Invited Article: Bragg stacks with tailored disorder create brilliant whiteness

Meiers, Dominic T.; Heep, Marie Christin; Freymann, Georg von

doi:10.1063/1.5048194

Cited by 19 publications

(19 citation statements)

References 25 publications

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“…In this respect, our simple BRW model provides a flexible platform to explore a larger configuration space, allowing to clarify the key features for efficient light scattering in network-like materials beyond the particular case of the Cyphochilus beetle. This stands in contrast with previously proposed models attempting to investigate its efficiency, which have been mainly limited to 2D projections, or involved ex-post deformations of the original structures rather than an ab-initio tuning of their growth parameters [8,24,33]. Despite its simplicity, in fact, our BRW model retains a number of desirable features.…”

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confidence: 91%

Optimized White Reflectance in Photonic‐Network Structures

Utel

Cortese

Wiersma

et al. 2019

Advanced Optical Materials

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freedom to tune light transport properties in these media. Indeed, shapes such as prolate ellipsoids or cylinders can be packed up to higher densities delaying the onset of spatial correlations at the cost of increased angular correlations. [10][11][12] Interestingly, both these aspects-namely, the high density and prevalent orientation exhibited by packed rods-can contribute to increase the overall turbidity, which makes cylinders particularly suited to realize highly turbid materials with a flat response over a broadband wavelength range. [13] Indeed, the smallest possible transport mean free path for a given refractive index contrast reported so far in the visible range has been obtained with GaP nanocylinders [14] and, for lower refractive index materials, claims of optimized scattering exist for the chitinous network structure of the Cyphochilus beetles. [5,13] In recent years, the latter has inspired an array of bio-mimicking materials attempting to reproduce its outstanding efficiency in terms of strong light scattering and limited material usage, taking advantage of a wide range of fabrication techniques including electro-spinning, super critical CO 2 foaming, polymer phase separation, and direct laser writing, [6,7,9,[15][16][17] to name a few.However, as opposed to nanoparticle systems, network materials are characterized by several additional aspects other than number density and spatial correlations, which makes it difficult to understand what key parameters should be optimized to design highly scattering network structures.Few notable examples include phase percolation, angular correlation, and network valence, all of which concur in determining their scattering properties. [13,[18][19][20] In this respect, simple generative models for photonic structures allowing to investigate the effects of these parameters separately are much needed to gain insight on their role and relevance.In this work, we describe a simple branching random walk (BRW) algorithm to generate random network structures inspired by that of the Cyphochilus beetle. Notably, the model allows to control and vary independently the volume fraction and degree of angular correlations without altering structural parameters such as the slab thickness, the shape, and aspect ratio of the constituent elements. The optical and transport properties of the generated structures have been investigated through finite difference time domain (FDTD) calculations and an inverse Monte Carlo (MC) approach, showing that the bright reflectance of the Cyphochilus beetle can be easily matched and surpassed by acting solely on the degree of anisotropy and the volume fraction of the network. To the best of our knowledge, this is the first rigorous demonstration of the key role played by structural anisotropy in highly reflective disordered samples.3D disordered networks are receiving increasing attention as they represent a versatile architecture for highly scattering materials. However, due to their complex morphology, little is known about the interplay betwee...

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confidence: 91%

Optimized White Reflectance in Photonic‐Network Structures

Utel

Cortese

Wiersma

et al. 2019

Advanced Optical Materials

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“…This is clearly illustrated in Parnell et al 15 , where Eurasian Jay feathers have a periodic gradient in colour from white to blue to black. Pseudo-photonic materials, which have a degree of disorder can be optically modelled using Bragg type reflecting structures, with a distribution of length scales in the layers and their spacings 16 . However, while such models may capture the optical properties well, they do not inform us as to the process by which these optically active nanomaterials form.…”

