it starts to decrease gradually. The decreasing of the efficiency could be attributed to the periodicity change of the gratings as shown in Fig. 4(d). Despite the highest reflectivity is obtained in a small range of fluence, we can modulate the fluence of the stable zone of the modulated Bessel beam to match this window so that a large stable modification can be obtained. As shown in Fig, 4(f), we show the fabricated structures on a tilted surface with an angle of 45°. Because the height of the sample is limited, after fabricating one stripe on the surface, the sample was raised by the translation stage for fabrication at another height. Therefore, each ribbon represents the fabricated results at a different height range, as marked on the left. Diffraction-induced-color can be obtained within the range of 90 to 270 mm, proving the large height range that can be processed by using this method.
At higher fluence, two-dimensional bump structures were fabricated instead of LIPSS structures, as shown in Figs. 5(a-c). The size of the bump structures and the periodicity increase with pulse fluence. The transition from the ripple structure to the bump structures may be attributed to the inhomogeneous nucleation and subsequent light redistribution [26]. It is worth noting that at high fluence the surface is covered by enormous nanostructures. This might be related to an ablation effect caused by side lobes of the Bessel beam. After the formation of bump structures, they were irradiated again by the side lobes of the Bessel beam, which has a weaker fluence. These side lobes may further increase the formation of nanostructures on the bump structures.
Fig. 5. (a-c) The SEM images of the fabricated structures with higher fluence ranges: 6 J/cm2, 12 J/cm2, and 18 J/m2. The scale bar is 50 μm. (d) The reflective intensity of the structures for three different fluences. The inset shows an image of the fabricated structures at different heights. (e) Contact angle and sliding angle of surfaces fabricated by different fluences.
This technique can be used for the fabrication of 3D anti-reflection structures. The reflectivity of the fabricated surfaces can be characterized by FT-IR spectroscopy in the range of 700 to 900 nm. The original untreated surfaces were set as the reference for all the measurements. The tested the reflecting properties at different fluence are shown in Fig. 5(d). Although the relative intensity varies as the height changes, the relative intensity at different height were kept below 10% from 700 nm to 900 nm for all the surfaces. Despite the micro/nano-structures are not the same, they have similar performance. This will highly benefit improving the 3D structuring range for this application. As a demonstration, anti-reflective surfaces can be obtained over a 20-mm height by the proposed methods, as shown in the inset in Fig. 5(d). This range is limited mainly because of the fluence value of our laser and the damage threshold of SLM.
In addition, 3D hydrophobic structures can be obtained using this method. For the fabrication of hydrophobic surfaces, the samples are first silanized using Trichloro (1H, 1H, 2H, 2H perfluorooctyl) silane with a concentration of 2% for 60 min to reduce the surface free energy. Then, the surface was heated at 200 ℃ for 60 min to stabilize the fluoroalkylsilane molecules. After the fluorination treatment to reduce the surface energy, highly hydrophobic surfaces can be obtained. The wetting parameters of the samples are measured by an imaging system consisting of a macro video lens (ZOOM7000, Navitar) and a charge-coupled device (CCD). The acquired images of droplets are treated using a snake-based algorithm [27] to accurately determine the contact angle. Figure 5(e) shows the measured contact angle and sliding angle at different fluences. The contact angle is much larger than that of unprocessed samples, which is reported to be around 96°. As the fluence increases from 5.9 J/cm2 to 9.8 J/cm2, the contact angle increases from 126° to 141° and the sliding angle decreases from more than 32° to less than 10°. As the fluence further increases, the contact angle and the sliding angle change more slowly. The contact angle stays larger than 140° and the sliding angle stays smaller than 10°. The fluence range for fabricating hydrophobic surfaces is much larger than that for LIPSS. Despite the micro/nano-structures are not the same, they have similar performance. Therefore, hydrophobic structures can be obtained more easily in a large height range by choosing proper fluence.
