Structural color points to the color reflected or transmitted by one physical structure [1]. In comparison to the chemical color produced by chemical dyes and pigments, structural color has better characteristics, such as controllable spatial resolution, long-term stability, and environmental friendliness [2,3]. The traditional methods to produce structural colors are performed with the help of multi-layered film [4], photonic crystal [5], bionic structure [6], and resonant scatterers [7], which usually need complex preparation process and strict restriction on material.

Optical metasurfaces consisting of nanometer scatterers show powerful light manipulation ability, and the parameters of the light field, including amplitude, phase, polarization, and frequency, may be regulated by changing the nanostructures [8]. Recently, just because of the compact structure, light weight, and high sensitivity, metasurfaces have been applied in the design of different optical components, such as metalenses [9,10], wave plates [11,12], circular polarizers [13,14], vector light generators [15–17], and color filters [18], and they have been widely utilized in many fields, including holographic imaging [19], biomedical sensing [20], information encrypting [21], and color displaying [22–25].

Metallic metasurfaces may produce structural colors with high purity [26,27], and dielectric metasurfaces possess the high efficiency [28,29]. In order to improve the tunability of a metasurface, stretchable substrates like PDMS are utilized to design the metasurface. Here, some researchers propose the use of aluminum nanoblocks patched on the PDMS substrate [30], TiO2 nanoblocks embedded into the PDMS substrate [31], and LiNbO3 nanodimers arranged on the PDMS substrate [32] to realize the tunable reflection structural colors. In comparison to the reflection style, the transmission mode is easy to detect and integrate, and it may provide more convenience for the practical applications [33–35].

In this work, we propose the stretchable transmission metasurface to generate the structural colors with high saturation and wide gamut. The transmission metasurface consists of nanoholes etched in silver film deposited on a PDMS substrate. The optimized parameters of the metasurface ensure the higher transmittance and larger color gamut area. Section 2 provides the design principle of the proposed metasurface. Section 3 shows the transmission spectra of the nanoholes and compares them with the results given in the reported works. We discuss and obtain the transmission spectra of the nanoholes etched on different substrates, with or without the matching layer of PMMA and for different incident polarization. Section 4 gives the experimental results for the structural colors of two samples and verifies the color displaying performance of the proposed metasurface. The ultra-thin and compact structure of the metasurface and the transmission mode facilitate the integration with other devices.

As we know, silver has a good response within the whole visible region, and the rectangular nanoholes etched in silver film take on the high wavelength sensitivity [26,34]. Here, we choose the rectangular nanohole arrays etched in silver film to realize the structural colors. The schematic diagram of our proposed metasurface is shown in Fig. 1(a), where the silver film is deposited on substrates like stretchable PDMS and glass. The thickness of the silver film is fixed at h=150nm, and the parameters of the nanoholes, including the length l and width w of the nanohole and the separation d of two adjacent nanoholes, along the two orthogonal directions change. The angle of θ with respect to the x-axis denotes the incident polarization angle.

Figures 1(b) and 1(c) give the transmittance of nanoholes varying with different parameters. The results in Fig. 1(b) show that the transmission spectra of the periodic nanoholes vary with the length and width of nanohole, where the separation of two adjacent nanoholes is set at d=300nm. One can see that the transmission intensity changes with the size of the nanohole, and the length and width of the nanohole also cause the weak wavelength. Figure 1(c) shows that the transmission intensity varies with the incident wavelength and the period of nanoholes, where the length and width of the nanohole are set at l=140nm and w=50nm. One can see that the transmission spectrum depends on the period of the nanoholes, and the peak wavelength gets larger with the increase of the period of the nanoholes. Moreover, the sharp peak wavelength means structural colors with high saturation can be obtained.

It needs to be pointed out that the transmission spectrum and the transmission intensity also change with the incident polarization angle. The results inserted in Fig. 1(b) show that the peak values of the transmission spectra for 45° and 90° are close, and the intensity for the former is lower than for the latter. The peak value of the transmission spectrum for 0° has undergone a blue shift and the intensity visibly decreases. Therefore, the structural color for one specific nanostructure is different for the different polarization angle. Considering these factors of the transmission spectrum, we can obtain the desired transmission spectrum with a controllable peak wavelength by optimizing the parameters of the nanoholes and choosing the illumination condition, and then realize controllable color displaying with high saturation and wide color gamut.

