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Abstract
Conventionally, optical coatings transmit and reflect the complementary colors due to the limitations of the resonant modes in multilayered structures. Here, we experimentally demonstrate a type of semi-transparent optical coating that can produce the same color in transmission and reflection based on Fano resonance. The results verify that when the metallic spacer is extremely thin, almost the same reflectance and transmittance spectra can be simultaneously achieved with the efficiency about 40%. Furthermore, we comprehensively explore the change of the colors by varying the thickness of the metal and the refractive index of the dielectric cap. Benefiting from the flexible tuning of the coupled resonator, a broad palette of colors and chromatic information display are presented. Our semi-transparent optical coatings exhibit viewing angle tolerance up to 40°, which may lead to novel display and optical anti-counterfeiting techniques.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Colors produced by traditional chemical dyes or pigments play an important role in human life. Inspired by these colors, researchers have long struggled to develop materials, devices and techniques to generate colors. Structural colors are produced by the interaction of light and nanostructures, and have many advantages such as environmental friendliness, high spatial resolution, and long durability [1,2]. Resulting from these, structural coloration has been used in a wide range of applications in color printing [3–9], display/imaging device [10–15], optical anti-counterfeiting [16–18], solar cells [19–21] and other fields. Recently, most reported works are based on nanostructured metamaterials such as plasmonic resonators [22–24], periodic arrangements of nanoparticles [25,26], and one-dimensional nanowires [27]. When the light of a certain wavelength strikes metallic nanoparticles, the incident electric field drives the free electrons in metal to resonantly oscillate inside the nanoparticles, which tailors the transmitted or reflected light spectra. Based on such coupled light-electron excitation, near-field coupled disk-holes array can achieve a high spatial resolution [28]. However, these coloring schemes require high-precision micro-nano-fabrication techniques such as electron beam lithography [29], direct laser writing [30], and focused ion beam milling [31].
As an indispensable optical component in most optical devices, optical coatings can break through such limitations and realize lithography free fabrication [32–35]. Typical thin-film nanostructures include photonic crystal models composed of high and low refractive index dielectric film or Fabry-Pérot (F-P) resonators composed of metal-dielectric-metal (MDM) layers. The MDM configuration is one of the choices that can realize spectral filtering with nearly perfect absorption, which is an effective way to generate colors in the visible region [36–40]. For example, a highly efficient color filter based on an asymmetric sandwiched Al-TiO2-Pt was proposed with a high reflection efficiency of 87.5% [41]. Based on similar principle, most works focused on the optical coatings consisting of lossless dielectric materials and metals. In 2013, Kats et al. showed that ultrathin film resonance can generate colors in reflected light [42]. They utilized a germanium (Ge) thin film deposited on gold substrate to realize a wide palette of colors by varying the thickness of Ge film. On the other hand, similar mechanism can also be used to generate transmissive structural color with multilayered films when the thickness of metallic layer is selected to be optically thin [36]. Low-refractive-index dielectric materials tend to cause an angular dependence of the resonance wavelength [43]. In order to solve this problem, Park et al. deposited an additional layer of TiO2 film on the surface of the F-P cavity structure, which further weakened the angle dependence of the transmission spectrum and achieved large viewing angles with performance of (-35° to 35°) [44]. However, the above discussed nanostructures are designed and optimized to generate colors in either transmission or reflection mode. As a result, their potential applications are limited or the colors in reflection and transmission are fixed to be complementary [45]. It seems to be a challenging problem to generate the same color rendering effect via artificial nanostructures. Recently, Mohamed et al. skillfully solved this problem by proposing a kind of optical coatings that exhibit Fano resonance [34]. Based on the coupling oscillator theory, unique semi-transparent optical coatings that behave as beam-splitting color filters are demonstrated. Moreover, based on the similar mechanism and employing an active electrochromic tungsten oxide (WO3), reversible switching between the Fano and F-P resonances to generate structural colors has also been demonstrated [46].