The proposed methods can be used in many applications that need functionization of complex curved surfaces [28–30]. Here we demonstrate a simple application of fabricating hydrophobic surfaces on complex 3D surfaces, which might be used for micro-droplet manipulation. Syringes and pipettes are widely used to transfer liquid droplets. However, if the volumes of the liquid droplet are too small, the liquid droplet will attach to the tip of the syringe/pipette because of the surface tension. This has been an obstacle for transferring small amount of liquid in many applications [31,32]. In order to solve this problem, the tip can be processed to be hydrophobic, so that the attraction can be substantially reduced. However, due to the complex 3D morphologies of the tip of a syringe, it is not easy to fabricate its surface by using a Gaussian laser beam. We solved the problem using the proposed method in this paper. Figure 6(a) shows the processed tip of a syringe with a diameter of 0.64 mm. Part of Fig. 6(a) is magnified in Figs. 6(d) and (e). Figure 6(b, c) is the image of the same position as Fig. 6(d, e), but before processing. It shows that the complex 3D surface is covered by micro/nano structures, essential for the hydrophobicity of the surface. The morphologies difference between Fig. 6(d) and (e) might be attributed to the surface quality difference before processing, which is shown in Fig. 6(b) and (c). Figure 6(f) compare the minimum amount of liquid that can emerge from a syringe. These two images are taken when the droplet begins to slide down from the syringe. To quantitively obtain the minimum volume of droplet that the syringe can manipulate, we used the syringe to create tens of droplet by slowly pushing the liquid out from the syringe. By measure the accumulated liquid volume divided by the droplet numbers, we obtain the minimum volume of the liquid droplet. It can be reduced from 45 μL to 22 μL by using the processed syringe. This reduction is attributed to the reduction of capillary force by using the hydrophobic surfaces. This improvement can lead to smaller droplet volume, thus benefitting precise transfer of the liquid [31,32].
Fig. 6. (a) An SEM image of the syringe tip. The scale bar is 0.5 mm. (b, c) Magnification of parts of (a) before processing. (d, e) shows the surfaces structures same position after processing. The scale bar is 50 μm. (f) Comparison of minimum volumes of droplets that can slide from the tip. The scale bar is 1 mm.
4. Conclusions
We have investigated freeform curved surface processing by using shaped beams in propagation axis. By using the strategy to enhance the Bessel beam length that we proposed recently, we increase the length of the stable-intensity zone with fluctuation smaller than 10% by more than 3 times compared to that of an unshaped Bessel beam. The energy deposition was also optimized to be less than 2% fluctuation based on simulations. By using optimized parameters, we obtained homogeneous micro/nano structures in different fluence ranges for functional surface applications: (1) Diffraction-induced-colorization can be fabricated on a surface with a height difference as high as 180 millimeters. (2) Relative reflective intensity can be reduced to 10% of its value for the untreated surfaces in the range of 700 nm to 900 nm. (3) Hydrophobic surfaces with a contact angle larger than 130° and sliding angle smaller than 10° can be obtained on 3D surfaces. The height range of the stable performance for anti-refection and hydrophobicity is about 20 millimeters in this research. This range is limited mainly because of the fluence value of our laser and the damage threshold of SLM. We demonstrated the applications of the proposed method in creating hydrophobicity on complex freeform syringe tip surfaces. This improved the minimum manipulatable volume of a liquid droplet to 2 times smaller compared with untreated syringe, thus extending its performance for micro-droplet manipulation. This method may offer a reliable and affordable approach for 3D surfaces fabrication in real-world applications. But it is a drawback for this method to deal with the 3D surfaces with strong angle differences. The reflection ratio may be different for the structuring process (especially for dielectric materials). Further improvements can be made to improve the effectiveness of this method for processing surfaces with higher angle difference.
Funding
National Key Research and Development Program of China (2018YFB1107200); Natural Science Foundation of Beijing Municipality (JQ20015); National Natural Science Foundation of China (52075041).
Disclosures
The authors declare no conflicts of interest.
References
1. A. Y. Vorobyev and C. Guo, “Direct femtosecond laser surface nano/microstructuring and its applications,” Laser Photon. Rev. 7(3), 385–407 (2013). [CrossRef] .
2. K. Yin, X. Dong, F. Zhang, C. Wang, and J. Duan, “Superamphiphobic miniature boat fabricated by laser micromachining,” Appl. Phys. Lett. 110(12), 121909 (2017). [CrossRef] .
3. X. Li, Z. Xu, L. Jiang, Y. Shi, A. Wang, L. Huang, and Q. Wei, “Creating a three-dimensional surface with antireflective properties by using femtosecond-laser Bessel-beam-assisted thermal oxidation,” Opt. Lett. 45(11), 2989 (2020). [CrossRef] .
