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Abstract
Lithography-free metal-dielectric-metal (M/D/M) configuration has attracted tremendous interest for vivid reflective color generation, especially the M/anodic aluminum oxide (AAO)/Al configuration due to its simple, cost-effective and flexible preparation processes. However, the physical mechanism of structural color generation in M/AAO/Al configuration has not been sufficiently discussed. Here, the Al/AAO/Al configurations with distinct vivid reflective colors were prepared. To reveal the color generation mechanism, the bumpy nanopore Al island film (BAlIpore) was proposed as the top meal layer. The optical properties of BAlIpore and BAlIpore/AAO/Al configurations are investigated by both experimental and finite-difference time-domain (FDTD) method. The results indicate that BAlIpore/AAO/Al configuration can generate vivid reflective colors due to the large enhancement and widening of the absorptive band in the reflective spectrum, which is the result of the strong coupling between the broadband local surface plasmon resonance (LSPR) and destructive interference (thin-film interference) supported by BAlIpore and BAlIpore/AAO/Al, respectively. Furthermore, the optical properties of the Al/AAO/Al configuration with a planar nanopore Al film (PAlpore) as the top metal layer are investigated by FDTD method and the results indicate that the PAlpore/AAO/Al configuration can’t result in vivid reflective colors because of the limit enhancement and widening of the absorptive band in the reflective spectrum, which resulted in the weak coupling between weak broadband non-resonance scattering and F-P resonance supported by the PAlpore and PAlpore/AAO/Al, respectively. The results contribute to our understanding of the vivid reflective color generation mechanism in the M/AAO/Al configuration and can be used to develop a reflective filter, color display, etc.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The structural colors arising from resonant interactions between light and specific structures, such as surface plasmon resonances (SPR) [1,2], Mie resonances [3,4] and Fabry-Perot (F-P) resonance [5,6], have received increasing interest recently owing to their potential applications in various areas including decoration, display/imaging, color printing, sensing, and so forth [7–11]. As compared with traditional colorant-based pigmentations, structural colors offer the advantage of high resolution, good sustainability, environmental friendliness, and monolithic integrated fabrication [12,13].
Reflective structural colors are particularly interesting in decorations and displays since they provide lower energy consumption and comfortable viewing under ambient light [14,15]. Various schemes, particularly plasmonic meta-surfaces or metal-dielectric-metal (M/D/M) configuration [16–21], have been proposed for achieving reflective colors. However, the fabrication of meta-surface generally require complicated procedures, such as electron beam lithography (EBL) and reactive ion etching (RIE), which are time-consuming and have high cost, and greatly limit their potential applications in large-area circumstances. The conventional MDM configuration (asymmetric reflective F-P cavity), which consists of a top ultrathin lossy metal layer, a middle dielectric layer and a bottom reflective metallic layer, can generate wide-range reflective colors by varying the dielectric layer thickness and can be fabricated on a large area through a lithographic-free manufacturing [22–27]. But there are at least two major drawbacks for conventional MDM configuration to produce vivid reflective color. First, the available structure parameters for regulation are limited because it can achieve distinct colors only by varying the thickness of dielectric layer. Second, multiple depositing steps are needed for the formation of different cavity thicknesses to simultaneously generate full colors on a single platform, which hinders their usage in practical applications.