In this work, we design and demonstrate a Fano-resonant optical coating with more spectral tunings by introducing various materials. The coating includes a broadband resonator composed by a SiO2 coated tungsten (W) layer and a narrowband absorber consisting of a SiO2 layer sandwiched by W cap layer and silver (Ag) bottom layer. For the dielectric layer, we use SiO2 due to the features such as high transmittance, easy to prepare, and low cost. For the absorption medium, W is a good choice since it exhibits broadband absorption performance in the visible band and well investigated in optical broadband absorber design [47]. Using such metal-dielectric-metal-dielectric (MDMD) configuration with a total thickness about 300 nm, we have verified that our optical coating could perform as beam-splitting color filter, namely transmitting and reflecting the same colors with a viewing angle tolerance of 40°. The thickness of metal layer is precisely controlled by magnetron sputtering technique and full colors can be generated by varying the thickness of the intermediate SiO2 layer. Furthermore, we explore the effect of different materials and thickness of each layer on the color hue and brightness. This optical coating with Fano resonance could exhibit almost the same reflection and transmission spectra. Such design principle opens up a new route to optimizing structural colors, which shows potential applications in transparent objects decoration, spectral filters and custom optical glasses.
2. Design principle and results
In recent decades, Fano resonance has been observed and investigated in plasmonic metamaterials and photonics crystals [48]. Based on coupled oscillator theory [34,49], we first realize a beam-splitting color filter which can simultaneously transmit and reflect the same colors based on specially designed Fano resonance. Figure 1(a) schematically presents the configuration of the Fano-type optical coating. The constructions of the broadband & narrowband resonators and the corresponding spectral and spatial electric field responses are presented in Fig. S1 in the supplementary material. Fano resonance usually occurs in the presence of interference between spectrally narrow and broad modes. In such multifilm nanostructure, the narrowband mode can be supported by the asymmetric F-P cavity resonance. By optimizing the thickness of Ag (h1), SiO2 (h2) and W (h3), the F-P cavity resonant mode of the tri-layer film could significantly enhance the transmission and reflection of the illuminating light at certain wavelength. The broadband mode lies in the construction consisting of the lossless film deposited on the absorbing medium, which is different from the reference [34]. The loss of W in visible region and the index-matching SiO2 (h4) layer flatten the transmission/reflection spectrum by causing non-perfect broadband absorption. The thickness of W determines the coupling strength between the narrowband F-P cavity resonator and the non-perfect broadband absorber. Considering that the reflected and transmitted colors are expected to be the same, the thicknesses of metallic layers are of great significance since they determine the transmittance. Using the finite-difference time-domain (FDTD) simulations, the thickness of W and Ag are both determined to be optimal at 13 nm. When the thickness of the metal layer is fixed at h1 = h3 = 13 nm, the reflection and transmission spectra could be manipulated to achieve the maximum value at approximate wavelength with almost the same efficiency. To have a better understanding of the impact of the structural parameters, the relations between spectra and different thickness of Ag, W and top SiO2 layer are presented in Fig. S2 in the supplementary material. Figure 1(b) depicts the simulated reflection and transmission spectra of our Fano-type optical coating and the corresponding field distribution at resonant wavelength (550 nm) is plotted as shown in Fig. 1(c).
Fig. 1. Construction and color performance of our optical coating. (a) Schematic diagrams of the optical coatings. (b) The reflection and transmission spectra of the Fano-type optical coating. The two insets are corresponding reflected and transmitted colors. The geometric parameters are determined to be h1(13 nm)-h2(150 nm)-h3(13 nm)-h4(105 nm). (c) Spatial distribution of electric field at calculated resonant wavelength (550 nm).
