Nanolithography Considerations for Multi-Passband Grating Filters

The placement accuracy and resolution of direct-write patterning tools, in particular the atomic force microscope (AFM), is considered for application to fabricating multi-passband integrated optical niters. Because of its simpler fabrication a grating structure is proposed that consists of identical stripes that are non-periodically spaced. The recently developed pseudorandom encoding method from the field of computer generated holography is modified to effectively assign analog reflectances at each point along the grating by selective withdrawal and offsetting of the stripes from a periodic spacing. An example filter designed by this method has two 1.5 nm bandwidth passbands and –23 dB of rejection for lightly coupled stripes. As with single band filters, the passbands broaden as the coupling increases. A calculation of the coupling coefficient of stripes on a fundamental mode, slab waveguide indicate that stripes on the order of 100 nm in depth and width support low insertion loss, multipassband filtering applications at visible wavelengths. Lines of these dimensions patterned with an AFM on (110) silicon indicates the feasibility of fabricating these filters. These conclusions are specific to current AFM’s that are limited to writing fields of 100 μm. Increased rejection and decreased passband widths will result from incorporating precise field stitching into future AFM’s.     

Low-dimensional electron gas at semiconductor surfaces

In recent years, it has become possible to create well-ordered semiconductor surfaces with metallic surface states by using self-assembly of metal atoms. Since these states lie in the band gap of the semiconductor, they completely decouple from the substrate. In addition to two-dimensional structures it is possible to obtain arrays of one-dimensional atomic chains, which may be viewed as the ultimate nanowires. The dimensionality can be varied systematically by using vicinal surfaces with variable step spacing. Angle-resolved photoemission and scanning tunnelling spectroscopy reveal surprising features, such as a fractional band filling, nanoscale phase separation into doped and undoped chain segments, and a spin-splitting at a non-magnetic surface. Prospects for one-dimensional electron gas physics in atomic chains are discussed.     

Estimating the parameters of a truncated normal distribution

Let ø(x) be a truncated normal pdf over the interval [a,b], that is, assume ø(x)=exp[-(x–μ)²/2σ²]/∝^b_aexp[-(x–μ)²/2σ²]dx for - ∞<a?x?b?< + ∞ and zero elsewhere. Suppose that X₁,X₂,…,X_n is a random sample of size n from this truncated distribution. Using known properties of exponential families of distributions and the system of Legendre polynomials over the interval [-1,1], we examine the maximum likelihood estimation of the parameters μ and σ².     

Low-dimensional electronic states at silicon surfaces

Self-assembled atomic chains can be triggered at stepped Si(111) surfaces by adding sub-monolayer amounts of metals, such as gold, silver, platinum, alkali metals, alkaline earths, and rare earths. A common feature of all these structures is the honeycomb chain, a graphitic strip of Si atoms at the step edge that is lattice matched in the direction parallel to the edge but completely mismatched perpendicular to it. This honeycomb chain drives one-dimensional surface reconstructions even on the flat Si(111) surface, breaking its three-fold symmetry. Particularly interesting are metallic chain structures, such as those induced by gold. The Au atoms are locked rigidly to the Si substrate but the electrons near the Fermi level completely decouple from the substrate because they lie in the band gap of silicon. The electronic structure of one-dimensional electrons is predicted to be qualitatively different from that of higher dimensions, since electrons cannot avoid each other when moving along the same line. The single-electron picture has to be abandoned, making way for collective excitations, such as spinons and holons, where the spins and charges of electrons become separated. Although such excitations have yet to be confirmed definitively, the band structure seen in angle-resoled photoemission exhibits a variety of unusual features, such as a fractional electron count and a doublet of nearly half-filled bands. Chains of tunable spins can be created with rare earths. The dimensionality can be controlled by adjusting the step spacing with intra- and inter-chain coupling ratios from 10:1 to >70:1. Thus, metal-induced chain structures on stepped silicon provide a versatile class of low-dimensional materials for approaching the one-dimensional limit and exploring the exotic electronic states predicted for one dimension. PACS 73.20.At; 71.10.Pm; 79.60.Jv; 81.07.Vb; 73.21.Hb     

Room temperature UV adhesive bonding of CE devices

A simple low temperature adhesive 'stamp-and-stick' bonding procedure for lab-on-a-chip glass devices has been tested for capillary electrophoresis applications. This technique involves use of a mask aligner to transfer a UV-curable adhesive selectively onto the top CE substrate which is then aligned with and bonded to the bottom CE wafer. The entire bonding process can be carried out at room temperature in less than 30 minutes, involved only user-friendly laboratory operations, and provided a near 100% success rate. CE microchips made in this manner exhibited similar electroosmotic flow and separation characteristics as ones made via conventional high temperature thermal bonding. Equally important, the devices provided stable long-term performance over weeks of use, encompassing hundreds of individual CE runs without structural failure or any apparent change in operating characteristics. Finally, these devices exhibited excellent chip-to-chip reproducibility. Successful adaptation of the stamp-and-stick approach did require the development and testing of new but easily implemented structural features which were incorporated into the chip design and whose nature is described in detail.     

