Nanophotonic Structural Colors

[Tina Popielarczyk]

Colorsplay important roles in human life since they are capable of carrying information for people to distinguish objects and perceive the world [1], [2], [3], [4]. In contrast to traditional colors generated from chemical dyes or pigments by the selective absorption and reflection of specific wavelengths of light, structural colors that arise from the interaction between light and nanostructures of objects rely strongly on the arrangement and shape of the nanostructures rather than their chemical properties, and have attracted great interest in recent decades due to their advantages of lower toxicity, superior stability, and higher anti-fading capacity [5], [6], [7]. During the past years, many types of optical engineering structures, such as, photonic crystals [8], [9], [10], [11], [12], metallic gratings [13], optical antennas [14], [15], plasmonic metamaterials, and metasurfaces [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], have been introduced to generate structural colors. While a wide gamut of color space can be covered based on such artificially engineered nanostructures, the realization of such nanostructures generally requires high precision micro-nano-fabrication techniques such as, electron-beam lithography [20], [21], nanoimprint lithography [22], focused ion beam milling [23], and direct laser writing [24], etc. This would result in high production cost and thus severely limiting their applications. Another strategy for generating structural color is based on planar thin-film stacks, including semiconductor-metal dual-layer [26], [27], [28], metal-insulator-metal triple-layer [29], [30], and multilayered structures [31], [32], [33], [34], [35]. Although such stratified media can be more easily realized in production and leading to the reduction of processing costs, their performances are usually sensitive to the angle of vision [27], [30]. This feature is also unfavorable for some specific applications such as angle-independent displays and color filters.

The fabrication procedure of the bilayer structure for producing structural colors is schematically illustrated in Figure 1A. Polished silicon (Si) wafers were first ultrasonically cleaned in sequence with acetone, alcohol, and deionized water (19.3 MΩ), followed by dried with nitrogen gas. A 150 nm gold (Au) layer was then deposited on the cleaned Si wafers by thermal evaporation (PZF-300, KYKY). In order to improve the adhesion between the Si wafer and Au film, a 10 nm chromium (Cr) film was deposited on the Si wafer before depositing the Au layer. Next, the gold-coated Si wafers were placed into the chamber of a high vacuum sputtering coater system (BAL-TEC, SCD 500) to grow a 5 nm thickness of copper (Cu) thin film. Then, a 3 nm thick Cu film was continued to grow by covering the first area of the sample with a mask. Through repeating the last step for several times by covering the deposited area with mask, at last, a series of thicknesses of Cu films (5, 8, 11, 14, and 17 nm) were obtained on a single substrate. After that, the ultrathin Cu films were annealing on a LED constant temperature heating table (SET-217) in air at 400C for 2 h on a hot plate. As a result, the Cu thin films were oxidized to CuO thin films with the corresponding thickness of 12, 20, 26, 30, and 36 nm. Finally, a series of deep-subwavelength planar nanostructures with vivid structural colors were obtained.

Fabrication process and characterization of the proposed bilayer thin-film nanostructures. (A) Schematic of the fabrication process of the proposed CuO/Au bilayer nanostructure. Surface and cross-sectional (insert) SEM images of a fabricated sample (B) before annealed (with 14 nm of Cu thin film) and (C) after annealed (with 30 nm of CuO thin film). XRD patterns of a structure consisting of a 50 nm thick Cu film deposited on Si substrate, before annealed (D) and after annealed (E).

The morphologies of the fabricated samples were characterized by scanning electron microscope (SEM, FEI Sirion 200) at high-resolution mode. Figure 1B, C shows the surface/cross section SEM images of a fabricated sample before annealed (with 14 nm of Cu thin film) and after annealed (with 30 nm of CuO thin film), respectively. As one can see, after annealed at 400C in ambient air for 2 h, well defined uniform artifacts with regular shapes are revealed on the surface of the sample (see Figure 1C and Figure S1 in Supplementary Material), indicating the existence of nanocrystallites.