To address the issues mentioned above, the modified MDM configuration based on the porous anodic aluminum oxide (AAO) membrane supported on an optical thickness Al substrate (AAO/Al template) was proposed. Porous AAO membrane is one of the cost-effective nanopore self-assembled material that consists of a hexagonal array of cells with uniform and parallel straight cylindrical nanopores perpendicular to membrane surface and its surface is of bumpy nanopore morphology (nanopores surrounded by small Al2O3 protrusions, see Fig. 1(a)), and its geometry parameters, such as thickness, pore diameter and period, can be easily tuned by controlling the anodic anodization conditions [28–30]. In addition, porous AAO structure can large tolerate the thickness of top metal film for retaining high color saturation, which was confirmed by Yue et al. [31]. By simply depositing a thin metal layer on the surface of AAO/Al template with various AAO geometries, the M/AAO/Al configuration can be constructed and the vivid distinctive reflective colors can be achieved on a single platform [32–34]. Although various M/AAO/Al structures have been investigated for achieving reflective colors [31–40], the exact underlying physical mechanism of the vivid reflective color generation was not sufficiently discussed. Choi et al. used a planar nanopore Au film as a top metal layer and proposed the coupled effect between surface plasmons and interference to explain the reflective color generation mechanism, however the plasmonic properties of the planar nanopore Au film were not investigated and the simulated reflective spectra were not matched well with the measured one [39]. Li et al. used a planar nanopore Al film (PAlpore) as a top metal layer and attributed the enhancement and widening of absorptive band in reflective spectrum to the plasmonic effect of nanopore, but the types of plasmonic resonances of PAlpore were not discussed and the simulated reflective spectra of the PAlpore/AAO/Al configuration did not match well with the measured one [40].
Fig. 1. SEM images and the measured reflective spectra of Al/AAO/Al configuration with top Al thickness 0 nm to 50 nm in a step 10 nm: (a)-(f) SEM images, (g) measured reflective spectra.
In this paper, Al/AAO/Al configuration is prepared by depositing a thin Al on the AAO/Al template and the vivid distinctive reflective colors are obtained by controlling the thickness of AAO membrane. To reveal the reflective colors generation mechanism, in addition to the PAlpore mentioned above, the bumpy nanopore Al island film (BAlIpore) is proposed as plasmonic layer, and the optical properties of the both plasmonic layers on AAO as well as the corresponding Al/AAO/Al configuration are investigated by finite-difference time-domain (FDTD) method. The investigated results show that PAlpore/AAO/Al configuration can’t achieve vivid reflective color due to the weak coupling between the broadband non-resonance scattering and F-P resonance in visible region. However, for the BAlIpore/AAO/Al configuration, the strong coupling between the broadband local surface plasmon resonance (LSPR) and the destructive interference result in the generation of vivid reflective colors. The results contribute to our understanding of vivid reflective colors generation mechanism and can be used to develop a reflective filter, color display, etc.
2. Experiment and methods
2.1 Fabrication of the AAO/Al template and Al/AAO/Al configuration
Self-ordered AAO/Al templates with various AAO membrane thicknesses were fabricated through a two-step anodization process in 0.3 M oxalic acid solution under constant voltage of 40 V at 3 °C for different duration. The Al/AAO/Al configuration with distinct vivid reflective colors were prepared by electron beam evaporating appropriate thickness Al onto the AAO/Al template with different AAO membrane thicknesses at the rate of $0.1\; \textrm{nm/s}$ in a vacuum of approximately $3{\times}10^{- 3}\;\textrm{Pa}$. The temperature of AAO/Al template during evaporation was kept at room temperature (300 K).
2.2 Characterization of the AAO/Al template and Al/AAO/Al configuration
The surface morphology of AAO/Al template and Al/AAO/Al configurations were obtained from the field-emission scanning electron microscope (SEM: JEOL JSM-6700F). The optical reflective spectra were measured by a fiber optic spectrometer (QE6500) with a 50 mm reflectance-integrating sphere (SPLISP-50) coupled to fiber optics and a deuterium/halogen non-polarized light source (DH-2000-BAL) under 5° incident angle.
2.3 Simulation method
The three-dimensional finite-difference time-domain (FDTD) method, commercial software Lumerical FDTD Solutions, was used to investigate the optical characteristics of top Al layer on AAO membrane and the corresponding Al/AAO/Al configuration. The dielectric function of Al and Al2O3 used for simulation were taken from Palik’s data. The normalized plane wave with broadband frequency, which is bloch/periodic plane wave type, is incident vertically to the surface of the simulated structure and the polarization is parallel to its surface. The periodic boundary conditions (PBCs) and perfectly matched layer (PML) boundary condition were adopted in horizontal and vertical direction, respectively. The mesh grid size is 1 nm in simulation volume. The convergence testing was done and the result indicated that simulations were computationally feasible.
