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Abstract
Surfaces with strong anti-reflection properties have attracted the wide attention of scientists and engineers due to their great application potential in many fields. Traditional laser blackening techniques are limited by the material and surface profile, which are not able to be applied to film and large-scale surfaces. Inspired by the rainforest, a new design for anti-reflection surface structures was proposed by constructing micro-forests. To evaluate this design, we fabricated micro-forests on an Al alloy slab by laser induced competitive vapor deposition. By controlling the deposition of the laser energy, the surface can be fully covered by forest-like micro-nano structures. The porous and hierarchical micro-forests performed a minimum and average reflectance of 1.47% and 2.41%, respectively, in the range of 400-1200 nm. Different from the traditional laser blackening technique, the micro-scaled structures were formed due to the aggregation of the deposited nanoparticles instead of the laser ablation groove. Therefore, this method would lead to little surface damage and can also be applied to the aluminum film with a thickness of 50 µm. The black aluminum film can be used to produce the large-scale anti-reflection shell. Predictably, this design and the LICVD method are simple and efficient, which can broaden the application of the anti-reflection surface in many fields such as visible-light stealth, precision optical sensors, optoelectronic devices, and aerospace radiation heat transfer device.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Surfaces with high anti-reflection properties have attracted wide attention of scientists and engineers due to their great application potential in many fields, such as visible-light stealth [1–4], precision optical sensors and detectors [5–8], optoelectronic devices [9,10], photovoltaic solar energy cells [11–15], and aerospace radiation heat transfer devices [16,17]. Generally, the anti-reflection properties are mainly determined by the surface microstructural characteristics [18,19]. When the size of the surface microstructure is much larger than the wavelength of the incident light, the surface reflectance reduction is dominated by the geometric light-trapping effect [20–22]. In this case, the light can be entrapped in the crevices of the structures due to multiple reflections, which can significantly reduce the surface reflectance. On the other hand, when the structural size is smaller than the optical wavelength, the reflectance of the surface can be reduced due to an effective graded refractive index layer at the solid and air interface [23–25]. Hierarchical porous structures could not only enhance the geometric light-trapping effect but also construct a solid-air interface with a graded refractive index. Therefore, it is effective to construct surfaces with these structures to achieve anti-reflection properties.
Scientists have used many methods to achieve anti-reflection surfaces. Kong [26] et al. fabricated ultra-low reflectance silicon surfaces by reactive ion etching (RIE) combined with chemical etching. In their work, the microstructures were obtained by chemical etching while the nanostructures were obtained by RIE. The processed surface showed an average reflectance down to 1.16% in the wavelength range of 400-1000 nm. Tulli [27] et al. constructed nanopillar arrays on the surface of anti-glare glass via the RIE method. The reflectance was effectively reduced to about 4.5% after surface treatment. Shen [28] et al. constructed double-sided molded moth-eye structures on glass slides by a replica molding method. The surface performed a reflectance of 0.4% with normal incident light. Yamaguchi [29] et al. produced flower-like alumina thin films on soda-lime-silica glass substrates by the sol-gel method. The reflectance of the surface was effectively reduced to 0.5% by controlling the height of the flower-like alumina films. Although these methods can achieve a remarkable anti-reflection performance, there are still several remaining problems. The fabrication process is complex for these methods, which is hard to realize large-area production. Moreover, these methods perform a poor material-adaptability and may be harmful to the environment.
