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Abstract
The high-quality imaging and easy cleaning property of microlens array (MLA) are two very important factors for its outdoor work. Herein, a superhydrophobic and easy-to-clean full-packing nanopatterned MLA with high-quality imaging is prepared by thermal reflow together with sputter deposition. Scanning electronic microscopy (SEM) images demonstrate that the sputter deposition method can improve 84% packing density of MLA prepared by thermal reflow to 100% and add nanopattern on the surface of microlens. The prepared full-packing nanopatterned MLA (npMLA) possess clear imaging with a significant increase of signal-to-noise ratio and higher transparency compared with the MLA prepared by thermal reflow. Besides for excellent optical properties, the full-packing surface displays a superhydrophobic property with a contact angle of 151.3°. Further, the full-packing contaminated by chalk dust become easier to be cleaned by nitrogen blowing and deionized water. As a result, the prepared full-packing is considered to be potential for various applications in the outdoor.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Compound eyes of some arthropods are composed of full-packing microlens arrays (MLA) with nanostructures on their surfaces. The specific structure not only possesses excellent optical performance, but also imparts superhydrophobic and easy-to-clean capability of the compound eye for normal visual capability in high humidity and dusty environments [1]. Inspired by natural compound eyes, researchers have actively developed their fabrication processes in the past decade, such as thermal reflow [2–4], laser ablation wet or dry etching [5–7], laser direct writing [8–10], imprinting [11,12], soft lithography [13], inkjet printing [14,15], electrodynamic deformation [16,17], and so on [18]. These methods can effectively control the geometric profile and/or packing density of MLA. Although these MLA preparation technologies have made great progress, the prepared MLAs are still difficult to meet a wide range of application requirements. For example, in outdoor environment, MLAs with hydrophobic and easy-to-clean properties can effectively avoid the contamination of water droplets and dust and prevent from deterioration of optical properties [19]. So, a hydrophobic and easy-to-clean MLA with high-quality imaging property is acquired to be developed.
Recently, the group of Wang et al. has realized superhydrophobic MLAs with a high-quality imaging, which is comprised of smooth-surface microlenses and dewetting hairs on the top of micropillars around the microlens [20,21]. The micropillars with dewetting hairs is much higher than microlens and thus isolate water droplet from microlens [22], which might make it difficult for water droplets to carry away dust particles on the surface of the MLA when rolling on the surface. Some researchers have directly added nanopatttern on the surface of MLA [23,24]. both microlens and nanopattern on its surface effectively prevent the water droplets from spreading and significantly improve the contact angle of water droplets [25,26]. However, the above superhydrophobic MLAs with a low packing density lead to a low signal-to-noise (SN) ratio of imaging and lack the investigation of easy-to-clean capability and further restrict their outdoor work [11,27]. On other hand, some superhydrophobic MLAs containing optical gain materials possess a relatively high light absorption and significantly reduce the transmittance of MLA although their reflections are diminished obviously [28,29].
In this work, a full-packing nanopatterned MLAs (npMLAs) were fabricated by successively employing photolithography, thermal reflow, sputter deposition and duplication. By sputter deposition, the packing density of close-packing smooth-surface MLA (smMLA) prepared by thermal reflow was improved to 100% and nanopattern was fabricated on its surface. High-quality imaging with a sharply increase of SN ratio of microlens was thus realized. By adding packing density and introducing nanopattern on the surface of close-packing smMLA, the as-prepared full-packing npMLA was demonstrated that the SN ratio of imaging was sharply improved without effecting imaging definition. Not only that, the surface hydrophobic and easy-to-clean properties of full-packing npMLA were significantly improved with a small increase of light transmittance compared with the close-packing smMLA. The full-packing npMLA surface became much easier to be clean in a nitrogen flow. And dusts were removed when water droplet rolling on the surface of full-packing npMLA. These findings are of great significance for many outdoor applications of MLA.