3. Results and discussion
3.1 Morphology and optical properties of the Al/AAO/Al configuration
The top view SEM images of Al/AAO/Al configuration formed by depositing various Al thicknesses (0 nm to 50 nm in a 10 nm step) onto the AAO/Al template with 370 nm thickness of AAO are shown in Fig. 1 and the insets are the corresponding cross-sectional images. The SEM image of AAO/Al template shown in Fig. 1(a) indicates that AAO membrane consists of a hexagonal array of cells with uniform and parallel straight cylindrical nanopores perpendicular to membrane surface and its surface is of bumpy nanopore morphology, i.e. nanopores surrounded by small Al2O3 protrusions. The diameter and interval of nanopores are about 45 and 100 nm, respectively. Figure 1(b) and (c) indicate that as the deposited Al thickness is no more than 20 nm, the BAlIpore which consists of Al nanoparticles with various sizes and shapes, is formed. Figure 1(d-f) indicate that as the deposited Al thickness is larger than 20 nm, the bumpy nanopores continuous Al film of coarse surface is formed and its thickness increases and nanopore size decreases with the increase of deposited Al thickness. The measured reflective spectra of as-prepared Al/AAO/Al configuration shown in Fig. 1(g) indicate that as the deposited Al thickness is in the range of 10-20 nm, the reflective spectra represent a maximal difference between peak and valley, resulting in the vivid reflective colors, which means that as the top metal layer is of the bumpy nanopore Al island film morphology, the Al/AAO/Al configuration can generate the vivid reflective structural colors.
Fig. 2. (a) Measured reflective spectra and the optical photographs of Al/AAO/Al configuration with AAO thickness 260, 300 and 370 nm, respectively. The thickness of deposited Al film is 15 nm. (b) Corresponding chromaticity coordinates in the CIE 1931 chromaticity diagram in response to the measured spectra on the left.
The reflective spectra and the corresponding photographs of Al/AAO/Al configuration formed by depositing 15 nm thickness Al onto the surface of AAO/Al template with AAO thickness of 260, 300 and 370 nm, respectively, are shown in Fig. 2(a), which indicate that the vivid blue, green and magenta reflective colors are obtained. To estimate the color performance, the chromaticity coordinates that correspond to the reflective spectra on the left are calculated and mapped into the standard International Commission on Illumination (CIE) 1931 chromaticity diagram, as depicted by the black stars symbols in Fig. 2(b), which indicated that the appropriately enhanced color saturation was acquired for the three configurations.
3.2 Optical properties of the planar nanopore Al film on AAO and the corresponding Al/AAO/Al configuration
To reveal the reflective colors generation mechanism of Al/AAO/Al structures, the PAlpore had been used as top metal layer [40], but its optical property was not investigated. Here, the optical property of the PAlpore on thick AAO membrane (AAOthick) and the PAlpore/AAO/Al configuration were investigated by FDTD method. The schematic diagrams of the PAlpore/AAOthick and the PAlpore/AAO/Al configuration are shown in Fig. 3(a) and (b), respectively. The thickness of Al film is 15 nm. The period and diameter of pore are 100 nm and 45 nm, respectively. The red dashed box shows the simulation region that contains one cell in the center and four a quarter cells around it. The plane wave is incident vertically to the surface of the simulated structure and the polarization is parallel to its surface.
Fig. 3. (a) 3D structure diagrams of PAlpore/AAOthick; (b) 3D structure diagrams of PAlpore/AAO/Al configuration.
The simulated transmissive and absorptive spectra of PAlpore/AAOthick are shown in Fig. 4, and for comparison, that of the planar Al film (PAl) on AAOthick are also shown. Compared with the spectra of PAl/AAOthick, in addition to the slight increase in transmittance and absorptance in visible light region, the narrow transmissive valley (suppressed transmission) and absorptive peak in the ultraviolet light region are observed. And as indicated in the inset, the PAlpore/AAOthick presents broadband absorption in visible region with an absorptance larger than 15%.
