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Abstract
The modulation of structural color through various methods has attracted considerable attention. Herein, a new modulation method for the structural colors in all-dielectric photonic crystals (PCs) using energetic ion beams is proposed. One type of periodic PC and two different defective PCs were experimentally investigated. Under carbon-ion irradiation, the color variation primarily originated from the blue shift of the optical spectra. The varying degrees of both the reflection and transmission structural colors mainly depended on the carbon-ion fluences. Such nanostructures are promising for tunable color filters and double-sided chromatic displays based on PCs.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In daily life, many of the brilliant hues are primarily obtained from pigments and dyes. In addition, colors can be obtained from periodic micro- or nanostructures with different materials. These colors, which depend on the structure, are called structural colors. Compared with pigment colors, structural colors can effectively resist photobleaching and preempt the use of toxic dyes. Therefore, they exhibit good fading resistance and stability [1], which makes them attractive potential substitutes for eco-friendly pigments, dyes, and inks for decoration, inking, and photon printing [2–4]. In addition, structural colors have considerable potential for application in other fields; they can play important roles in sensors [5,6], visual information coding [7], and information encryption and hiding [8], and they are promising candidates for eco-friendly color applications.
Structural colors were first observed in nature. As early as the 17th century, Hooker [9] and Newton [10] discovered colorful feathers of birds such as peacocks and ducks and ascribed the colors to the unique structures of the feathers. Subsequently, Lord Rayleigh analyzed the origin of the colors of some butterfly and beetle wings using electromagnetic theory. In [11], structural colors were reviewed. In [12], the concept of structural color was developed, and the authors denied the existence of surface colors, laying a foundation for the research and development of structural colors. Nowadays, several nanostructures of different scales, including photonic crystals (PCs), are used to generate structural colors. PCs are generally composed of different materials with varying refractive indices that are arranged periodically [13,14], and these materials have unique optical properties, such as a photonic bandgap (PBG) and photonic localization [15,16]. When the incident light falls on the PBG region, it is strongly reflected and prevented from propagating through the PCs [17]. Accordingly, if the PBG is located in the visible region, brilliant structural colors can be observed. There are many colorful organisms and minerals whose surface structures comprise various PCs; these can be divided into one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) nanostructures. For example, 1D PCs can be found in the wings of the morpho butterfly [18–20] and Hoplia coerulea (beetles) [21,22]. More sophisticated nanostructures can be found in peacocks, whose feathers contain 2D PCs [23]. In addition to these 1D and 2D nanostructures, some colorful mineral gemstones, such as opals, contain 3D PCs [24,25]. Inspired by these vivid structural colors in nature, 1D, 2D, and 3D periodic nanostructures have been fabricated to generate structural colors. For example, synthetic 1D multilayers are used to recreate the structural colors of insect surfaces [26,27] and produce colorful solar selective absorbers [28] and 2D nanorod arrays composed of silver [29]; furthermore, nanowire arrays with silicon [30] are used to fabricate color filters. Additionally, 3D PCs have been used in color displays and patterns [31–35].
However, simply producing stable colors may not satisfy the current requirements of modern applications. Many applications require visual color modulation with external stimuli. Dynamic structural colors can respond to environmental changes and transform different external stimuli, such as mechanics, temperature, humidity, and electromagnetism, into visual color changes for monitoring the external environment instantaneously, which has potential applications in medical and traffic safety. However, as external stimuli can cause changes in structural colors, external environmental stimuli can be exploited to tune the colors. Inspired by the chameleon [36], Zhu et al. prepared a self-healing photonic elastomer that changed its structural color through mechanical stretching and bending [37]. Furthermore, the structural colors can be regulated by applying an electric or magnetic field [33,38–40]. Researchers have used thermo- or hygroresponsive PCs to tune structural colors by controlling the temperature [41,42] and humidity [43]. Generally, changes in structural color are caused by changes in the nanostructure, e.g., the lattice distance, refractive index, and material thickness. However, once PCs have been fabricated, it is typically difficult to change their structural colors. Therefore, new methods have been proposed for structural color regulation. Furthermore, it has been found that structural features, including shape, size, and orientation, can be controlled by modifying the irradiation intensity, duration, and polarization of light [44]. Such effects can also be induced by ion irradiation. After the ion beams are accelerated by the high-voltage electric field of a large accelerator, they uniformly irradiate the surface or inside of the target materials, effectively modulating the optical properties of the materials, which have broad application prospects [45–48]. Recent studies have indicated that ion irradiation can change the refractive indices and thicknesses of different materials in PCs simultaneously [49]. Thus, structural colors can be modulated effectively by ion irradiation.