2. Experimental section
Figure 1(a) shows the preparation procedure of full-packing npMLA. The brief description is as follows. First, a micropillar array was prepared using the maskless lithography technique (Fig. 1(a1)) [30]. Then the micropillar array was placed on a baking table at 130°C for 1 min to melt into a micro-droplet array (Fig. 1(a2)). Under the action of surface tension, the microdroplet presented a spherical crown shape and a smooth MLA (smMLA) was obtained after cooled at room temperature. Subsequently, a layer of Al nanoparticle film was deposited on the surface of smMLA using sputter deposition to improve the packing density to 100% (Fig. 1(a3)) and obtain a convex full-packing rough MLA (npMLA). Then the prepared full-packing npMLA with Al-nanoparticle layer was transferred to obtain a high-transparency convex full-packing npMLA. Polydimethylsiloxane (PDMS) and its crosslink agent with a mass ratio of 10:1 were poured onto the surface of the full-packing npMLA together with a cured process (Fig. 1(a4)). After peeled off from the full-packing npMLA with Al-nanoparticle layer, the PDMS concave npMLA was duplicated using epoxy resin and its crosslink agent with mass ratio of 3:1 (Fig. 1(a5)). After cured at 80°C for 4 h, an epoxy resin full-packing npMLA was obtained by directly peeling off the PDMS concave npMLA (Fig. 1(a6)). Finally, the epoxy resin full-packing npMLA was soaked in sodium hydroxide solution with a mass ratio of 2% to remove residual Al particle film. Figure 1(b) shows SEM image of the epoxy resin full-packing npMLA with high transparency.
3. Result and discussion
3.1 Morphology of full-packing npMLA
As shown in Fig. 2(a), a smMLA is prepared using thermal reflow technique, and its width, height and gap are 19.2, 9.6 and 0.8 µm. The packaging density and height of smMLA are achieved by controlling the height and gap of micropillars prepared by the maskless lithography technique, which are described in detail in Ref. 2. The packing density of smMLA is calculated to be about 84%. A layer of Al nanoparticle film is deposited on the surface of the smMLA by high-vacuum magnetron sputtering device (Fuzhou yingfeixun Photoelectric Tech Co., Ltd, China). Figure 2(b)-(e) clearly shows that the deposition time (t) gradually increases, and the packaging density of the MLA gradually increases to 100%. Beside for the increase of packing density, the surface of microlenses shows nanopyramid pattern. The nanopyramid size is about 100 nm. This could be explained by the following rationale. As shown in Fig. 2(f), during the deposition process, aluminum nanoparticles collide with microlens inelastically. Some of the Al nanoparticles adhere to the surface of the microlens, and the other part splash on the surface of the adjacent microlens and are adhered by the adjacent microlens, which lead to an increase in the packing density. Further, the packing densities of MLA under different deposition time are calculated, as shown in Fig. 2(g). The packing density of npMLA at t = 0 h is only 84.3% while the packing density reaches to 100% at t > 18 h. The results effectively confirm that film deposition method can convert close-packing MLA to full-packing one.
Fig. 2. (a) SEM images of MLA at t = (a) 0, (b) 3, (c) 6, (d) 9, 12 and (e) 18 h. (f) Schematic of the dynamic Al nanoparticle deposition process and (g) packing density of MLA function as t. Inset image in (a) is the section of microlens. Inset image in (e) is amplified SEM image of naopattern on microlens surface.
In addition, the geometric morphology of prepared full-packing npMLA is analyzed using a LEXT OLS4100 three-dimensional (3D) confocal microscopy (Olympus Co., Japan). Figure 3(a) shows the hexagonal distribution of full-packing npMLA. And the geometric morphology of full-packing npMLA marked by the red dotted line is analyzed and plotted in Fig. 3(b) and (c). The green curve clearly presents the geometric morphology of the main part of the npMLA, but the edge morphologies of microlenses are not completely presented. This is because the reflected light on the edge of microlens cannot enter the 3D confocal microscope system, causing abnormal imaging of microlense edge, as shown in Fig. 3(d). The loss of reflected light signal is mainly ascribed to the steep edge of microlens, which reflects incident light to other directions. Further, the measured geometric morphologies are fitted by quadratic polynomial and the fitted geometric morphologies are plotted using curved black dotted lines, meaning the prepared nanopyramid is distributed on the paraboloid surface. Radius of the vertex of these fitted parabolic lines are calculated to be 12.56, 12.63, 12.58, 12.71 µm in the x direction and 12.56, 12.63, 12.52 µm in the y direction. And the height is increased to 5.5µm. The variation changes in width and height are mainly due to the fact that nanoparticles are easy to be deposited at low positions. Consequently, the thickness of microlens top is smaller than that of the lower position of microlens and the Al nanoparticle deposition leads to an increase in width and decrease in height and is an effective method to improve packing density of MLA.
