Wing Yim Tam

Direct and Seamless Coupling of TiO2 Nanotube Photonic Crystal to Dye-Sensitized Solar Cell: A Single-Step Approach

A TiO(2) nanotube layer with a periodic structure is used as a photonic crystal to greatly enhance light harvesting in TiO(2) nanotube-based dye-sensitized solar cells. Such a tube-on-tube structure fabricated by a single-step approach facilitates good physical contact, easy electrolyte infiltration, and efficient charge transport. An increase of over 50% in power conversion efficiency is obtained in comparison to reference cells without a photonic crystal layer (under similar total thickness and dye loading).

Three-dimensional photonic crystals fabricated by visible light holographic lithography

In this report, we present three-dimensional photonic crystals fabricated by a four-beam holographic lithography method using visible photoinduced polymerization. High-quality face-centered-cubic single crystals with a large range of polymeric matrix volume fraction were fabricated using optimal conditions obtained from computer simulations. Optical measurements of the crystals showing photonic band-gap-like behavior are presented for different polymeric matrix volume fractions.

Chiral microstructures (spirals) fabrication by holographic lithography.

We present an optical interference model to create chiral microstructures (spirals) and its realization in photoresist using holographic lithography. The model is based on the interference of six equally-spaced circumpolar linear polarized side beams and a circular polarized central beam. The pitch and separation of the spirals can be varied by changing the angle between the side beams and the central beam. The realization of the model is carried out using the 325 nm line of a He-Cd laser and spirals of sub-micron size are fabricated in photoresist.

Sustained spiral waves in a continuously fed unstirred chemical reactor

Previous experiments on the self‐organization of spatial patterns in chemical reactors have generally been restricted to closed (batch) reactors. In such reactors the system relaxes irreversibly and uncontrollably towards thermodynamic equilibrium. It is difficult to make comparisons with existing theories which address asymptotic (long time) behavior because of the transient nature of the spatial patterns and the lack of well‐defined control parameters in such experiments. We report a novel disk‐shaped reactor that can be maintained far from thermodynamic equilibrium indefinitely by a continuous feed of reagents. Chemical patterns are formed inside a thin layer of inert gel that suppresses any convective motion. The feed to the gel is uniform and normal to the plane in which pattern can form. The reactor is used to conduct the first quantitative study of transitions between well‐defined states with different patterns.

Realization of optical periodic quasicrystals using holographic lithography

We have fabricated three-dimensional periodic quasicrystals exhibiting quasiperiodicity of the Penrose structure in the x-y plane and periodic along the z axis using ten-beam visible light holographic lithography. The quasicrystals show photonic band gaps in the visible range. The band gaps are found to follow a simple relation as a function of periodicity and polymeric volume fraction, in accord with the Bragg’s diffraction relation.

Dielectric electrorheological fluids: theory and experiment

Electrorheological (ER) fluids are a class of materials whose rheological properties are controllable by the application of an electric field. A dielectric electrorheological (DER) fluid is the simplest type of ER fluid, in which the material components follow a linear electrostatic response. We review and discuss the progress of the studies on physics of this type of material. A first-principles theory of DER fluids, along with relevant experimental verifications, are presented in some detail. In particular, the properties presented include static equilibrium structure, shear modulus, static yield stress and its variation with applied electric field frequency, and structure-induced dielectric nonlinearity.

Statistical properties and shell analysis in random cellular structures

We investigate the statistical properties of two-dimensional random cellular systems ~froths! in terms of their shell structure. The froth is analyzed as a system of concentric layers of cells around a given central cell. We derive exact analytical relations for the topological properties of the sets of cells belonging to these layers. Experimental observations of the shell structure of two-dimensional soap froth are made and compared with the results on two kinds of Voronoi constructions. It is found that there are specific differences between soap froths and purely geometrical constructions. In particular these systems differ in the topological charge of clusters as a function of shell number, in the asymptotic values of defect concentrations, and in the number of cells in a given layer. We derive approximate expressions with no free parameters which correctly explain these different behaviors. @S1063-651X~96!13111-1#

Icosahedral quasicrystals for visible wavelengths by optical interference holography.

Quasicrystals, realized in metal alloys, are a class of lattices exhibiting symmetries that fall outside the usual classification for periodic crystals. They do not have translational symmetry and yet the lattice points are well ordered. Furthermore, they exhibit higher rotational symmetry than periodic crystals. Because of the higher symmetry (more spherical), they are more optimal than periodic crystals in achieving complete photonic bandgaps in a new class of materials called photonic crystals in which the propagation of light in certain frequency ranges is forbidden. The potential of quasicrystals has been demonstrated in two dimensions for the infrared range and, recently, in three-dimensional icosahedral quasicrystals fabricated using a stereo lithography method for the microwave range and direct laser writing for the IR range. Here, we report the fabrication and optical characterization of icosahedral quasicrystals using a holographic lithography method for the visible range. The icosahedral pattern, generated using a novel 7-beam optical interference holography, is recorded on photoresists and holographic plates. Electron micrographs of the photoresist samples show clearly the symmetry of the icosahedral quasicrystals in the submicron range, while the holographic plate samples exhibit bandgaps in the angular-dependent transmission spectra in the visible range. Calculations of the bandgaps due to reflection planes inside the icosahedral quasicrystal show good agreement with the experimental results.

Spatiotemporal patterns in a one-dimensional open reaction-diffusion system

Abstract We have developed a novel spatially extended chemical reactor that can be maintained indefinitely in a well-defined nonequilibrium state. The system is effectively one-dimensional. Reagents of the Belousov-Zhabotinsky reaction are fed at both ends, the oxidizer at one end and the reducer at the other end, but there is no met mass flux in the reactor. The system is designed so that the effective diffusion coefficient, which is the same for all species (typically 0.1 cm2/s), can be varied; spatiotemporal patterns were studied for several values of the diffusion coefficient. The following sequence of well-defined dynamical regimes was observed as the gradient in oxidizer concentration was increased with other control parameters held fixed: steady, periodic, quasiperiodic, frequency-locked, period-doubled, and chaotic. The transitions to different regimes occurred with no observable hysteresis. This is the first observation of a sequence of spatiotemporal regimes in a laboratory reaction-diffusion system. These observations are described qualitatively by a simple reaction-diffusion model with only two species.

Wide band gap photonic structures in dichromate gelatin emulsions

The authors report the fabrication of wide band gap photonic crystals with planar structures in dichromate gelatin emulsions using a two-beam holographic method. By exploiting the differential swelling of the gelatin, planar structures with gradient spacing are fabricated. The crystals exhibit high efficiencies and wide band gaps in the visible range. The authors model the planar gelatin system by an effective medium approach and use transfer matrix to calculate the reflectance and transmittance. Good agreement is obtained between theory and experiment.