Colors, generated by the interaction of light and matter, play an essential role in our world [1,2]. Compared to organic dyes and chemical pigments, which are formed by absorbing certain wavelength in visible light, structural colors generated through the interference or scattering effects of light with nanostructures offer the advantages of high resolution, environmental friendliness, and great durability [3–11]. Examples of structural colors are widespread in nature, where the vivid color is found such as the wings of butterflies and the feathers of peacocks [12]. Recently, man-made structural color has attracted much attention in a variety of research fields and found important applications in digital display [13,14], imaging security certification [15], optical data storage [16], dynamic full-color display [17–23], and so on. In particular, beneficial from the development of advanced nanofabrication technologies, structural colors based on plasmonic and high-index dielectric materials offer a new route for color printing technologies because of the enhanced light–matter interaction due to plasma and Mie resonances [24–29]. Sun et al. reported full color printing with TiO2 metasurfaces, achieving high reflection and high contrast bright field structural colors in the entire visible region [30]. Moreover, the plasmonic structural colors possess an extremely high resolution for color printing and the angle independence [31–33]. More recently, Elkabbash et al. developed a Fano resonant optical coatings platform [34], and Chen et al. successfully designed and manufactured an optimized Fano resonance thin film structure [35], both of which provide structural colors with high purity, high brightness, and full color access. These works most have recently achieved various colors with large gamut, high saturation, and high resolution.

All colors in the practical world are defined by its three inherent characteristics: the hue, saturation, and brightness (HSB). Our brain distinguishes colors according to the three characteristics, which is a more intuitive color mode and closer to people’s visual principles than decomposing the color light into red, green, and blue individual colors. The color brightness can give the right chiaroscuro of an image, and thus the generation of HSB color is essential for the display of the 3D stereoscopy [6,36,37]. Thus, a basic indispensable requirement for a structural color technique is the on-demand generation of colors in the whole three-dimensional (3D) HSB space [38–40]. However, most works have recently been devoted to generating structural colors with large gamut and high saturation, which brings about only the control of the color in the HS plane but lacks brightness (B) control [25,30,41–43]. In these structures, once the H and S values of a color are fixed, the B value is also fixed correspondingly, indicating that the colors cannot cover the whole 3D HSB space. Some approaches have been proposed to control the brightness of structural colors, which unfortunately require large complex pixels (even larger than that in commercial displays) composed of multiple units and restrict the spatial resolution [44,45]. To create high-resolution artificial structural colors, Bao et al. reported a dielectric metasurface made of crystal silicon nanoblocks, showing that the dielectric metasurface could exhibit HSB color nanoprinting [46]. However, the metasurface involved surface feature with high aspect ratio (the ratio of height to lateral dimension) beyond 15, bringing difficulty to lithography and etching precision. Therefore, it remains a challenging task to create easily fabricated color pixels with high saturation and brightness control, which are required for vivid, chiaroscuro images.

In this work, we present a general strategy for structural color generation based on localized surface plasmon resonances (LSPRs), which consists of thin anisotropic patterned PMMA film clamped by two metallic thin films, featuring a low aspect ratio and resulting in easy fabrication. In contrast to most previous reports, which can hardly exhibit the right chiaroscuro of an image due to the lack of brightness control, our strategy can independently control the brightness by varying the ratio between the major and minor axes of the elliptical hole without changing hue and saturation. Moreover, the larger gamut is achieved since the structural colors are from the reflection spectra of cross-polarization, where the narrow reflection peaks can be easily realized by the polarization conversion [37,47]. The measured reflection spectra show that the resonant wavelengths remain invariant but the reflection peaks change, leading to the control of the brightness of colors, by varying the length of minor axis and fixing the length of major axis. As a result, the structural color is expanded from the general two-dimensional HS color space to the three-dimensional HSB color space. Moreover, the high purity is obtained by the sharp resonance of polarization conversion. The structural colors from reflection spectra of cross conversion cover approximately 120% of the area of the sRGB gamut. The resolution of the fabricated images reaches 15,000 pixels per inch (PPI), which is high enough for various display and imaging applications [48]. These exciting findings are believed to facilitate the development of ultra-high-resolution 3D nanoprinting, making images more vivid and realistic.