Fig. 3. (a) 3D-confocal image. (b) and (c) Morphology of full-packing npMLA marked by the red dotted line in the x and y direction of (a). (d) Confocal microscopy image of full-packing npMLA.
3.2 Optical performance of full-packing npMLA
The experimental setup for investigating the imaging performance of the full-packing epoxy resin npMLA is shown in the Fig. 4(a). A brief description is as follows. A mask of transparent letter ‘a’ is placed in front of a white-light light-emitting diode (LED) with controllable lumination intensity (L). The light emitted by the LED goes through the transparent letters ‘a’, reaches MLA, and forms reduced letter ‘a’ images on the image plane of the MLA. The letter ‘a’ images are captured by a CCD camera with a 50× magnification objective with a numerical aperture of 0.8. As shown in Fig. 4(b) and (c), under three illumination of L = 400, 800 and 1200 Lx, the image plane of full-packing npMLA presents clear letter ‘a’ on without any noise. By contrast, the smMLA image plane only displays clear letter ‘a’ at L = 200 Lx and presents blurry letter ‘a’ near clear letter ‘a’ at L = 800 and 1200 Lx. This is because noise ‘a’ disappears in the CCD camera at a low light intensity below its detection threshold and is captured by the CCD camera with a high light intensity beyond its detection threshold. To find out the origin of the noise, images formed by three MLAs with t = 0, 6 and 18 h under the condition of L = 1200 Lx is shown in Fig. 4(d). The image plane of the npMLAs at t = 6 h still displays blurry letter ‘a’ near clear letter ‘a’, however, the blurry letter ‘a’ becomes less noticeable compared to the smMLA at t = 0 h. These results confirmed that improving the packing density of MLA can effectively suppress the noise of imaging and thus enhance imaging property and the nanostructure on the microlens surface has no direct inhibitory effect on the noise. Figure 5(a) shows the experimental setup for analyzing focusing performance of npMLA. The mask with high-transparency letter ‘a’ is removed for light to pass through full-packing npMLA directly. The focusing spots on the focusing plane were capture by the CCD camera. Figure 5(b) and (d) shows the focusing spot images under three luminance of L = 400, 800 and 1200 Lx. The focusing spot images on the focusing plane of full-packing npMLA have no any noise at any luminance. By contrast, the focusing plane of close-packing smMLA displays a negligible noise at L = 400 and 800 Lx and however a very obvious noise at L = 1200Lx. Figure 5(b) and (e) shows that the intensity distribution of focusing spots in Fig. 5(b) and (d). It can be found that the focusing plane of both the MLAs present uniform intensity distribution, verifying that both the MLAs have high uniformity and the proposed prepared method is effective for high-property MLA.
Fig. 4. (a) Experimental setup of imaging for the letter ‘a’. (b) and (c) Photos of letter ‘a’ formed by full-packing npMLA and close-packing smMLA at different lumination intensity (L) of 400, 800 and 1200 Lx. (d) Photos of letter ‘a’ formed by close-packing smMLA at different deposition time.
Fig. 5. (a) Experimental setup of analyzing focusing performance. Energy distribution of focal plane at different lumination intensity of 400, 800 and 1200 Lx: (b) and (c) Full-packing npMLA, (d) and (e) close-packing smMLA.
Figure 6(a) and (b) shows the intensity distribution of focusing plane of both close-packing smMLA and full-packing npMLA. Apparently, there are many low-energy bright spots near focusing spots with a high energy. By contrast, in the focal plane of full-packing npMLA, these low-energy spots are completely lost, meaning that the SN ratio is significantly improved. Further, the normalized intensity distribution marked by white dashed line in Fig. 6(a) and (b) is extracted, as shown in Fig. 6(c) and (d). The intensity of low-energy spot was calculated to be 0.056 for the close-packing smMLA, 0.0042 for the full-packing npMLA. Herein, the SN ratio is defined as the intensity ratio of low-energy spot and focusing spot. It can be seen the SN ratio of full-packing npMLA is 13.33 times higher than that of close-packing smMLA. The improvement of SN ratio is mainly attributable to the variation of packing density. A MLA with a low packing density can not block light from the gap between microlenses while there are no gaps for a MLA with 100% packing density and thus the noise is completely lost. Moreover, the shapes of focusing spot formed by both the MLA are investigated. As shown in Fig. 6(e) and (f), both the focusing spots enclosed by the white dashed circle in Fig. 6(a) and (b) have symmetrical shapes in x and y direction, meaning that these microlenses have good symmetrical characteristic. It should be noted that both the focusing spots show different sizes. This is due to the deposition which decreases the height-to-width ratio of microlens and results in the decrease of the numerical aperture of microlens. Furthermore, the numerical aperture and focal distance of microlens can be calculated by the following equation:
where H, D and R are height, width and radius of microlens, respectively. n is refractive index of epoxy resin measured to be 1.58. The radius of microlens is equal toThe numerical apertures of smMLA and npMLA are calculated to be 0.58 and 0.48. And the focal lengths of smMLA and npMLA are 17.27 and 21.72 µm. Obviously, the position of object and image remains unchanged, and the image with short focal length is smaller. Therefore, the smMLA has a smaller imaging ‘a’ and focal spot.
