We report the mechanism of color production in peacock feathers. We find that the cortex in differently colored barbules, which contains a 2D photonic-crystal structure, is responsible for coloration. Simulations reveal that the photonic-crystal structure possesses a partial photonic bandgap along the direction normal to the cortex surface, for frequencies within which light is strongly reflected. Coloration strategies in peacock feathers are very ingenious and simple: controlling the lattice constant and the number of periods in the photonic-crystal structure. Varying the lattice constant produces diversified colors. The reduction of the number of periods brings additional colors, causing mixed coloration. C olor production in nature takes advantage of either structural coloration (1, 2) or pigmentation. Structural colors result from the interaction of light waves with a featured structure having the same order of size as the light wavelength. Structural colors in avian feathers have been usually qualitatively understood by thin-film interference (3Ϫ5) or the scattering from a spongy matrix structure incoherently (6, 7) or coherently (8, 9). Although the structural colors of avian feathers have been studied for a long time (10Ϫ14), many questions remain to be answered. In particular, the precise physical mechanism that produces the diversified colors in peacock tail feathers has not been established. Materials and MethodsThe male peacock tail contains spectacular beauty because of the brilliant, iridescent, diversified colors and the intricate, colorful eye patterns. Peacock feathers serve as an excellent canonical example for investigating structural colors in avian feathers. The structures of the blue, green, yellow, and brown barbules in the eye pattern of a male green peacock (Pavo muticus) feather were characterized by using an optical microscope and a scanning electron microscope. The peacock tail feather has a central stem with an array of barbs on each side. On each side of a barb there is an array of flat barbules. Each barbule has round indentations of typically Ϸ20-30 m, which disperse the incident light, causing coloration. The round indentation has a smoothly curved crescent-like profile in transverse cross section (14).To understand the detailed mechanisms of color production in peacock feathers, a plane-wave expansion method (15) was used to calculate the photonic band structure of the periodic photonic structures. A transfer matrix method (16) was adopted to compute the reflectance spectra to compare with experimental results. Fig. 1 shows the submicron structures of barbules. The transverse cross sections reveal that a barbule consists of a medullar core of Ϸ3 m enclosed by a cortex layer. Interestingly, the cortex of all differently colored barbules contains a 2D photonic-crystal structure (14, 17Ϫ19) made up of melanin rods connected by keratin. The longitudinal cross section shows that the melanin rod length is Ϸ0.7 m. Melanin is created by melanocyte cells, deposited in developing feathers,...
Raman shifts of Si nanocrystals versus size were studied theoretically by a bond polarizability model. Zero-dimensional spheres and one-dimensional columns were considered. The relation between the Raman shift and the size for Si spheres and columns was established, from which the size of Si nanocrystals can be obtained for a given Raman shift or vice versa.
Materials with massless Dirac fermions can possess exceptionally strong and widely tunable optical nonlinearities. Experiments on graphene monolayer have indeed found very large third-order nonlinear responses, but the reported variation of the nonlinear optical coefficient by orders of magnitude is not yet understood. A large part of the difficulty is the lack of information on how doping or chemical potential affects the different nonlinear optical processes. Here we report the first experimental study, in corroboration with theory, on third harmonic generation (THG) and four-wave mixing (FWM) in graphene that has its chemical potential tuned by ion-gel gating. THG was seen to have enhanced by ~30 times when pristine graphene was heavily doped, while difference-frequency FWM appeared just the opposite. The latter was found to have a strong divergence toward degenerate FWM in undoped graphene, leading to a giant third-order nonlinearity. These truly amazing characteristics of graphene come from the possibility to gate-control the chemical potential, which selectively switches on and off one-and multi-photon resonant transitions that coherently contribute to the optical nonlinearity, and therefore can be utilized to develop graphene-based nonlinear optoelectronic devices.
A transfer matrix method is developed for optical calculations of non-interacting graphene layers. Within the framework of this method, optical properties such as reflection, transmission and absorption for single-, double- and multi-layer graphene are studied. We also apply the method to structures consisting of periodically arranged graphene layers, revealing well-defined photonic band structures and even photonic bandgaps. Finally, we discuss graphene plasmons and introduce a simple way to tune the plasmon dispersion.
The combination of graphene with noble-metal nanostructures is currently being explored for strong light-graphene interaction enhanced by plasmons. We introduce a novel hybrid graphene-metal system for studying light-matter interactions with gold-void nanostructures exhibiting resonances in the visible range. Strong coupling of graphene layers to the plasmon * To whom correspondence should be addressed † DTU Fotonik ‡ CNG ¶ Fudan University § DTU Nanotech CINF 1 modes of the nanovoid arrays results in significant frequency shifts of the underlying plasmon resonances, enabling more than 30% absolute light absorption in a single layer of graphene and up to 700-fold enhancement of the Raman response of the graphene. These new perspectives enable us to verify the presence of graphene on gold-void arrays and the enhancement even allows us to accurately quantify the number of layers. Experimental observations are further supported by numerical simulations and perturbation-theory analysis. The graphene gold-void platform is beneficial for sensing of molecules and placing R6G dye molecules on top of the graphene, we observe a strong enhancement of the R6G Raman fingerprints. These results pave the way toward advanced substrates for surface-enhanced Raman scattering (SERS) with potential for unambiguous single-molecule detection on the atomically well-defined layer of graphene.Graphene is an atomic monolayer formed by carbon hexagons, whose extraordinary electrical and optical properties have led to a range of promising optoelectronic devices, 1-3 such as photodetectors, 4 optical modulators, 5 and ultra-fast lasers. 6 However, all such devices suffer from the inherently weak interaction between pristine graphene and light (2.3% light absorption at normal incidence), therefore imposing substantial challenges and restrictions for many electro-optical and all-optical applications. 7,8 Doped graphene nanostructures which support surface plasmons in the teraherz and infrared regions offer an exciting route to increase the light-graphene interaction by confining the optical fields below the diffraction limit. 9-13 However, graphene is less attractive when the interband loss becomes large, and it effectively mimics a dielectric material in the visible and near-infrared frequencies. 14 One alternative way to enhance the light-graphene interaction in short wavelengths is the combination of graphene with conventional plasmonic nanostructures based on noble metals. 15 These graphene-plasmonic hybrid structures could be beneficial for both fields of investigation: first of all, graphene can influence the optical response of plasmonic structures leading to graphene-based tunable plasmonics, 16 and in turn, plasmonic nanostructures can dramatically enhance the local electric field, leading to strong light absorption and Raman signature of graphene layers.In this Letter, a novel platform based on graphene-covered gold nanovoid arrays (GNVAs) is 2 proposed to enhance the light-matter interaction in graphene-plasmonic hybrid str...
Non-iridescent structural colors of high color visibility are produced by amorphous photonic structures, in which -natural cuttlefish ink is used as an additive to break down the long-range order of the structures. The color hue and its spectral purity can be tuned by adjusting the diameter of the polystyrene (PS) spheres and the proportion of ink particles.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.