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Structural Colors: from Natural to Artificial Systems Advanced Review Structural colors: from natural to artificial systems Yulan Fu,1 Cary A. Tippets,2 Eugenii U. Donev3 and Rene Lopez1* Structural coloration has attracted great interest from scientists and engineers in recent years, owing to fascination with various brilliant examples displayed in nature as well as to promising applications of bio-inspired functional photonic structures and materials. Much research has been done to reveal and emulate the physical mechanisms that underlie the structural colors found in nature. In this article, we review the fundamental physics of many natural structural colors displayed by living organisms as well as their bio-inspired artificial counterparts, with emphasis on their connections, tunability strategies, and proposed applica- tions, which aim to maximize the technological benefits one could derive from these photonic nanostructures. © 2016 Wiley Periodicals, Inc. How to cite this article: WIREs Nanomed Nanobiotechnol 2016. doi: 10.1002/wnan.1396 INTRODUCTION vegetation to look green. On the other hand, light can be strongly reflected and/or deflected from reach- umans perceive the world in a colorful way ing the eye because of the interaction of light with because of the selective spectral sensitivities of the structure of an object, as is the case with so-called H ’ the light receptors in our eyes retinas. Objects pre- structural colors. A familiar example of structural senting different light-spectrum distributions appear color is the atmospheric rainbow, which is caused by as possessing distinct colors. Several mechanisms for reflection, refraction, and dispersion of light in water generating colors in materials are known, including droplets resulting in a spectrum of colors appearing preferential absorption, emission, birefringence, and in the sky. photochromism. For a non-luminous object, we see a There is a rich variety of structural colors in fi fl speci c color if the object strongly re ects only a par- biology, such as the brilliant blue of the wings of ticular range of visible wavelengths. This can happen Morpho butterflies, the iridescent colors of some bee- in two ways. On the one hand, the object absorbs tles, and the vibrant metallic blue of the Pollia – part of the light spectrum due to interactions between fruit.1 3 Many efforts have been undertaken in the the illuminating photons and electrons in the mate- past decade or so to both reveal the origins of biolog- rial, which is the general coloration mechanism of ical structural colors as well as replicate them. pigments, dyes, and metals. For example, the chloro- Inspired by the natural creatures sporting these col- phyll pigment found in many bacteria, algae, and ors, researchers have achieved substantial progress in plants absorbs most of the red and blue light from developing functional photonic materials with vivid fl the sun and re ects green light, thus causing most structural colors that could find applications in sen- sing technologies, security, light-emitting sources, – paints, and other areas.4 6 * Correspondence to: [email protected] Previous review articles have cataloged and 1 Department of Physics and Astronomy, University of North Caro- explained in detail the structural colors found in lina at Chapel Hill, Chapel Hill, NC, USA 7–9 2 nature. In addition, other recent works have Department of Applied Physical Sciences, University of North fi 10–16 Carolina at Chapel Hill, Chapel Hill, NC, USA reviewed arti cial structural coloration. In this 3Department of Physics and Astronomy, The University of the article, we focus on the connection between these South, Sewanee, TN, USA two fields and explore the bridge between natural structural colors and the artificial structural colors Conflict of interest: The authors have declared no conflicts of inter- est for this article. they have inspired. The article is organized as © 2016 Wiley Periodicals, Inc. Advanced Review wires.wiley.com/nanomed follows: Structural Color Physics section briefly If light is incident from a medium with a smaller reviews the basic physics behind structural refractive index onto a medium with a higher one, coloration; Structural Colors in Nature section dis- the reflected wave shifts its phase at the interface by cusses various manifestations of structural color in an additional π radians. For a thin film in air, the biological species; and Inspired Structural Color condition for constructive interference of order section highlights the artificial structural colors gen- m and wavelength λ then becomes: erated by bio-inspired, artificially patterned, func- tional micro-/nanostructures. 