Fig. 6. Focusing spots of (a) full-packing npMLA and (b) close-packing smMLA under the luminance of 800Lx. (c) and (d) Intensity distribution of focusing spot obtained along dotted line in Fig. 6(a) and (b), respectively. (e) and (f) Intensity of focusing spot enclosed by the white dashed circle in Fig. 6(a) and (b), respectively.
Following the above analysis, the focusing spot area of full-packing npMLA is larger than that of close-packing smMLA while the energy density of the focusing spot of full-packing npMLA is smaller than that of close-packing smMLA. So, the energy (E) was introduced to evaluate the focusing capability of microlens:
σ(r, θ) is the energy density, r is the radius of focusing spot and θ is the polar angle. Due to the circular symmetry of focusing spot, Eq. (4) is converted as:The gray value g(r) of focusing spot photo is linear relation to the energy density, so the total energy can be expressed as:
k is the scaling factor of gray value to energy density. The ratio of total energy of the focusing spots of full-packing npMLA and close-packing smMLA is: where Er and Es are the total energy of the focusing spot of full-packing npMLA and close-packing smMLA, respectively. In the calculation, the energies of 50 focusing spots were calculated using Eq. (6) and averaged even though the differences in the energies of these focusing spots are small. Notably, the energy of the incident light should be suitable for obtaining focusing spots with appropriate energy: a high energy of focusing spot will cause the exposure of the CCD camera to be saturated, so that the gray values of the focusing spots are all equal in the center; a low energy of focusing spot may be close to the sensitivity threshold of the CCD camera, making it difficult for the gray value of the focusing spot to accurately measure the energy distribution of focusing spot. Figure 7(a) shows the ratio of total energy between the focusing spots of full-packing npMLA and close-packing smMLA under different luminance. It can be seen that the intensity of focusing spot of npMLA is 1.27 times higher than that of smMLA.Fig. 7. (a) Ratio of total energy of the focusing spots of the full-packing npMLA and the close-packing smMLA under different luminance. (b) Light ray tracing of hemispherical microlens. Energy distribution of the focusing spots of both (c) full packing and (d) close-packing smMLAs.
To confirm the credibility of high focusing spot energy of npMLA, the optical properties of microlens are simulated using light-ray tracing method. Figure 7(b) shows a parallel beam incident from the bottom to the curved surface of the microlens is refracted. It should be noted that the microlens is a hemisphere and its refractive index and radius (R) are 1.58 and 9.6 µm. It can be seen that light rays close to the optical axis are refracted and converged, while the light rays far away from the optical axis cannot pass through the microlens due to total reflection. This illustrates that the actual aperture diameter of the microlens is closely related to total reflection angle (α) and equal to 2Rsinα. So the aperture diameter of the microlens is only 12.15 µm although its size is equal to 19.2 µm. For a npMLA, the light-ray tracing method is not suitable for nanopattern smaller than wavelength. In this regard, a full-packing smMLA is simulated, in which all structural parameters of geometric morphology are the same as the prepared npMLA. The radius of full-packing smMLA and its aperture diameter are 11.77 and 14.90 µm, respectively. Further, the focusing properties of both close-packing and full-packing smMLA are analyzed using the light-ray tracing method. In the simulation, due to aperture difference, the number of light rays irradiated on the surface of full-packing smMLA is much larger than that irradiated on the surface of close-packing smMLA and thus the ratio of incident-ray energy irradiated on the surface of full-packing and close-packing smMLA is equal to Sf/Sc, Sf and Sc are the aperture size of full-packing and close-packing smMLA. Figure 7(c) and (d) shows focusing spots of both full-packing npMLA and close-packing smMLA. Both the focusing spots display different sizes and energy distribution, which are well accord with experimental results. The simulated results show the energy densities of focusing spot center of full-packing and close-packing smMLA are 1.40 and 2.19 W/m2 and η is calculated as 1.24, which confirm that a full-packing npMLA without nanopattern can improve significantly focal intensity.
