The limitations of extending nature’s color palette in correlated, disordered systems

Living organisms have developed a wide range of appearances from iridescent to matte textures. Interestingly, angular independent structural colors, where isotropy in the scattering structure is present, only produce coloration in the blue wavelength region of the visible spectrum. One might, therefore, wonder if such observation is a limitation of the architecture of the palette of materials available in nature. Here, by exploiting numerical modeling, we discuss the origin of isotropic structural colors without restriction to a specific light scattering regime. We show that high color purity and color saturation cannot be reached in isotropic short-range order structures for red hues. This conclusion holds even in the case of advanced scatterer morphologies, such as core-shell particles or inverse photonic glasses — explaining recent experimental findings reporting very poor performances of visual appearance for such systems

Manufacturing Large-scale Materials with Structural Color

Living organisms frequently use structural color for coloration as an alternative mechanism to chemical pigmentation. Recently there has been a growing interest to translate structural color into synthetic materials as a more durable and less hazardous alternative to conventional pigments. Efforts to fabricate structurally colored materials take place in different fronts, from 3D printing to spray-coating and roll-to-roll casting. Stability, performance, and quality of the color, the environmental impact of the materials or their manufacturing methods are some of the heavily researched topics we discuss. First, we highlight recent examples of large-scale manufacturing technologies to fabricate structurally colored objects. Second, we discuss the current challenges to be tackled to create perfect appearances which aim at the full color gamut while caring for environmental concerns. Finally, we discuss possible scenarios that could be followed in order to involve other manufacturing methods for creating structurally colored objects

Tunable high-index photonic glasses

Materials with extreme photonic properties such as maximum diffuse reflectance, high albedo, or tunable band gaps are essential in many current and future photonic devices and coatings. While photonic crystals, periodic anisotropic structures, are well established, their disordered counterparts, photonic glasses (PGs), are less understood despite their most interesting isotropic photonic properties. Here, we introduce a controlled high index model PG system. It is made of monodisperse spherical TiO$_2$ colloids to exploit strongly resonant Mie scattering for optimal turbidity. We report spectrally resolved combined measurements of turbidity and light energy velocity from large monolithic crack-free samples. This material class reveals pronounced resonances enabled by the possibility to tune both the refractive index of the extremely low polydisperse constituents and their radius. All our results are rationalized by a model based on the energy coherent potential approximation, which is free of any fitting parameter. Surprisingly good quantitative agreement is found even at high index and elevated packing fraction. This class of PGs may be the key to optimized tunable photonic materials and also central to understand fundamental questions such as isotropic structural colors, random lasing or strong light localization in 3D.Comment: Main text: 8 pages, 4 figures; Supporting Information: 5 pages, 5 figure

Disordered wax platelets on Tradescantia pallida leaves create golden shine

Plants have various strategies to protect themselves from harmful light. An example of such aprotective mechanism is the growth of epicuticular nanostructures, such as a layer of hair or waxcrystals. Most nanostructures are optimised to screen UV radiation, as UV light is particularlydamaging for cellular tissue. We find that, contrary to the commonly found UV reflectance, theepicuticular wax crystals onTradescantialeaves reflect strongly in the higher visible wavelengthregime. Thus, they give the leaves a golden shine. We characterize the optical appearance ofTradescantia pallida‘purpurea’ leaves by angularly resolved spectroscopy and compare the resultsto finite difference time domain simulations. We find that it is the disordered assembly of the waxplatelets that is the crucial parameter to obtain the observed reflected intensity increase for higherwavelengths.This work was funded by by EU’s Horizon 2020 research and in-novation programme under the Marie Skłodowska-Curie grantagreement No. 722842 (ITN Plant-inspired Materials and Sur-faces–PlaMatSu), the Swiss National Science Foundation underproject P2ZHP2_183998 and BBSRC David Phillips Fellowship(BB/K014617/1) and the European Research Council (ERC-2014-STG H2020 639088

Light Management with Natural Materials: From Whiteness to Transparency

Abstract: The possibility of structuring material at the nanoscale is essential to control light–matter interactions and therefore fabricate next‐generation paints and coatings. In this context, nature can serve not only as a source of inspiration for the design of such novel optical structures, but also as a primary source of materials. Here, some of the strategies used in nature to optimize light–matter interaction are reviewed and some of the recent progress in the production of optical materials made solely of plant‐derived building blocks is highlighted. In nature, nano‐ to micrometer‐sized structured materials made from biopolymers are at the origin of most of the light‐transport effects. How natural photonic systems manage light scattering and what can be learned from plants and animals to produce photonic materials from biopolymers are discussed. Tuning the light‐scattering properties via structural variations allows a wide range of appearances to be obtained, from whiteness to transparency, using the same renewable and biodegradable building blocks. Here, various transparent and white cellulose‐based materials produced so far are highlighted