We start by the mechanism of the enhanced cross-polarization reflection of the anisotropic metasurface in Fig. 1(a), which is composed of a PMMA film patterned by arrays of elliptical-shaped holes clamped by two aluminum (Al) thin films. Here, two aluminum (Al) thin films are adopted since the polarization conversion efficiency can be enhanced by the Al layer on the Si substrate. And the Al material was selected since it is superior to Au or Ag. First, Al has higher stability than Ag since the Al2O3 self-oxidation layer forms a durable protective layer and preserves the metal while Ag will be oxidized in the air and becomes black AgO [49]. Second, the absorption of Au at short wavelengths (400–550nm) is larger than that of Al since its surface plasmon resonant wavelength is about 550 nm (Al∼250nm, Ag∼350nm). Third, Al material is the third most abundant material on the Earth and much cheaper than Au and Ag. The simple structure has a low depth-to-width ratio, where the thicknesses of Al and PMMA films, and the height of the Al nanodisk, are, respectively, 80 nm, 110 nm, and 100 nm. The elliptical-shaped nanoholes in Fig. 1(a) are arranged in square lattices with period P, the major and minor axes of ellipse are 2a and 2b, and the ratio of the major axis to the minor axis is defined as rab=2a/2b. The ratio of major axis to the period P is defined as raP=2a/P. In this work, unless stated, there is a 45° angle between the long axis of the ellipse and the x axis, the incident linearly polarized light is parallel to the x axis, and the reflective linearly polarized light along the y axis is measured. Thus, the ellipse receives two orthogonal components (ES and EP) of incident light with equal contribution. In contrast to previous reports where plasmon or Mie resonances are excited in an isotropic nanostructure [2,31,43,50], our structural colors are from the reflection spectra of cross-polarization, where the anisotropic metasurfaces can tune the position and the intensity of the resonant peak [51–53]. The narrow reflection peaks can be easily realized by the polarization conversion since anisotropic nanostructures are able to efficiently rotate the polarization of normally incident linearly polarized light only at specific design wavelengths [37,47]. At some specific wavelengths, one of the two components of the reflected light incident on anisotropic metasurfaces undergoes a π phase shift so that it rotates the total polarization vector by π/2, and passes through the analyzer exactly, as shown in Fig. 1(a) [47,53,54]. Consequently, the cross-polarization structural color of bilayer anisotropic metasurfaces is achieved by the high-performance linear polarization conversion in reflection at narrow band range.

We numerically investigate the effect of the ratio of the major axis to the minor axis rab on the efficiency of the polarization conversion. Figure 1(b) presents the influence of the different minor axis on the cross-polarization reflection spectra of anisotropic metasurfaces with the period of 440 nm, the major axis of 275 nm, and the raP of 0.625. It is shown that the resonant wavelengths of the reflection spectra almost remain invariant with 12 different minor diameters from 165 to 275 nm. As the diameter of the minor axis changes, rab varies from 0.6 to 1. The inset shows that the peak value of the reflection spectrum changes with the length of the minor axis, indicating that the polarization conversion efficiency first increases and then decreases with the increase of the minor axis. Obviously, the polarization conversion efficiency is 0 when the ratio of the major and minor axes is 1 (the ellipse is circular). It is also seen that the cross-polarized conversion efficiency is highly dependent on the ratio of the major axis to the minor axis rab, reaching the maximum when the short axis diameter is 185 nm. For the even shorter minor axis, the peak value of cross-polarization diminishes. This is attributed to the reduction in the interaction between nanoholes as the minor axis length decreases. The increasing minor diameter reduces the anisotropy of the metasurface, leading to a reduction in cross-polarization conversion efficiency, which is demonstrated by the decrease in the cross-polarized electric field |Ey| distributions in Fig. 1(c) (left, the resonant peak 1 at 516 nm, the minor diameter of 185 nm; right, the resonant peak 2 at 504 nm, the minor diameter of 225 nm). Further, to more intuitively observe the effect of the diameter of the minor axis, the left panel in Fig. 1(c) demonstrates the field distributions |Ey| at the cross section parallel to the minor axis, which presents two “hot spots,” signifying a strong plasmon resonance near the aluminum surface. This is because surface plasmon polaritons (SPPs) will cause the electric field to be strongly localized and enhanced on the metal surface [55–58]. All cross-polarized electric field |Ey| distributions (not shown here) in the cross section parallel to the minor axis direction for 12 different minor diameters from 165 to 275 nm show that the field first increases and then decreases with the minor diameter increasing, which agrees with the cross-polarization reflection spectra in Fig. 1(b).

