Open Access
Add to BibSonomy
Abstract
In this paper, we investigated the material redistribution phenomenon controlled by spatially modulated femtosecond laser pulses on a silicon surface. The intensity distribution was shaped by using a spatial light modulator. The material was first selectively melted and then redistributed by the laser-induced plasma. Thus, complex surface patterns were formed conformal to the laser intensity distribution. Sub-diffraction-limit size can be achieved due to the nanoscale material redistribution. Only one pulse was needed in the surface patterning process, thus greatly favoring the efficiency improvement. Combined with multibeam interference, a large-scale nanostructure array can be fabricated with high efficiency of 1600 μm2/pulse. This method offers a simple, flexible and efficient alternative approach for nanoscale surface patterning applications.
© 2017 Optical Society of America
1. Introduction
Surface patterning have been continually attracting wide attention of researchers due to its wide applications in surface functionalization, such as anti-reflection surfaces [1, 2], wettability control [3, 4], structure colorization [5], and grating fabrication [6, 7]. Although several techniques can be used for silicon surface patterning, including lithography [8], chemical etching [9, 10], reactive ion etching [3, 11]. These methods have certain limitations such as complex procedures, low efficiency, and substrate contamination by the processing chemicals. The increasingly higher requirements in industry have posted considerable challenges for fabrication methods to be more efficient, lower-cost, and higher-resolution.
One of the most promising alternatives for surface patterning is laser induced materials redistribution [12–15]. No mask or vacuum system are needed in the process, thus greatly reduced the operation costs. In addition, this method is environment-friendly. Comparing with conventional processes which are based on the removal of materials, less wasted material would be created in this process. Many researches concerning laser induced materials redistribution have been reported recently. By using high-fluence long-duration laser pulses, Temmler et al. reported metal surface structuring of materials based on laser-controlled melt pool due to surface tension. High quality surface structures can be generated with precision on micron-scale [12, 13]. Recently, higher precision of sub-microns can be achieved by using spatially modulated laser pulses. For example, Toyoda et al. used vortex beam to generate needle-like nanostructures with chirality on metal surface. Curvature radius of 36 nm can be achieved based on this method [14]. Yoo et al. controlled the dewetting process of amorphous silicon film by using donut-shaped laser pulse. Nanoparticle structures can be fabricated with diameter as small as 190 nm [15]. Girdauskas et al. reported laser ablation of thin metal films by using interfered ultrafast laser beam [16, 17]. Although higher precision has been achieved by using spatially modulated laser pulses, the fabricated structures are limited to nanoparticle or nanopillar structures. In addition, the phenomenon of material redistribution induced by the spatially modulated laser pulses were still not clear. Recently, spatially modulated femtosecond pulses with different polarization [18, 19] have also been employed to produce complex patterns. However, multi-pulses are needed in the fabrication process, and the structure morphologies are still limited.st. These challenges greatly restrain the applications of this method.
Due to the fast advances of laser shaping techniques, the intensity distribution of femtosecond laser pulses can be modulated flexibly, thus offering new opportunities in controlling the laser-material interactions. In this research, we investigated the laser induced material redistribution process using spatially modulated femtosecond laser. We chose silicon as the target material due to the wide applications of silicon surface structuring. The material redistribution results versus different types of shaped pulses were carefully researched. Complex patterns with sub-wavelength precision can be flexibly fabricated by this method. Combined with interference, arrays of high-precision surface structures can be processed in a single flash. Our researches may offer a new approach for high-precision, high-efficiency and high-flexibility surface structuring.
2. Patterning mechanism
We used spatially modulated femtosecond laser to control the material redistribution process, as shown in Figs. 1(a)-1(c). The spatial intensity distribution was digitally modulated by using liquid crystal spatial light modulators (SLM). The adopted fluence was much higher than the ablation threshold. After irradiated by the spatially modulated femtosecond laser pulses, a series of phenomenon were induced, including electrons ionization, plasma formation, shock wave propagation, electron-phonon coupling, etc.
Fig. 1 (a, b, c) Schematic of material redistribution induced by a spatially modulated laser pulse; (d, e) AFM measurement of crater morphologies ablated by a single pulse Gaussian beam.
