arXiv subjects

Ullrich Steiner

Publications and source records attributed to Ullrich Steiner.

16 recordsLinked to original sources

arXiv2026

From rings to resonance: an inverse method links biophotonic structural color to inverse photonic glasses

Structural color arises from the interaction of light with nanoscale structures and is widespread in nature. As structural complexity increases, the mechanisms governing coloration become progressively less understood. The optical response of periodic photonic crystals with a spatially periodic refractive index is well described by Bloch theory, whereas that of photonic glasses composed of randomly assembled uniform spheres is more subtle yet well studied. In contrast, disordered photonic networks found in many beetles are among the most complex natural photonic architectures, and the fundamental relationships between their structure and color remain unclear. Here, we use an inverse method to identify the structural features encoded in the reflectance spectrum. By comparing the spectrum of an unknown system with a database of simulated spectra from computer-generated photonic networks, we infer its structural properties. Applying this approach to both simulated networks and the biophotonic network responsible for the blue coloration of the weevil Pachyrhynchus congestus mirabilis, we identify rings and pores as the key local scattering motifs. Their characteristic sizes govern the spectral position of the reflectance peak, whereas short-range disorder controls its width. The inverse method reveals clear spectral signatures of short-range order, whereas the influence of hyperuniformity and primitive similarity appears comparatively weak. This suggests that blue structural coloration is governed by local scattering mechanisms rather than photonic band-gap effects. We propose an analogy to an inverse photonic glass, in which pores and rings act as correlated local resonators. This perspective provides new design principles for bio-inspired structural-color materials.

physics.optics↗

arXiv2026

Scalable Size- and Shape-Selective Purification of Colloidal Building Blocks via Excluded Volume Interactions

Excluded-volume interactions, arising solely from steric constraints, play a crucial role in determining the structure, dynamics, and phase behaviour of colloidal suspensions. This is particularly important for non-spherical particles, where orientation-dependent effects also become significant. In this study, we employ depletion-driven phase separation to develop a scalable, size-selective method for purifying spherical and non-spherical colloidal clusters that exhibit an interplay of concave and convex surface areas. Phase diagrams of charge-stabilised polystyrene spheres ranging in size from 267 to 1008 nm demonstrate that the mixing-demixing transition occurs across a range of surfactant concentrations rather than at a single threshold. Taking advantage of this transition width enables the purification of a single component from binary mixtures at size ratios as low as 1.6 in a single step. When the same approach is applied to tetrameric colloidal clusters, these are enriched fifteenfold relative to uncoordinated spheres. Importantly, the efficiency of sorting depends not only on the effective size but also on the geometry of the aggregate. For instance, anisotropic, weakly fused clusters separate more efficiently than spherical aggregates because their concave surface curvature is reduced compared to unfused clusters. These findings establish excluded-volume-driven sorting as a practical and scalable route for purifying colloidal building blocks for hierarchical assembly.

cond-mat.soft↗

arXiv2026

Self-Assembly of Lipid-Biopolymer Periodic Nanostructures on Photonic Length Scales

The self-assembly of photonic nanostructures in insects involves chitin, proteins, and lipids. While synthetic photonic systems have been extensively studied, current lipid-based self-assembly systems are limited in periodicity to $68\,\text{nm}$ compared to photonic length scales ($\approx 450\,\text{nm}$) observed in biological organisms. We hypothesise that lipids facilitate how structural colour arises in vivo by acting as templates for the self-assembly of biopolymers via lipidic lyotropic liquid crystal mesophases. Here, we aim to understand and identify how structural colour is produced in insects by the co-assembly of lipids and biopolymers. We study the effect of biopolymers, pH, temperature, surface charge, and stability on lipid vesicles using dynamic light scattering, X-ray scattering, and zeta potential analysis. Using cryo-electron microscopy, we demonstrate that these vesicles interact with the biopolymers and generate periodic nanostructures with periodicities ranging from $700\,\text{nm}$ to $1.2\,μ\text{m}$ (more than ten times larger than for purely lipidic systems) and dimensionalities ranging from 1D to 3D. Our results establish that lipid mesophases and biopolymers can induce reorganisation into ordered nanostructures, overcoming key limitations of periodicities achieved by lipid-only systems, and providing a methodology for recreating the physicochemical mechanisms underlying biophotonic structural colour.