The antireflective property of full-packing npMLA was investigated in visible light range by using a spectrophotometer (Lambda 950, Perkin Elmer INC). As shown in Fig. 8, the npMLA surface has the lowest Fresnel reflection than epoxy resin surface with and without smMLA. Further, the average reflectivity and transmittance are calculated using the following equation:
Fig. 8. Reflected and transmitted spectra of epoxy resin with a planar, clos-packing smMLA and full-packing npMLA in the visible light band.
3.3 Hydrophobic and easy-to-clean property of full-packing
The hydrophobic property of full-packing epoxy resin npMLA was verified. A 2 µL deionized water droplet injected by the mechanical pump falled onto the surface of planar surface without any microstructure, close-packing smMLA and full-packing npMLA, respectively. And the geometric morphologies of droplet were photographed using a camera with a magnifying lens. As shown in Fig. 9, the contact angle of droplet on a planar surface is only 89.5°. By contract, on the surface of close-packing smMLA and full-packing npMLA, the contact angles display 134.8° and 151.3°. For the close-packing smMLA and full-packing npMLA, the contact angle significantly increases, meaning that a noticeable improvement in hydrophobic property compared with planar surface. This can be explained by Cassie−Baxter equation:
where θc is the Cassie−Baxter contact angle, θ is the contact angle for an ideal surface, and f is the proportion of the air−liquid interface. For the case of the close-packing smMLA and full-packing npMLA, the introduction of the microlens can generate a significantly increase in f as comparison to the planar surface without any microstructure. It is noteworthy that the contact angle on the surface of full-packing npMLA has a significant increase compared with the close-packing smMLA. This is because nanopattern on the surface of microlens can further improve f and effectively prevent water droplet from spreading around.Fig. 9. Geometric morphologies of 2 µL deionized-water droplets on (a) planar surface without any microstructure, surface of (b) close-packing smMLA and (c) full-packing npMLA.
The easy-to-clean property of full-packing npMLA was investigated. Figure 10(a)-(c) shows optical microscope photographs of clean surface of planar epoxy resin without any microstructure, close-packing smMLA and full-packing npMLA. These surfaces were smudged by a lot of chalk dust and then cleaned in a nitrogen flow with a rate of 60 dm3/min to remove the chalk dust, as shown in Fig. 10(d)-(f). Apparently, the close-packing smMLA and the full-packing npMLA display a cleaner surface as comparison to the planar epoxy resin, which implies the addition of microstructure can reduce adhesion of chalk dust, which makes dust easy to be removed by the nitrogen flow. Noticeably, compared with the surface of close-packing smMLA, the full-packing npMLA surface displays a slightly cleaner surface. This might be due to the nanopattern on the microlens surface which reduces the contact area between chalk dust and microlens and thus improves the easy-to-clean property. Further, these contaminable samples were further cleaned by rolling deionized-water drops on the surfaces. As shown in Fig. 10(g)-(i), the planar surface without any microstructures still has some dust particles while dust particles are completely removed on the surfaces of both the close-packing smMLA and the full-packing npMLA. These results verify that the full-packing npMLA surface possesses a good easy-to-clean property.
Fig. 10. Microscopy images of (a)-(c) clean surfaces and (d)-(i) surfaces contaminated by chalk dust and then cleaned by nitrogen wind bath and deionized water.
4. Conclusion
In summary, we have proposed an effective method for improving packing density of MLA prepared by thermal reflow technology together with sputtering deposition. The sputtering deposition method improved packing density of MLA from 84% to 100%, added nanostructure on the surface of microlenses with a decreased height-to-width ratio. In the demonstration, the prepared full-packing npMLA possesses a high-quality imaging property with a high SN ratio and high transparency. Due to the decrease of height-to-width ratio of MLA, the size of focusing spot is increased with an insignificant increase of total energy. Besides for the above excellent optical properties, the nanopattern on the surface of microlenses can further improve hydrophobic and easy-to-clean property as comparison to the close-packing smMLA. As a result, we believe the prepared full-packing is potential application for outdoor high-quality imaging.
Funding
National Natural Science Foundation of China (61805179, 61905180).
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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