The beauty of opals results from a densely packed, highly ordered arrangement of silica spheres with a diameter of several hundred nanometers. Such ordered nanostructures are typical examples of materials called photonic crystals, which can be formed by known microstructuring methods and by self-assembly. Opals represent a self-assembly approach to these structured media; such an approach can lead to novel materials for photonics, photocatalysis, and other areas. Although self-assembly leads to many types of defects, resulting in the surprising and very individual appearance of natural opals, it causes also difficulties in technological applications of opal systems.

Opal films fabricated by the CDM (capillary deposition method, a special flow induced technique) have been studied with respect to their lattice alignment and defect distribution in the film. The substrate plane as well as the growth direction controls the orientation of the lattice. Both together allow two possible densely packed sphere arrangements in the film, very likely occurring simultaneously and interconnected either by stacking faults or by disordered regions. Parallel oriented cracks are formed randomly near open edges and propagate stepwise inward the cell. The distance between two adjacent cracks was found to be nearly proportional to the thickness of opal film. The UV/vis transmission spectra of the opal films indicate a homogeneous defect distribution and a very broad scattering background. The background has a 1/λ behavior pointing to a Mielike scattering phenomenon. The investigations show indications for possible defect control and reveal differences in opals fabricated by other methods.

Die Schönheit von Opalen beruht auf einer dichten, hochgeordneten Packung von SiO2‐Kugeln mit einem Durchmesser von einigen hundert Nanometern. Solche geordneten Nanostrukturen sind typische Beispiele für so genannte photonische Kristalle, die sowohl durch die bekannten Mikrostrukturierungsmethoden als auch durch Selbstorganisation hergestellt werden können. Opale stehen für den Selbstorganisationszugang zu diesen strukturierten Medien, die zu neuen Materialien in der Photonik, Photokatalyse und anderen Gebieten führen können. Die Selbstorganisation hat viele Arten von Defekten zur Folge, die zu den überraschenden und sehr individuellen Erscheinungsformen natürlicher Opale führen, aber auch Schwierigkeiten bei der technischen Anwendung von Opalsystemen verursachen.

Two-dimensional arrays of split ring resonators for near-infrared frequencies are embedded in a liquid crystal (LC) and the influences of LC alignment, temperature, and electric fields on the resonance frequencies are studied. The results show that tunability can not only be achieved by influencing the state of polarization of the incident radiation, but also by direct interaction of the evanescent field of the resonant modes with the LC. Depending on the LC alignment, the field-induced shift of the resonance frequency is found to vary for different excited modes. Some guidelines for the design of tunable frequency selective metasurfaces can be deduced from these experimental results.

We investigate the phase-dependent excitation of localized surface plasmon polaritons in coupled nanorods by using nonlinear spectroscopy. Our design of a coupled three-nanorod structure allows independent excitation with cross-polarized light. Here, we show that the excitation of a particular plasmon mode can be coherently controlled by changing the relative phase of two orthogonally polarized light fields. Furthermore, we observe a phase relation for the excitation that is dominantly caused by damping effects.

The drying of colloidal crystals is connected with a continuous shrinkage process. However, several minutes after starting the drying, the system seems to take a breath before it shrinks monotonously until its final state after about one day. This short period we call "v"-event because of the shape of the curve characterizing the lattice constant: a decrease followed by a counter-intuitive increase which ends after one hour. This event is found in time-dependent optical spectra. It is assigned to the start of a nano-dewetting process occurring at the colloidal particles.

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