Measurement of wire diameter by optical diffraction

A combined interference and diffraction pattern, in the form of equidistant interference fringes, resulting from illuminating a vertical metallic wire by a laser beam is analyzed to measure the diameter of four standard wires. The diameters range from 170 to 450 μm. It is found that the error in the diameter measurements increases for small metallic wires and for small distances between the wire and the screen due to scattering effects. The intensity of the incident laser beam was controlled by a pair of sheet polaroids to minimize the scattered radiation. The used technique is highly sensitive, but requires controlled environmental conditions and absence of vibration effects. The expanded uncertainty for k=2 is calculated and found to decrease from U(D)=±1.45 μm for the wire of nominal diameter 170 μm to ±0.57 μm for the diameter 450 μm.     

Quarks and gluons in dense two-colour QCD

We compute quark and gluon propagators in 2-colour QCD at large baryon chemical potential μ. The gluon propagator is found to be antiscreened at intermediate μ and screened at large μ. The quark propagator is drastically modified in the superfluid region as a result of the formation of a superfluid gap.     

Tuning of long-wavelength emission in InxGa1−xAs quantum dot structures

This work explores the conditions to obtain the extension of the PL emission beyond 1.3 μm in InGaAs quantum dot (QD) structures growth by MOCVD. We found that, by controlling the In incorporation in the barrier embedding the QDs, the wavelength emission can be continuously tuned from 1.25 μm up to 1.4 μm at room temperature. However, the increase in the overall strain of the structures limits the possibility to increase the maximum gain in the QD active device, where an optical density as high as possible is required. By exploring the kinetics of QD surface reconstruction during the GaAs overgrowth, we are able to obtain, for the first time, emission beyond 1.3 μm from InGaAs QDs grown on GaAs matrix. The wavelength is tuned from 1.26 μm up to 1.33 μm and significant improvements in terms of line shape narrowing and room temperature efficiency are obtained. The temperature-dependent quenching of the emission efficiency is reduced down to a factor of 3, the best value ever reported for QD structures emitting at 1.3 μm.     

Weak-field magnetoresistance anomaly and Shubnikov–de Haas oscillations in Sn-doped InSb thin films on GaAs (100) substrates

Distinctive magnetoresistance (MR) effects in weak magnetic fields before the appearance of Shubnikov–de Haas (SdH) oscillations at low temperatures in Sn-doped (7×10¹⁶ cm⁻³) InSb films grown on GaAs(100) substrates by MBE have been investigated with decreasing film thickness d from 1 μm. The negative MR found in weak magnetic fields for d0.5 μm can be broadly divided into two regimes: T-sensitive negative MR below B_c observed with anisotropy between parallel and perpendicular magnetic field and a T-insensitive parabolic one above B_c observable only under in-plane magnetic fields. The latter is ascribable to the skipping orbit effect due to surface boundary scattering. In vanishing magnetic fields far below B_c, the negative MR reduces with decreasing d and the different positive MR overlaps with it below 0.5 μm, eventually dominating the positive MR at d0.2 μm. These results have been analyzed using a two-layer model for the films, where the composition of the upper layer under the surface and the lower one adjacent to the InSb/GaAs interface is assumed. The MR data in the extremely weak magnetic fields below 100 G for each layer have been successfully fitted to the two-dimensional (2D) weak localization (WL) theory. These results explain that the crossover from the 2D WL to the weak anti-localization (WAL) occurs when the interface is approached with the increase of SO interaction in the layers caused by the increased influence of the asymmetric potential at the hetero interface (Rashba term) and the SO rate in the intrinsic InSb film due to the crystal field of the zinc-blende structure (Dresselhaus term) is as small as τ_so⁻¹3×10⁸ s⁻¹.     

Influence of microsupersonic gas jets on nanosecond laser percussion drilling

This paper reports on etching rates and hole quality for nanosecond laser percussion drilling of 200-μm thick 316L stainless steel performed with micro supersonic gas jets. The assist-gas jets were produced using nozzles of 200, 300 and 500 μm nominal throat diameters. Air and oxygen were used separately for the process gas in the drilling trials and the drilling performance was compared to drilling in ambient conditions. The highest etch rate of 1.2 μm per pulse was obtained in the ambient atmosphere condition, but this was reduced by about 50% with assist-air jets from the 200 μm nozzle. Increasing the jet diameter and/or using oxygen assist gas also decreased the etching rate and increased the hole diameter. The 200 μm nozzle using air-assist jets produced the least amount of recast and gave the best compromise for etching rate. A combination of plasma shielding and different gas dynamic conditions inside the holes and at the surface are correlated to the observations of different drilling rates and hole characteristics.     