cond-mat.soft↗

arXiv2026

Computer Generation of Disordered Networks with Targeted Structural Properties

Disordered spatial networks describe structures and interactions across multiple length scales. The scattering and interference of waves within these networks result in structural phase transitions, localization, diffusion, and band gaps. Studying these phenomena requires efficient numerical methods for generating disordered networks with specific structural properties. The Wooten-Weaire-Winer algorithm is an established method that introduces disorder into an initial network through a series of bond switch moves. However, the strain energies that govern this evolution are conventionally limited to three-dimensional networks with coordination numbers of no more than four. We here introduce a maximum bond repulsion to produce networks with an arbitrary coordination number. We control the degree and type of disorder by adjusting the bond-bending force constant in the strain energy and the temperature profile. The effects of these variables are quantified through a list of order metrics that capture both direct and reciprocal space. A feedforward neural network predicts the structural characteristics from the algorithm inputs, enabling efficient targeted network generation. As a case study, we statistically reproduce four disordered biophotonic networks that exhibit structural color. This work presents a versatile method for generating disordered networks with tailored structural properties. It will provide new insights into structure-property relations.

cond-mat.dis-nn↗

arXiv2025

Termination-Driven Control over BIC Q-Factors and Frequencies in Plasmonic Double Net Metamaterials

Interlaced metallic wire meshes are 3D metamaterials consisting of two intertwined metallic networks. These plasmonic double nets give rise to otherwise unobserved longitudinal, weakly dispersive and broadband electron acoustic modes from the effective plasma frequency of the double net down to arbitrarily low frequencies. These modes have recently been shown to generate confined slab modes with extremely long lifetimes (high quality factors), so-called quasi-bound states in the continuum. This work reveals the central role of the double net termination in determining the mode's resonant frequency and quality factor. We compare two limiting cases, a tennis net termination recently studied experimentally by others and a protruding column array with a much lower quality factor, as demonstrated by microwave transmission experiments and full-wave simulations. Our work thus vividly demonstrates the failure of a homogenisation approach to explain and quantify the physics of terminated plasmonic network materials. We introduce a new approach, in which additional evanescent bulk states are included in the scattering problem, yielding a qualitative understanding of the slab's optical response. The resulting engineering principles pave the way for the design and exploitation of these materials for applications such as coherent light generation.

physics.optics↗

arXiv2023

Anoplophora graafi Longhorn Beetle Coloration is due to Disordered Diamond-like Packed Spheres

While artificially photonic materials are typically highly ordered, photonic structures in many species of birds and insects do not possess a long-range order. Studying their order-disorder interplay sheds light on the origin of the photonic band gap. Here, we investigated the scale morphology of the Anoplophora graafi longhorn beetle. Combining small-angle X-ray scattering and slice-and-view FIB-SEM tomography with molecular dynamics and optical simulations, we characterised the chitin sphere assemblies within blue and green. A. graafi scales. The low volume fraction of spheres and the number of their nearest neighbours are incompatible with any known close-packed sphere morphology. A short-range diamond lattice with long-range disorder best describes the sphere assembly, which will inspire the development of new colloid-based photonic materials.

physics.optics↗

arXiv2023

X-ray nanotomography reveals formation of single diamonds by block copolymer self-assembly

Block copolymers are recognised as a valuable platform for creating nanostructured materials with unique properties. Morphologies formed by block copolymer self-assembly can be transferred into a wide range of inorganic materials, enabling applications including energy storage and metamaterials. However, imaging of the underlying, often complex, nanostructures in large volumes has remained a challenge, limiting progress in materials development. Taking advantage of recent advances in X-ray nanotomography, we non-invasively imaged exceptionally large volumes of nanostructured soft materials at high resolution, revealing a single diamond morphology in a triblock terpolymer composite network. This morphology, which is ubiquitous in nature, has so far remained elusive in block copolymers, despite its potential to create materials with large photonic bandgaps. The discovery was made possible by the precise analysis of distortions in a large volume of the self-assembled diamond network, which are difficult to unambiguously assess using traditional characterisation tools. We anticipate that high-resolution X-ray nanotomography, which allows imaging of much larger sample volumes than electron-based tomography, will become a powerful tool for the quantitative analysis of complex nanostructures and that structures such as the triblock terpolymer-directed single diamond will enable the generation of advanced multicomponent composites with hitherto unknown property profiles.

cond-mat.soft↗

arXiv2023

High-resolution three-dimensional imaging of topological textures in single-diamond networks