In order to obtain structural colors with high performance, we first simulate the transmission spectra of the nanohole arrays by sweeping the length and width of the nanohole and separating the nanoholes. For comparison, we also explore the influence of the substrate and the coating. The simulations are performed using the finite-difference time-domain technique with the incident wavelength taking from 380 to 780 nm, where the periodic boundary condition is utilized, and the minimum mesh takes 0.25 nm. The refractive indices of the silver and glass are taken from the values given by Ref. [36], and the refractive indices of the PMMA choose the values given by Ref. [37], and the ones of the PDMS take the values given by Ref. [38]. The observation plane is located at 3 μm above the metasurface. Figure 2 shows the chromaticity coordinates for a part of the obtained structural colors for the periodic nanoholes etched on the glass and PDMS substrates with the incident polarization angle taking 90° and the transmission spectra for seven different colors.

The chromaticity coordinates (x, y) of the structural colors are obtained in terms of x=X/(X+Y+Z) and y=Y/(X+Y+Z) with X, Y, and Z denoting the trichromatic stimulus values. These stimulus values satisfy the integration relations of X=A∫T(λ)Sx(λ)dλ, Y=A∫T(λ)Sy(λ)dλ, and Z=A∫T(λ)Sz(λ)dλ, respectively, where A is the constant, T is the transmission spectrum of the periodic nanoholes, Sx, Sy, and Sz are the CIE standard chrominance observer spectra, and the integral limits are limited from 380 to 780 nm. The positions of the structural colors in the chroma space of CIE1931 are shown in Figs. 2(a) and 2(c). The gamut area of the metasurface with the glass substrate, which is covered by the circles and surrounded by the solid black lines in Fig. 2(a), can reach 120.7% sRGB, which is surrounded by yellow dashed lines. The output gamut area of the metasurface with the PDMS substrate, surrounded by the black solid lines in Fig. 2(c), is about 146.9% sRGB. These results indicate that the proposed metasurface can realize the color displaying with high saturation and wide gamut.

Figures 2(b) and 2(d) give the transmission spectra with high saturation for the silver nanoholes deposited on glass and PDMS substrates, respectively. For the glass substrate, the full-widths at half-maximum (FWHMs) for the spectra with respect to the colors of green, cyan, blue, and purple are about 50 nm, and the FWHMs with respect to the other colors are also smaller than 100 nm. For the PDMS substrate, since the refractive indices of the PDMS in the visible light range are similar to those of the glass, the transmission spectra for the PDMS substrate are similar to the ones for the glass substrate. Certainly, the transmission spectra for the PDMS can realize the tunable structural colors because of its stretchable characteristic.

For further illustrating the advantages of our designed metasurface in structural color generation, we give the comparison of this work to the reported results from four aspects, such as the working mode, the shape of unit, the structure of material, and the color gamut, and Table 1 shows the comparison terms. It can be seen that the color gamut for the reflection mode is larger than that of the transmission mode. Yet our proposed transmission metasurface has a color gamut that is comparable to that of the reflection metasurface. Meanwhile, the shape of the nanounits and the structure of material in our design are simpler, and the manufacture of the metasurface is easier to achieve.Table 1.

Comparison for Structural Colors Obtained in Many Works

As we know, silver film is easily oxidized. Thus, we may cover the PMMA layer on the metasurface structure so as to prevent oxidization. We also simulate the transmission spectra of the metasurfaces with the PMMA coating. For conveniently observing the change of transmission spectrum with or without the PMMA, Fig. 3(a) shows the transmission spectra of the compound silver nanoholes on the glass substrate with and without the PMMA coating together. The transmission spectra without the PMMA coating are denoted by the solid lines, and the transmission spectra with the PMMA coating are denoted by the dash lines. The parameters of the nanoholes in Figs. 3(a) and 3(b) are the same as the ones in Figs. 2(b) and 2(d).

The results of Fig. 3(a) show that covering the PMMA makes the spectrum red shift, and the structural color changes accordingly. This is because the addition of the PMMA changes the coupling effect of the light field and the nanostructures. Moreover, as the metasurface deposited on the PDMS substrate is stretched, the transmission spectrum is also changed. During the simulation, suppose the area of the nanohole is unchangeable during the stretching process, and the width increases with the decrease of the length and vice versa. In order to see clearly the change of the transmission spectrum before and after the stretch, the transmission spectra and the structural colors for two cases are put together. Figure 3(b) shows seven results for a 20% elongation along the x-direction. Obviously, after the stretch, the peak value of the spectrum moves to the large wavelength except for the one that is red, and the intensity also changes.