Magnetic-field effects on one-dimensional Anderson localization of light

Transport of coherent waves in multiple-scattering media may exhibit fundamental, nonintuitive phenomena such as halt of diffusion by disorder called Anderson localization. For electromagnetic waves, this phenomenon was observed only in one and two dimensions so far. However, none of these experiments studied the contribution of reciprocal paths nor their manipulation by external fields. In order to weaken the effect of reciprocity of coherent wave transport on Anderson localization in one dimension, we studied light propagation through stacks of parallel Faraday-active glass slides exposed to magnetic fields up to 18 T. Measurements of light transmission statistics are presented and compared to one-dimensional (1D) transfer- matrix simulations. The latter reveals a self-organization of the polarization states in this system leading to a saturation of the Faraday rotation-induced reciprocity breaking, an increase of the localization length, and a decrease of transmission fluctuations when reciprocity is broken. This is confirmed experimentally for samples containing small numbers of slides while for larger samples a crossover from a 1D to a quasi-1D transport regime is found

Cytoklepty in the plankton: A host strategy to optimize the bioenergetic machinery of endosymbiotic algae

Endosymbioses have shaped the evolutionary trajectory of life and remain ecologically important. Investigating oceanic photosymbioses can illuminate how algal endosymbionts are energetically exploited by their heterotrophic hosts and inform on putative initial steps of plastid acquisition in eukaryotes. By combining three-dimensional subcellular imaging with photophysiology, carbon flux imaging, and transcriptomics, we show that cell division of endosymbionts (Phaeocystis) is blocked within hosts (Acantharia) and that their cellular architecture and bioenergetic machinery are radically altered. Transcriptional evidence indicates that a nutrient-independent mechanism prevents symbiont cell division and decouples nuclear and plastid division. As endosymbiont plastids proliferate, the volume of the photosynthetic machinery volume increases 100-fold in correlation with the expansion of a reticular mitochondrial network in close proximity to plastids. Photosynthetic efficiency tends to increase with cell size, and photon propagation modeling indicates that the networked mitochondrial architecture enhances light capture. This is accompanied by 150-fold higher carbon uptake and up-regulation of genes involved in photosynthesis and carbon fixation, which, in conjunction with a ca.15-fold size increase of pyrenoids demonstrates enhanced primary production in symbiosis. Mass spectrometry imaging revealed major carbon allocation to plastids and transfer to the host cell. As in most photosymbioses, microalgae are contained within a host phagosome (symbiosome), but here, the phagosome invaginates into enlarged microalgal cells, perhaps to optimize metabolic exchange. This observation adds evidence that the algal metamorphosis is irreversible. Hosts, therefore, trigger and benefit from major bioenergetic remodeling of symbiotic microalgae with potential consequences for the oceanic carbon cycle. Unlike other photosymbioses, this interaction represents a so-called cytoklepty, which is a putative initial step toward plastid acquisition

Magneto-optical Faraday effect in multiple-scattering media

When waves get multiply scattered in 3D random media, a disorder driven phase transition from diffusion to localization can be observed. This phase transition was fi rst predicted by P. W. Anderson for electronic systems [1]. For light waves, this transition was recently measured in 3D by Störzer et al. with time of flight measurements [2] and by Sperling et al. analyzing the 2D transmission pro file [3]. While the origin of Anderson localization is claimed to be the interference between time-reversed scattering paths, experimental evidence is still missing. The Faraday effect can be used to destroy time-reversal symmetry in multiple scattering media [4, 5]. To affect light localization by magnetic fields via the Faraday effect, disordered media that show both signs of localization and strong Faraday rotation has to be used. This thesis reports the characterization of a two component material made of a strongly scattering powder (TiO2) and a Faraday active powder (CeF3). The samples were characterized in a speckle interferometer regarding their degree of Faraday rotation. This was done by measuring the decay of the speckle intensity correlation function with increasing magnetic field, and comparing the experimental data with a theoretical description developed by F. Erbacher [6]. A time of flight setup was used to determine their light transport properties, namely the diffusion coeffcient and the absorption length, and the disorder parameter was characterized by measuring the coherent backscattering cone. The obtained results lead to the prediction that this two component material can be prepared to show the required properties for observing a magnetic field driven transition from light localization to diffusion. This would be an evidence that the origin of Anderson localization is the constructive interference on time-reversed scattering paths.publishe