To date, most studies have focused on structural colors based on reflection, and few have focused on transmission structural colors. In addition, two-side color regulation by PCs has recently been investigated [50,51]. Notably, tunable structural colors on both sides endow PCs with powerful projection and display capabilities, making them promising for applications in transflective color filter devices [51–54]. In this study, three types of all-dielectric 1D PCs were prepared, and the reflection and transmission two-way structural colors were tuned by C5+ ion irradiation with different fluences, which is significant for the design of transmissive and reflective two-way color filters.
The remainder of this paper is organized as follows. Section 2 describes the structural color modification of periodic PCs by ion irradiation. Section 3 describes the structural color modulation of different defective PCs with a SiO2 layer and a TiO2 layer as defects. The conclusions are presented in Section 4.
2. Structural color modulation of periodic PCs by ion irradiation
First, a truncated periodic 1D PC, which is denoted as (AB)nS in Fig. 1, is considered. A and B denote silicon dioxide (SiO2) and titanium dioxide (TiO2) with refractive indices of nA = 1.431 and nB = 2.123, respectively [55]. S denotes the substrate (BK7) with a refractive index of 1.52, and n is the periodic number.
We set nAdA = nBdB = λ0/4, where λ0 = 654.0 nm is the center wavelength of the PBG. Hence, the thicknesses of layers A and B are dA = 114.3 nm and dB = 77.0 nm, respectively. The periodic number is n = 15. In the experiment, we utilized a box-type vacuum coating machine to prepare 1D PCs through electron-beam evaporation. Then, the structural colors of the PCs were measured after C5+ ion irradiation. Different PCs were irradiated with C5+ ions with constant energy of 15 MeV at two different fluences (1.0 × 1014 and 2.0 × 1014 ions/cm2) through a 3-MV tandem accelerator at the Helmholtz-Zentrum Dresden-Rossendorf, where C5+ ions were tilted by 7° off their normal to minimize the channeling effect. During the irradiation, the current density remained low (<10 nAcm−2) to avoid charging and heating the samples. A schematic of the PCs irradiated by C5+ ions is shown in Fig. 2.
Figures 3(b) and 4(b) show the measured reflection and transmission spectra of (AB)15S, respectively, measured at different incidence angles of 7.5°, 15°, 30°, 45° and 60°. It is considered that the natural light is composed of 50% TE and 50% TM waves in this paper. The black line indicates the experimental reflection spectrum of PC without ion irradiation, and the red (blue) line indicates the optical spectrum under C5+ ion irradiation with a fluence of 1.0 × 1014 (2.0 × 1014) ions/cm2. The band edges of the PBG shifted to shorter wavelengths with an increase in the irradiation fluence and incident angle. This is because the total optical thickness of the PC decreased, primarily owing to the reduction in the material thickness with ion irradiation [49]. Therefore, the reflection and transmission structural colors of the PCs were significantly changed after the ion irradiation, as shown in Figs. 3(a), 3(c), 4(a) and 4(c).
Fig. 3. (a, c) Digital photographs of reflection structural colors of PC (AB)15S under different conditions. (b) Reflection spectra of (AB)15S under different conditions. (d, e, f) Chromaticity coordinates of reflection colors of (AB)15S indicated in chromaticity diagrams with different irradiation fluences. The incidence angles are 7.5° and 60° for (a, c); 7.5°, 15°, 30°, 45° and 60° for (b, d, e, f).