Compared with traditional methods of producing anti-reflection surface structures, ultrafast laser micro-nano manufacturing is more targeted, highly effective, environment-friendly, and with excellent material adaptivity. Therefore, anti-reflection structures fabricated via laser manufacturing, also known as the laser blackening technique, have become an emerging research direction in laser machining. Vorobyev et al. fabricated black platinum [30] and silicon [31] directly via the femtosecond laser processing technique. By constructing micro/nano structures, the reflectance of the blackened surface can be reduced to below 5% both on platinum and silicon. Li [32] et al. fabricated alumina with different colors by controlling the overlapping rate of the femtosecond laser pulses. In their work, the micro/nanos tructures were constructed via laser ablation, and the black alumina was achieved by 100 µm grooves covered with nanoparticles. Yang [17] et al. enhanced the absorptance of a titanium surface via femtosecond laser irradiation. The efficiency of light-trapping effect was improved up to 90% within the ultraviolet-visible-infrared wavelength region for coral-like surface structures. Fan [33] et al. constructed anti-reflection nanowires on the laser-structured copper surface via the thermal oxidation method. The surface performed a reflectance down to 0.6% at the wavelength of around 17 µm through the effect of nanowires and microstructures. Chen [34] et al. fabricated multi-scale micro-nano structures with broad-band ultra-low-reflectance both on silicon and copper surfaces by combining femtosecond laser ablation with in situ deposition. An average reflectance of 2.21% and 3.33% was achieved for Si and Cu surfaces, respectively, in the broad-band spectrum from UV to NIR.
Although anti-reflection surfaces via ultrafast laser processing on multiple materials have been achieved, some problems remain and limit further development of anti-reflection surface applications. For example, the ultrafast laser blackening technique mainly depends on the laser ablation effect to construct micron-scaled structures. For this technique, the beam spot should be focused on the surface, maximizing the laser fluence to realize surface ablation. Therefore, the application of this technique requires a high surface quality, and it is hard to be adapted to complex curved surfaces. Moreover, the traditional laser blackening technique is hard to be adapted to some film materials. For the traditional laser blackening technique, micro-scaled grooves, cracks, or holes are fabricated to enhance the light-trapping effect. However, the depth of the grooves could be closed or beyond the thickness of the film, which would damage the substrates and unable to fabricate anti-reflection film.
In the rainforest, the space is filled with hierarchical branches and leaves due to phototaxis, Therefore, most solar energy is absorbed by the plant and barely can reach the ground. Inspired by the rainforest, we propose a new design of light-trapping microstructures. In this design, the microstructures consist of discrete stacked nanoparticles, similar to the branches and leaves in the forest, and perform a stronger hierarchy and porosity compared with common micro-nano structures [34] whose interior is solid, which allows the photons to enter the microstructures to be trapped due to multireflection and shows better anti-reflection properties.
To obtain the micro-forest and evaluate its anti-reflection properties, we employed a simple laser induced competitive vapor deposition (LICVD) method which can directly fabricate hierarchical porous micro-forests. The evolution process of the micro-forests was also systematically investigated to clarify the production and deposition behavior of the laser-induced nanoparticles. This novel method provides a new strategy to construct micro/nano structures via laser irradiation. Predictably, this simple and efficient method can further broaden the application field of anti-reflection surfaces.
2. Experimental equipment, materials, and methodology
2.1 Experimental equipment
A picosecond laser system (Edgewave, Germany) is employed to generate laser pulses at a wavelength of 1064 nm with a pulse duration of 10 ps. The linearly polarized laser is expanded to 12 mm in diameter after passing through a 4× beam expander. After passing through reflectors, the laser beam is delivered into a 2D galvanometer scanner and a focused F-Theta lens (Linos, Germany) with a 100 mm focal length. The beam was incident onto the surface of the plates perpendicularly. The diameter of the focal spot defined by an intensity drop to e−2 of the maximum value was approximately 20 µm. All experimental studies were performed in an atmospheric environment. In this study, different laser scanning intervals and defocus distances were applied to modify the micro−nano structure.
2.2 Characterization and measurements
The 6061-aluminum alloy (Al alloy) plates with a dimension of 50 × 50 × 2 mm3 were used as samples in this work. Before laser irradiation, the plates were cleaned ultrasonically with ethanol to remove the oxide and grease on the surfaces.
The sample surface morphologies were characterized by scanning electron microscopy (Nova nano, FEI, USA). A confocal laser scanning microscopy (VK-1000, Keyence, Japan) was utilized to measure and analyze the three-dimensional microstructure and topography of the sample surfaces. The wavelength dependence of the total reflectance (specular and diffuse reflection) in the visible (vis), and near-infrared (NIR) regions (400−1200 nm) was measured at an 8° angle of incidence using an ultraviolet spectrophotometer (Shimadzu, Japan). For the reflectance measurement, three samples for each parameter are measured, and the measurement would be repeated three times.
