Andreas Kontogeorgos

3D bulk ordering in macroscopic solid opaline films by edge-induced rotational shearing.

A breakthrough in the fi eld of large area photonic structures is reported, based on permanent ordering of solid polymeric fi lms of sub-micrometer spheres by edge rotational-shearing. The resulting high-quality polymer opal thin-fi lms exhibit strikingly intense structural color, as confi rmed by combining a number of spectroscopic approaches. This induced self-assembly on macroscopic length scales represents a step-change away from current surface lithographies, presenting new routes for assembling solid ordered photonic materials. Despite previous reports of shear-ordering in sedimentary colloids in solution, [ 1 , 2 ] no precedents exist for the application of such techniques to these granular solvent-free systems, which allow formation of permanent composite structures in the solid-state. Full 3D ordering of sub-micrometer components into defi ned architectures is a major challenge for bottom-up nanophotonics, nano-electronics, plasmonics and metamaterials. [ 3‐5 ] Even simple structures, such as opaline photonic crystals based on fcc colloidal lattices, have optical properties dominated by defects and cannot be fabricated in any scalable fashion. [ 6‐9 ] Here, we report a signifi cant advance in high-quality polymer opal thin-fi lms exhibiting tunable structural color across visible wavelengths. An edge-induced rotational shearing (EIRS) process produces reproducible highly uniform samples with bulk-ordering of sub-micrometer components, greatly enhancing both the intensity and chromaticity of the observed structural color. The demonstration of reproducible scale-up of these elastomeric synthetic opaline fi lms to industrial length scales makes them very attractive as a route to a wide range of large-area photonics applications, including sensors and coatings [ 10 ] as well as metamaterials when combined with metallic core‐shell particles. The advance reported here is based on a recently developed technique to produce fl exible opals using melting and shearordering under compression of core/shell polymer nanoparticles. [ 11‐13 ] So far, this produces fl exible polymer opals with

Ordering in stretch-tunable polymeric opal fibers.

We demonstrate the production of high-quality polymer opal fibers in an industrially-scalable process. These fibers exhibit structural color, based on the self-assembly of sub-micron core-shell particles, with a spectrum which is stretch-tunable across the visible region. The internal substructure and ordering of fibers, as inferred from variations in spectral bandwidth, is studied using dark-field microscopy. We employ a granular model to examine flow and shear forces during the extrusion process, and the effects on particle ordering. In both theory and experiment, a concentric zone of the fiber near the exposed surface develops particularly strong structural color. Such elastically-tuned structurally colored fibers are of interest for many applications.

Interplay of index contrast with periodicity in polymer photonic crystals

We report how the strength of resonant Bragg reflection from polymeric photonic crystals (polymer opals) varies linearly with the refractive-index contrast, Δn, in contrast to the quadratic buildup of Fresnel reflections scaling as (Δn)2. This occurs due to the interplay of disorder and periodicity, in agreement with a simple 1-dimensional periodic model. Goniometry experiments show that opal films exhibit “cones” of resonantly scattered light, which extend to ±20° angular deviation from the specular direction. The intensity of the scattering cones varies super-linearly with Δn. Such medium contrast photonic crystals are of significant interest for understanding structural colors exhibited in nature, by structures with inherent disorder.

Extruding opals: Self-assembling active soft nanophotonics on the kilometre scale

New photonic properties are produced in materials which are assembled from diverse combinations of metals, semiconductors and dielectrics that are periodically structured on the 100nm-scale, with a wealth of potential applications ranging from communications to bio-sensing. However producing such nanomaterials on the mass-scale is far from trivial as three-dimensional structures are very hard for traditional lithographies and self-assembly has so far been a lab-scale tricky process.

Opto-elastic anisotropy in stretched polymer photonic crystals

Using a new technique for single-domain shear-ordering of elastomeric photonic crystals we demonstrate novel opto-elastic properties. Tensile stress experiments demonstrate coupled mechanical and optical anisotropy, producing striking colour tuning depending on the stretch direction.

Thermochromic polymer opals

Highly unusual thermochromic properties of large-scale shear-ordered photonic crystals are demonstrated. A simple theoretical model of the temperature dependence of this resonant Bragg scattering based structural colour is developed.

Cavity-enhanced structural colour in extrudeable photonic crystals

Photonic crystals remain of significant interest because of the opportunity to modify a host of optical properties by structuring the material at the sub-wavelength scale, including enhanced light emission and absorption, superprism performance, and negative refraction. However until now photonic crystals have remained expensive to fabricate and control, and inconceivable to adopt in industrial processes because their assembly requires expensive and slow lithographies. In addition, most work has concentrated on high refractive index contrast systems which offer the best chance of light localisation. Here we discuss a novel range of polymer photonic crystals that we have successfully assembled into ≫100 m long films using the process of shear assembly. Core-shell polymer spheres are designed in such a way that they lock into 3D fcc opals when suitably sheared, and we will demonstrate how this process can work in the extrusion geometry to give single domain samples [1,2]. The optical properties of these elastomeric opals are highly unusual, since they are dominated by scattering which is enhanced by the photonic crystal environment. We will discuss how new coherent backscatter measurements taken across all wavelengths show how the photon mean free path is modified in these materials, and how scattering is forward directed in a cone at specific resonant colours [3]. A host of applications currently under investigation will be discussed.