Next, we analyze the effect of the period on the reflection spectra and the coloration hue. Figure 1(d) shows the calculated cross-polarized reflection spectra and virtual colors of the arrays of elliptical-shaped holes with the period varying from 380 nm to 600 nm with a step of 20 nm under the incident x-polarized light and the analyzer angle at 90° from the incident light. Here, the raP/rab is fixed at 0.6/1.15. The cross-polarization peak red shifts significantly across the full visible range with the period increasing, indicating that the hue is mainly determined by the period. The calculated result shows a 195 nm red shift from 448 to 643 nm. The corresponding structural colors calculated from the reflection spectra exhibit distinct vivid colors such as red, green, and blue [see inset in Fig. 1(d)], showing the ability to generate colors that may be feasible across the entire visible spectrum. Moreover, the cross-polarized reflection peak carries more than 50% of the incident power, which is much higher than the reported polarization conversion of the single reflection of the monolayer structure [56].

We also analyzed the performance of the co-polarization reflection spectra as shown in Fig. 2. It is indicated that an absorption peak appears on the co-polarization reflection spectra, which is quite different from the cross-polarization case. Thus, the co-polarization reflection spectra can be another dimension used for information storage. However, the co-polarization reflectance spectra remain almost constant as the rab increases from 1.2 to 1.5 as shown in Fig. 2(a). The corresponding brightness of structural colors changes a little with the rab increasing, as shown in the inset in Fig. 2(a). Moreover, the cross-polarization reflectance spectra can be covered by the total ones (the cross-polarization and co-polarization) as shown in Fig. 2(b), which present the similar behavior to the co-polarization ones. Therefore, the encryption is obtained if the analyzer is removed. Figure 2(c) shows the co-polarized reflection spectra and virtual color of the metasurfaces with periods ranging from 380 nm to 600 nm and the step size of 20 nm. It is shown that an absorption peak appears at each co-polarization reflection spectrum and redshifts in the visible light range with the period increasing. The corresponding subtractive structure color calculated from the reflection spectrum is shown in the inset in Fig. 2(c). It is obvious that the subtractive structural colors have less saturation than that from cross-polarization. Therefore, the cross-polarization reflection spectra are focused on in the following due to their ability of brightness control and high saturation.

Figure 3(a) shows the fabricated color palettes consisting of arrays of elliptical-shaped holes with different periods and ratios of the major/minor axes. We can see that the brightness of the color obviously increases with rab increasing, and the hue and saturation of each column almost do not change. At the same time, the arrays present blue to green and to red when the period spans from 380 to 620 nm while exhibiting almost the same color for the same period. This indicates that the rab mainly affects the color brightness, but the color phase strongly depends on the period. The extension of structural color from the general two-dimensional space (HS) to the three-dimensional space (HSB) can be achieved by changing the rab. On the other hand, we calculated the CIE coordinates of the colors with the same rab and different periods, varying from 380 nm to 620 nm with step of 20 nm. As shown in Fig. 3(b), when the rab changed from 1.2 to 1.7, the position of the CIE coordinates did not change drastically. So rab has little effect on the hue and saturation of color.

Figure 3(c) shows the calculated and measured zeroth-order cross-polarized spectra of period 400, 500, and 580 nm, respectively. One can see that, for the three periods, both the calculated and measured reflections of each nanohole array increase from about 10% to above 30%, with the rab increasing from 1.2 to 1.5. And with the linear increase of rab, the increase of reflectance gradually becomes saturated, which is due to a certain polarization conversion efficiency. The discrepancy between the measured and simulated reflection spectra possibly comes from the fabrication imperfections and measurement errors. The measured reflection spectra are converted into the HSB color space coordinates as shown in Fig. 3(d). Here, the hue parameter is divided into 15 segments, and the saturation parameter is divided into 14 segments. That is to say, the top view of the obtained color space is a 15-hue circle, each sector covering 24°, and the sector from inner to outer saturation increases by 1/14. When the period is 400 nm, the coordinates of the four measured points are all in the outermost layer of the conical HSB space and in the same column. For convenience, we set the highest value of the converted brightness to 100%, all values are linearly transformed with the highest value, and the resulting final value is represented in Fig. 3(d). It is shown that the brightness gradually increases for the three periods of 400 nm, 500 nm, and 580 nm.