Fig. 4. (a, c) Digital photographs of transmission structural colors of PC (AB)15S under different conditions. (b) Transmission spectra of (AB)15S under different conditions. (d, e, f) Chromaticity coordinates of transmission colors of (AB)15S indicated in chromaticity diagrams with different irradiation fluences. All other parameters are the same as those used in Fig. 3.
To further investigate the color changes of the PC samples (AB)15S due to ion irradiation, we measured the tristimulus values of the CIE 1931 XYZ chromaticity system of the PC and drew chromaticity diagrams of the reflected and transmitted structural colors under five incidence angles (7.5°, 15°, 30°, 45° and 60°), as shown in Figs. 3(d-f) and Figs. 4(d-f). Here, the dots represent the chromaticity coordinates of the structural color of (AB)15S without ion irradiation, and the squares (stars) represent the chromaticity coordinates with a fluence of 1.0 × 1014 (2.0 × 1014) ions/cm2. It is obvious that structural colors are changed with the increase of the incident angles. More intriguingly, as can be seen from Figs. 3(a, c, d, e and f) and Figs. 4(a, c, d, e and f), the reflected and transmitted structural colors are modulated obviously with an increase of the irradiation fluence at same incident angles.
As shown in Fig. 3(a), when the incidence angle was 7.5°, the reflected color of the unirradiated sample was orange. After ion irradiation at a dose of 1.0 × 1014 ions/cm2, the sample exhibited a slight goldenrod color, and with an increase in the irradiation fluence, the color faded. This is because, with an increase in the irradiation intensity, the proportion of short-wavelength light reflected by the PCs increased, while the proportion of long-wavelength light reflected by the PCs decreased. Therefore, the mixed light shifted to a shorter wavelength, and the color changed from orange to a light goldenrod. At the incidence angle of 60°, as shown in Fig. 3(c), with an increase in the irradiation fluence, the reflected color of the PC surface gradually changed from light yellow to yellowish-green. Figures 4(a) and 4(c) show the transmission structural colors of the PC surface. At the incidence angle of 7.5°, the PBG shifted to a shorter wavelength with an increase in the irradiation dose, and the light in the wavelength range of 550–600 nm was gradually covered by the PBG. Therefore, the proportion of short-wavelength transmitted light continued to increase so that the transmitted mixed light continuously shifted to a shorter wavelength, leading to a dark blue color. However, at the incidence angle of 60°, the structural color of the transmitted mixed light shifted to the red wavelength region with an increase in the irradiation fluence. This is because the PBG completely covered the green light, and the transmitted light was a mixture of red and blue light. With an increase in the irradiation dose, the proportion of red light increased and that of blue light decreased. Therefore, the overall mixed light exhibited a trend of shifting to the red wavelength region. These changes correspond to the chromaticity diagram, as shown in Figs. 3(d-f) and 4(d-f), clearly observed in the experiment.
3. Structural color modulation of defective PCs by ion irradiation
The structural colors of former periodic PCs originate from the PBG. Besides, they also can be generated based on the photonic localization, such as by metal-insulator-metal nanocavities [56,57]. If a defect layer is introduced into PCs, the photonic localization of the defect mode (also called as the cavity mode) can be generated within the defect layer as a nanocavity [58,59], and periodic stacks on both sides of the defect layer have the role of optical barriers. It is expected that structural colors of defective PCs also can be modulated considerably.
Furthermore, we investigated the structural colors of two different defective PCs. The PC (AB)mD(BA)mS containing a SiO2 defect layer was considered first, as shown in Fig. 5. Here, A and D represent SiO2, and B represents TiO2. S represents the BK7 substrate, and m is the periodic number. We set the periodic number as m = 6 and the thicknesses of layers A, B and D as dA = 95.6 nm, dB = 64.4 nm and dD = 213.1 nm, respectively. After the PC samples were prepared and irradiated with different fluences of C5+ ions as shown in Fig. 6, we measured their reflection and transmission spectra and chromaticity data. The results are presented in Fig. 7 and Fig. 8. Figures 7(b) and 8(b) present the measured reflection and transmission spectra of the PCs with a SiO2 defect layer at five incidence angles (7.5°, 15°, 30°, 45° and 60°) under same fluence conditions used for the periodic PCs. As shown, the PBG and defect mode of (AB)6D(BA)6S shifted to shorter wavelengths with an increase in the C5+ iron irradiation fluence and incident angle.
