Preparation of 3D photonic crystals from opals

Light scattering in opal heterojunctions

Light propagation in photonic hetero-crystals, consisting of two opal films with different lattice constants, has been investigated in the realistic regime of weak scattering, when the main light flux is transported by ballistic photons. The light scattering at the photonic bandgap interface is studied in combination with transmission data. The spectra and directionality diagrams of scattered light of single- and hetero-opal films are compared. The rate of decrease of the scattering intensity as a function of the scattering angle has been used to quantify the scattering strength. The anisotropy of the interface scattering is identified by deducing the trajectories of single scattered photon…

Exploring integration prospects of opal-based photonic crystals

Different methods of functionalisation of thin opal films are discussed, including synthesis of opals on pre-patterned substrates, post-synthesis electron beam lithography, preparation of opals with heterogeneous photonic band gap structure and integrating opals with light sources. These approaches have been tested experimentally and key technological problems have been identified.

Surfactant-Free Emulsion Polymerization of Various Methacrylates: Towards Monodisperse Colloids for Polymer Opals

A systematic investigation of the surfactant-free emulsion polymerization (SFEP) on three different methacrylates (MMA, tert-butyl methacrylate and 2,2,2-trifluoro ethyl methacrylate) is described to obtain monodisperse colloids for the preparation of artificial polymer opals. The experimental results are in agreement with a model, in which seed particles are formed very early during the polymerization process. The particles are formed very early during the polymerization process. The particles grow afterwards into the colloids until all monomer is consumed. This gives rise to a linear relationship between the volume of the colloids and the ratio of monomer to water in the reaction batch. T…

Micromoulding of three-dimensional photonic crystals on silicon substrates

International audience; The growth of three-dimensional photonic crystals (PhCs) on patterned silicon substrates is reported. It is shown that deep trenches can be uniformly filled by a self-assembly of polymer microspheres, in a close-packed face-centred cubic lattice. The crystalline quality is compared for different channel widths. These observations are confirmed by optical reflectance measurements in the visible range, showing a bandwidth of enhanced reflection. The possibility to detach the PhC, i.e. to use the substrate as a mould, is also demonstrated. The potential of this approach for building PhC-based complex architectures is discussed.

Force control and powder dispersibility of spray dried particles for inhalation

This study aims towards a deeper understanding of the correlation between particle morphology, cohesion forces, and aerosol performance of spray dried powders for inhalation. Therefore, forces affecting cohesion and dispersion are consid- ered and some novel contact models are introduced to explain the improved powder dispersibility of corrugated particles. Particles with different degrees of corrugation are prepared by spray drying and characterized. Powder dispersibility is measured by positioning a dry powder inhaler in front of the laser diffraction device. The particle sizes of all powders are in the range of x50 ¼ 2.11 � 0.15 mm. The ratio of mass specific surface area Sm to volume sp…

Tuning the Properties of Photonic Films from Polymer Beads by Chemistry

This paper describes the preparation of monodisperse colloids from various methacrylates. These colloids sediment well on glass slides and formin the dried statelarge three-dimensional face-centered cubic photonic crystal films with Bragg reflection in the visible region. Depending on crystallization conditions, the domain sizes vary between 50 and 300 μm. By cross-linking the polymer within the colloids, it is possible to increase the thermal stability of the polymeric photonic structure tremendously (up to 1 h at 200 °C). These photonic structures soften above Tg (rubbery photonic structure), but they cannot fuse. Cross-linked colloids from poly(tert-butyl methacrylate) can either thermal…

Cover Picture: Macromol. Mater. Eng. 2/2004

Artificial Opals as Effect Pigments in Clear-Coatings

In this paper, we describe the use of artificial opals from polymer beads as effect pigments in transparent industial and automotive coatings. For this purpose, we synthesized monodisperse colloids from fluorinated methacrylates by surfactant-free emulsion polymerization. The fluorinated monomers, in combination with crosslinking, lead to a refractive index contrast, thermal stability, and solvent resistivity necessary for use as effect pigments. After crystallization of the monodisperse polymer beads, crystal flakes with iridescent colors can be obtained. The crystal flakes can act as effect pigments in various transpartent industrial and automotive coatings. due to photonic crystal behavi…

Propagation and scattering of light in opal heterojunctions

The directionality of the polarised light transmission has been studied in heterogeneous photonic crystals based on double-film opal in comparison to homogeneous crystals on single-film opals. In spite of additional scattering at the photonic bandgap interface, ballistic propagation has been preserved. Changes of the polarisation anisotropy in hetero-opal compared to homogeneous opal have been assigned to specific mode matching conditions at the photonic bandgap interface.

Light transport in hetero-opal photonic crystals

The effect of photonic bandgap interface upon the light scattering was studied in hetero-opals consisting of two opal thin films with different lattice constant. It is shown that the weak scattering regime is preserved in thin hetero-opal films. By comparing scattering spectra of single and hetero-opal films recorded under reversing angles of light incidence and detection it was demonstrated that the interface scatters stronger the light at oblique incident angles. Squeezing of the scattering diagram of hetero-opals compared to single opal films is also assigned to the interface scattering.

Large Photonic Films by Crystallization on Fluid Substrates

Cracks, which appear due to shrinkage during drying of artificial opaline films, strongly limit the use of these materials. Crystallization on fluid substrates (liquid metals) is a way to circumvent this problem. By this method it is possible to prepare crack-free three-dimensional “monocrystals” with a very low defect density and the size in millimeters.