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Abstract
Although structural colors based on nanostructures have attracted many researchers’ attentions due to their superior durability and high resolution, most previous reports focused on the static and dynamic structural colors in reflection mode and few researchers focus on the static and dynamic transmission colors for high-saturation RGB models. Here, the hybrid Al-Si3N4 nanogratings with the top SiO2 capping layer and the bottom MgF2 layer that can switch full-hue and high-saturation transmitted structural colors on and off completely by changing the polarization state are theoretically demonstrated. Meanwhile, the hybrid Al-Si3N4 nanogratings with the top capping layer and the bottom layer also achieve the transmittance spectra with the full width at half maximum of ∼58 nm and the transmittance efficiency of over 70% in the on state. The added top capping layer and bottom layer can suppress the sideband of transmittance spectra in the on state and maintain the near-zero transmittance in the off state, thus improving the switching performance between bright and dark states. The realizable high-saturation colors in the on state can take up 125% sRGB space and 80% Adobe sRGB space. More interestingly, with the incident angle varying from 0° to 50°, full-hue color can be also realized in the on state and nearly black color can be also maintained in the off state. The strategy will provide potential applications in advanced color encryption and multichannel imaging.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Structural colors, originating from the interactions between visible light and plasmonic or dielectric nanostructures, have attracted many researchers’ attentions for several years due to their widespread applications in security tags, color displays, high-resolution images, and so on [1–5]. Up to now, many valuable studies on structural colors including static structural colors and dynamic structural colors in reflection mode have been developed to modulate the color characteristics (e.g., hue, saturation, and lightness) [6–10]. In our previous work, a large plasmonic color gamut covering 70% sRGB (standard RGB) color space can be realized by introducing the strategy of mixing colors in the reflective plasmonic color filters [8].
For the structural colors (e.g., static and dynamic colors) in transmission mode, most of previous researches mainly focus on the CMYK color module [11–13]. In order to realize the transmitted colors in RGB module, the arrayed nanostructures with optically-thick metallic films were widely used [11,14–16]. However, due to the high ohmic loss of metal materials, the spectra with low transmittance and monochromaticity will be obtained, which seriously hinders their further applications. Recently, the configurations of hybrid metal-dielectric nanostructures have been applied to improve the efficiency and monochromaticity of the transmittance spectra, thus achieving the high-saturation structural colors in RGB modules [17–19]. The hybrid Titanium dioxide (TiO2)-silver (Ag) core-shell nanowires with relatively high transmittance, proposed by Lee et al., can achieve a large color gamut covering about 80% sRGB space in the on state and insufficiently diminished colors in the off state, where the on or off state depends on the polarization state of the incident light [17]. Improving the structural colors with insufficiently attenuated in the off state is important for the performance of image encryptions and security tags. In this sense, nanostructures that can produce full-hue and high-saturation transmitted structural colors in the on state and near-zero transmittance in the off state have been rarely reported.
In this work, we proposed the polarization-tunable color optical switches (PTCOSs), which can switch transmitted structural colors on and off completely by tuning the polarization state. Compared with the unimproved polarization-tunable color optical switches (UPTCOSs) composed of metal-dielectric nanogratings on a glass substrate, the PTCOSs with the top capping grating layer and the bottom layer can basically eliminate the minor transmittance peaks at short wavelengths for the transverse magnetic (TM) polarization and also maintain near-zero transmittance for the transverse electric (TE) polarization. Meanwhile, the transmittance spectra of the PTCOSs for TM polarization have the full width at half maximum (FWHM) of ∼58 nm and the transmittance efficiency of over 70% (Table 1). With the help of the metal-dielectric nanogratings with the top capping grating layer and the bottom layer, the PCPOSs can achieve a wide color gamut covering 125% sRGB space and 80% Adobe sRGB space for TM polarization and the nearly black color can be also maintained for TE polarization, which can be explained by the hybridized resonance for TM polarization and the electric field blocked for TE polarization. Meanwhile, as the incident angle increases from 0° to 50°, full colors that are from blue to red can be realized by a PTCOS for TM polarization and nearly black colors for TE polarization can be maintained by the PTCOS. Such excellent properties can indicate that the PCPOSs can be used in advanced color encryption, dynamic color display applications, and so on.