Spatial and temporal mapping of water content across Nafion membranes under wetting and drying conditions

Water transport and water management are fundamental to polymer electrolyte membrane fuel cell operation. Accurate measurements of water content within and across the Nafion layer are required to elucidate water transport behavior and validate existing numerical models.We report here a direct measurement of water content profiles across a Nafion layer under wetting and drying conditions, using a novel magnetic resonance imaging methodology developed for this purpose. This method, multi-echo double half k-space spin echo single point imaging, based on a pure phase encode spin echo, is designed for high resolution 1D depth imaging of thin film samples. The method generates high resolution (<8 μm) depth images with an SNR greater than 20, in an image acquisition time of less than 2 min. The high temporal resolution permits water content measurements in the transient states of wetting and drying, in addition to the steady state.     

Type-II InAs/GaSb superlattices for very long wavelength infrared detectors

Type-II superlattices (SLs) can be designed for semiconductor band gaps as large as 400 meV down to semimetallic. This flexibility in design makes them an excellent candidate for infrared photodiodes with cut-off wavelengths beyond 15 μm. There are relatively few options for high-performance infrared detectors to cover wavelengths longer than 15 μm, especially for operating temperatures above 15 K. In the past few years, excellent results have been obtained on photoconductive and photodiode samples designed for infrared detection in the very long wavelength infrared (VLWIR) range (λ>15 μm). There is a variety of possible designs for these SLs which will produce the same narrow band gap by adjusting individual layer thicknesses, or indium content, in the InGaSb layer. Several of these different design options have been grown and characterized. These designs often require monolayer control per layer over hundreds of repeats in the SL. Photoresponse spectra for type-II SLs are compared to show how the design choices not only change the band gap but also the band structure, as reflected in features observed in the spectra. Theoretical modeling results are used to interpret the photoresponse spectra. SLs with cut-off wavelengths ranging from 15 to 25 μm are covered.     

Quantitative Imaging by Optical Diffraction Tomography

Optical diffraction tomography (ODT) is applied to reconstruct the cross-sectional, complex refractive index distribution of cylindrical objects. Experimental reconstructions showing noticeable image contrast for refractive index variations of about 0.001, and having a spatial resolution of about 2 μm, are obtained for objects with cross-sectional diameters of about 100 μm. The results demonstrate that ODT can be used for quantitative imaging of semitransparent fibers.Presented at the International Commission of Optics Topical Meeting, Kyoto, 1994.     

Fast electro-optical switching and high contrast ratio in epoxy-based polymer dispersed liquid crystals

We report the observation of an electro-optical switching effect from an opaque to a transparent state occurring at a threshold value of the applied field in a polymer dispersed liquid crystal (PDLC). Optical responses of the composite film under the conditions of an externally applied ac electric field (2–5 V_p−p/μ) and a film thickness (50 μm), were determined using an Argon laser (wavelength 514 nm). The experimental results showed promising switching times with a rise time of 190 μs and a decay time of 2 ms and an exceptionally high contrast ratio up to 410. These results demonstrate the validity of employing this new PDLC in electro-optical devices.     

Optimization of InAs/GaSb type-II superlattices for high performance of photodetectors

The optimum growth conditions and strain balancing processes have been studied using molecular beam epitaxy (MBE) grown 51 Å InAs/40 Å GaSb type-II superlattices (SLs) designed to have cut-off wavelength of 10 μm. The most dominant factor in reducing the defect level in the SL structure was buffer growth temperature evidenced by transmission electron microscopy. In the study of the strain balancing process, the SLs could be lattice matched to the GaSb substrate by increasing the thickness of the InSb interfaces (IFs) from a nominal value of 1.0 to 1.4 ML, however, the structural quality degraded dramatically when the thickness of IFs reached beyond 1.0 ML. By optimizing the growth condition and MBE shutter sequences, micron thick InAs/GaSb SLs with a reduced lattice mismatch were routinely obtained with the full-width half-maximum of 18 arcsec, and the root mean square values of surface roughness of 2 Å in 5 μm area scan of atomic force microscopy demonstrating high quality. Correlation between material quality and photoresponse signal strength in photoconductivity measurements was made on SL samples with cut-off wavelength on the order of 10 μm.     