We then experimentally demonstrate the color control by changing the period. Figure 4(a) presents the “hue circle” consisting of anisotropic metasurfaces with low etching aspect ratio and different periods, which are fabricated on PMMA film (110 nm) by using standard electron-beam lithography followed by electron-beam evaporation as detailed in Section 4.C. The period is 380 nm, 390 nm, 420 nm, 430 nm, 450 nm, 460 nm, 480 nm, 500 nm, 520 nm, 580 nm, 600 nm, and 620 nm, respectively. The ratio of the major axis and the period raP is 0.84, and the ratio of the major/minor axes rab is 1.3. Anisotropic metasurfaces are characterized by using a home built-in spectroscopy and imaging system (see Section 4.A). The optical microscopy image of “hue circle” in Fig. 4(a) demonstrates that anisotropic metasurfaces exhibit distinct vivid colors such as blue, green, and red. The hue changes with the variation of the period indicate that the optical response is primarily determined by the lattice constant. Scanning electron microscopy (SEM) images in Fig. 4(a) clearly present the array of elliptical holes of the “hue circle,” with periods of 380 nm (blue), 460 nm (green), and 620 nm (red), respectively. The long axis of the ellipse is 45° from the horizontal.

Figure 4(b) presents the measured and calculated reflection spectra of 12 sections of the “hue circle.” In simulations, we adopted the measured geometric parameter. The measured reflection spectra show that the resonance wavelength is redshifted with the array period increasing, covering almost the visible region, which are corresponding to the simulated ones. Moreover, the full widths at half maximum of all reflection spectra are almost 80 nm, which leads to the structural colors with high saturation. To more intuitively evaluate the color control, we calculate the corresponding CIE 1931 chromaticity diagram from measured and simulated reflection spectra in Fig. 4(b). Both the solid dive-pointes stars (measured) and hollow diamond (simulated) in Fig. 4(c) show the full-color display can be achieved by changing the period, which covers approximately 120% of the sRGB color gamut.

Figure 5(a) shows the HS and HSB modes of the same image; one can see that the right chiaroscuro allows the stereoscopic impression of the figure, making the HSB image much more vibrant and artistic. Owing to the lack of a brightness dimension, the HS image cannot display the chiaroscuro information. We extract the H, S, and B values of each pixel of HSB picture in Fig. 5(a) to match the pixels in the fabricated color palette in Fig. 5(a) and then convert the matched pixels to the corresponding parameter values into the manufacturing structure. Here, the raP is fixed to 0.84, and the periods of red, yellow, green, and blue are 620, 480, 460, and 380 nm, respectively. Figure 5(b) shows the optical microscope image of the fabricated HSB color printing, which clearly demonstrates the reproduction of the HSB image with varied colors and chiaroscuro information. The fabricated HSB image almost identically reproduces the digital image. The diameters of the minor axis are 276 and 322 nm according to the SEM measurement of the green region, respectively. It indicates that the SEM images of areas of different light and dark colors on the windmill present different ratios of the major and minor axes (smaller ratio, darker color), which can accurately control the color brightness.

Figure 5(c) shows high spatial resolution images of red, green, and blue patterns with different metasurfaces, whose square subpixels are reduced in size from 20 μm (mobile display levels, 400 PPI) to 1.2 μm (ultrahigh density displays, more than 15,000 PPI). The nanoholes in the red, green, and blue metasurfaces have identical raP=0.84 and rab=1.2 but different periods of 580, 440, and 380 nm, respectively. The optical microscope image in Fig. 4(c) shows the clear red, green, and blue colors in large pixels (10 to 20 μm). More importantly, no appreciable color distortion is observed in small RGB pixels downscaling to 2 and 1.2 μm, although it appears somewhat blurry owing to the resolution limit of optical microscopy. It is shown that the resolution is high enough for various display and imaging applications [48].

