Polarity inverted gallium nitride for photonic crystal biosensors.

Photonic crystals are periodic nanostructures used to control the propagation of electromagnetic waves. Depending on geometry and refractive index contrast between adjacent regions, periodic variation of the refractive index can result in a photonic band gap or non-allowed set of frequencies that cannot propagate through the crystal. Defects can be introduced at photonic crystal lattice sites, resulting in localized modes that lie within the photonic gap. Such defect cavities can be tuned to resonant frequencies of a defect mode and whole planes or lines of defects can be fabricated in photonic crystals resulting in optical confinement of defect modes. These properties of photonic crystals make them useful in a wide variety of applications such as chemical and biological sensors, high Q lasers, and optical wave guiding. With its transparency in the visible wavelength regime of the electromagnetic spectrum, GaN is a candidate for photonic crystal structures with photonic band gaps corresponding to visible wavelengths. GaN is a wide, direct band gap semiconductor which exists primarily in the wurtzite crystal structure. The wurtzite crystal structure lacks inversion symmetry, resulting in two distinct crystal polarities or crystal growth directions, the Ga-polar or [0001] and N-polar or [0001¯]. Through choice of substrate or growth conditions, GaN can be grown with either polarity. An unusual, but potentially useful, result is that by generation of near-monolayer surface coverage of Mg, the crystal polarity can be inverted during growth from gallium polar to nitrogen polar without introducing any additional defects at the domain boundary. Subsequent patterning and etching of the inversion layer, followed by re-growth, results in periodically poled GaN. This changes the nonlinear optical response of the material and such a structure can be used in a variety of applications. This study uses a subsequent highly anisotropic wet etch of polarity inverted GaN to selectively etch N-polar regions, where Ga-polar regions remain unaffected without introducing any additional structural damage. This wet etching technique for fabricating nanostructures has potential advantages over other dry etching techniques, such as inductively coupled plasma and reactive ion etching. The specific aim of this work is to develop the knowledge and techniques to allow fabrication of GaN photonic crystals via wet etching of periodically poled GaN. Growth conditions for polarity inversion by Mg doping during Molecular Beam Epitaxy growth of GaN, as well as process development for fabrication of photonic crystal structures on both the micron and nanometer scales are investigated. This study also involves theoretical modeling using MIT photonic bands software to determine photonic crystal geometries for the fabrication of GaN photonic crystals with photonic band gaps in the visible as well as the infrared wavelength regimes for future optical characterization. This work is part of a larger collaborative effort at West Virginia University for the design, fabrication, and testing of a flow-though, resonant florescence based GaN photonic crystal biosensor

As-built design specification for MISMAP

The MISMAP program, which is part of the CLASFYT package, is described. The program is designed to compare classification values with ground truth values for a segment and produce a comparison map and summary table

Sensory substitution for space gloves and for space robots

Sensory substitution systems for space applications are described. Physical sensors replace missing human receptors and feed information to the interpretive centers of a different sense. The brain is plastic enough so that, with training, the subject localizes the input as if it were received through the missing receptors. Astronauts have difficulty feeling objects through space suit gloves because of their thickness and because of the 4.3 psi pressure difference. Miniature force sensors on the glove palm drive an electrotactile belt around the waist, thus augmenting the missing tactile sensation. A proposed teleoperator system with telepresence for a space robot would incorporate teleproprioception and a force sensor/electrotactile belt sensory substitution system for teletouch

Experimental evaluation of joint designs for a space-shuttle orbiter ablative leading edge

The thermal performance of two types of ablative leading-edge joints for a space-shuttle orbiter were tested and evaluated. Chordwise joints between ablative leading-edge segments, and spanwise joints between ablative leading-edge segments and reusable surface insulation tiles were exposed to simulated shuttle heating environments. The data show that the thermal performance of models with chordwise joints to be as good as jointless models in simulated ascent-heating and orbital cold-soak environments. The suggestion is made for additional work on the joint seals, and, in particular, on the effects of heat-induced seal-material surface irregularities on the local flow

Randomized Extended Kaczmarz for Solving Least-Squares

We present a randomized iterative algorithm that exponentially converges in expectation to the minimum Euclidean norm least squares solution of a given linear system of equations. The expected number of arithmetic operations required to obtain an estimate of given accuracy is proportional to the square condition number of the system multiplied by the number of non-zeros entries of the input matrix. The proposed algorithm is an extension of the randomized Kaczmarz method that was analyzed by Strohmer and Vershynin.Comment: 19 Pages, 5 figures; code is available at https://github.com/zouzias/RE

Influence of random roughness on the Casimir force at small separations

The influence of random surface roughness of Au films on the Casimir force is explored with atomic force microscopy in the plate-sphere geometry. The experimental results are compared to theoretical predictions for separations ranging between 20 and 200 nm. The optical response and roughness of the Au films were measured and used as input in theoretical predictions. It is found that at separations below 100 nm, the roughness effect is manifested through a strong deviation from the normal scaling of the force with separation distance. Moreover, deviations from theoretical predictions based on perturbation theory can be larger than 100%.Comment: 18, 5 figure

Perpendicular-to-grain compression behaviour of screw reinforced timber and a novel application of digital image correlation

This project investigates various factors which influence the behaviour of a timber roof structure designed by Smith & Wallwork Engineers. The first part of the project focused on material of sweet chestnut, where Digital Image Correlation (DIC) was employed to obtain values for the shear modulus. DIC was able to implement the shear field test method prescribed in BS EN 408:2010, along with two new methods developed based on additional information available through DIC. These methods were accurate, fast to implement, and potentially more robust than the shear field test method. A method to estimate the true value of the Timoshenko shear coefficient was also developed. The second section of the project involved physical testing of portions of the roof structure to investigate the behaviour of screw-reinforced and unreinforced timber loaded perpendicular to grain. Together with finite element modelling, it was noticed that a key aspect of the connection's behaviour was the axial force transfer between overlapping screws via shear in the timber, and the resulting relative displacement between the screws. A simple spring model was developed to characterise the compression stiffness of the roof which can now be used in reverse to calculate the forces due to moisture expansion or contraction