1 2n d⁢cos⁡θ = m− λ: 2 2 2 STRUCTURAL COLOR PHYSICS If the film is attached to a material with a Thin-film Interference higher refractive index, the constructive interference Thin-film interference is perhaps the simplest source condition is: of structural color. It occurs when an incident light ⁢ ⁡θ λ: wave is reflected by each boundary of a thin film and 2n2d cos 2 = m the two reflected waves interfere with each other to form a new wave.17 This mechanism can be seen if These simple relationships illustrate a depend- fi one considers a plane wave of light incident from a ence between the geometric structure of the lm and the preferential wavelengths it reflects, giving rise to medium of refractive index n1 on a thin film of refractive index n , as depicted in Figure 1(a). The the perception of color when white light is incident 2 fi incident and refracted angles are θ and θ , respec- upon the lm. A soap bubble is a familiar manifesta- 1 2 fi tively. The optical path difference (OPD) between the tion of this mechanism and exempli es the basis of two reflected waves becomes: more complicated structural coloration systems. The next stage of sophistication can be considered when multiple films are stacked on top of each other. OPD = 2n2d⁢cos⁡θ2: (a) (b) n A A nB D B 1 A n nA dB 1 1 A A C n B d B A B 2 d n2 n A A B n B B (c) (d) ω ω=vk 1D 2D 3D k 2ω ω ω 2ω – a – a 0 a a FIGURE 1 | (a) Two-beam interference in a single thin film. (b) Interference in multilayers (neglecting multiple reflections). (c) Schematics of 1D, 2D, and 3D photonic crystals. (d) Band dispersion diagram of a 1D photonic crystal. © 2016 Wiley Periodicals, Inc. WIREs Nanomedicine and Nanobiotechnology Structural colors Multilayer Interference in all directions, and can confine and control light in Multilayer photonic structures are composed of peri- three spatial dimensions via complete photonic odically stacked thin films with alternating high and bandgaps. low refractive indices, as shown in Figure 1(b). When a light wave is reflected from a multilayer consisting STRUCTURAL COLORS IN NATURE of different films A and B, with refractive indices nA and nB and thicknesses dA and dB, the condition for Despite their complexity, photonic architectures are constructive interference is17: found in naturally formed structures such as opal stones and certainly in many biological materials. 2ðÞnAdA⁢cos⁡θA + nBdB⁢cos⁡θB = mλ: Their occurrence in a wide range of living species is indeed remarkable, as these animals, insects and This equation shows that, for incident sunlight, the plants manage to utilize the fundamental physical apparent color of the multilayer reflector varies with mechanisms described above to create variegated col- the observation angle: as the angle of observation ors for their biological needs. increases, the reflected color blueshifts toward shorter wavelengths. This effect occurs because the Morpho fl net OPD actually decreases even as the reflected Butter ies waves travel longer distances through their respective Among the structural colors displayed in nature, a layers, and thus constructive interference occurs for truly spectacular example is the brilliant blue on the 1,20 shorter wavelengths. The coloration tends to be more wings of the Morpho butterflies. Figure 2 selective as the number of layers increases, resulting (a) shows a photograph of a Morpho rhetenor, which 21 in very sharp reflection peaks with a strong angular gives off a bright metallic blue color. Under a low- dependence. This multilayer only has periodic index power optical microscope, it can be observed that the variation along one spatial dimension; one can intui- back sides of the fore- and hind-wings are covered tively expect similar physics could be observed in 2- with tilted scales (Figure 2(b)). Closer optical inspec- and 3-dimensional structures. tion (Figure 2(c)) indicates that a single scale consists of seemingly parallel rows, while transmission elec- tron microscopy (TEM) of a cross-section of a scale Photonic Crystals (Figure 2(d)) reveals that the rows themselves have Photonic crystals (Figure 1(c)) are ordered nanostruc- complex nanostructures of ridges and lamellae. tures of two media with different refractive indices The arrays of ridges and lamellae constitute – arranged in a spatially periodic fashion.18,19 The three types of photonic crystal-like structures.22 25 multilayer periodic stacks discussed above can be Firstly, thin films with air gaps and lamellae form a considered a 1-dimensional (1D) photonic crystal. In multilayer structure that gives rise to the blue color higher dimensions, photonic crystals exhibit analo- according to the interference condition. Secondly, a gous behaviors to crystalline solids, providing a peri- ridge is composed of a set of staggered lamella odic potential for photons via the refractive index (‘shelves’) on the right and left sides that form two contrast as solids do for electrons through the lattice phase-shifted photonic crystals. When incident white arrangement of ions. Figure 1(d) shows the band light interacts with the two photonic crystals, the structure of a 1D photonic crystal, in which there is blue component scatters back to the source. Thirdly, no electromagnetic field near k ≈ π/a, where k is the the ridges of random height form a grating-like struc- wave vector and a is the crystal lattice constant.
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