First, the photons of the materials were mainly absorbed by electrons. The laser energy absorption is completed before the lattice changes, resulting in a significantly nonequilibrium state between electrons and lattices. Then, in picosecond regime, the energy absorbed by electrons can be transmitted to the lattice through electron-phonon coupling. Although the thermal effects were minimized for ultrafast laser, materials heating effect could be observed in high fluence regime, as shown in Fig. 1(b). Plasma and shockwave may be generated in the laser-material interaction process, as shown in Fig. 1(c). The plasma generation and shockwave expansion may offer strong force to redistribute the melted materials. By using spatially modulated laser pulses, plasma and shockwave can be formed simultaneously in different area, redistributing the materials into different patterns. By adjusting the intensity distribution, complex structures can be fabricated.
The materials heating effect induced by conventional femtosecond laser have been investigated in several previous literatures [20–22]. Yaker et al. irradiated borosilicate glass using a single femtosecond laser pulse. They found that nanoscale thin rims surrounding the laser ablated craters can be formed [20,21]. They attributed this phenomenon to a pressure-driven fluid motion of the molten material outward from the center of laser focal point. The high pressure mainly resulted from dense plasma induced by the high-fluence ultrafast laser pulses. Goya et al. investigated the melting phenomenon of different irradiation numbers. They concluded that thermocapillary force and hydrodynamic force dominated in the high-fluence regime [23]. In our researches, we also found nanoscale rim formed by single pulse irradiation on silicon surfaces. Figures 1(d) and 1(e) shows the structures fabricated using 20 × objective lens (N.A. 0.45) with pulse energy of 50 nJ, including the three-dimensional morphologies and cross-sections of the fabricated structures measured by atomic force microscopy (AFM). The diameter of the crater is 1.67 μm and the width of the ridge is approximately 326 nm, which is one-fifth of the crater size.
Laser fabrication has long been restricted by the diffraction limit, thus it is typically difficult to fabricate structures smaller than the wavelength by using the traditional direct ablation method. However, in the aforementioned case, the structures induced by materials redistribution were much smaller than the diffraction limit. Under the inspiration of this phenomenon, we suggested that laser induced material redistribution might be a good alternative for fabricating nanoscale surface structures if the redistribution process can be well controlled. By using a simple Gaussian beam, only round-shaped structures can be generated. A more complex intensity distribution should be used to fabricate more complex structures. In addition, by combining with multibeam interference, large scale of nanostructures can be fabricated simultaneously, greatly improve the fabrication efficiency.
3. Experimental setup
We used a SLM to shape the original laser pulses to change the intensity distribution of the original laser pulses. The experimental setup is shown in Fig. 2. First, a Ti:sapphire laser regenerative amplifier system (Spitfire Ace-35F, Spectra Physics) provides a fundamental Gaussian mode with a central wavelength of 800 nm, a pulse duration of 35 fs, and a repetition rate of 1 kHz. The beam diameter(1/e2) of the laser is 10 mm. The phase pattern is loaded on the liquid crystal on a silicon SLM (Pluto-NIR, Holoeye) to change the phase of the laser pulses. The laser polarization direction is parallel to the variable axis of the liquid crystal which is shown by blue double-headed arrow in Fig. 2. Then the modulated laser beam passes through a 4f relay system, which comprises two plano-convex lenses (L1, L2). The distance between the SLM and lens L1 is equal to the focal length of L1. The distance between the two lenses is the sum of their focal lengths. The laser beam can then be transmitted to the focal plane of the second lens L2 without any distortion. At the end plane of the 4f system, a slit was placed at the beam center. Finally, after focusing by the objective lens (20 × , NA 0.45), we obtain the shaped laser beam at the focusing plane. A highly-polished silicon (100) sample (10 × 10 × 0.5 mm3) is mounted on a computer-controlled, six-axis translation stage (M-840.5DG, PI, Inc.), which has a positioning accuracy of 1 μm in the x and y directions and 0.5 μm in the z direction. All experiments were carried out in air at the same atmosphere pressure and room temperature.
Fig. 2 Schematic of the experimental setup. The incident Gaussian beam was phase-modulated by SLM, transmitted by a 4f relay system comprising two convex lenses (L1, L2) and reflected by a dielectric mirror (DM). An in-vivo video system comprising a white-light source (WS), a beam splitter (BS), a focusing lens (L3) and a charge-coupled device (CCD) was used to monitor the fabrication process.