Fig. 4. Simulated transmissive and absorptive spectra of PAl/AAOthick and PAlpore/AAOthick. The inset is the amplificatory absorptive spectra of PAlpore/AAOthick in visible region.
The simulated localized electric field intensity distribution of the PAlpore/AAOthick in the X-Z plane at center of the nanopore at specific wavelength are shown in the top panel in Fig. 5, and for comparison, that of the PAl/AAOthick are also shown in the bottom panel. One can see that in ultraviolet region, at wavelength of 242 nm, the electric field is confined at the interfaces of Al film. In addition, the angle-dependent transmissive optical properties as well as the dispersion relation of PAlpore/AAOthick shown in Fig. S2 indicate that the transmissive valley at 242 nm is related to the resonant excitation of the short-range surface plasmon polariton (SR SPP). At wavelength of 270, 286 and 350 nm, the electric field is mainly confined in the edges of pore and gradually decrease with the increase of wavelength, which is attributed to the resonant excitation of the localized surface plasmon resonance (LSPR) in nanopore. At peak wavelength of absorption (252 nm), the maximal electric field enhancement appears at the interfaces of Al film and the edges of pore, which attributes to the resonant excitation of both SR SPP and the LSPR in nanopore. In the visible region, compared with the electric field distribution of PAl/AAOthick, the very weak electric field enhancement around the nanopore are observed and slightly decrease with the increase of wavelength, which are related to the non-resonance scattering of nanopore and results in the slight increase in transmittance and absorptance in spectra. In conclusion, the PAlpore can only support the narrowband SPR in ultraviolet region and weak broadband non-resonance scattering in visible region.
Fig. 5. Localized electric field intensity distribution (X-Z plane at center of the nanopore) of PAlpore/AAOthick and PAl/AAOthick at different wavelength.
The simulated reflective spectrum of the PAlpore/AAO/Al configuration is shown in Fig. 6, and for comparison, that of the PAl/AAO/Al configuration is also shown in Fig. 6. The thickness of the Al film and AAO membranes are 15 and 370 nm, respectively. The reflective spectra of two configurations are of the similar profile, but the depth and bandwidth of reflective valleys (i.e. the intensity and width of absorptive peaks) of the PAlpore/AAO/Al configuration (olive solid line) are much larger than that of the PAl/AAO/Al (magenta short dashed line) and gradually decrease and widening, respectively, with the increase of wavelength, which means that the enhancement and widening of the absorptive band in the reflective spectrum are related to the nanopore in the top Al layer.
To reveal the enhancement and widening mechanism of absorptive band, the localized electric field intensity distribution of the PAlpore/AAO/Al configuration in the X-Z plane at center of the nanopore at reflective peaks and valleys were simulated and the results are shown in the middle panel in Fig. 7. For comparison, the localized electric field intensity distribution of the PAlpore/AAOthick and PAl/AAO/Al configurations are also shown in the top and bottom panel, respectively, in Fig. 7. As indicated in middle and bottom panels, except for nanopore in the top Al layer, the localized electric field distribution of the PAlpore/AAO/Al is of the similar standing field characteristic with that of PAl/AAO/Al because of the F-P resonance supported by the MDM configuration. However, at reflective peaks, compared with the PAl/AAO/Al configuration, the localized electric field enhancement in PAlpore/AAO/Al configuration is observed not only within the configuration but also within the nanopore in the top Al layer and the total localized electric field enhancement is only a little large than that in PAl/AAO/Al, corresponding to a slight decrease of reflective peaks in reflective spectrum, because the destructive interference pattern (corresponding to the constructive interference in reflective spectrum) is positioned under the top Al layer (as indicated by the black shot dashed line), resulting in the attenuation of non-resonance scattering compared with that in PAlpore/AAOthick shown in the top panel. At reflective valleys, the total localized electric field enhancement in PAlpore/AAO/Al is large than that in PAl/AAO/Al, corresponding to large decrease and widening of reflective valleys in reflective spectrum, because the constructive interference pattern (corresponding to the destructive interference in reflective spectrum) is positioned near the top Al layer (as indicated by the black shot dashed line), resulting in the enhancement of non-resonance scattering compared with that in PAlpore/AAOthick shown in the top panel. Therefore, for PAlpore/AAO/Al configuration, the enhancement and widening of the absorptive band in the reflective spectrum are related to the weak coupling between the weak broadband non-resonance scattering and interference rather than between the SPR and interference. And the limited enhancement and widening of absorptive band in reflective spectrum can’t generate the vivid reflective colors.