Table 1. Representative works on transmission colors in RGB color model
2. Results and discussion
Figure 1 shows the geometric diagram of the PTCOSs, which is comprised of a one-dimensional hybrid metal-dielectric grating structure. Each unit of the PTCOSs is composed of a vertically stacked 20 nm thick (H1) silica (SiO2) capping grating layer, a 40 nm thick aluminum (Al) grating layer, and a 120 nm thick hafnium oxide (HfO2) grating layer from top to bottom with varying periods (p) and widths (w) of gratings on the SiO2 substrate with a 200 nm thick (H2) Magnesium fluoride (MgF2) layer. The gap size (g) between adjacent stacked-layer gratings is defined as g = p – w and is set as 80 nm. With the help of the bottom MgF2 layer and the top SiO2 capping layer, the PTCOSs can realize a large color gamut that exceeds sRGB color space at TM polarized light and near-zero transmittance at TE polarized light. Meanwhile, as shown in Figs. S1 and S2, the effects of thickness (H1) of the top SiO2 grating layer, the thickness (H2) of the bottom MgF2 layer, and the gap size (g) between adjacent gratings on the transmittance spectra of the PTCOSs are studied to obtain the corresponding optimum structural parameters. Furthermore, the optical properties of the PTCOSs are calculated and analyzed by utilizing the finite difference time domain (FDTD) solutions commercial software [20]. The Bloch boundary conditions are used for the x- and y-axis and the perfectly matched layers (PML) are used for the z-axis. Notably, the plane wave source is used in all simulations. The optical constants of Al, SiO2, MgF2, and HfO2 are presented in Fig. S3.
Fig. 1. Geometric diagram of the PTCOSs. Each unit of the PTCOSs consist of a 20 nm thick (H1) SiO2 capping grating layer, a 40 nm thick Al grating layer, and a 120 nm thick HfO2 grating layer from top to bottom with varying periods (p) and widths (w) of gratings on the SiO2 substrate with a 200 nm thick (H2) MgF2 layer.
As presented in Fig. 2, the optical performances of the PTCOS and UPTCOS are compared to prove the role of the configuration of the PTCOSs. The geometric drawings of the PTCOS and UPTCOS are illustrated in Figs. 2(a) and 2(b). The PTCOS consists of a vertically stacked 20 nm thick SiO2 grating layer, a 40 nm thick Al grating layer, and a 120 nm thick HfO2 grating layer on the SiO2 substrate with a 200 nm thick MgF2 layer; the UPTCOS is composed of a vertically stacked 40 nm thick Al grating layer and a 120 nm thick HfO2 grating layer on the SiO2 substrate. And, for easy comparison, the periods and grating widths of the PTCOS and UPTCOS keep the same and their period and grating width are set as p = 340 nm and w = 260 nm, respectively. Figures 2(c) and 2(d) present the transmittance spectra of the PTCOS and UPTCOS for TM and TE polarizations under normal incidence. For TE polarization, the PTCOS and UPTCOS can achieve near-zero transmittance. However, the transmittance spectra of the PTCOS and UPTCOS for TM polarization are obviously different. When the UPTCOS is illuminated at TM polarized light, a minor transmittance peak is excited at 462 nm, which has a strong impact on the monochromaticity of transmittance spectrum. On the contrary, the minor transmittance peak vanishes when the PTCOS is illuminated at TM polarized light. To clearly analyze the reason for the difference between the transmittance spectra of the PTCOS and the UPTCOS for TM polarization, the magnetic field distributions |H| at the wavelength of 462 nm are studied, as shown in the insets of Figs. 2(c) and 2(d). At the wavelength of 462 nm, the magnetic field excited between the grating structure of the UPTCOS and the SiO2 substrate (indicated in blue dashed box) is stronger than that excited between the grating structure of the PTCOS and the SiO2 substrate with a MgF2 layer (indicated in red dashed box), which means that the strong magnetic field excited between the grating structure and the SiO2 substrate is the main reason why the minor transmittance peak is excited at 462 nm when the UPTCOS is illuminated at TM polarized light.