3. Results and discussion
3.1 Formation mechanism of the micro-forests fabricated via LICVD
In this experiment, a line-by-line laser scanning path was applied to investigate the formation process of micro-nano structures. The laser scanned in the x direction and the processing would be executed in the Y direction, as shown in Fig. 1(a). To simplify the description, the nearside represents the start side of the processing, while the farside represents the end side. As shown in Fig. 1(b), I and N represented the interval between two adjacent scanning lines and the total number of the scanning lines, respectively. The defocus distance and the interval of two adjacent scanning lines were kept at a constant value of 4.5 mm and 5 µm, respectively. The input power of the laser was fixed at 30 W. The repetition rate of the picosecond laser is 200 kHz. The laser spot follows a pre-set scanning path at a constant speed of 1 m/s. N was modulated from 1 to 50 to reflect the formation process of the micro-nano structures. Figure 2 and Fig. 3 show the SEM and optical images of the surface morphologies as N increases from 1 to 50, respectively. The red arrows in these figures point to the processing direction. When N = 1, as shown in Fig. 2(a), the width of the laser-irradiated area is about 170 µm where periodical nano-ripples were generated, and the surface roughness is enhanced compared with the original surface. Besides, some nanoparticles can be observed in the periphery region of the irradiated surface, as shown in Fig. 2(a). As shown in Fig. 3(a), the periphery of the irradiated region is darker than other regions, which implies a stronger absorption of light. When N increases to 5, the irradiated surface becomes rougher and performed a darker surface color, especially in the periphery region. As shown in Fig. 2(b) and Fig. 3(b), the nano-ripples are covered by numerous nanoparticles on the nearside of the irradiated surface. Moreover, some micro-clusters are generated and arranged irregularly near the periphery region. The number of the micro-cluster gradually decreases as the position from the periphery to the center. In contrast, on the farside of the irradiated surface, the number of the micro-cluster and nanoparticles is less than that on the nearside. As N increases to 10, more micro-cluster with larger sizes are formed in the periphery region of the irradiated surface. The distribution range of these micro-clusters further widened and spread to the center region gradually, as shown in Fig. 2(c). Furthermore, more nanoparticles and micro-clusters are generated on the nearside of the irradiated surface compared with that on the farside, which leads to stronger light absorption, as shown in Fig. 3(c).
Fig. 2. SEM images of the surface structures with different N, the red arrows indicate the processing direction: (a) N = 1, (b) N = 5, (c) N = 10, (d) N = 30, (e) N = 50 nearside, and (f) N = 50 farside.
Fig. 3. Optical images of the surfaces with different N, the red arrows indicate the processing direction: (a) N = 1, (b) N = 5, (c) N = 10, (d) N = 30, (e) N = 50 nearside, and (f) N = 50 farside.
When N increases to 30, the more irradiated surface was covered by the micro-nano structures, as shown in Fig. 2(d). Nanoparticles and micro-clusters can be observed in the periphery region of the nearside. As the position closes to the center region, the size of the micro-clusters grows larger gradually and reforms tree-crown-like micro-structures, and most area of the irradiated surface is covered by these micro-clusters and micro-crowns. In the center region, the tightly-arranged micro-crowns further shelter the surface, forming a range of frost-like micro-structures. These micro-frosts could reduce the reflectance of the surface significantly, as shown in Fig. 3(d). As the position is further closed to the farside region, the size of the micro-structures decreases, and the distribution becomes loose. Besides, as shown in Fig. 3(d), the micro-clusters in the farside region performed a strong reflectance, which indicated that these structures have smoother surfaces compared with other regions.
As N further increased to 50, the micro-clusters and micro-crowns are distributed more widely and densely that a little region of the substrate was bare in the nearside and center region. Besides, the region covered by the micro-forest was widened. Similar to the surface structures at N = 30, the clusters in the farside of the irradiated surface were smaller and looser with stronger reflection, as shown in Fig. 2(f) and Fig. 3(f).