4. P. Fan, M. Zhong, L. Li, P. Schmitz, C. Lin, J. Long, and H. Zhang, “Sequential color change on copper surfaces via micro/nano structure modification induced by a picosecond laser,” J. Appl. Phys. 114(8), 083518 (2013). [CrossRef] .
5. X. Yu, D. Qi, H. Wang, Y. Zhang, L. Wang, Z. Zhang, S. Dai, X. Shen, P. Zhang, and Y. Xu, “In situ and ex-situ physical scenario of the femtosecond laser-induced periodic surface structures,” Opt. Express 27(7), 10087–10097 (2019). [CrossRef] .
6. A. Wang, L. Jiang, X. Li, Z. Xu, L. Huang, K. Zhang, X. Ji, and Y. Lu, “Nanoscale material redistribution induced by spatially modulated femtosecond laser pulses for flexible high-efficiency surface patterning,” Opt. Express 25(25), 31431–31442 (2017). [CrossRef] .
7. K. Ding, C. Wang, Y. Zheng, Z. Xie, Z. Luo, S. Man, B. Wu, and J. Duan, “One-step fabrication of multifunctional fusiform hierarchical micro/nanostructures on copper by femtosecond laser,” Surf. Coat. Technol. 367, 244–251 (2019). [CrossRef] .
8. Y. Song, C. Wang, X. Dong, K. Yin, F. Zhang, Z. Xie, D. Chu, and J. Duan, “Controllable superhydrophobic aluminum surfaces with tunable adhesion fabricated by femtosecond laser,” Opt. Laser Technol. 102, 25–31 (2018). [CrossRef] .
9. A. Wang, L. Jiang, X. Li, Q. Xie, B. Li, Z. Wang, K. Du, and Y. Lu, “Low-adhesive superhydrophobic surface-enhanced Raman spectroscopy substrate fabricated by femtosecond laser ablation for ultratrace molecular detection,” J. Mater. Chem. B 5, 777–784 (2017). [CrossRef] .
10. D. Wu, Z. Zhang, Y. Zhang, Y. Jiao, S. Jiang, H. Wu, C. Li, C. Zhang, J. Li, Y. Hu, G. Li, J. Chu, and L. Jiang, “High-Performance Unidirectional Manipulation of Microdroplets by Horizontal Vibration on Femtosecond Laser-Induced Slant Microwall Arrays,” Adv. Mater. 32(48), 2005039 (2020). [CrossRef] .
11. K. Yin, H. Du, X. Dong, C. Wang, J. A. Duan, and J. He, “A simple way to achieve bioinspired hybrid wettability surface with micro/nanopatterns for efficient fog collection,” Nanoscale 9(38), 14620–14626 (2017). [CrossRef] .
12. Z. Wu, K. Yin, J. Wu, Z. Zhu, J. Duan, and J. He, “Recent advances in femtosecond laser-structured Janus membranes with asymmetric surface wettability,” Nanoscale (2021). [CrossRef]
13. J. A. Duan, X. Dong, K. Yin, S. Yang, and D. Chu, “A hierarchical superaerophilic cone: Robust spontaneous and directional transport of gas bubbles,” Appl. Phys. Lett. 113(20), 203704 (2018). [CrossRef] .
14. D. Qi, S. Tang, L. Wang, S. Dai, X. Shen, C. Wang, and S. Chen, “Pulse laser-induced size-controllable and symmetrical ordering of single-crystal Si islands,” Nanoscale 10(17), 8133–8138 (2018). [CrossRef] .
15. W. Xiong, Y. Liu, L. J. Jiang, Y. S. Zhou, D. W. Li, L. Jiang, J.-F. Silvain, and Y. F. Lu, “Laser-Directed Assembly of Aligned Carbon Nanotubes in Three Dimensions for Multifunctional Device Fabrication,” Adv. Mater. 28(10), 2002–2009 (2016). [CrossRef] .
16. R. Tian, J. Liu, X. Li, X. Wang, and Y. Wang, “Design and fabrication of complicated diffractive optical elements on multiple curved surfaces,” Opt. Express 23(26), 32917 (2015). [CrossRef] .
17. J. Chen, J. Cheng, D. Zhang, and S.-C. Chen, “Precision UV imprinting system for parallel fabrication of large-area micro-lens arrays on non-planar surfaces,” Precis. Eng. 44, 70–74 (2016). [CrossRef] .