The morphologies of the patterned silicon surfaces were characterized by using a scanning electron microscope (SEM, XL30 S-FEG from FEI and S-4800 from Hitachi Limited). We used a focused ion beam (FIB, JEOL JIB-4610F) to cut the silicon. Energy dispersion X-ray spectroscopy (Oxford Instrument) was conducted to analyze the elementary component of the silicon structures.
4. Results and discussion
4.1 Spatial light modulation
We used Fresnel diffraction theory to obtain the intensity distribution at the focusing point under paraxial approximation [20]. The incident beam was a Gaussian beam with a waist of W0. After passing through the SLM, the field of the phase-modulated beam can be expressed as
where A0 is field amplitude and τ(x, y) is the phase pattern loaded on the SLM. The 4f relay system transferred the laser field from the SLM to the objective lens without distortion; thus, in this calculation the diffraction of this process can be omitted. For some cases, a slit with a width of 1 mm was placed at the beam center at the end plane of the 4f system, which is the back focal plane of the second lens of the 4f system. After the slit, the intensity can be expressed aswhere M(x, y) represents the intensity shape induced by the slit, defined as:Using the slow varying envelope approximation, the field after the objective can be written as
where k = 2π/λ and f is the focal length of the objective lens. The laser field at the focal point can be obtained by using the Fresnel diffraction integral under the paraxial approximation [20]:where R is the radius of the objective lens aperture, and xf, yf, and zf are the coordinates on the focusing plane. The intensity distribution of the focused phase-modulated beam can be obtained by combining the aforementioned equations. To validate the effectiveness of the pulse shaping, we measured the intensity distribution at the focusing plane of a plano-convex lens (focal length, 600 mm). A different focusing lens was used because the focal spot of the objective lens was too small to be detected by our camera. The intensity distribution of the objective lens has a similar profile to that of the plano-convex lens except for the size.4.2 Line-shape structures fabricated using a two-spots beam
As a simplest case, we studied the case of material redistribution by using a two-spots laser beam as shown in Fig. 3. The two-spot beam was generated by loading a 0-π phase step on the spatial light modulator, which is shown in upper row of Fig. 3(a). The theoretical and experimental intensity distribution was shown in the middle row and bottom row of Fig. 3(a). The figure shows that an intensity slit with a relatively small intensity exists in the center of the two-spot laser beam. By using this type of laser pulse, the material in the center of the beam is pushed from both sides, thus forming line-shape structures. The fabrication results under different pulse energies are shown in Fig. 3(b). The ablation threshold of the silicon (defined as Fth) was measured to be 88.9 mJ/cm2. At a lower pulse energy of 2.3 Fth, which are shown in Fig. 3(b1), two separate craters exist with no superposition. As the pulse energy increased, which are shown in Fig. 3(b2-b9), a line-shaped structure with a bump in the center was formed between the two craters. The size of the bump first increased when the pulse energy increased from 2.7 Fth to 5.4 Fth, which are shown in Fig. 3(b2-b6), and decreased when the pulse increased from 6.6 to 9.6 Fth, which are shown in Fig. 3(b7-b9). When the pulse energy was higher than 11.1 Fth, which are shown in Fig. 3(b10-b12), the bump in the center of the beams disappeared. Longer and more uniform line-shaped structures were formed.
Fig. 3 (a) The phase pattern and intensity distribution for two-spots beam; (b) The line-shape structures under different pulse energies. Insets (1-12) are the center parts of the craters. The entire parts of (1, 6, 7, 12) are shown in (I-IV) respectively.
The high-fluence ultrafast laser pulses may lead to a dense plasma. The plasma may result in a high pressure, which drove the molten material outward from the center of laser focal point. When the material was irradiated by two-spot beam, the molten material would encounter in the center of the shaped beam. Under the pressure between two plasma, a line-shape structures were formed in the center. The formation of the bump structures may be related to the material aggregation and Rayleigh-Plateau instability. Since the laser intensity is higher in the center part, more material was melted, thus more material was aggregated in the center. This may contribute to the bump formation. In addition, Rayleigh-Plateau instability can play an important role in the formation of central bump structure. The phenomenon was predicted by the Plateau–Rayleigh instability. According to Rayleigh-Plateau instability, when the length of the line-shape structure is longer than the circumference of the line-shape structure, the line-shape structures tends to be broken into droplets. Gedvilas reported nano/micro sphere formation in the molten and re-solidified ridge during laser ablation process [24]. In our experiments, similarly, the Rayleigh-Plateau instability may contribute to the bump structures formation.