Figure 2A displays the experimental reflectance spectra for five different thickness of Cu thin films (5, 8, 11, 14, and 17 nm) deposited on Au at normal incidence. It is noted that for such intermediate structures there is no significant difference in the reflectance spectra, meaning that they can only cover a very limited range of color space. A photograph of these intermediate structures is shown in the top panel of Figure 6A, which was taken from an optical camera (Nikon, D7000). As expected, weak color changes are observed from this panel. Figure 2B shows the experimental reflectance spectra for five different thickness of CuO thin films (12, 20, 26, 30, and 36 nm) deposited on Au at normal incidence. Compared with Cu/Au intermediate structures, these CuO/Au samples possess completely different optical properties, all of them have much deeper and broader absorption resonances. More importantly, as the thickness of CuO increases, the resonance dip becomes redshift obviously. This makes our structures to probably generate different colors by modifying the reflection in a portion of the visible spectrum. The bottom panel of Figure 6A shows a photograph of these five fabricated samples. It is obvious that five distinct colors are really achieved: light yellow (12 nm of CuO), orange (20 nm), dark red (26 nm), dark purple (30 nm), and cyan (36 nm).

Reflectance spectra of a series of samples at normal incidence. (A) The experimental reflectance spectra of a series of Cu/Au intermediate structures at normal incidence, where the thicknesses of Cu thin films are 5, 8, 11, 14, and 17 nm, respectively. (B) The experimental reflectance spectra of a series of CuO/Au nanostructures at normal incidence, where the thicknesses of CuO thin films are 12, 20, 26, 30, and 36 nm, respectively. (C, D) The calculated reflection spectra respectively correspond to the experimental results in Figure 2A, B.

Angular responses of the optical reflection properties. Experimental (A, B, C) and calculated (D, E, F) reflectance spectra as a function of the wavelength and incident angle for s, p, and unpolarized light, respectively. Here, hCuO = 30 nm.

Angular responses of the color effects. (A) Top panel: A photograph of a series of Cu/Au intermediate structures (before annealed). Bottom panel: Photographs of a series of CuO/Au nanostructures (after annealed) taken at different angles. (B) The CIE 1931 chromaticity coordinates of the colors generated by our structures. Solid symbols and stars are the results of Cu/Au intermediate structures (before annealed) and CuO/Au nanostructures (after annealed), respectively. The black line denotes sRGB color space.

To gain insight into the characteristic of our structure, theoretical analyses were performed. We consider light incident from air (N1 = 1) upon an absorbing film with thickness h and complex refractive index N2 = n2 + ik2, deposited on a metallic substrate with complex refractive index N3 = n3 + ik3 at an angle θ (see Figure 3), according to Ref. [37], the reflection coefficient can be written as:

To provide further theoretical insights, we performed the partial reflected wave calculations to analyze the optical behaviors of our structures [39], [40]. As schematically shown in Figure 3, the reflection coefficient for such a system can be achieved by the coherent sum of the partial reflection waves, namely

Structural colors traditionally refer to colors arising from the interaction of light with structures with periodicities on the order of the wavelength. Recently, the definition has been broadened to include colors arising from individual resonators that can be subwavelength in dimension, for example, plasmonic and dielectric nanoantennas. For instance, diverse metallic and dielectric nanostructure designs have been utilized to generate structural colors based on various physical phenomena, such as localized surface plasmon resonances (LSPRs), Mie resonances, thin-film Fabry-Prot interference, and Rayleigh-Wood diffraction anomalies from 2D periodic lattices and photonic crystals. Here, we provide our perspective of the key application areas where structural colors really shine and other areas where more work is needed. We review major classes of materials and structures employed to generate structural coloration and highlight the main physical resonances involved. We discuss mechanisms to tune structural colors and review recent advances in dynamic structural colors. In the end, we propose the concept of a universal pixel that could be crucial in realizing next-generation displays based on nanophotonic structural colors.

N2 - Structural colors traditionally refer to colors arising from the interaction of light with structures with periodicities on the order of the wavelength. Recently, the definition has been broadened to include colors arising from individual resonators that can be subwavelength in dimension, for example, plasmonic and dielectric nanoantennas. For instance, diverse metallic and dielectric nanostructure designs have been utilized to generate structural colors based on various physical phenomena, such as localized surface plasmon resonances (LSPRs), Mie resonances, thin-film Fabry-Prot interference, and Rayleigh-Wood diffraction anomalies from 2D periodic lattices and photonic crystals. Here, we provide our perspective of the key application areas where structural colors really shine and other areas where more work is needed. We review major classes of materials and structures employed to generate structural coloration and highlight the main physical resonances involved. We discuss mechanisms to tune structural colors and review recent advances in dynamic structural colors. In the end, we propose the concept of a universal pixel that could be crucial in realizing next-generation displays based on nanophotonic structural colors.

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