In order to verify the performance of our designed metasurface and obtain the structural colors, two samples deposited, respectively, on the glass substrate and the PDMS substrate are manufactured. One sample etched on the glass substrate is to display four letters of SDNU with different colors separated by different color lines [see Fig. 4(b)]. The other sample etched on the PDMS substrate is to display four letters of LOVE with different colors, like the patterns shown in Fig. 4(f). The scanning electron microscope (SEM) images for two metasurface samples are shown in Figs. 4(c) and 4(g). The total size is 11.5μm×12.5μm for the SDNU image and 10μm×10μm for the LOVE image. The deposition of 150-nm-thick silver film is performed using the magnetic sputtering method, and the metasurface structure is fabricated using the focused ion beam etching technique. The sputtering rate is 0.22 nm per second, the sputtering power is 10 W, and the vacuum degree is 10−3Pa. The working voltage is set at 30 kV, and the current is set at 92 pA during the fabrication process. After the measurement, the 200-nm-thick PMMA is coated on the first sample using the spin-coating machine. The rotational speed is 200 r/s for the first 30 s and 4000 r/s for the last 60 s.

The sample is placed in the optical path shown in Fig. 4(a) to observe the color of the target image. The white light emitted from one light-emitting diode (LED) is converged by the lens and then changed into linearly polarized light through one polarizer. The light beam illuminates onto the metasurface sample from the transparent substrate. The color charge-coupled device (CCD) receives the diffraction pattern, which is amplified by the microscope objective (MO). The dense filter is used to control the intensity. Figures 4(d) and 4(h) show the measured results with the incident polarization angle taking 90°.

Comparing the measured patterns of Figs. 4(d) and 4(h) to the target patterns of Figs. 4(b) and 4(f), one can see that the patterns and the colors are almost the same, except for the smaller color difference. As for the smaller difference, we think it mainly comes from the nonuniform spectrum of the light source, the fabrication error of the sample, and the color aberration of the CCD. The extracted spectra from the printed pictures of the experimental and target patterns that verify their color difference are very close. Moreover, we also study the influence of the coating, and Fig. 4(e) shows the color pattern as the PMMA covers the first sample. It is obvious that the color of the pattern changes from the one of Fig. 4(d) because the transmission spectra have undergone red shifts. Certainly, the incident polarization also influences the transmission spectrum of the metasurface. Figure 4(i) shows the color pattern for the second sample with the incident polarization angle taking 0°. One can see that the transmission intensity and the color of the pattern are different from the one of Fig. 4(h). The change trend of the color pattern is consistent with the simulated spectrum change inserted in Fig. 1(b).

In addition, we also explore the tunable structural colors by stretching the sample deposited on the PDMS substrate in the experiment. Due to the influence of the ultra-thin thickness of the metasurface, the intrinsic strain of the PDMS material, and the large external stress, the metasurface is damaged as the sample is stretched. Although we observe the color change during the stretch process, the results are not recorded. Even so, it is still affirmative in terms of the simulated and observed results that the stretchable substrate can produce the tunable structural colors. We believe that the high purity, large gamut, and convenient operation are benefits to the applications of structural colors based on the metasurfaces in the fields of color displaying, optical anti-counterfeiting, and information storage.

Tunable structural colors are studied based on the transmission metasurface deposited on the stretchable substrate and coated by the matching layer. The stretchable substrate can realize the tunable structural colors, and the anti-oxidation coating ensures the long-term stability of the metasurface structure. The proposed metasurface has high resolution, and the resolution is up to 63,500 dpi. The structural colors generated by the proposed metasurface have a large gamut, and the color gamut of the nanostructures reaches 146.9% sRGB, which is larger than the reported results in the transmission mode, and it can be comparable to the ones in the reflection mode. The transmission mode is convenient for the operation and integration with other devices. The provided examples fully confirm the high performance of the designed metasurfaces for the color displaying. The structure characteristics of the metasurface, including ultra-thin thickness and high resolution, the generation of high-performance structural colors, and the convenient working mode, provide broad application prospects for metasurfaces in color displaying, color printing, and color holographic imaging.