Fig. 2. (a) (b) Configuration of the PTCOS and the UPTCOS. (c) (d) Transmittance spectra of the PTCOS and UPTCOS for TM and TE polarizations under normal incidence. The insets present the magnetic field distributions of the PTCOS for TM polarization at 462 nm (red dashed box) and the UPTCOS for TM polarization at 462 nm (blue dashed box), respectively. It is worth noting that the color bars of two magnetic field distributions have the same upper limits.
Compared with the strong magnetic field excited between the grating structure of the UPTCOS and the SiO2 substrate for TM polarization, a 200 nm thick MgF2 layer in the PTCOS is added to act as a spacer layer to suppress the excitation of magnetic field between the grating structure of the PTCOS and the SiO2 substrate, thus making the excitation of the minor transmittance peak at short wavelengths vanishing. Also, a 20 nm thick SiO2 capping layer in the PTCOS is added to confine the magnetic field excited between the Al grating layer and the top dielectric layer to the grating ridge structure, which limits the leakage of the incoming light from the grating groove to further suppress the sideband. Apparently, the monochromaticity of transmittance spectrum for TM polarization can be dramatically enhanced by adding the bottom MgF2 layer and the top SiO2 capping layer in the configuration of the PTCOS, most of energy of the whole transmittance spectrum can be confined in the transmittance peak. According to the calculated equation of the XYZ tristimulus values [21],
Fig. 3. Calculated saturation and hue values of the corresponding colors produced by the UPTCOSs and PTCOSs with different periods for TM polarization under normal incidence. The insets are the corresponding CIE 1931 chromaticity diagrams.
The color characteristics (e.g., saturation and hue) of colors produced by the UPTCOSs and PTCOSs with different periods for TM polarization under normal incidence are shown in Fig. 3. The saturation is considered in the CIE 1931 chromaticity diagram (the solid color on the CIE-space outline has the highest saturation (100%), but the white point in the center has the lowest saturation (0%)). And the hue is calculated in Lch (lightness, chroma, and hue) module (0° (360°) is red, 90° is yellow, 180° is green, and 270° is blue). Apparently, when two kinds of nanostructures are illuminated at the TM polarized light, the hue value from 50° to 300° can be covered as the period varies from 280 nm to 420 nm. But, compared with the UPTCOSs, the saturation of colors produced by the PTCOS for TM polarization can be significantly enhanced when the period varies from 320 nm to 420 nm. Meanwhile, the phenomena that full hue coverage and dramatic saturation enhancement are more intuitively observed from the insets with the corresponding chromaticity coordinates.
Figure 4 shows the transmittance spectra and color performances of the PTCOSs for TM and TE polarizations under normal incidence. Here, the gap size is fixed at 80 nm as the period of the PTCOS changes. Figure 4(a) shows the transmittance spectra of the PTCOSs for TM polarization, indicating that the transmittance peak with stable efficiency of over 70% and high monochromaticity can be flexibly tuned from 442 nm to 631 nm by increasing the period from 280 nm to 420 nm. It is worth noting that the transmittance spectra of the PTCOSs for TM polarization have the FWHM of ∼58 nm and the transmittance efficiency of over 70% (The details are shown in the Part. 3 in the Supplement 1). According to the illumination D65 and the transmittance spectra in Fig. 4(a), the corresponding tristimulus values are calculated and plotted as a black dashed line on the CIE 1931 chromaticity diagram in Fig. 4(b), where the white dots represent the chromaticity coordinates when the period increases from 280 to 420 nm in 20 nm increments. Obviously, for TM polarization, the color gamut of the PTCOSs can take up 125% sRGB space and 80% Adobe sRGB space, which means that the high saturated structural colors can be obtained when the PTCOSs are illuminated at TM polarized light. The transmittance spectra of the PTCOSs for TE polarization are shown in Fig. 4(c), and the PTCOSs with different periods for TE polarization can achieve near-zero transmittance that is less than 6%. Therefore, based on the high monochromaticity of transmittance spectra of the PTCOSs for TM polarization and the near-zero transmittance achieved by the PTCOSs for TE polarization, the PTCOSs with different periods can realize full-hue and high-saturation structural colors for TM polarization and nearly black colors for TE polarization, which is presented in Fig. 4(d).