Fig. 7. The localized electric field intensity distribution (X-Z plane at center of the nanopore) of PAlpore/AAO/Al, PAl/AAO/Al and PAlpore/AAOthick at reflective peaks and valleys.
3.3 Optical properties of the bumpy nanopore Al island film on AAO and the corresponding Al/AAO/Al configuration
As discussed in section 3.2, using the PAlpore as a top metal layer does not explain the generation mechanism of vivid reflective colors in Al/AAO/Al configuration. Therefore, according to the SEM image shown in Fig. 1, we use a BAlIpore as the top metal layer and the optical property of the BAlIpore/AAOthick and the BAlIpore/AAO/Al configuration were investigated by FDTD method and compared with that of experimental one. The geometrical model and parameters of BAlIpore/AAO/Al configuration used for the simulations were determined by SEM image in Fig. 1, in which the BAlIpore is formed by distributing many Al nanoparticles with various sizes and shapes on the bumpy surface of AAO membrane. The schematic diagrams of the model for BAlIpore/AAOthick and the BAlIpore/AAO/Al configuration are shown in Fig. 8(a) and (b), respectively. The thickness of Al film is 15 nm. The period and diameter of nanopore are 100 and 45 nm, respectively. The red dashed box shows the simulation region that contains one cell in the center and four a quarter cells around it. The plane wave is incident vertically to the surface of the simulated structure and the polarization is parallel to its surface.
Fig. 8. (a) 3D structure diagrams of BAlIpore/AAOthick; (b) 3D structure diagrams of BAlIpore/AAO/Al configuration.
The simulated absorptive spectrum of BAlIpore/AAOthick is shown in Fig. 9 (red solid line) and the experimentally measured absorptive spectrum of Al/AAOthick prepared by depositing 15 nm Al on the thick AAO membrane is also shown in Fig. 9 (red hollow circle symbol line). The simulated absorptive spectrum of BAlIpore/AAOthick presents broadband absorption with an absorptance larger than 25% in visible region, which matches well with measured one except for the decrease in short wavelength because the dielectric function of Al2O3 used for simulation was taken from Palik’s data and did not include the absorption of AAO membrane.
Fig. 9. Simulated absorptive spectrum of BAlIpore/AAOthick and measured absorptive spectrum of prepared Al/AAOthick.
The simulated localized electric field distribution of the BAlIpore/AAOthick in the X-Z plane at center of the nanopore and in the X-Y plane at Z = 375 nm at specific wavelength are shown in the top panel in Fig. 10. It can be seen that the localized electric field enhancement occurs around the Al nanoparticles and in the gaps between adjacent nanoparticles, which is attributed to the resonant excitation of the LSPR in Al nanoparticles with different sizes and the LSPR coupling of adjacent nanoparticles. Therefore, the BAlIpore can support the broadband LSPR in visible region.
Fig. 10. The localized electric field intensity distribution (X-Z plane at center of the nanopore and X-Y plane at Z = 375 nm, respectively) of BAlIpore/AAOthick and BAlIpore/AAO/Al, and that (X-Z plane at center of the nanopore) of the AAO/Al template at reflective peaks and valleys.