Figure 4 demonstrates the cross profile of the surface for N = 10 and N = 50. When N = 10, the thickness of the removed materials is about 1 µm, Fig. 4(a). As N increases to 50, the removed layer is increased to about 5 µm, as shown in Fig. 4(b). This result indicates that only a thin layer of materials is removed and no deep cracks exist after the processing.
According to the formation procedure of the micro-nano structures, the formation of the surface structures can be mainly attributed to the competitive deposition rate due to the co-action of laser ablation and laser-induced deposition. When N is low, the ablation of the laser dominates, and numerous nano-ripples are formed after being irradiated by the first scanning laser beam, as shown in Fig. 5(b). When the surface is irradiated by the laser beam, the interference of the incident laser light and with the excited surface plasmon polaritons could result in a spatial periodic energy distribution on the surface [6,19,35,36]. The inhomogeneous distribution of the laser energy led to recoil pressure, surface tension variance, and temperature gradients [37]. These effects can lead to a Marangoni-driven flow and capillary waves, resulting in ripples forming on the surface due to resolidification [38,39]. As the laser scanning continued, the ablation effect is strengthened. The rippled surface could enhance the absorption of the subsequent laser beam. Therefore, the surface is further vaporized with more laser power deposition, and part of the materials would be separated from the substrates. However, these separated materials can not be removed totally. The metallic vapor generates due to the laser irradiation could absorb the energy of the subsequence laser pulse, being excited and forming plasma. When the metallic vapor is approaching to or contacting with metal surface, it would cool rapidly and then solidify, resulting in the formation of nanoparticles. Therefore, the nanoparticles remain regular spherical due to the surface tension. Driven by the pressure, some of the nanoparticles are deposited back onto the irradiated surface, especially on the periphery region, covering the nano-ripples uniformly to form the grass-like nanoparticles [40,41], as shown in Fig. 5(c). As N increases, more materials are ablated, producing more nanoparticles. These newly produced nanoparticles would stack on the grass-like nanoparticles, reforming into flower-like nanoparticles, as shown in Fig. 5(d). On the other hand, a part of the grass-like nanoparticles would be ablated by the subsequent laser beam. However, these pre-deposited structures won’t be removed totally and the residue can play a role like seeds, accelerating the adsorption of nanoparticles. The re-ablated residue could block the diffusion of the metallic vapor between the structures, which is conducive to the deposition of nanoparticles produced by the subsequent laser irradiation. Thus, the nanoparticles perform higher local deposition rate on these residue regions compared with the substrates, result in a competitive deposition effect. These directional deposited nanoparticles further stack, reforming into micro-clusters, as shown in Fig. 5(e). With further increase of N, the micro-clusters further adsorb nanoparticles to form micro-crowns due to the competitive deposition effect, as shown in Fig. 5(f). Since the nanoparticles remain regular spherical, the micro-crowns consisting of nanoparticles can perform strong porosity. These porous micro-crowns can efficiently strengthen the surface roughness and enhance the anti-reflection performance. As the size of the micro-crowns increases, the micro-crowns distributes more widely and densely, forming micro-forests on the surface. Thus, like the rainforest which is eager to absorb the solar energy, the surface covered with micro-forests can perform a strong anti-reflection effect.
Fig. 5. SEM images of typical surface structures: (a) flat surface, (b) nano-ripples, (c) grass-like nanoparticles, (d) flower-like nanoparticles, (e) micro-cluster consisting of nanoparticles, and (f) micro-crown consisting of nanoparticles.