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confidence: 99%

Liquid–liquid phase separation morphologies in ultra-white beetle scales and a synthetic equivalent

et al. 2019

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Cyphochilus beetle scales are amongst the brightest structural whites in nature, being highly opacifying whilst extremely thin. However, the formation mechanism for the voided intrascale structure is unknown. Here we report 3D x-ray nanotomography data for the voided chitin networks of intact white scales of Cyphochilus and Lepidiota stigma. Chitin-filling fractions are found to be 31 ± 2% for Cyphochilus and 34 ± 1% for Lepidiota stigma, indicating previous measurements overestimated their density. Optical simulations using finitedifference time domain for the chitin morphologies and simulated Cahn-Hilliard spinodal structures show excellent agreement. Reflectance curves spanning filling fraction of 5-95% for simulated spinodal structures, pinpoint optimal whiteness for 25% chitin filling. We make a simulacrum from a polymer undergoing a strong solvent quench, resulting in highly reflective (~94%) white films. In-situ X-ray scattering confirms the nanostructure is formed through spinodal decomposition phase separation. We conclude that the ultra-white beetle scale nanostructure is made via liquid-liquid phase separation.

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“…The non-iridescent blue coloration of the Morpho butterflies as well as the brilliant whiteness of Lepidiota stigma and beetles of genus Cyphochilus are very well studied and the underlying processes understood. [12,18,[42][43][44]114,115] Here, we want to discuss how to transfer these optimized microstructures to a simple model, [45] which allows reproducing non-iridescent coloration as well as brilliant whiteness with a material of low refractive index (n = 1.55) by accounting for the underlying disorder mechanisms. This encompasses the optimized microstructure of the scales, including the filling fraction, the size, and the distancing of the individual scattering elements as well as their size distribution and spatial arrangement.…”

Section: Structural Coloration and Brilliant Whitementioning

confidence: 99%

Tailored Disorder in Photonics: Learning from Nature

Rothammer

Zollfrank

Busch

et al. 2021

Advanced Optical Materials

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Through natural selection, optimally designed nano-and microstructures have arisen for tailored light-matter interactions, even despite a rather limited repertory of materials. [3] Hierarchy is a major reason for the multifunctionality of natural materials such as feathers, which keep the birds body warm and make it water repellant, create wings and tails for aerodynamic lift during flying, and furthermore, provide coloration for camouflage as well as for intraspecific sexual communication. [3][4][5][6][7][8] In comparison, mankind just very recently developed fabrication technologies, which can be separated into bottom-up and top-down approaches. In bottom-up approaches, small building blocks (e.g., nanoparticles or molecules) are organized using self-assembly strategies. Bottom-up approaches are scalable and, hence, are suitable for mass fabrication. However, self-assembly always faces the unavoidable risk of introducing unwanted defects (e.g., stacking faults, missing particles), as these processes are not fully controllable. Top-down processes on the other hand allow for full control (within the limits of the precision of the technique). Here, different techniques have been developed for 1D (e.g., atomic-layer deposition, chemical-vapor deposition, molecular beam epitaxy), 2D (e.g., photo-lithography or electron-beam lithography) as well as 3D (e.g., direct laser writing, direct inkjet printing, laser induced forward transfer) structures. With increasing number of dimension, suitability for mass fabrication reduces. However, top-down fabrication reaches the required precision of few tens of nanometers even for 3D approaches to address the length scales present in the intricate structures found in nature.Due to the growing worldwide interest in photonics, researchers study natural systems as many concepts are resilient against disorder or even utilize certain amounts of disorder to achieve the desired functionality. Coloration derived from the microstructure and the underlying photonic dispersion relations and transport processes play an important role, as these properties can be designed or tailored by controlling the microstructure. Combining the precision of top-down approaches with the flexibility of bottom-up strategies opens new avenues for structures with controlled amounts of disorder. Here, we want to review the mechanisms found in natural structures, briefly discuss the underlying theoretical principles before we provide a broader overview of materials and methods for fabrication. We will conclude with a detailed exposition of four illustrative examples that showcase different aspects of tailored disorder.

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Invited Article: Bragg stacks with tailored disorder create brilliant whiteness

Cited by 19 publications

References 25 publications

Optimized White Reflectance in Photonic‐Network Structures

Optimized White Reflectance in Photonic‐Network Structures

Liquid–liquid phase separation morphologies in ultra-white beetle scales and a synthetic equivalent

Tailored Disorder in Photonics: Learning from Nature

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