The simulated reflective spectra of AAO/Al template and BAlIpore/AAO/Al configuration are shown in Fig. 11 (blue short dot symbol and olive solid line, respectively). And the experimentally measured reflective spectra of AAO/Al template and Al/AAO/Al configuration prepared by depositing 15 nm Al film on the AAO/Al template are also shown in Fig. 11 (blue spherical symbol and magenta semi solid circle symbol line, respectively), in which the thickness of AAO membrane is 370 nm. The simulated reflective spectra of AAO/Al template and BAlIpore/AAO/Al configuration match well with measured one. The depth and bandwidth of reflective valleys (i.e. the intensity and width of absorptive peaks) of the BAlIpore/AAO/Al configuration are much larger than that of the AAO/Al template, which means that the large enhancement and widening of the absorptive band in the reflective spectrum are related to the BAlIpore.
Fig. 11. Simulated and measured reflective spectra of AAO/Al template and BAlIpore/AAO/Al configuration.
The simulated localized electric field intensity distribution of the BAlIpore/AAO/Al configuration in the X-Y plane at Z = 375 nm and X-Z plane at center of the nanopore at reflective peaks and valleys are shown in the middle panel in Fig. 10, and, for comparison, that of the AAO/Al template in the X-Z plane at center of a nanopore are shown in the bottom panel in Fig. 10. One can see that the similar interference field distribution is observed within two configurations because of the thin-film interference. However, for BAlIpore/AAO/Al configuration, at reflective peaks, only the localized electric field enhancement within BAlIpore layer is a little large than that within the corresponding position in AAO/Al template, corresponding to a slight decrease of reflective peaks in reflectvie spectrum, because the destructive interference pattern (corresponding to the constructive interference in reflective spectrum) match with BAlIpore layer (as indicated by the black shot dashed line), resulting in the weakening of LSPR compared with that in BAlIpore/AAOthick shown in the top panel. At reflective valleys, the localized electric field enhancement mainly appears in BAlIpore layer and is larger than that in BAlIpore/AAOthick shown in the top panel, corresponding to a large decrease and widening of reflective valleys in reflective spectrum, because the constructive interference pattern (corresponding to the destructive interference in reflective spectrum) is positioned in BAlIpore layer (as indicated by the black shot dashed line), resulting in the enhancement of LSPR compared with that in BAlIpore/AAOthick shown in the top panel. The strong coupling between the broadband LSPR and the thin-film interference, i.e. the interference and LSPR co-enhanced effect result in the enhancement and widening of absorptive band in reflective spectrum, which contributes to the generation of vivid reflective colors in Al/AAO/Al configuration.
4. Conclusions
In summary, Al/AAO/Al configuration formed by depositing a thin Al film on the AAO/Al substrate with different AAO thicknesses can generate various vivid reflective colors. To reveal the reflective colors generation mechanism, the PAlpore/AAO/Al and BAlIpore/AAO/Al models were proposed and their optical properties were investigated by FDTD method. The investigated results revealed that PAlpore/AAO/Al can’t generate vivid reflective colors because the PAlpore can only support the narrowband SPR (SR SPP and LSPR) in ultraviolet region and weak broadband non-resonance scattering in visible region. The weak coupling between the non-resonance scattering and F-P resonance in visible region can limitedly enhance and broaden the absorptive band in the reflective spectrum. However, for the BAlIpore/AAO/Al configuration, the BAlIpore can support the broadband SPR (LSPR) in visible region and the strong coupling between the broadband LSPR and the destructive interference results in the large absorption enhancement and widening of the absorptive band in reflective spectrum and the generation of vivid reflective colors. The results obtained in this article provide a more comprehensive understanding of vivid reflective color generation mechanism in Al/AAO/Al configurations.
Funding
National Natural Science Foundation of China (61890961); Key R&D project in the Shaanxi Province of China (2020GY-274); National Major Scientific Instruments and Equipments Development Project of National Natural Science Foundation of China (62127813).
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.
Supplemental document
See Supplement 1 for supporting content.
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