The inhomogeneity of the distribution of the surface structures on the irradiated surface can be ascribed to the difference in the relationship between ablation and deposition in different regions, as shown in Table 1. Figure 6 shows the schematic diagram of the microstructure formation processing on different regions of the surface. For the first laser scanning, the nanoparticles would be deposited uniformly on both the nearside and farside periphery of the irradiated region. In the nearside periphery, since the region is behind the processing, these nanoparticles won’t be ablated by the subsequent laser beam. Therefore, in this region, the competitive deposition effect does not occur and the nanoparticles would cover the surface uniformly. In the nearside region, the ablation and deposition would be gradually enhanced synchronously. More nanoparticles would be produced and deposited, reforming into micro-clusters. Unlike the structures that grow in the nearside periphery, these micro-clusters would be irradiated and ablated by the subsequent laser beam, resulting in surface melting and resolidification. Therefore, interference fringes would appear at the top of micro-clusters, while floccule structures appear at the bottom, as shown in Fig. 7(a). After the scanning laser moves ahead, the re-ablated micro-clusters fell behind the laser scanning. The re-ablated micro-clusters could play a role like seeds where the micro-crowns are easier to grow on. The re-deposition of the subsequent nanoparticles makes the micro-clusters grow wider and higher due to the competitive deposition effect, transiting to the micro-crowns. For the center region, both the ablation and deposition are developed to a stable state, which leads to the strongest competitive deposition effect. Therefore, the micro-crowns in the center distribute more densely compared with that in the nearside, as shown in Fig. 3(e).These densely-distributed micro-crowns shelter the substrate and form micro-forests, which enhanced the anti-reflection performance. Therefore, the micro-forests are mainly formed in the center of the irradiated surface.
Table 1. Formation mechanism of different regions
However, in the farside, since the structures grow near the periphery of the irradiated region, the re-deposition would not occur after the micro-clusters are re-ablation. Consequently, the ablation effect was dominant, and the micro-clusters in the farside periphery region are distributed loosely and performed a strong reflection, as shown in Fig. 3(d) and Fig. 3(f).
In summary, in the nearside periphery, since the deposited nanoparticles are behind the laser scanning, the pre-deposited structures won’t be irradiated by the subsequent laser beam. Therefore, nanoparticles are deposited on this region uniformly, forming grass-like and flower-like surface structures. for other regions in the irradiated surface, the pre-deposited structures would be ablated by the subsequent laser scanning. However, these structures won’t be removed totally. The residue structures can block the spread of plasma and accelerate the deposition of nanoparticles on them. Thus, the newly produced nanoparticles perform a higher deposition rate on the residue compared with the substrate, resulting in a competitive deposition effect. Due to this competitive deposition effect, nanoparticles can be deposited directionally, resulting in the formation of micro-clusters and micro-crowns. For the nearside region, the ablation and deposition would be gradually enhanced synchronously, driving the micro-crowns to grow larger and denser. In the center region, the processing is developed to a stable state so that the micro-crowns are fully-grown and distributed densely, which forms the main part of the micro-forests. These hierarchical porous micro-forests can the anti-reflection performance effectively. In the farside, after being ablated, no newly produced nanoparticles can be deposited on the residue due to the end of processing. Therefore, the micro-structures in the farside periphery were distributed loosely and performed a strong reflection. According to the results, for anti-reflection surface fabrication, N is suggested to be larger than 50 to achieve stable and uniform performance. When N is larger than 50, the center region would be widened to expand the distribution area of the micro-forests to cover the whole target surface of the sample.
3.2 Anti-reflective performance of different micro-nano structures
As previously described, the formation of the anti-reflection surface structures is caused by the aggregation of the deposited nanoparticles. The energy density of the laser pulse should be controlled to ensure that the nanoparticles can be produced and deposited back to the surface without damaging the substrate. Therefore, the laser beam needed to be defocused to reduce the energy density. Figure 8 demonstrates the morphology of the irradiated surface with different defocusing distances when the scanning interval is kept constant at 5 µm. When the defocusing distance varies from 4 to 5 mm, all the surfaces are covered by the micro-crowns stacked by the nanoparticles after laser irradiation. However, the characteristic size of the micro-crowns on the surface varies significantly as the defocus distance increases, as shown in Table 2. When the defocusing distance is 4 mm, the characteristic size of the micro-crowns is about 7.26 µm, as shown in Fig. 9(a). As the defocusing distance decreases, the size of the micro-crowns narrows gradually. When the defocusing distance rises to 5 mm, the characteristic size of the micro-crowns narrows to about 4.53 µm, as shown in Fig. 9(e).