Using magnetron sputtering technique, we deposit multilayer films on the glass substrate. The experimental and simulated results of the reflection and transmission spectra from the proposed multilayer nanofilms are shown in Figs. 2(a) and 2(b), respectively. Figure 2(c) showcases the cross-sectional view of the MDMD optical coating (Ag(13 nm)-SiO2(110 nm)-W(13 nm)-SiO2(105 nm)) captured by scanning electron microscope (SEM). The position of the resonant peak can be adjusted by changing the thickness of the SiO2 spacer (h2), and the best result seems to occur when the thickness of the top layer SiO2 is h4 = 105 nm. Based on this, when the thickness of the SiO2 spacer is h2 = 110 nm, 150 nm, 170 nm and 190 nm, the corresponding resonant wavelengths of the optical coatings are 440 nm, 550 nm, 610 nm and 660 nm, corresponding to blue, green, orange, and red, respectively. Interestingly, the spectral profiles of reflection and transmission almost keep the same with efficiency of ∼ 40%.
Fig. 2. Demonstration of rich color modulation and structural characterizations. Measured (a) and simulated (b) reflectance (solid lines) / transmittance (dashed lines) spectra with different colors. (c) Cross-sectional SEM image of Ag(13 nm)-SiO2(110 nm)-W(13 nm)-SiO2(105 nm) configuration. (d) The samples with blue, green, orange and red colors. (e, f) Color coordinates of reflection and transmission modes mapped in the CIE 1931 chromaticity diagram. Experimentally photographed (g) reflected and (h) transmitted images of color patterns.
Figure 2(d) presents four large-area samples (1 × 1cm2) of different colors by changing the thickness h2. Photographs of the samples, with black text on a white background, are taken with a digital camera under illumination of white light. Vivid colors can be observed in these four subfigures. The word ‘color’ in the background can be clearly seen through the fabricated sample, indicating that our device has a certain transmittance and reflectance. Figures 2(e) and 2(f) show the corresponding simulated and measured color coordinates. Obviously, the coordinates of the reflected and transmitted colors produced by the same structure are nearly the same, and the measured and simulated results are in good agreement. In addition, film deposition can also be combined with the mask to create multicolor images. We generate a multicolor pattern containing three letters of ‘NJU’ on a glass substrate, corresponding to three colors: blue (h2 = 110 nm), green (h2 = 150 nm) and orange (h2 = 170 nm). As shown in Fig. 2 g and 2 h, the patterns present almost the same color information in transmission and reflection mode as expected.
Besides, considering the application of color in display devices, we also explore the angular dependence of the nanostructure when it interacts with light. These coatings are thinner than the wavelength of light and have little accumulation of phases transmitted through the film, resulting in strong stability with respect to the incident angles. Figure 3 presents the reflection and transmission spectra as a function of oblique angle of incidence. The simulation results agree well with the measurements when the incident angle is increased from 0° to 40° for different thicknesses (110 nm, 150 nm, 170 nm, 190 nm). It could be observed that the resonant wavelengths and amplitudes of the reflection and transmission spectra remain relatively unchanged even when the incident angle is tilted to 40°. Most importantly, the reflection spectrum still keeps a high degree of coincidence with the transmission spectrum, featuring that such a Fano resonant optical coating shows superior performance in viewing angles.
Fig. 3. Angle-insensitive optical responses. Measured reflection/transmission spectra of (a, e) blue, (b, f) green, (c, g) orange, and (d, h) red for various incident angles ranging up to 40°.
In addition to generating the same color in transmission and reflection mode, we also evaluate other possibilities by testing these optical coatings with other geometrical parameters and materials. Thicker metallic layer will naturally increase the effect of reflection and reduce the efficiency of transmission. Here we calculate and measure the effect of increasing metal thickness on color. We measure the reflection and transmission spectra for h1 = h3 = 20 nm, h4 = 105 nm, and different h2 (110 nm, 150 nm, 170 nm and 190 nm). Compared to the results in Fig. 2, the thickness change of metallic layers has little effect on the corresponding resonant wavelength. However, the maximum reflectance increases to 60% while the transmission efficiencies decrease to 20%, leading to the difference between the reflected and transmitted colors (Figs. 4(a)–4(d)). However, since the resonant peaks of reflection and transmission are in the same position, the color still exhibits similar hue values, but the brightness of the color is quite different, as shown in Fig. 4(e). Such an effect is better to be understood when the colors are transferred to coordinates in the CIE diagram as shown in Fig. 4(f). It can be concluded that such optical coating can be flexible to tailor reflection and transmission spectrum by altering the material or thickness.