Fig. 7. (a, c) Digital photographs of reflection structural colors of PC (AB)6D(BA)6S under different conditions. (b) Reflection spectra of (AB)6D(BA)6S under different conditions. (d, e, f) Chromaticity coordinates of reflection colors of (AB)6D(BA)6S indicated in chromaticity diagrams with different irradiation fluences. The incidence angles are 7.5° and 60° for (a, c); 7.5°, 15°, 30°, 45° and 60° for (b, d, e, f).
Fig. 8. (a, c) Digital photographs of transmission structural colors of PC (AB)6D(BA)6S under different conditions. (b) Transmission spectra of (AB)6D(BA)6S under different conditions. (d, e, f) Chromaticity coordinates of transmission colors of (AB)6D(BA)6S indicated in chromaticity diagrams with different irradiation fluences. All other parameters are the same as those used in Fig. 7.
The PC sample images corresponding to the experimental reflection and transmission spectra of (AB)6D(BA)6S are shown in Figs. 7(a), 7(c), 8(a) and 8(c), and the CIE 1931 chromaticity diagrams are shown in Figs. 7(d-f) and Figs. 8(d-f). The colors are also changed with the increase of the incidence angle and irradiation fluence as shown in Figs. 7(d-f) and Figs. 8(d-f). Besides, compared to the reflected colors, transmitted colors are modulated more obviously by increasing the incidence angle under the same irradiation fluence. At the incidence angle of 7.5°, the color of the reflected light from the sample without ion irradiation was pale yellow. With an increase in the irradiation fluence, the structural color faded. Finally, the structural color of the sample irradiated by ions with an irradiation dose of 2.0 × 1014 ions/cm2 was close to white because of the large PBG region of (AB)6D(BA)6S as shown in Fig. 7(b). In the range of 450–700 nm, the light was completely reflected by the PCs, except for those with defect-mode wavelengths; thus, the color of the mixed light formed was close to white. When the incidence angle changed to 60°, as shown in Figs. 7(c-f), similar to the periodic PC samples, with an increase in the irradiation fluence, the reflected light shifted to a shorter wavelength, and the color changed from light blue to dark blue. This is because the PBG and defect mode also had blue-shifted effects, as shown in Fig. 7(b). The transmission structural color of the defective PC at an incidence angle of 7.5° changed from purplish-blue to reddish-purple with an increase in the ion irradiation fluence, as shown in Fig. 8(a). At a large incidence angle of 60°, as shown in Fig. 8(c), the transmitted structural color changed from reddish-orange to yellow-orange with the increasing ion irradiation fluence. At the incidence angle of 7.5°, the transmission structural color of defective PC surface changed from blue to red wavelength region parallel the purple line of the chromaticity diagram because the PBG of the PC covers most part of the visible region except red and blue light. Therefore, the mixed light transmitted from the PC contained red and blue lights and a narrow band of light from the defect mode, as shown in Fig. 8(b). With an increase in the irradiation fluence, the PBG shifted to a shorter wavelength, reducing the proportion of blue light and increasing the proportion of red light. However, at the incidence angle of 60°, the structural color shifted to the short-wavelength region and gradually approached the spectral locus with an increase in the irradiation fluence because the short-wavelength visible light primarily fell onto the PBG region, as shown in Fig. 8(b). Therefore, the mixed transmitted light of the PC mainly contained long-wavelength light in the range of 550–780 nm.