The size variation of the center bump structures is an interesting phenomenon which deserves more discussions. Borowiec et al. [25] reported the surface morphologies of silicon ablated by femtosecond laser with Gaussian types intensity distribution. In their research, at a lower fluence, recast part was formed at the rim of the ablated crater. After increasing the pulse fluence above the critical threshold, the recast part was break down into small-scale debris and redeposited to the nearby of the crater. They attributed the break-down of the recast part to the high pressure induced by the high fluence laser pulses.
Our results can be explained within this picture. At a lower laser fluence (smaller than 5.4 Fth), the pressure was relatively small thus no material breakdown happened which are shown in Fig. 3(b1-b6). In Fig. 3(bI-bII), the rim of the fabricated structures showed no breakdown, which offers clear evidence of our deduction. Within this regime, the bump size will increase as the pulse fluence increase because of more melted materials and smaller line width of the line-shape structures. The materials cannot diffuse to nearby areas and gather in the center. However, if the pulse fluence was increased to be larger than the threshold, the high laser-induced pressure would lead to materials break-down from the surface, and debris would be generated, which are shown in Fig. 3(b7-b12). Within this regime, the bump size will decrease as the pulse fluence increase, which is contrary to previous phenomenon. This is because the material removal dominates the process. In Fig. 3b(III-IV), silicon nanoparticles appeared near the fabricated structures, which clearly support this deduction. Although more materials would be created, the high pressure induced by the high-fluence laser pulses would remove the melted material away from the structures. At laser fluence of 13.6 Fth, uniform and long nanowires were formed.
The obtained intensity slit obtained shown in Fig. 3(a) was very short, and we attempted to elongate it by using a slit-mask. Combination modulation of both amplitude and phase was proposed to shape the laser pulses in this research. Here, the slit-mask was placed in the center of the phase pattern, which is shown in the upper row of Fig. 4(a). The width of the slit is 1 mm. The simulation results are shown in the middle row of Fig. 4(a) and measured results, are shown in bottom row of Fig. 4(a). They clearly show that the slit was elongated. Figure 4(b) illustrates the structures fabricated under different pulse energies. The morphology evolution is very similar to the ordinary two-spots beam. First, a clean line-shaped structure was generated in the center of the beam at a low pulse energy of 2.5 Fth. As the pulse energy increased, the width reduced and the length increased. In addition, bumps appeared in the beam center. At the final stage, a clean and long-lined structure appeared in the center of the beam. One interesting phenomenon is the shape of the bump, which is similar to a Y-shape. This experiment demonstrates that a three-dimensional shaping ability can be obtained by this method.
Fig. 4 (a) The experimental setups, simulation and experimental results for elongated two-spots beam; (b) The line-shape structures under different pulse energies; (c) The cross section of the fabricated structures; (d) The EDX-spectrum of the fabricated structures and the reference surface.
We conducted several measurements to further characterize the cross section of the fabricated structures and the material properties. First, we used an FIB to cleave a thin layer of the line-shaped structure, as shown in Fig. 4(b) (8.0 Fth). The cross section is shown in Fig. 4(c). The dark area in the figure is the metal thin-film deposited to protect the surfaces. Two cleaves were formed because of the air bubbles formed by the deposition defect. The bright area in the upper left part is the silicon structure. As shown in the figure, the line-shaped structure has a “mushroom” shape with a larger head than the bottom. This mushroom shape may be attributed to the material redistribution process, bringing more material to the center part. In addition, we can observe that the superficial layer of the fabricated structure had a darker color than the unprocessed one, implying that an amorphous type silicon was formed in this process. We further performed energy-dispersive X-ray spectroscopy (EDXS) to analyze whether the silicon was oxidized. The spectra of the line-shape structures of 8 Fth are shown in Fig. 4(d). The spectrum of the line-shaped structure is shown in red color and a reference spectrum of untreated silicon is shown in black. No obvious peak appears at the position of oxygen, revealing that little oxidization occurred in the structuring process.
