Frank Marlow

Opals: Status and Prospects

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.

Doped Mesoporous Silica Fibers: A New Laser Material

632 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 0935-9648/99/0806-0632

Simulation of powder diffraction patterns of modified ordered mesoporous materials

17.50+.50/0 Adv. Mater. 1999, 11, No. 8 After this the polymer was rigid and could be peeled off carefully from the silicon master. The suspensions were prepared by adding powder to distilled water under constant stirring. The pH was always kept between 4 and 5 by dropwise adding a 2 M solution of hydrochloric acid (Titrisol, Merck, Darmstadt, Germany). Thereafter the suspensions were ball milled with alumina milling balls for 18 h. Two drops of 1-octanol (puriss, Fluka, Buchs, Switzerland) were added to reduce the surface tension of the suspension before degassing for 15 min at 90 mbar under constant rotation. For the formation of the ceramic samples a few drops of the particle suspension were poured onto the PDMS template and dried in room atmosphere. No separating agent was applied onto the mold. The suspension layer on the template structure was kept relatively thin (about 1 mm) in respect to its diameter (about 5 mm) to get quasi uniaxial shrinkage in the thickness direction to prevent high lateral shrinking rates during drying. After drying, the ceramic parts were carefully lifted from the molds and sintered. The PDMS templates were cleaned in an ultrasonic bath after processing and could be reused. The alumina samples were heated with 300 K/h to 1550 C and held there for 3 h. The boehmite samples were heated with 300 K/h to 300 C, then with 60 K/h to 600 C to allow the phase transition of boehmite from g-AlOOH to g-Al2O3 and then with 300 K/h to 1250 C and held there for 4 h. Suspensions of three powders with different particle sizes were prepared. Their properties are listed in Table 2. For a good pattern replication the suspensions needed to have a relatively low viscosity of about 0.5 Pa s which limits the amount of powder that can be dispersed. The achieved solids loadings decreased with decreasing particle size of the powders. Solids loadings in volume percent are shown in Table 2.

Large CoAPO-5 single crystals: Microwave synthesis and anisotropic optical absorption

Powder diffraction patterns of ordered mesoporous materials are simulated with a newly developed program, which allows investigation of the influence of any desired matter distribution in the unit cell on the diffraction pattern. The simulation process can be subdivided into two major steps. First, a unit cell is generated from SiO2 and, optionally, other building units. A weighted random placement of atoms is used to simulate the distribution of different atoms in different parts of the unit cell. This is done by a Fermi-type function, by which the probability of finding an atom on a site depends on the distance of a point from the center of a pore, leading to a smooth, continuous transition from wall to pore. Secondly, structure factors and then intensities of reflections are calculated, using the Lorentz correction and a geometric correction for powder data. The use of this program is demonstrated by the simulation of diffraction patterns, mainly for unmodified and modified SBA-15 as well as for MCM-41. Good agreement of simulated and experimental data is observed.

Ultralow refractive index substrates–a base for photonic crystal slab waveguides

Abstract Large CoAPO-5 crystals were synthesized by microwave heating of a fluoride-containing aluminophosphate gel. By polarization microscopy and polarization-dependent single-crystal ultraviolet—visible (UV—VIS) spectroscopy, the as-synthesized crystals show an optical anisotropy of light absorption. It is proposed that this effect originates from structure-related symmetry distortions of the tetrahedral Co2+ environment in the AFI framework. Consequently, the effect of anisotropic optical absorption can be taken as a proof for the incorporation of cobalt in the AFI framework.

Hollow mesoporous silica fibers: tubules by coils of tubules

Out of plane radiation losses in two-dimensional (2D) photonic crystal (PC) waveguides occur due to a lack of total internal reflection at the core-substrate∕superstrate boundaries. In order to minimize these losses, either a high vertical refractive index contrast or deep etching into the substrate is required [G. Boettger, C. Liguda, M. Schmidt, and M. Eich, Appl. Phys. Lett. 81, 2517 (2002)]. The maximum vertical contrast is achieved in air bridge type PC waveguides, which are inherently fragile. In this article, we introduce a concept which combines the advantages of a high vertical index contrast of an air bridge with those of a solid substrate. This approach consists of mesoporous silica as substrate material with an ultralow refractive index (n=1.14 at 1.3μm), close to that of air. Finite 2DPC line defect resonators consisting of an optical polymer as core and mesoporous silica as substrate were fabricated. Compared to ordinary substrates with higher refractive indices like silica or amorphous Tefl...

Photoinduced switching of nanocomposites consisting of azobenzene and molecular sieves: investigation of the switching states

Hollow mesoporous silica fibers have been synthesized under acidic conditions in a two-phase static system. The fibers always have a circular internal architecture with hexagonally arranged channels running perpendicularly around the fiber axis. The inner radius of the hollow fibers is variable and the fibers are connected with solid fibers without inner cavity. This points to a novel formation mechanism for the central cavity of the hollow fibers.

Three-dimensional fabrication of optically active microstructures containing an electroluminescent polymer

Abstract Cis – trans photoisomerization of azobenzene in the molecular sieve hosts AlPO 4 -5 and ZSM-5 causes large and reversible changes of the refractive index of the composite systems. This process strongly depends on both the irradiation wavelength and the molecular sieve host. The rate constant of the thermal relaxation of the cis -state composites increases from 2.0×10 −6 s −1 in AlPO 4 -5 to 3.1×10 −5 s −1 in ZSM-5. Both switching states are stable over hours or days. The reversibility of the switching process exceeds several hundred switching cycles indicating that the cis – trans photoisomerization proceeds without significant side reactions in both matrices.

Ordered and disordered pNA molecules in mesoporous MCM-41

Microfabrication via two-photon absorption polymerization is a technique to design complex microstructures in a simple and fast way. The applications of such structures range from mechanics to photonics to biology, depending on the dopant material and its specific properties. In this paper, we use two-photon absorption polymerization to fabricate optically active microstructures containing the conductive and luminescent polymer poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV). We verify that MEH-PPV retains its optical activity and is distributed throughout the microstructure after fabrication. The microstructures retain the emission characteristics of MEH-PPV and allow waveguiding of locally excited fluorescence when fabricated on top of low refractive index substrates.

Optical characterization of organized adsorbates in zeolite microcrystals: Polarized absorption spectroscopy

Para-nitroaniline (pNA) has been incorporated into the one-dimensional channels of mesoporous MCM-41, the hexagonal member of the M41S family of ordered mesoporous silicate materials. The composite material shows a number of remarkable features, depending on the sorption procedure for the organic component, the after treatment, and the annealing time of the composite. In X-ray diffraction experiments different states of one-dimensional ordering of the sorbate molecules in the channel pores are observed. In agreement with the molecular dipole, chain-like coherent domains with the long axis of the sorbate molecule parallel as well as perpendicular to the host channel axis show up. In aged samples the periodic ordering of the pNA molecules is lost, and nonlinear optical properties of the composite have been measured in second harmonic generation experiments indicating the presence of a polar direction in the sample.