Fig. 4. (a) Transmittance spectra of the PTCOSs with different periods for TM polarization under normal incidence. (b) Chromaticity coordinates corresponding to the transmittance spectra of (a). (c) Transmittance spectra of the PTCOSs with different periods for TE polarization under normal incidence. (d) Color performances of the PTCOSs with different periods for TM and TE polarizations under normal incidence.
Since the incident angle-dependent properties are important for the applications of color devices, the incident angle-dependent properties of the PTCOSs for TM and TE polarizations are studied. The transmittance spectra and color performances of the PTCOS for TM and TE polarizations are illustrated in Fig. 5 as the incident angle (θ) varies from 0° to 50°, where the period and grating width of the PTCOS are 280 nm and 200 nm, respectively. As shown in Fig. 5(a), the transmittance peak of the PTCOS for TM polarization appears an obvious redshift from 442 nm to 611 nm with the incident angle increasing. Corresponding to the dramatic redshift of transmittance peak for TM polarization at different incident angles, a large color gamut realized by the PTCOS for TM polarization at different incident angles is shown in Fig. 5(b) and the color changes from blue to green to red as the incident angle increases from 0° to 50°. Moreover, as the incident angle increases from 0°to 50°, the changes of three different transmittance peaks for TM polarization and the spectral monochromaticity for TM polarization are more clearly shown in Fig. S5. In contrast, as presented in Fig. 5(c), the PTCOS can achieve near-zero transmittance for TE polarization when the incident angle is varying within 50°, indicating the PTCOS holds weak angular dependence for TE polarization. Also, the color performances of the PTCOS for TM and TE polarizations at different incident angles are shown in Fig. 5(d). With the incident angle increasing from 0° to 50°, the PTCOS can produce bright colors including red, green, and blue colors for TM polarization and keep nearly black colors for TE polarization.
Fig. 5. (a) Transmittance spectra of the PTCOS for TM polarization at different incident angles. Notably, the period and grating width of the PTCOS are 280 nm and 200 nm. (b) Chromaticity coordinates corresponding to the transmittance spectra of (a). (c) Transmittance spectra of the PTCOS for TE polarization at different incident angles. (d) Color performances of the PTCOS for TM and TE polarizations at different incident angles.