To quantitatively analyze of the features of the structures obtained through the laser shaping technique, we measured the length of the line-shape structures and the minimum width of line-shape structures as a function of the fluence changes (shown in Fig. 5). Figure 5(a) corresponds to the structures of Fig. 3. The width of the line-shape structures showed a stable decrease as the pulse energy increased from 0.1 μJ to 0.4 μJ. However, then the width didn’t show an obvious change anymore when the pulse energy increased from 0.45 μJ to 0.6 μJ. The minimum width of the line-shape structures can reach 180 nm. The length of the line-shape structures showed a continuous increase v.s. the pulse energy. Figure 5(b) corresponds to the structures of Fig. 4. The width and length change of the line-shape structures is similar to that of Fig. 5(a). The minimum width can reach 275 nm. It is worth noting that the part marked by shadow blue is the fluence range where the bump structures occurred.
Fig. 5 The length of the line-shape structures and the minimum width of line-shape structures as a function of the fluence changes. (a, b) corresponds to measurements from Fig. 3, 4, respectively. The part marked by shadow blue is the fluence range where the bump structures occurred.
4.3 Complex patterns fabricated by spatially modulated pulses
We further fabricated complex patterns by shaping the laser pulses into more complex intensity distributions. Materials were effectively redistributed to the area where the intensity was low. Figure 6 illustrates that flower-shaped edges can be fabricated using the spatially modulated laser pulses. The fundamental Gaussian beam was divided into four, six, and eight parts. Each spot resulted in material redistribution to the periphery of the laser spots. When the two spots were nearby, short line-shaped structures were generated. By using this method, the redistributed materials can be shaped into different types of structures.
Fig. 6 Flower-shaped edges fabricated by laser beam comprising four (a), six (b), and eight (c) spots, respectively; (a–c) share the same scale bar.
4.4 High-efficiency nanostructures array fabrication using interfered laser pulses
Currently, the efficiency of fabricating nanostructures array was highly limited by the two factors. First, the nanostructures need to be fabricated one by one, which greatly limited the processing efficiency. Second, the translation stages need to stop and start back and forth, thus much time was wasted on this process. Here we propose non-stop flash printing strategy for large-scale nanostructures array fabrication. Because only one pulse is required to fabricate the structures, the moving stage may move very fast without stop; thus, this method has a very high efficiency. In addition, we used multibeam interference to fabricate the structures in parallel.
The setup is the same with that of Fig. 2. A phase pattern of multifaceted prism was loaded on the SLM (the phase pattern is the same as that in [25]). Due to deflection induced by the SLM, the two 4f lenses and the objective lens generated microscale interference intensity patterns 260 μm above the original focusing plane. Periodic structures were generated by the laser-induced redistribution process. Figure 7(a) shows the hexagonal structures fabricated by three-beam interference at difference pulse energies. At a pulse energy of 40 μJ, craters with no intersection were fabricated. After increasing the pulse energy to 55 μJ, the peripheries of the craters intersected with each other, resulting in the formation of line-shaped structures. When the pulse energy was further increased to 82 μJ, the materials aggregated at the intersection points with a triangular shape. Figure 7(b) shows the square-grid structures fabricated by four beam interferences at different pulse energies. The silicon surface was located 120 μm above the original focusing plane. The structural evolution is similar to that in the case of three-beam interference.
Fig. 7 (a, b) Surface structures fabricated by three-beam interference (a) and four-beam interference (b).
We fabricated a silicon surface with an area of 1 × 1 cm. The process is illustrated in Figs. 8(a)-8(b). In order to stitch different patches of structures, the distance between different patches should be controlled precisely. In addition, to make the structure more uniform, we overlapped the surrounding areas with weaker intensity. In our case, every single pulse can fabricate an area of 40 μm × 40 μm, comprising more than 400 grids. The processing speed can be as high as 1600 μm2/pulse. With a repetition rate of 40 Hz for the laser system and a moving speed of 2000 μm/s for the moving stage, the fabrication of the 1 × 1 cm structure was completed in 23 min. However, it is expected to take 140 h for single spot fabrication of the same area. The efficiency can be considerably improved if a larger moving speed and higher repetition rate are employed. Considering that a moving speed of 1 m/s and a repetition rate of 1 MHz can be easily achieved, the patterning speed can reach as high as 40 mm2/s (1 m/s, 1 MHz), presenting excellent potential for practical industrial applications.
Fig. 8 (a) Schematic of fabrication strategies; (b) SEM images of the fabricated surfaces. The dotted line represents the edge of single pulse patterns; (c) Comparison of the surface color between the patterned surface (right part) and the unpatterned surface (left part); (d) Diffraction pattern of the fabricated surfaces.