Design, analysis and optimization of silicon-on-insulator photonic crystal dual band wavelength demultiplexer

A highly efficient photonic crystal dual band wavelength demultiplexer (DBWD) using silicon-on-insulator (SOI) substrates is proposed for demultiplexing two optical communication wavelengths, 1.31 μm and 1.55 μm. Demultiplexing of two wavelength channels is obtained by modifying the propagation properties of guided modes in two arms of Y type photonic crystal structure. Propagation characteristics of proposed DBWD are analyzed utilizing 3D finite difference time domain (FDTD) method. Enhancement in spectral response is further obtained by optimizing the Y junction of demultiplexer giving rise to high transmission and extinction ratio for the wavelengths, 1.31 μm and 1.55 μm. Hence it validates the efficiency of proposed optimized DBWD design for separating two optical communication wavelengths, 1.31 μm and 1.55 μm. Tolerance analysis was also performed to check the effect of variation of air hole radius, etch depth and refractive index on the transmission characteristics of the proposed design of SOI based photonic crystal DBWD.     

Classical in-plane negative magnetoresistance and quantum positive magnetoresistance in undoped InSb thin films on GaAs (1 0 0) substrates

Magnetoresistance (MR) effects have been investigated in perpendicular and parallel magnetic fields at 300, 80 K and liquid He temperatures for undoped InSb thin films 0.1–2.3 μm thick grown on GaAs(1 0 0) substrates by MBE. At high temperatures, the intrinsic carriers show the parabolic negative MR observable only in magnetic fields parallel to the film. The skipping-orbit effect due to surface boundary scattering in the classical orbits in the plane vertical to the film has been argued to be responsible for the negative MR. At low temperatures (T=80 K), the transport is dominated by the two-dimensional (2D) electrons in the accumulation layers at the InSb/GaAs(1 0 0) hetero interface; MR is positive and shows a logarithmic increase with anisotropy between parallel and perpendicular field orientation, arising from the 2D weak anti-localization (WAL) that reflects the interplay between the spin-Zeeman effect and strong spin–orbit interaction caused by the asymmetric potential at the interface (Rashba term). The zero-field spin splitting energy of Δ₀13 meV, the electron effective mass of m^*0.10m₀ seven times of the band edge mass in bulk InSb and the effective g-factor of |g^*|15 in the accumulation layer have been inferred from fits of MR for the 0.1 μm thick film to the 2D WL theory.     

Spectroscopy of non-equilibrium electrons in quantum Hall conductors

Spectroscopy of local cyclotron emission from the hot spots is carried out on a GaAs/AlGaAs heterostructure two-dimensional electron gas system at B=6 T (ν=2.5) by applying a terahertz scanning microscope. The spectra of CE at the current entry and exit corners (hot spots) are remarkably broadened towards lower frequencies with increasing I up to 300 μA, indicating substantial relevance of non-equilibrium electrons generated in higher-level LLs; in terms of effective electron temperature, T_E reaching as high as 300 K is suggested while T_E=25–30 K on an average in the surrounding region (within a distance of 50 μm) about the hot point.     

Determining NMR flow propagator moments in porous rocks without the influence of relaxation

Flow propagators, used for the study of advective motion of brine solution in porous carbonate and sandstone rocks, have been obtained without the influence of Nuclear Magnetic Resonance (NMR) relaxation times, T₁ and T₂. These spin relaxation mechanisms normally result in a loss of signal that varies depending on the displacement ζ of the flowing spins, thereby preventing the acquisition of quantitative propagator data. The full relaxation behaviour of the system under flow needs to be characterised to enable the implementation of a true quantitative measurement. Two-dimensional NMR correlations of ζ − T₂ and T₁ − T₂ are used in combination to provide the flow propagators without relaxation weighting. T₁ − ζ correlations cannot be used due to the loss of T₁ information during the displacement observation time Δ. Here the moments of the propagators are extracted by statistical analysis of the full propagator shape. The measured displacements (first moments) are seen to correlate with the expected mean displacements for long observation times Δ. The higher order moments of the propagators determined by this method indicate those obtained previously using a correction were overestimated.     