To clearly explore the working principle for the PTCOSs and the physical reason for the incident angle-dependent properties of the PTCOSs, the magnetic and electric field distributions of the PTCOS with the period of 340 nm for TM and TE polarizations at the incident angles of 0° and 50° are selected as an example to study in Fig. 6. For TM polarization, the magnetic field distribution of the PTCOS at the resonance wavelength of 530 nm under normal incidence is shown in Fig. 6(a). The hybridized forms of localized surface plasmon resonances (LSPRs), propagating surface plasmon resonances (PSPRs), and waveguiding mode resonances (WGMRs) are excited in the grating ridge of the PTCOS and there are also strong couplings between adjacent gratings of the PTCOS. It is worth noting that the extensive details of these resonances (LSPRs, PSPRs, and WGMRs) and supplementary calculations are presented in Figs. S6 and S7. The electric field distribution of the PTCOS for TE polarization at the wavelength of 485 nm under normal incidence is shown in Fig. 6(b). Apparently, the electric field is blocked by the PTCOS and the resonance is barely excited in the PTCOS structure, which is due to the fact that the free oscillation of electrons is blocked on the surface of metal layer when the period of the metal grating is smaller than the wavelength of the incident light. The hybridized resonances excited in the PTCOS structure for TM polarization and the electric field blocked by the PTCOS structure for TE polarization are the main reasons for the working principle of the PTCOS. Furthermore, the magnetic and electric field distributions of the PTCOS for TM and TE polarizations at the incident angle of 50° are shown in Figs. 6(c) and 6(d). As shown in Figs. 6(a) and 6(c), the magnetic field distribution of the PTCOS for TM polarization at the wavelength of 530 nm appears obviously changes with the incident angle increasing, which is attributed to the strong angle-dependent property of the PTCOS for TM polarization. As shown in Figs. 6(b) and 6(d), the electric field distributions of the PTCOS for TE polarization at the wavelength of 485 nm keeps almost invariant as the incident angle varies from 0° to 50°, which is the reason why the PTCOS holds weak angular dependence for TE polarization.
Fig. 6. (a) (c) Magnetic field distributions of the PTCOS with the period of 340 nm for TM polarization at the incident angles of 0° (a) and 50° (c) with the wavelength of 530 nm. (b) (d) Electric field distributions of the PTCOS with the period of 340 nm for TE polarization at the incident angles of 0° (b) and 50° (d) with the wavelength of 485 nm.
It is worth noting that the smile face pattern combining of three different PTCOSs is shown in Fig. 7(a), where area I consists of the PTPOS with the period of 280 nm, area II consists of the PTPOS with the period of 340 nm, and area III consists of the PTPOS with the period of 420 nm. As shown in Fig. 7(b), the smile face pattern generated by three different color pixels can be clearly presented when the TM polarized light is illuminated at the different incident angles; the smile face pattern becomes unclear and the colors in the smile face pattern are nearly black color when the TE polarized light is illuminated at different incident angles. Compared with the traditional thermal/electric-driven tunable color devices, the PTCOSs can switch the color pixels on and off by changing the polarization state of incident angle and do not require other power consumption.
Fig. 7. (a) Smile face pattern combining of three different PTCOSs. (b) Color performances of the smile face pattern for different polarizations at different incident angles.
3. Conclusions
In summary, it is theoretical that the PTCOSs can completely switch full-hue and high-saturation transmitted structural colors by changing the polarization state. The PTCOSs consist of a 20 nm thick SiO2 grating layer, a 40 nm Al grating layer, and a 120 nm Si3N4 grating layer on a SiO2 substrate with a 200 nm MgF2 layer. Compared with the UPTCOSs composed of Al-Si3N4 nanogratings on a SiO2 substrate, the PTCOSs with the top SiO2 capping grating layer and the bottom MgF2 layer can suppress the minor transmittance peaks at short wavelengths for TM polarization and also maintain near-zero transmittance for TE polarization, thus drastically enhancing the switching performance between bright and dark states of the PTCOSs. Furthermore, it is worth noting that the transmittance spectra of the PTCOSs for TM polarization have the FWHM of ∼58 nm and the transmittance efficiency of over 70%. With the period of the PTCOS changing, a wide color gamut covering 125% sRGB space and 80% Adobe sRGB space can be achieved for TM polarization and the near-zero transmittance can be also maintained for TE polarization, which is caused by the hybridized resonance for TM polarization and the electric field blocked for TE polarization. Furthermore, full and vivid colors for TM polarization and nearly black colors for TE polarization can be realized by the PTCOSs with the incident angle varying from 0° to 50°. These results lead us to believe that high-performance PTCOSs are expected to provide a potential platform for dynamic color display applications and advanced color encryption.
Funding
National Natural Science Foundation of China (62275160); Shanghai Pujiang Program (21PJD048); National Key Research and Development Program of China (2022YFB2804602).
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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