Fig. 8. The morphology of the irradiated surface with different defocusing distances. (a) 4.0 mm, (b) 4.2 mm, (c) 4.5 mm, (d) 4.7 mm, and (e) 5.0 mm.
Fig. 9. SEM image of the micro-crowns with different defocusing distances. (a) 4.0 mm and (b) 5.0 mm.
Table 2. Sa, Sz, and characteristic size of the surface with different defocusing distances
The shrinkage in the characteristic size of the micro-crowns can be attributed to the change in the energy density, as shown in Table 2. When the defocusing distance was low, the energy density of the laser pulse is relatively high. Therefore, the ablation effect dominated the interaction between the laser beam and materials, and most of the materials in the affected zone were vaporized and removed directly. As the defocusing distance increased, the energy density of the laser pulse decreased gradually. Thus, the ablation effect is weakened so that the vaporized metal could not be removed totally. Part of these metallic vapors is cooled and solidified, reforming into nanoparticles. Driven by the competitive deposition effect, the nanoparticles can be deposited on the substrate directionally, forming hierarchical micro-forests instead of uniform grass-like nanoparticles layer. Furthermore, the decrease in energy density could lead to a drop in the substrate temperature, which prevented the sintering of the nanoparticles and the formation of large-sized micro-crowns. Therefore, with the decrease in the defocusing distance, the characteristic size of the micro-crowns narrows gradually. However, further increasing the defocusing distance could further weaken the ablation effect so that inadequate nanoparticles can be produced during the procedure. Consequently, the surface is covered only by a layer of nanoparticles instead of micro-structures stacked by the nanoparticles.
Figure 10 demonstrates the morphology of the irradiated surface with different scanning intervals at a constant defocus distance of 4.5 mm. The width of the irradiated region with single scanning lines is about 170 µm. Therefore, the scanning path would not overlap if the interval is larger than 200 µm. In contrast, when the interval is narrower than the width of the irradiated region, the adjacent scanning laser beam would overlap. The overlapping rate can be calculated as $O = 1 - I/D$, where I is the interval and D was the effective width of the scanning line. That is, the narrower the interval is, the higher the overlapping rate is, and the more times the surfaces would be repeatedly irradiated. Besides, if I is larger than D, the overlapping rate O should be regarded as 0 since no overlapping exists.
Fig. 10. The morphology of the irradiated surface with different scanning intervals. (a) 2 µm, (b) 5 µm, (c) 10 µm, (d) 50 µm, (e) 100 µm, and (f) 200 µm.
As shown in Fig. 10(f), the width of the irradiated region is about 170 µm, which was narrower than I = 200 µm. Therefore, the scanning laser beam irradiated the surface singly, and the surface morphology is the same as a result at N = 1, as previously described. When the interval decreases to 100 µm, the overlapping rate O increases to about 40%. The laser beam would irradiate the surface repeatedly. Repeated irradiation can enhance the efficiency of absorption and ablation, producing more nanoparticles during the processing. Therefore, the irradiated surface is covered by the deposited nanoparticles, as shown in Fig. 10(e). When the interval further decreases to 50 µm, O further increases to 70%, so that more nanoparticles are produced, and shelter the substrate. Moreover, some of the deposited nanoparticles can aggregate and form micro-clusters with a size of 5 µm, as shown in Fig. 10(d). As the interval decreases to 10 µm, O grew to 94%, more micro-clusters are formed, covering most areas of the substrate, and the size of the micro-clusters enlarged to about 5 µm. When the interval is narrower than 5 µm, O increased to 97%. Thus, the surface is irradiated by the laser beam over 30 times, and numerous nanoparticles would deposit back on the surface. Consequently, the surface is covered with micro-forests, and hardly the substrate is bare. The enhanced deposition effect could also enlarge the size of the micro-crowns, which increases to about 6 µm. However, furtherly decreasing the interval will lead to an obvious change in the morphology. Since O furtherly increased to about 99%, much more energy is deposited upon the surface via repeatedly laser irradiation. The excessive energy deposition could lead to a further increase in the substrate temperature, resulting in the melting and recrystallization of the micro-nano structures. Therefore, the sintered micro-structures would lose their porosity and form irregular micro-ridge, as shown in Fig. 10(a). Moreover, the nanoparticles produced due to the subsequent laser scanning can still be deposited upon the micro-ridges, enhancing their surface roughness.