Fast DNA and protein microarray tests for the diagnosis of hepatitis C virus infection on a single platform

Hepatitis C virus (HCV) is a major cause of chronic liver disease and liver cancer, and remains a large health care burden to the world. In this study we developed a DNA microarray test to detect HCV RNA and a protein microarray to detect human anti-HCV antibodies on a single platform. A main focus of this study was to evaluate possibilities to reduce the assay time, as a short time-to-result (TTR) is a prerequisite for a point-of-care test. Significantly reducing hybridisation and washing times did not impair the assay performance. This was confirmed first using artificial targets and subsequently using clinical samples from an HCV seroconversion panel derived from a HCV-infected patient. We were able to reduce the time required for the detection of human anti-HCV antibodies to only 14 min, achieving nanomolar sensitivity. The protein microarray exhibited an analytical sensitivity comparable to that of commercial systems. Similar results were obtained with the DNA microarray using a universal probe which covered all different HCV genotypes. It was possible to reduce the assay time after PCR from 150 min to 16 min without any loss of sensitivity. Taken together, these results constitute a significant step forward in the design of rapid, microarray-based diagnostics for human infectious disease, and show that the protein microarray is currently the most favourable candidate to fill this role.     

Membrane mediated regulation in free peptides of HIV-1 gp41: minimal modulation of the hemifusion phase

Membrane-mediated structural modulation in two short fragments of the human HIV-1 envelope protein gp41 is demonstrated. Derived from the C-terminal membrane proximal external (MPE) and N-terminal fusion peptide proximal (FPP) regions, these peptides are widely separated in the primary sequence but form tertiary contacts during the intermediate (hemifusion) phase of HIV infection. The structural perturbations observed at the membrane interface offer evidence of rudimentary regulatory mechanisms operating in the free peptides which may be relevant in the biological system. No such regulatory phenomena were observed for the individual peptides in a membrane environment or between the peptides in aqueous solutions. Structure determination is made using a combination of circular and linear dichroism spectroscopy (supported by calorimetric measurements) and molecular dynamics simulations. Specifically, we show that these peptides interact locally without the conformational support of helical heptad repeat regions in native gp41 and that the modulation is not mutual with the FPP peptide operating as a primary regulator of the MPE-FPP interactions in the hemifusion phase.     

Mechanical Properties of Thermo-Reversible Colloid Liquid Crystal Composites

Abstract

The mechanical properties of concentrated dispersions of sterically-stabilised colloidal particles in nematic hosts are explored using a combination of optical microscopy and viscoelastic shear measurements. Starting from an initially homogeneous dispersion in an isotropic host, it is found that the kinetics of the isotropic-nematic transition lead to the formation of a percolated particle network that imparts surprising mechanical rigidity to the resulting colloid/liquid crystal composite. Specifically, the viscoelastic storage modulus G′ is observed to rise by approximately 5 orders of magnitude within a few degrees of the bulk isotropic-nematic transition temperature. Using 4-4′pentyl cyano-biphenyl as the solvent, the composite at room temperature is a self-supporting soft solid over a wide range of particle concentrations. The particle microstructure depends sensitively on thermal treatment and particle concentration and it is found that faster cooling enhances the visoelastic moduli of the composite. Also, several classes of network topology have been identified. In all cases studied to date, the particles can be redispsersed upon heating the solvent through the nematic-isotropic transition. The formation of the composite is therefore thermally reversible. A rigorous theory to account for the observed mechanical properties is not currently available.     

Electronic effects in the length distribution of atom chains

Gold deposited on Si(553) leads to self-assembly of atomic chains, which are broken into finite segments by defects. Scanning tunneling microscopy is used to investigate the distribution of chain lengths and the correlation between defects separating the chains. The length distribution reveals oscillations that indicate changes in the cohesive energy as a function of chain length. We present a possible interpretation in terms of the electronic scattering vectors at the Fermi surface of the surface states. The pairwise correlation function between defects shows long-range correlations that extend beyond nearest-neighbor defects, indicating coupling between chains.