Another defective PC (BA)5E(AB)5S was considered, where E represents a defect layer, A represents SiO2, B and E represent TiO2, and S represents the substrate BK7. The thicknesses of layers A, B and E are dA = 93.5 nm, dB = 63.0 nm and dE = 133.5 nm, respectively. This type of defective PC was prepared and irradiated by C5+ ions with fixed irradiation energy and the aforementioned fluence conditions. We also measured the reflection and transmission spectra of the PC with a TiO2 defect layer at five incidence angles (7.5°, 15°, 30°, 45° and 60°) and different ion irradiation conditions, as shown in Figs. 9(b) and 10(b). With an increase in the C5+ ion irradiation fluence, the PBG and defect modes exhibited blue shifts.
Fig. 9. (a, c) Digital photographs of reflection structural colors of PC (BA)5E(AB)5S under different conditions. (b) Reflection spectra of (BA)5E(AB)5S under different conditions. (d, e, f) Chromaticity coordinates of reflection colors of (BA)5E(AB)5S indicated in chromaticity diagrams with different irradiation fluences. The incidence angles are 7.5° and 60° for (a, c); 7.5°, 15°, 30°, 45° and 60° for (b, d, e, f).
Fig. 10. (a, c) Digital photographs of transmission structural colors of PC (BA)5E(AB)5S under different conditions. (b) Transmission spectra of (BA)5E(AB)5S under different conditions. (d, e, f) Chromaticity coordinates of transmission colors of (BA)5E(AB)5S indicated in chromaticity diagrams with different irradiation fluences. All other parameters are the same as those used in Fig. 9.
The CIE 1931 chromaticity diagrams of the structural colors corresponding to the reflection and transmission spectra of PC (BA)5E(AB)5S are shown in Figs. 9(d-f) and 10(d-f), respectively. It can be seen that both of the reflection and transmission structural colors are also altered dramatically by increasing the incidence angle and irradiation fluence. Digital photographs of the PC samples are shown in Figs. 9(a), 9(c), 10(a) and 10(c). At an incidence angle of 7.5°, as shown in Figs. 9(a), 9(d), 9(e) and 9(f), the reflected structural color gradually changed from light yellow to white with an increase in the irradiation fluence. At a large incidence angle of 60°, as shown in Figs. 9(c) and 9(d), 9(e) and 9(f), the structural color changed from pale green to greenish-blue. The color variation trend was similar to that of the former defective structure PC (AB)6D(BA)6S, but the chromaticity points of the colors were different. As shown in Figs. 10(a), 10(d), 10(e) and 10(f), at the incidence angle of 7.5°, the transmitted structural color of PC (BA)5E(AB)5S shifted from violet to reddish-purple with an increase in the irradiation fluence. At the incidence angle of 60°, as shown in Figs. 10(c), 10(d), 10(e) and 10(f), the transmitted structural color of PC (BA)5E(AB)5S varied from yellowish pink to orange with the increasing irradiation fluence. Thus, the structural color modulation of PCs containing a TiO2 defect layer was analyzed by changing the fluence of C5+ ion irradiation.
4. Conclusion
We experimentally prepared periodic and defective PCs via electron-beam evaporation. The PCs were irradiated by C5+ ion beam with fixed energy and different fluences. The experimental results indicated that the PBG and defect mode of the PCs shifted to shorter wavelengths, which significantly affected the transmission and reflection structural colors, with the increasing ion irradiation fluence. The results of this study provide guidance for the fabrication of tunable color devices using energetic ion beams.
Funding
Natural Science Foundation of Shandong Province (ZR2019MA055, ZR2020QA071); National Natural Science Foundation of China (12047536); Taishan Scholar Project of Shandong Province (tspd20210303); State Key Laboratory of Surface Physics and Department of Physics (KF2022_01); Key Laboratory of Micro-and Nano-Photonic Structures (Ministry of Education).
Acknowledgments
This work is supported by Physical-Chemical Materials Analytical & Testing Center of Shandong University at Weihai.
Disclosures
The authors declare no conflicts of interest.
Data availability
The 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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