Tilmann Ruhl

Nanoparticle-tuned structural color from polymer opals.

The production of high-quality low-defect single-domain flexible polymer opals which possess fundamental photonic bandgaps tuneable across the visible and near-infrared regions is demonstrated in an industrially-scalable process. Incorporating sub-50nm nanoparticles into the interstices of the fcc lattice dramatically changes the perceived color without affecting the lattice quality. Contrary to iridescence based on Bragg diffraction, color generation arises through spectrally-resonant scattering inside the 3D photonic crystal. Viewing angles widen beyond 40 masculine removing the strong dependence of the perceived color on the position of light sources, greatly enhancing the color appearance. This opens up a range of decorative, sensing, security and photonic applications, and suggests an origin for structural colors in Nature.

Compact strain-sensitive flexible photonic crystals for sensors

A promising fabrication route to produce absorbing flexible photonic crystals is presented, which exploits self-assembly during the shear processing of multi-shelled polymer spheres. When absorbing material is incorporated in the interstitial space surrounding high-refractive-index spheres, a dramatic enhancement in the transmission edge on the short-wavelength side of the band gap is observed. This effect originates from the shifting optical field spatial distribution as the incident wavelength is tuned around the band gap, and results in a contrast up to 100 times better than similar but nonabsorbing photonic crystals. An order-of-magnitude improvement in strain sensitivity is shown, suggesting the use of these thin films in photonic sensors.

Colloidal Crystals in Latex Films: Rubbery Opals

Full Paper : Elastometric films were prepared, by uniaxial compression, from core-shell latex particles with a rigid thermoplastic core and a soft elastometric shell. The films are rubbery, yet well ordered. The latex spheres form a fcc lattice, the 111 plane of which is oriented parallel to the film plane.This colloidal crystallinity is also found in opals. The films are colored due to selective reflection of the light wavelength corresponding to the lattice spacing, but they are not opaque. The crystallization process is surprisingly simple and fast because the core-shell latex spheres flow in the melt pratically like a regular polymer melt. Under uniaxial compression, this flowing melt deposits crystalline layers of the latex spheres along the plates of the press. The order in the films was characterized with transmission electron microscopy and UV spectroscopy. Deformation of the films results in a shift of the reflected light wavelength.

Large-area photonic crystals

Materials with a periodically modulated refractive index, with periods on the scale of light wavelengths, are currently attracting much attention because of their unique optical properties which are caused by Bragg scattering of the visible light. In nature, 3d structures of this kind are found in the form of opals in which monodisperse silica spheres with submicron diameters form a face-centered-cubic (fcc) lattice. Artificial opals, with the same colloidal-crystalline fcc structure, have meanwhile been prepared by crystallizing spherical colloidal particles via sedimentation or drying of dispersions. In this report, colloidal crystalline films are introduced that were produced by a novel technique based on shear flow in the melts of specially designed submicroscopic silica-polymer core-shell hybrid spheres: when the melt of these spheres flows between the plates of a press, the spheres crystallize along the plates, layer by layer, and the silica cores assume the hexagonal order corresponding to the (111) plane of the fcc lattice. This process is fast and yields large-area films, thin or thick. To enhance the refractive index contrast in these films, the colloidal crystalline structure was inverted by etching out the silica cores with hydrofluoric acid. This type of an inverse opal, in which the fcc lattice is formed by mesopores, is referred to as a polymer-air photonic crystal.

Photonics of stretchable opals based on polymers

Novel optical properties of stretchable polymer-based opals are presented. Nanoparticle doping strongly enhances the sharpness and depth of transmission filtering. Angle-dependent chromatic scattering exhibits dispersion characteristics beyond the Bragg scattering regime.