Mechanisms of the central mode particle formation during pulverized coal combustion

Ash particles produced from pulverized coal combustion are considered to be tri-modally distributed. These include the well-known ultrafine and coarse modes, and a central mode that is less reported but attracts increasing attention. This work presents a preliminary study on the formation mechanisms of the central mode particles during pulverized coal combustion. Experiments of four sized and density-separated coal samples were carried out in a laboratory drop-tube furnace under various controlled conditions. Experimental data show that the ash particle size distributions have an evident central mode at 4 μm for all coal samples. Increasing combustion temperature leads to an increase in the central mode particle formation, which is thought to be due to enhanced char fragmentation. The small-size coal sample produces a larger amount of the central mode particles, reasonably due to abundant fine particles in the parent coal sample. Under similar combustion conditions, both the Heavy (>2.0 g/cm³) and Light (<1.4 g/cm³) coal fractions produce a central mode, indicating that not only the included minerals but also the excluded minerals contribute to the formation of the central mode particles.     

Influence of substrate misorientation and growth temperature on N incorporation in InGaAsN/GaAs quantum wells grown on (1 1 1)B GaAs

In this work, the effects of the substrate misorientation in the nitrogen incorporation in InGaAsN (1 1 1)B p–i–n diode structures grown by molecular beam epitaxy are discussed. The (1 1 1)B surfaces misoriented towards [2–1–1] are found to be more suitable to enhance the optical quality of the samples. We also found that the nitrogen incorporation is highly dependent on the growth temperature as well as on the V–III flux ratio. In addition to this, the optical properties and crystal quality of these structures depend strongly on the nitrogen content, as in the case of similar samples grown on (1 0 0) surfaces. High nitrogen contents (up to 3%) in InGaAsN layers grown on two different misoriented (1 1 1)B GaAs substrates are reported. Besides, low-temperature photoluminescence emission wavelengths longer than 1.4 μm are achieved using (1 1 1)B misoriented substrates.     

Echo-Planar Imaging of Porous Media with Spatial Resolution below 100 μm

A modified version of the echo-planar imaging technique incorporating a Carr–Purcell train of 180° rf pulses (PEPI) has been implemented on a standard spectrometer. It is demonstrated that artifacts in the image due to cumulative errors in the rf field can be reduced by replacing each 180° pulse by a composite sequence of three rf pulses. Artifact-free 3D images at 94 μm voxel resolution are obtained within 15 min. This technique has been applied to study the drying process in an initially water-saturated model porous medium with characteristicT^*₂of order 700 μs.     

Continuous-Wave Magnetic Resonance Imaging of Short T2 Materials

There is growing interest in the use of magnetic resonance imaging (MRI) to examine solid materials where the restricted motion of the probed spins leads to broad lines and short T₂ values, rendering many interesting systems invisible to conventional 2DFT pulsed imaging methods. In EPR T₂ seldom exceeds 0.1 μs and continuous-wave methods are adopted for spectroscopy and imaging. In this paper we demonstrate the use of continuous-wave MRI to obtain 2-dimensional images of short T₂ samples. The prototype system can image samples up to 50 mm in diameter by 60 mm long and has been used to image polymers and water penetration in porous media. Typical acquisition times range between 10 and 40 min. Resolution of 1 to 2 mm has been achieved for samples with T₂ values ranging from 38 to 750 μs. There is the possibility of producing image contrast that is determined by the material properties of the sample.     

Microrods based on nanocubes of Prussian blue

Using a facile dynamic vacuum evaporation method, a novel microrod with diameters of ca. 1–2 μm and lengths of up to 80 μm has been constructed using uniform Prussian blue (PB) nanocubes as the building blocks. The PB nanocubes are arranged fairly orderly in the rod-like superstructures. The assembled architecture can be transformed from one-dimensional microrods to two-dimensional layers via a fish-bone-like structure by tuning the evaporation rate. The formation of the PB superstructures follows an oriented-attachment mechanism and this provides a simple approach to fabricate hierarchical nanostructures and self-assembled superstructures using nanosized building blocks. Magnetic studies indicate that the PB microrods have a Curie temperature (T_c) of 4.9 K and a coercivity of ca. 26 Oe at 1.8 K. The photoluminescence (PL) spectra of the PB microrods and the dispersed nanocubes show an UV emission band at 358 and 367 nm respectively, suggesting an interesting assembly effect.