18. Y. Su, S. Wang, D. Yao, Y. Fu, H. Zang, H. Xu, and P. Polynkin, “Stand-off fabrication of irregularly shaped, multi-functional hydrophobic and antireflective metal surfaces using femtosecond laser filaments in air,” Appl. Surf. Sci. 494, 1007–1012 (2019). [CrossRef] .
19. F. Courvoisier, J. Zhang, M. K. Bhuyan, M. Jacquot, and J. M. Dudley, “Applications of femtosecond Bessel beams to laser ablation,” Appl. Phys. A 112(1), 29–34 (2013). [CrossRef] .
20. Q. Xie, X. Li, L. Jiang, B. Xia, X. Yan, W. Zhao, and Y. Lu, “High-aspect-ratio, high-quality microdrilling by electron density control using a femtosecond laser Bessel beam,” Appl. Phys. A 122(2), 136 (2016). [CrossRef] .
21. Z. Yao, L. Jiang, X. Li, A. Wang, Z. Wang, M. Li, and Y. Lu, “Non-diffraction-length, tunable, Bessel-like beams generation by spatially shaping a femtosecond laser beam for high-aspect-ratio micro-hole drilling,” Opt. Express 26(17), 21960–21968 (2018). [CrossRef] .
22. J.-W. Yao, C.-Y. Zhang, H.-Y. Liu, Q.-F. Dai, L.-J. Wu, S. Lan, A. V. Gopal, V. A. Trofimov, and T. M. Lysak, “High spatial frequency periodic structures induced on metal surface by femtosecond laser pulses,” Opt. Express 20(2), 905 (2012). [CrossRef] .
23. S. Hou, Y. Huo, P. Xiong, Y. Zhang, S. Zhang, T. Jia, Z. Sun, J. Qiu, and Z. Xu, “Formation of long- and short-periodic nanoripples on stainless steel irradiated by femtosecond laser pulses,” J. Phys. D: Appl. Phys. 44(50), 505401 (2011). [CrossRef] .
24. J. Bonse, S. Hohm, S. V. Kirner, A. Rosenfeld, and J. Kruger, “Laser-Induced Periodic Surface Structures-A Scientific Evergreen,” IEEE J. Sel. Top. Quantum Electron. 23(3), 109–123 (2017). [CrossRef] .
25. J. Yao, C. Zhang, H. Liu, Q. Dai, L. Wu, S. Lan, A. V. Gopal, V. A. Trofimov, and T. M. Lysak, “Selective appearance of several laser-induced periodic surface structure patterns on a metal surface using structural colors produced by femtosecond laser pulses,” Appl. Surf. Sci. 258(19), 7625–7632 (2012). [CrossRef] .
26. G. D. Tsibidis, C. Fotakis, and E. Stratakis, “From ripples to spikes: A hydrodynamical mechanism to interpret femtosecond laser-induced self-assembled structures,” Phys. Rev. B: Condens. Matter Mater. Phys. 92(4), 041405 (2015). [CrossRef] .
27. A. F. Stalder, G. Kulik, D. Sage, L. Barbieri, and P. Hoffmann, “A snake-based approach to accurate determination of both contact points and contact angles,” Colloids Surf., A 286(1-3), 92–103 (2006). [CrossRef] .
28. T. Bieker and S. Dietrich, “Wetting of curved surfaces,” Phys. A 252(1-2), 85–137 (1998). [CrossRef] .
29. M. Shi, X. Ji, S. Feng, Q. Yang, T. J. Lu, and F. Xu, “Self-Propelled Hovercraft Based on Cold Leidenfrost Phenomenon,” Sci. Rep. 6(1), 28574 (2016). [CrossRef] .
30. T. Sun, Q. Shi, Q. Huang, H. Wang, X. Xiong, C. Hu, and T. Fukuda, “Magnetic alginate microfibers as scaffolding elements for the fabrication of microvascular-like structures,” Acta Biomater. 66, 272–281 (2018). [CrossRef] .
31. L. Wu, Z. Dong, N. Li, F. Li, L. Jiang, and Y. Song, “Manipulating Oil Droplets by Superamphiphobic Nozzle,” Small 11(37), 4837–4843 (2015). [CrossRef] .
32. Z. Dong, J. Ma, and L. Jiang, “Manipulating and Dispensing Micro/Nanoliter Droplets by Superhydrophobic Needle Nozzles,” ACS Nano 7(11), 10371–10379 (2013). [CrossRef] .