Highly periodic structures are often said to convey the beauty of nature. However, most material properties are strongly influenced by the defects they contain. On the mesoscopic scale, molecular self-assembly exemplifies this interplay; thermodynamic principles determine short-range order, but long-range order is mainly impeded by the kinetic history of the material and by thermal fluctuations. For the development of self-assembly technologies, it is imperative to characterise and understand the interplay between self-assembled order and defect-induced disorder. Here we used synchrotron-based hard X-ray nanotomography to reveal a pair of extended topological defects within a self-assembled single-diamond network morphology. These defects are morphologically similar to the comet and trefoil patterns of equal and opposite half-integer topological charges observed in liquid crystals and appear to maintain a constant separation across the thickness of the sample, resembling pairs of full vortices in superconductors and other hard condensed matter systems. These results are expected to open new windows to study defect formation in soft condensed matter, particularly in biological systems where most structures are formed by self-assembly.

cond-mat.soft↗

arXiv2022

3D tomographic analysis of the order-disorder interplay in the Pachyrhynchus congestus mirabilis weevil

The bright colors of Pachyrhynchus weevils originate from complex dielectric nanostructures within their elytral scales. In contrast to previous work exhibiting highly ordered single-network diamond-type photonic crystals, we here show by combining optical microscopy and spectroscopy measurements with 3D FIB tomography that the blue scales of P. congestus mirabilis differ from that of an ordered diamond structure. Through the use of FIB tomography on elytral scales filled with Pt by electron beam-assisted deposition, we reveal that the red scales of this weevil possess a periodic diamond structure, while the network morphology of the blue scales exhibit diamond morphology only on the single scattering unit level with disorder on longer length scales. Full wave simulations performed on the reconstructed volumes indicate that this local order is sufficient to open a partial photonic bandgap even at low dielectric constant contrast between chitin and air in the absence of long-range or translational order. The observation of disordered and ordered photonic crystals within a single organism opens up interesting questions on the cellular origin of coloration and studies on bio-inspired replication of angle-independent colors.

physics.optics↗

arXiv2021

Enhancing the Refractive Index of Polymers with a Plant-Based Pigment

Polymeric materials are prized for their formability, low density, and functional versatility. However, the refractive indices of common polymers fall in a relatively narrow range between 1.4 and 1.6. Here, we demonstrate that loading commercially-available polymers with large concentrations of a plant-based pigment can effectively enhance their refractive index.For polystyrene loaded with 67w/w\% $β$-carotene, we achieve a peak value of 2.2 near the absorption edge at $531~\mathrm{nm}$, while maintaining values above 1.75 across longer wavelengths of the visible spectrum. Despite high pigment loadings, this blend maintains the thermoforming ability of polystyrene, and $β$-carotene remains molecularly dispersed. Similar results are demonstrated for the plant-derived polymer ethyl cellulose. Since the refractive index enhancement is intimately connected to the introduction of strong absorption, it is best suited to applications where light travels short distances through the material, such as reflectors and nanophotonic systems.We experimentally demonstrate enhanced reflectance from films, as large as seven-fold for ethyl cellulose at selected wavelengths. Theoretical calculations that highlight that this simple strategy can significantly increase light scattering by nanoparticles and enhance the performance of Bragg reflectors.

cond-mat.soft↗

arXiv2020

Strong Circular Dichroism in Single Gyroid Optical Metamaterials

Over the past two decades, metamaterials have led to an increasing number of biosensing and nanophotonic applications due to the possibility of a careful control of light propagating through subwavelength features. Chiral nanostructures (characterized by the absence of any mirror symmetry), in particular, give rise to unique chiro-optical properties such as circular dichroism and optical activity. Here, we present a gyroid optical metamaterial with a periodicity of 65 nm exhibiting a strong circular dichroism at visible wavelengths. Our bottom-up approach, based on metallic replication of the gyroid morphology in triblock terpolymer films, generates a large area of periodic optical metamaterials. We observe a strong circular dichroism in gold and silver gyroid metamaterials at visible wavelengths. We show that the circular dichroism is inherently linked to the handedness of the gyroid nanostructure, and demonstrate its tuneability. The optical effects are discussed and compared to other existing systems, showing the potential of bottom-up approaches for large-scale circular filters and chiral sensing.

physics.optics↗

arXiv2020

Designing Refractive Index Fluids using the Kramers-Kronig Relations

For a number of optical applications, it is advantageous to precisely tune the refractive index of a liquid. Here, we harness a well-established concept in optics for this purpose. The Kramers-Kronig relation provides physical connection between the spectral variation of the (real) refractive index and the absorption coefficient. In particular a sharp spectral variation of the absorption coefficient gives rise to either an enhancement or reduction of the refractive index in the spectral vicinity of this variation. By using bright commodity dyes that fulfil this absorption requirement, we demonstrate the use of the Kramers-Kronig relation to predictively dial-in refractive index values in water solutions that are otherwise only attained by toxic specialised liquids.

