Phase transitions in two-dimensional colloidal systems
2007, von Grünberg, Hans Hennig, Keim, Peter, Maret, Georg
This chapter is an introduction to phase transitions in two-dimensional (2D) systems. In contrast to three dimensions (3D), microscopic theories of melting exist in 2D. The most well known of them was developed more than 30 years ago by Kosterlitz, Thouless, Halperin, Nelson and Young (KTHNY theory). This theory predicts the unbinding of topological defects to break the symmetry in two steps at two distinct temperatures. Dissociation of dislocation pairs first melts the crystal into a still orientationally ordered (hexatic) phase and, in the second step, dissociation of free dislocations causes the system to go over to an isotropic fluid. Colloidal systems are used to verify experimentally the predictions of KTHNY theory in detail as colloids provide the possibility to visualize the change in symmetry on an "atomic" level by simple video-microscopy. Elastic moduli like Young's modulus and Frank's constant are deduced from microscopic trajectories of colloids in order to quanify the softening of the 2D ensemble in the vicinity of the phase transitions.
Dark speckle imaging of colloidal suspensions in multiple light scattering media
1997, Heckmeier, Michael, Maret, Georg
Quasielastic multiple light scattering experiments have been performed on suspensions of particles in brownian motion embedded inside a solid turbid medium. The photon transport mean free path of the suspension and of its solid environment where adjusted to be identical. We show that by selecting a minimum intensity spot of the speckle pattern generated by the solid medium, it is possible to visualize objects which would be undetectable with common optical techniques. Our results are shown to be in agreement with a diffusion theory for the location of strongly absorbing objects inside multiple scattering media.
Influence of additives on size and porosity in the synthesis of uniform TiO2 Nanoparticles
2004-12-07, Widoniak, Johanna, Eiden-Assmann, Stefanie, Maret, Georg
Monodisperse spherical titania particles of variable sizes are produced in a sol-gel synthesis from Ti(EtO)4 in ethanol with addition of a salt or a polymer solution. The influence of different salt ions or polymer molecules on the size and on the size distribution of the final particles was investigated. The amorphous hydrous titania particles were characterized by electron microscopy, thermogravimetry, 1H-MAS-NMR and X-ray absorption spectroscopy and electrophoresis. Nitrogen absorption measurements revealed that the addition of polymers yields hollow and porous titania colloids.
Coherent backscattering of light in multiple scattering media
1997, Lenke, Ralf, Maret, Georg
We will give a short overview of the phenomenon of coherent backscattering of light, essentially with respect to its possible application for the characterization of multiple scattering samples. We will present some basic experiments and their evaluation in comparison to Monte-Carlo simulations.
Preparation of 3D photonic crystals from opals
2004, Egen, Marc, Zentel, Rudolf, Ferrand, Patrick, Eiden, Stefanie, Maret, Georg, Caruso, Frank
The majority of the contributions in this topically edited book stems from the priority program SPP 1113 "Photonische Kristalle" run by the Deutsche Forschungsgemeinschaft (DFG), resulting in a survey of the current state of photonic crystal research in Germany. The first part of the book describes methods for the theoretical analysis of their optical properties as well as the results. The main part is dedicated to the fabrication, characterization and modeling of two- and three-dimensional photonic crystals, while the final section presents a wide spectrum of applications: gas sensors, micro-lasers, and photonic crystal fibers.
Illustrated in full color, this book is not only of interest to advanced students and researchers in physics, electrical engineering, and material science, but also to company R&D departments involved in photonic crystal-related technological developments.