Figure 11(a) demonstrates the reflectance spectra of the surface with different scanning intervals. After laser irradiation, the absorption properties of the Al alloy surfaces are all enhanced compared with the original surface. For the original surface, it performs the lowest reflectance at a wavelength of 400 nm. The reflectance increases gradually as the wavelength rises to 1200 nm, except for a little drop at 730 nm. When the scanning interval is 200 µm, its reflectance varies, following a similar trend to that of the original surface, and the average reflectance increases to 41.2%. When the interval increases to 100 µm, the minimum and the average reflectance decrease to 12.2% and 31.3%, respectively. Although the reflectance still grows gradually as the wavelength increases, a little rise occurs at 730 nm instead of a drop. As the interval decreases, the reflectance was restrained. When the interval increases to 10 µm, the minimum and average reflectance decrease to 5.4% and 8.7%, respectively. When the interval increases to 5 µm, the trend of the reflectance changes. The minimal reflectance of 2.81% is achieved at 843.5 nm, and the average reflectance from 400 to 1200 nm is 4.06%. However, when the interval finally reached 2 µm, the reflectance decreases from 10.82% at 400 nm to 7.52% at 850.5 nm and then rises to a maximum value of 12.51% at 1200 nm. During this range, the surface performed an average reflectance of 9.93%.
The changes in the anti-reflection performance can be mainly ascribed to the difference in surface structures. When the interval is wider than 100 µm, the surface structures mainly consist of nano-ripples with a characteristic scale of about 730 nm. The surface can perform a strong light-trapping effect for optical with a wavelength below 730 nm due to multiple reflections. When the optical wavelength is above 730 nm, the surface could reduce reflectance due to the effective graded refractive index layer at the solid/air interface. Therefore, the reflectance of the surface is restrained after laser irradiation. When the interval increases to 50 µm, nanoparticles and micro-crowns on the surface could enhance the light-trapping effect, which decreases the reflectance, especially for optical with wavelength above 730 nm. As the interval decreases to 5 µm, the surface is almost completely covered by the micro-crowns, forming the micro-forests, as shown in Fig. 12(b). Figure 13 demonstrates the mechanism of anti-reflection surface structures. Compared with the common solid structures, the hierarchical porous structures can construct a graded refractive index layer more effectively, enhancing the anti-reflection performance. The crevices between micro-crowns can significantly enhance the anti-reflection and geometry light-trapping effect. Besides, the porosity of the micro-crowns allows the trajection of the photons inside the structures, which can entrap the photons effectively due to the multi-reflection. Thus, the surface performs an optimal anti-reflection property when the interval was 5 µm. However, when the interval further decreases to 2 µm, the excessive ablation and energy deposition sintered the micro-crowns together and destroyed the porosity, which weakened the light-trapping effect, especially when the optical wavelength was below 600 nm.
Fig. 13. Schematically illustrating the anti-reflection of the (a) micro-forest and (b) common micro-nano structures.
Figure 11(b) demonstrates the reflectance spectra of the surface with different defocusing distances. When the defocusing distance is 4 mm, the surface performs a reflectance of 6.0% at 450 nm. The reflectance drops to the minimal value of 5.35% at 837.5 nm, and then it grows to 10.53% as the optical wavelength rises to 1200 nm. When the defocusing distance increases to 4.2 mm, the absorptance varies, following a similar trend to that at 4 mm. The minimum and average reflectance is 2.83% and 3.96%, respectively. As the defocusing distance rises to 4.5 mm, the variation trend of the reflectance is almost the same as that at 4.2 mm from 400 to 850 nm. However, when the wavelength is above 850 nm, the reflectance is weakened slightly. When the defocusing distance reaches 4.7 mm, the surface performed a lower reflectance from 400 to 850 nm compared with that at 4.2 mm. The reflectance drops to the minimal value of 2.81% at about 850 nm and then rises rapidly to the maximal value of 7.02% at 1200 nm. As the defocusing distance finally reaches 5 mm, the anti-reflection effect is further weakened. The surface performs a minimal and an average reflectance of 3.54% and 4.76%, respectively.