physics.optics↗

arXiv2019

Self-Rolled Multilayer Metasurfaces

Multilayer metasurfaces (MLMs) represent a versatile type of three-dimensional optical metamaterials that could enable ultra-thin and multi-functional photonic components. Herein we demonstrate an approach to readily fabricate MLMs exploiting a thin film self-rolling technique. As opposed to standard layer-by-layer approaches, all the metasurfaces are defined within a single nanopatterning step, significantly reducing fabrication time and costs. We realize two MLMs platforms relying on widely used nanopatterning techniques, namely focused ion-beam and electron-beam lithographies. A first example are MLMs comprised of nanohole patterns structured into metal-dielectric seed bilayers. The second platform is comprised of vertical stacks of angled plasmonic nanorod arrays separated by thin dielectric layers. Such angled MLMs exhibit a selective response to circularly polarized light, in agreement with previous works relying on layer-by-layer processes. Our approach can pave the way for the efficient prototyping of novel MLMs, such as devices with varying number of layers and configurations that can be fabricated on a single chip.

physics.optics↗

arXiv2019

Surface Reconstruction Limited Conductivity in Block-Copolymer Li Battery Electrolytes

Solid polymer electrolytes for lithium batteries promise improvements in safety and energy density if their conductivity can be increased. Nanostructured block copolymer electrolytes specifically have the potential to provide both good ionic conductivity and good mechanical properties. This study shows that the previously neglected nanoscale composition of the polymer electrolyte close to the electrode surface has an important effect on impedance measurements, despite its negligible extent compared to the bulk electrolyte. Using standard stainless steel blocking electrodes, the impedance of lithium salt-doped poly(isoprene-b-styrene-b-ethylene oxide) (ISO) exhibited a marked decrease upon thermal processing of the electrolyte. In contrast, covering the electrode surface with a low molecular weight poly(ethylene oxide) (PEO) brush resulted in higher and more reproducible conductivity values, which were insensitive to the thermal history of the device. A qualitative model of this effect is based on the hypothesis that ISO surface reconstruction at the different electrode surfaces leads to a change in the electrostatic double layer, affecting electrochemical impedance spectroscopy measurements. As a main result, PEO-brush modification of electrode surfaces is beneficial for the robust electrolyte performance of PEO-containing block-copolymers and may be crucial for their accurate characterization and use in Li-ion batteries.

physics.app-ph↗

arXiv2018

Halogen-Bond Driven Self-Assembly of Perfluorocarbon Monolayers

The self-assembly of a single layer of organic molecules on a substrate is a powerful strategy to modify surfaces and interfacial properties. The detailed interplay of molecule-to-substrate and molecule-molecule interactions are crucial for the preparation of stable and uniform monomolecular coatings. Thiolates, silanes, phosphonates and carboxylates are widely used head-groups to link organic molecules to specific surfaces study we show that self-assembly of stable and highly compact monolayers of perfluorocarbons. Remarkably, the lowest ever reported surface energy of 2.6 mJ m-2 was measured for a perfluorododecyl iodide monolayer on a silicon nitride substrate. As a convenient, flexible and simple method, the self-assembly of halogen-bond driven perfluorocarbon monolayers is compatible with several applications, ranging from biosensing to electronics and microfluidics. Compared to other methods used to functionalise surfaces and interfaces, our procedure offers the unique advantage to work with extremely inert perfluorinated solvents. We demonstrate that surfaces commonly unstable in contact with many common organic solvents, such as organic-inorganic perovskites, can be functionalized via halogen bonding.

physics.chem-ph↗

arXiv2017

Porous translucent electrodes enhance current generation from photosynthetic biofilms

We tested the enhancement of electrical current generated from photosynthetically active bacteria by use of electrodes with porosity on the nano- and micrometer length-scale. For two cyanobacteria on structured indium-tin-oxide electrodes, current generation was increased by two orders of magnitude and the photo-response was substantially faster compared to non-porous anodes. These properties highlight porosity as an important design strategy for electrochemical bio-interfaces. The role of porosity on different length scales was studied systematically which revealed that the main performance enhancement was caused by the increased surface area of the electrodes. More complex microstructured architectures which spanned biofilms as translucent 3D scaffolds provided additional advantage in the presence of microbial direct electron transfer (DET). The absence of a clear DET contribution in both studied cyanobacteria, Synechocystis and Nostoc, raises questions about the role of conductive cellular components previously found in both organisms.

q-bio.OT↗