We analyzed the optical properties of the fabricated surfaces by examining the surface color and diffraction patterns. The surface color is shown in Fig. 8(c). Moreover, two pieces of silicon are shown in the image. The left part shows untreated surfaces whereas the right part shows the patterned surfaces. The patterned surface exhibits a vivid color, whereas the untreated surface shows no color other than its original color. The structure color is due to the diffraction of white light by the periodic structures. A laser beam of 632 nm was incident on the patterned surfaces at an angle of 45°. Figure 8(d) shows the diffraction patterns, which indicates the possibility of using the patterned structures as gratings.
5. Summary
To conclude, we reported a new silicon surface structuring method based on material redistribution using programmable spatially modulated femtosecond laser pulses. By using a SLM, the intensity distribution at the focusing plane can be digitally modulated; thus, the surface patterns on the silicon surface can be flexibly controlled. By using laser pulses with intensity slits, the plasma-induced pressure drives the fluid motion of the molten material, thus forming a line-shaped structures in the slit. The minimum width of the line-shape structures can reach one fourth of the wavelength. The superficial layer of the fabricated structure was transformed into an amorphous type, and little oxidation occurred. By using complex intensity distribution, flower-shaped structures and grid-shaped structures can be easily fabricated. The structures can be formed by using only one pulse; thus, the efficiency is considerably enhanced. Combined with interference, the efficiency can be as high as 1600 μm2/pulse. This flexible and efficient method has excellent potential for various scientific and industrial applications involving silicon surface patterning, such as grating fabrication, structural coloration, etc.
Funding
National Natural Science Foundation of China (NSFC) (Grant No. 91323301 and 51675049); Beijing Municipal Natural Science Foundation (No. 3172027); and the Young Elite Scientists Sponsorship Program (No. 2016QNRC001).
References and links
1. H. Savin, P. Repo, G. von Gastrow, P. Ortega, E. Calle, M. Garín, and R. Alcubilla, “Black silicon solar cells with interdigitated back-contacts achieve 22.1% efficiency,” Nat. Nanotechnol. 10(7), 624–628 (2015). [PubMed]
2. J. Yang, F. Luo, T. S. Kao, X. Li, G. W. Ho, J. Teng, X. Luo, and M. Hong, “Design and fabrication of broadband ultralow reflectivity black Si surfaces by laser micro/nanoprocessing,” Light Sci. Appl. 3, e185 (2014).
3. Y. He, C. Jiang, H. Yin, J. Chen, and W. Yuan, “Superhydrophobic silicon surfaces with micro-nano hierarchical structures via deep reactive ion etching and galvanic etching,” J. Colloid Interface Sci. 364(1), 219–229 (2011). [PubMed]
4. L. Sainiemi, V. Jokinen, A. Shah, M. Shpak, S. Aura, P. Suvanto, and S. Franssila, “Non-Reflecting Silicon and Polymer Surfaces by Plasma Etching and Replication,” Adv. Mater. 23(1), 122–126 (2011). [PubMed]
5. L. Cao, P. Fan, E. S. Barnard, A. M. Brown, and M. L. Brongersma, “Tuning the color of silicon nanostructures,” Nano Lett. 10(7), 2649–2654 (2010). [PubMed]
6. B. Jin, J. Yuan, C. Yu, X. Sang, S. Wei, X. Zhang, Q. Wu, and G. Farrell, “Efficient and broadband parametric wavelength conversion in a vertically etched silicon grating without dispersion engineering,” Opt. Express 22(6), 6257–6268 (2014). [PubMed]
7. H. Qiu, Y. Su, P. Yu, T. Hu, J. Yang, and X. Jiang, “Compact polarization splitter based on silicon grating-assisted couplers,” Opt. Lett. 40(9), 1885–1887 (2015). [PubMed]
8. Y. Lu and A. Lal, “High-efficiency ordered silicon nano-conical-frustum array solar cells by self-powered parallel electron lithography,” Nano Lett. 10(11), 4651–4656 (2010). [PubMed]
9. B. M. Bang, J. I. Lee, H. Kim, J. Cho, and S. Park, “High‐Performance Macroporous Bulk Silicon Anodes Synthesized by Template‐Free Chemical Etching,” Adv. Energy Mater. 2, 878–883 (2012).
