All-dielectric metasurface for high-performance structural color

The achievement of structural color has shown advantages in large-gamut, high-saturation, high-brightness, and high-resolution. While a large number of plasmonic/dielectric nanostructures have been developed for structural color, the previous approaches fail to match all the above criterion simultaneously. Herein we utilize the Si metasurface to demonstrate an all-in-one solution for structural color. Due to the intrinsic material loss, the conventional Si metasurfaces only have a broadband reflection and a small gamut of 78% of sRGB. Once they are combined with a refractive index matching layer, the reflection bandwidth and the background reflection are both reduced, improving the brightness and the color purity significantly. Consequently, the experimentally demonstrated gamut has been increased to around 181.8% of sRGB, 135.6% of Adobe RGB, and 97.2% of Rec.2020. Meanwhile, high refractive index of silicon preserves the distinct color in a pixel with 2 × 2 array of nanodisks, giving a diffraction-limit resolution.


Supplementary Note 2:The effect of gamut with the different environmental refractive index (n)
The small gamut of previous silicon metasurfaces is caused by the reflection background from the substrate as well as the broad resonant modes. With the increasing of the refractive index of the media surrounding the nanodisks from 1 to 1.5, the reflection background through the whole spectrum range becomes lower (see inset in Supplementary Figure 2 diagram, which means the color is not pure and saturation is low. With the increase of the refractive index, the chromaticity coordinate of dot keeps moving towards the outside and more and more close to the boundary, indicating that the color becomes purer and more saturated.

Supplementary Note 6:The pure color of the "福"
In China traditional culture, the Chinese character "福" represents blessing. Here, We design two kinds of Chinese paper cutting pattern" 福" with plum blossom and two fish, respectively. When the two kinds of "福" in air, the dark orchid, dark cyan, Apple green, Bitter lime, and Carrot orange are presented in Supplementary Figure 6

Supplementary Note 7: The SEM image two kinds of "福"
We also characterize the sample of diameter/period (nm) 120/240 nm (Supplementary show that the high-resolution colors can still be distinguished when they are distributed closely in a pattern. 14 Supplementary Note 10: The fabrication process of the silicon nanodisk with PMMA photoresist The metasurfaces were fabricated with electron beam lithography technique followed by a lift-off process. Firstly, we cleaned the 100 nm silicon sapphire substrates in the ultrasound bath in acetone and isopropyl alcohol (IPA) for 10 min, respectively. Secondly, 80 nm PMMA film was spin-coated onto the silicon-coated sapphire substrate and the substrate is baked at 180 °C for an hour. After that, the PMMA resist was exposed to the electron beam (Raith E-line, 30 kV) and developed in MIBK/IPA solution for 60 s at 0 °C to form the PMMA nanostructures. Then the sample was transferred into an E-beam evaporator and directly coated with 25 nm Cr films (deposition rate 0.5 Å/s, base vacuum pressure 5 × 10 −7 Torr). After immersing the sample in acetone for 8 h, the PMMA was removed and the nanostructures were well transferred to Cr.
Then the silicon was etched away with reactive ion etch (RIE) performed in an Oxford Plasma System using CHF 3 and SF 6 gases. Finally, by immersing the sample into the chromium etchant for 10 min, Si metasurfaces were finally obtained.

ZEP photoresist
In order to save fabrication time, we also perform fabrication of silicon metasurface using ZEP520 photoresist as etching mask since ZEP520 typically has a 1:1 dry plasma etch selectivity relative to Si. We first spin-coat ZEP520 electron beam resist with thickness of 350 nm on top of the 100 nm silicon on sapphire substrate. After baking at 180 °C for 1 hour, Electron beam lithography (EBL) method is used to pattern the nanostructure on the resist. The reverse pattern of the nanodisks structure is wrote on the ZEP resist. After that, the resist is developed in the ND510 solution for 60 s at 0 °C. Since ZEP520 is a positive tone resist, nanodisks structure will remain in the resist and acts as the soft mask for silicon etching. Then the silicon structure is obtained by anisotropy etching with the reactive ion etching (RIE) process using SF 6 as etching gas and CHF 3 as protect gas at a RF power of 300 W. After immersing the sample in acetone for 8 h, the remaining ZEP is removed and the Si metasurfaces are formed. By means of the XYZ tristimulus values, we can define the chromaticity coordinates x y z.
that is: = + + In this paper, we use the FDTD Solutions software to simulate the optical reflectance spectrum from the silicon nanodisks with the different diameter and period. Then the simulated reflectance spectrum for each color pixel is converted to xy coordinate in the CIE 1931 chromaticity diagram.

Supplementary Note 17: The further optimization of structural color
In the main text, we have shown that the combination of refractive index matching layer and Si metasurface is a good way to approach the ultimate limits of structural color. However, it is important to note that the results in the main text are not extremely optimized. There are still rooms to improve the performances. To illustrate this statement, we perform more numerical calculations and experiments to optimize the reflection of the silicon metasurface at shorter wavelengths. As shown in Supplementary Figure 16(a), through optimizing five parameters in simulation, the highest reflection of a metasurface with diameter/period of 120/200 nm can reach as high as 65% at a wavelength around 500 nm (for green color) when the sample is placed in air. Similarly, the highest reflectance of Si metasurface with diameter/period of 80/140 nm can reach 53% at a wavelength around 430 nm (for blue color). After adding the refractive index matching layer, the highest reflectance peak at 500 nm and 430 nm only slightly deceases to 62% and 47% (see Supplementary Figure 16

Supplementary Note 18:The multipolar decomposition analysis
In the main text, we have observed numerically and experimentally that the reflection background is reduced and the reflection peak is narrowed. Here we employ the multipolar decomposition analysis to study the underlying mechanism for the improved color performances.
In the calculation of multipolar decomposition, we expand the total scattering power into electric dipole (ED), magnetic dipole (MD), electric quadrupole (EQ) and magnetic quadrupole (MQ).
In the calculation of multipolar decomposition. In the case of harmonic excitation ( ), the power of scattering cross section can be expressed as: The electric dipole moment is defined as = ∫ . (2) The magnetic dipole moment is = ∫ ( × ) . (3) The electric quadrupole moment is And the magnetic quadrupole moment is = ∫ ( × ) + ( × ) . (5) Here j represent current density and c is light speed.
To illustrate the universal of the mechanism, we take three Si metasurfaces with blue, and MD resonances lie in their electromagnetic field distributions. As shown in Fig. 2 of the main text, the ED resonance has larger portion of electromagnetic field that is localized outside the Si nanodisk. As a comparison, the MD resonance is well confined within the nanodisk.
Consequently, the former one shows more significant change when the refractive index of surrounding medium is changed.

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We have also studied the dependence of color performance of the pixels on the viewing angle. We simulate the scattering cross section of 2 × 2 pixels for three primary colors. The results are shown in Supplementary Figure 22. The diameter/period (nm) for these three primary colors are 80/140 nm, 120/240 nm, and 170/320 nm, respectively and the viewing angles are 0° and 60°. As shown in Supplementary Figure 22 It is important to note that the structural color can be well preserved after replacing the DMSO with PMMA or SU8. Supplementary Figure 25 shows the experimental results. We can see that the yellow, green and blue colors in air becomes distinct red, green, and blue colors (see Supplementary Figure 25 This kind of packaging can be repeated for many rounds. As shown in Supplementary Figure   25(d)- Supplementary Figure 25(l), the packing process has been repeated for another 9 rounds without obvious degradation in color impressions. This clearly shows the robustness of the packaging process as well as the durability of Si metasurface.