Since all the surface is covered by micro-crowns consisting of nanoparticles to form the micro-forest, the difference in reflectance can be mainly ascribed to the scale of the micro-crowns. Table 2 demonstrates the average height Sa and the maximal height Sz of the surface with different defocusing distances. When the defocusing distance is 5 mm, the Sa and Sz of the micro-crowns were relatively small. Therefore, the crevices between micro-crowns are shallow, which weakened their light-trapping effect to some degree. As the defocusing distance decreases, the increased laser fluence intensified the materials ablation and the nanoparticles deposition, resulting in the enlarging of the scale of the micro-crowns and deepening of the crevices. The deepened crevices could enhance the light-trapping effect, which decreases the reflectance of the surface on a macro scale. However, as the defocusing distance drops to 4 mm, although the increased laser fluence could further deepen the crevices, it also reduces the number of crevices. Therefore, the light-trapping effect is limited, and the reflectance increases.
According to the above analysis of the influence of the laser processing strategy on the anti-reflection property, we further optimized the method to achieve a surface with stronger anti-reflection property. The diameter of the defocused laser spot was enlarged from 170 µm to 240µm, while the scanning speed and interval were kept constant at 1 m/s and 5 µm. The surface covered by the optimized micro-forest performed an extreme anti-reflection property, with a minimum reflectance of 1.47%, as shown in Fig. 14(a). The optimized micro-forest is displayed in Fig. 14(b).
Fig. 14. (a) The reflectance spectra of the optimal anti-reflection micro-frost, and (b) the SEM image of the optimal micro-forest.
To verify that this LICVD method can be adapted to film materials, we applied the method to aluminum film with a thickness of 50 µm, as shown in Fig. 15(b). After processing, the aluminum film could be blackened effectively, and the film is still complete without breakage. Common methods to fabricate anti-reflection surfaces are hard to adapt to large-scale curved surfaces. By covering the black aluminum film on the large-scaled or curved shell, the device with anti-reflection can be achieved. Figure 15(a) demonstrates a curved shell covered with black aluminum. These anti-reflection shells can be used to produce visible-light stealth and aerospace radiation heat transfer devices.
4. Conclusions
In this paper, a new design has been proposed to achieve an anti-reflection surface by constructing micro-forests. To realize the micro-forests, we employed a simple laser induced competitive vapor deposition method. The evolution procedure of the surface structures has been systematically investigated to clarify the production and deposition behavior of the nanoparticles. The formation of the micro-forests is mainly determined by the competitive deposition effect under the co-action of laser ablation and laser induced deposition. During the processing, the pre-formed structures can play a role like seeds, resulting in a higher deposition rate compared with the substrate. This competitive deposition effect drives nanoparticles to aggregate directionally, forming hierarchical micro-forests instead of nanoparticles layer. Moreover, excessive ablation can lead to melting and sintering of the micro-structures, while insufficient ablation is not able to produce plenty of nanopar ticles to form micro-forests. Furthermore, these hierarchical porous forest-like structures can improve the anti-reflection and light-trapping effect significantly, which leads to ultra-low reflectance, especially for visible light. The average reflectance was only 2.5% in the visible light over a range of 400-800 nm, and the lowest reflectance of 1.47% can be achieved at 850 nm. By applying this method, black aluminum films with a thickness of 50 µm were achieved, which can be used for large-scale curved shells. This method for the fabrication of anti-reflection surfaces is simple and efficient, which has great application potential in many fields such as visible-light stealth, precision optical sensors and detectors, optoelectronic devices, and aerospace radiation heat transfer device.
Funding
National Natural Science Foundation of China (51675205, 52175405).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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