Fig. 4. The effect of the thickness h2 on the spectra when the metallic layers get thicker (h1 = h3 = 20 nm). Measured reflection (solid lines)/ transmission (dashed lines) spectra of Ag(20 nm)-SiO2((a)110 nm, (b)150 nm, (c)170 nm, (d)190 nm)-W(20 nm)-SiO2(105 nm). (e) The photographed colors corresponding to the spectra in (a)-(d). (f) CIE color coordinates in reflection and transmission modes.
Similarly, we replace the optical material based on the configuration of the Fano resonance and showcase the experimental results in Fig. 5. When the material in the optical coating is changed, the overall structure exhibits different color-filtering properties. To illustrate this situation, we alter the SiO2 in the top layer of the MDMD structure with Air, silicon nitride (Si3N4) and silicon carbide (SiC). The thickness of SiC and Si3N4 are determined to be 45 nm and 90 nm, respectively, since the corresponding reflection and transmission spectra show the clearest complementary (SiC) and overlapping (Si3N4) relationship under such condition. Based on similar principle, different materials of top layer lead to various color combinations (Fig. 5(a)). This is significantly different from the situation when SiO2 is used as the top layer material. For the convenience of illustration, color hue is used as a parameter for comparison of coloring performance (Fig. 5(b)). An MDM structure without a top-layer medium can produce colors of similar hue, but there is a large gap between the color of the transmission mode and the reflection mode at different wavelengths. For MDMD structures, taking the case where h2 = 150 nm as an example, their resonant wavelengths are all around 550 nm, but the refractive indices of the materials are quite different at 550 nm. It seems that the material with the highest refractive index SiC has the largest difference in hue value under reflection and transmission.
Fig. 5. Different coloration performances of the thin films. (a) Photographed optical images of the coloration with a Si3N4 (90 nm), SiC (45 nm), Air, or SiO2 (105 nm) dielectric layer, wherein the thicknesses of the metallic layer are fixed at 13 nm and the thickness h2 of the SiO2 spacer is 110 nm, 150 nm, 170 nm and 190 nm respectively. (b) Color hue values at h2 = 150 nm for three different top dielectric materials.
The total reflectance and transmittance of the optical coating are determined by the interference of the light reflected/transmitted at each interfaces between two kinds of materials. Therefore, not only the phase accumulation but also amplitude modulations in each layer have an influence on the total reflectance or transmittance. In order to realize the function of transmitting and reflecting the same color, the refractive index of the top layer material should be chosen to offer a necessary balance between the refractive index of the top layer dielectric and the absorbing tungsten to tune the amplitudes of the reflected and transmitted wave at the interface. From the experimental results, it can be considered that the higher refractive index of the top layer material compared to SiO2 could lead to larger color hue difference. Figure S3 in the supplementary material presents the simulated results when the material of the top layer changes to TiO2.
3. Conclusions
In summary, this study reproduces the color of pigments on transparent materials using optical coatings that reflect and transmit the same color simultaneously. We present a large-scale and high-performance MDMD nanostructure as an extended study of traditional MDM devices with structural coloring effect. We demonstrate that the broadband and narrowband absorbing cavities can be coupled to achieve Fano resonance at certain wavelengths. As a demonstration, we optimized the thickness of each layer, and successfully prepared samples on glass substrates by precisely controlling the coating rate. The measured results agree well with the simulation results, including the reflection/transmission spectrum and colors. The color performances of the samples keep consistent till incident angles up to 40°, which is crucial for optical device or some real-life applications. In addition, both the transmission and reflection spectrum can be tailored by only changing the thickness of the metal. We also explore wider color palette by introducing other materials and geometrical parameters. We believe that this work can broaden the application field of structural colors, and its unique advantages possibly open new avenues in the fields of display, intelligent camouflage, etc.
Funding
National Natural Science Foundation of China (62005117); Natural Science Foundation of Jiangsu Province (BK20220068).
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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