10. Z. Huang, N. Geyer, P. Werner, J. de Boor, and U. Gösele, “Metal-assisted chemical etching of silicon: a review,” Adv. Mater. 23(2), 285–308 (2011). [PubMed]
11. A. Rahman, A. Ashraf, H. Xin, X. Tong, P. Sutter, M. D. Eisaman, and C. T. Black, “Sub-50-nm self-assembled nanotextures for enhanced broadband antireflection in silicon solar cells,” Nat. Commun. 6, 5963 (2015). [PubMed]
12. A. Temmler, M. Küpper, M. Walochnik, A. Lanfermann, T. Schmickler, A. Bach, T. Greifenberg, O. Oreshkin, E. Willenborg, and K. Wissenbach, “Surface structuring by laser remelting of metals,” J. Laser Appl. 29, 012015 (2017).
13. A. Temmler, O. Pütsch, J. Stollenwerk, E. Willenborg, and P. Loosen, “Optical set-up for dynamic superposition of three laser beams for structuring and polishing applications,” Opt. Express 22(2), 1387–1393 (2014). [PubMed]
14. K. Toyoda, K. Miyamoto, N. Aoki, R. Morita, and T. Omatsu, “Using optical vortex to control the chirality of twisted metal nanostructures,” Nano Lett. 12(7), 3645–3649 (2012). [PubMed]
15. J.-H. Yoo, J. B. In, C. Zheng, I. Sakellari, R. N. Raman, M. J. Matthews, S. Elhadj, and C. P. Grigoropoulos, “Directed dewetting of amorphous silicon film by a donut-shaped laser pulse,” Nanotechnology 26(16), 165303 (2015). [PubMed]
16. E. Molotokaité, “Picosecond Laser Beam Interference Ablation of Thin Metal Films on Glass Substrate,” J. Laser Micro/Nanoengineering 5, 74– 79 (2010).
17. B. Voisiat, M. Gedvilas, S. Indrišiūnas, and G. Račiukaitis, “Flexible Microstructuring of Thin Films Using Multi-beam Interference Ablation with Ultrashort Lasers,” J. Laser Micro/Nanoengineering 6, 185– 190 (2011).
18. J. J. Nivas, S. He, A. Rubano, A. Vecchione, D. Paparo, L. Marrucci, R. Bruzzese, and S. Amoruso, “Direct Femtosecond Laser Surface Structuring with Optical Vortex Beams Generated by a q-plate,” Sci. Rep. 5, 17929 (2015). [PubMed]
19. E. Skoulas, A. Manousaki, C. Fotakis, and E. Stratakis, “Biomimetic surface structuring using cylindrical vector femtosecond laser beams,” Sci. Rep. 7, 45114 (2017). [PubMed]
20. A. Ben-Yakar, R. L. Byer, A. Harkin, J. Ashmore, H. A. Stone, M. Shen, and E. Mazur, “Morphology of femtosecond-laser-ablated borosilicate glass surfaces,” Appl. Phys. Lett. 83, 3030–3032 (2003).
21. A. Ben-Yakar, A. Harkin, J. Ashmore, R. L. Byer, and H. A. Stone, “Thermal and fluid processes of a thin melt zone during femtosecond laser ablation of glass: the formation of rims by single laser pulses,” J. Phys. D Appl. Phys. 40, 1447 (2007).
22. T. Schwarz-Selinger, D. G. Cahill, S.-C. Chen, S.-J. Moon, and C. Grigoropoulos, “Micron-scale modifications of Si surface morphology by pulsed-laser texturing,” Phys. Rev. B 64, 155323 (2001).
23. K. Goya, M. Shiraishi, Y. Fuchiwaki, K. Watanabe, and T. Ooie, “Femtosecond Laser-Induced Surface Modification and its Application,” in Applications of Laser Ablation-Thin Film Deposition, Nanomaterial Synthesis and Surface Modification (InTech, 2016).
24. M. Gedvilas, B. Voisiat, K. Regelskis, and G. Račiukaitis, “Instability-triggered transformations in thin chromium film on glass under laser irradiation,” Appl. Surf. Sci. 278, 26–32 (2013).
25. Q. Zhou, W. Yang, F. He, R. Stoian, R. Hui, and G. Cheng, “Femtosecond multi-beam interference lithography based on dynamic wavefront engineering,” Opt. Express 21(8), 9851–9861 (2013). [PubMed]