As summarized in Table 1, the proposed metasurface realizes structural colors with the large gamut, high saturation, and independent brightness control, which can achieve better or comparable performance compared to the reported works [39,40,46,56,59]. Jiang et al. presented a structural color based on cross-polarized reflection, which realized the large color gamut and high saturation in the HS space, but lacked brightness control [56]. Hail et al. realized the brightness control of the plasma structural color by changing the period of color pixels in the y direction, but the color gamut is very limited [40]. In the works of Shang et al. and Jang et al. color brightness can be independently controlled by changing the incident polarization angle, while the saturation was not high [39,59]. Our strategy has realized the high saturation of structural colors by the polarization conversion since anisotropic nanostructures can efficiently rotate the polarization of normally incident linearly polarized light only at specific design wavelengths. Moreover, the proposed metasurfaces have a low-aspect-ratio surface feature, benefiting the fabrication, compared to the silicon nanocuboid [46].Table 1.

Color Performance of Related Works

In summary, we theoretically and experimentally demonstrated a structural color printing at HSB space employing low-aspect-ratio anisotropic metasurfaces based on cross-polarized reflection. This approach offers an independent control over the gamut and luminance of the generated colors. It is experimentally and numerically demonstrated that the brightness of colors can be tuned by simply varying minor axis of nanoholes, and, at the same time, the hue and saturation of the colors are almost unaffected. The fabricated 3D windmill image shows the realization of a real HSB color printing technique to customize the brightness of colors at the nanoscale. In addition, the full colors, covering approximately 120% of the sRGB color gamut, are generated by simply tuning the periods. The sub-wavelength resolution for our metasurfaces can at least reach 15,000 PPI, which is high enough for various display and imaging applications. We believe this research could serve as a new inspiring option for structural colors generation, opening possibility to potential applications of ultra-high-resolution 3D nanoprinting future imaging and high-density data storage.

Measurement results are obtained by using an optical microscope (Olympus BX51), a charge-coupled device (CCD) camera (Canon EOS 750D), a fiber-coupled spectrometer (Ocean Optics HR4000), and a scanning electron microscope (ZEISS SUPRA 55). The light source of the microscope is non-polarized white halogen light (100 W), which is focused on the sample surface through a 5× (NA: 0.15) objective lens. The color reflection image in the cross-polarization state can be obtained through the CCD. At the same time, the spectrometer measures the reflectance spectrum through a pinhole, which collects the light reflected from a specific area. The polarizer and analyzer used are the default rotatable polarizer and analyzer in the microscope. The cross-polarization (incident with polarization along the x direction and exiting with polarization along the y direction) spectrum is measured by first obtaining a reference spectrum of a clean aluminum surface in co-polarization (incident with polarization along the x direction and exiting with polarization along the x direction). The sample is then measured in cross polarization and normalized by the reference spectrum. The geometrical dimensions of the sample are obtained by the SEM.

We calculated the cross-polarized reflection spectrum of the structure and the electric field profile at the various cut-planes by using the FDTD method. For the cross-polarized reflection spectrum simulation, we set the periodic boundary condition to both x and y boundaries of the simulation region. The simulation regions have x and y lengths spanning the length of the specified periodicity P and z-dimension span from −300 to 2000 nm. The refractive index of PMMA was taken from the ellipsometer. The refractive indices of aluminum, alumina, and silicon were taken from the material data of the software. An x-polarized plane wave was placed at 1200 nm, illuminating from elliptical nano-hole side to the substrate, and the reflected light was detected by a monitor at 1500 nm. For the reflected light, reflectance can be taken as the squared modulus of the y component of electric field intensity. Because wavelengths are shorter than the effective period, diffraction is inevitable, and the reflected field does not impinge the monitor normally; we have to only collect the zeroth-order reflected wave and calculate the y component of the electric field. For the electric field distribution, three frequency-domain field and power profile monitors, two for the x−y cut-plane and for the y−z cut plane and field time monitoring, were set up.

Aluminum was first deposited on a silicon substrate using an evaporator (Denton vacuum explorer 14). Subsequently, 110 nm of PMMA electron beam resist (A3, Microchemistry) was spin-coated on the substrate at 4000 r/min for 1 min and baked on a hotplate at 180°C for 2 min. The ellipses were patterned using electron beam lithography (JEOL JBX-6300FS). After development, the sample was washed with DI water and dried with nitrogen gas. Finally, 80 nm aluminum was deposited on sample using the evaporator (Denton vacuum explorer 14) again.