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As a promising technology, refractive index (RI) sensing has been developed for biosensing and chemosensing applications as well as for multiple industrial applications. Nevertheless, conventional RI sensors are confined by their operating principle or detection devices in terms of high sensitivity or a wide detection range. This study experimentally demonstrates metasurface-based spectrometer-free RI sensing through a spatial weighted variance (SWV)-based analysis of metasurface-generated vortex beams. The proposed liquid-immersed metasurface device creates a focused vortex beam at a single wavelength of 1550 nm, whose focusing effect varies according to the surrounding medium. The images, captured by a position-fixed near-infrared camera, exhibit divergent beam profiles determined by certain RI values. The SWV is used to assess the degree of divergence and is calculated based on the recorded beam images to quantify the variation of the RI. Thus, the proposed RI sensing scheme concurrently achieves high sensitivity (113 598/RIU), wide detection range (1.3164–1.3825), and high resolution (8.8 × 10−6 RIU). The proposed image-based spectrometer-less RI sensing methods overcome the constraints of instruments that are unavoidable in conventional detection techniques. Compared to the existing methods, the proposed strategy is simpler, less costly, more user-friendly, and usable in a broader range of sensing applications. [ABSTRACT FROM AUTHOR]

We investigated a growth behavior of highly oriented and columnar grained cuprous oxide (Cu2O) films, which were obtained through a chelate-assisted electrochemical solution approach. It was demonstrated that the electrochemical growth of Cu2O films followed a layer-to-island growth mode with a critical thickness of ∼190 nm. The chelating agent induced the layer-growth of flat-surfaced films consisting of single-crystalline planar grains, and influenced the preferred orientation of films maintained within the island-growth mode. In particular, the single-crystalline columnar grains with stable interfaces and diameters of 100–200 nm provided highly localized areas of linear grain boundaries for filamentary resistive switching. We measured different conduction behaviors of flat-surfaced films showing nonswitching Ohmic conduction and unipolar memory switching in as-deposited and annealed films, respectively. These different conduction behaviors were found to originate from the microstructure changes generated by the annealing process, which may suggest a clue for the origins of filamentary resistive memory switching in single-channeled grain boundaries. [ABSTRACT FROM AUTHOR]

The optical properties of shape-engineered InAs/InAlGaAs quantum dots (SEQDs) were investigated by temperature-dependent and excitation-power-dependent photoluminescence (PL) spectroscopy and compared with those of the conventionally grown InAs QDs (CQDs). The emission wavelength of the InAs/InAlGaAs SEQDs at 240 K was redshifted by 18 nm from that at 15 K, which was relatively smaller than that of the InAs CQDs (97 nm). The PL yield at 240 K was reduced to 1/86 and 1/65 of that measured at 15 K for the InAs CQDs and the InAs/InAlGaAs SEQDs, respectively. The emission wavelength for the InAs CQDs was blueshifted by 76 nm with increasing excitation power from 0.56 to 188 mW, compared to only by 7 nm for the InAs/InAlGaAs SEQDs. These results indicated that the InAs/InAlGaAs SEQDs were optically more stable than the InAs CQDs mainly due to the enhancement of the carrier confinement in the vertical direction and the improvement in the size uniformity. [ABSTRACT FROM AUTHOR]

The introduction of a thin (In)GaAs layer between an InAs quantum dot (QD) layer and an InAlGaAs layer altered the structural and optical properties of self-assembled InAs/InAlGaAs QDs grown on InP (001) substrates. The emission peak of the InAs QDs on a thin In0.32Ga0.68As layer grown on InAlGaAs was redshifted with increasing thickness of the In0.32Ga0.68As layer. However, the photoluminescence (PL) peak of the InAs QDs on a thin GaAs layer grown on InAlGaAs was blueshifted with increasing thickness of the GaAs layer, and the PL yield was also drastically decreased. Although the lattice constants of both In0.32Ga0.68As and GaAs are smaller than that of InAlGaAs, which would be expected to have a similar type of strain modulation, the structural and optical properties of the InAs QDs were significantly different because of the different nucleation characteristics of QDs. [ABSTRACT FROM AUTHOR]

This article reports the structural and optical properties of shape-engineered InAs/InAlGaAs quantum dots (QDs) on InP substrates formed using an alternate growth method (AGQDs), where a thin InAs layer (0.8 or 1 monolayer) and a thin InAlGaAs layer (1 monolayer) were deposited alternately. For five-stacked AGQD layers, a double-peak feature was observed in the photoluminescence (PL) spectra. The two peaks were related to two different QD branches as confirmed by excitation-power-dependent PL. Observation of a double-peak feature in the PL spectra indicated that the growth conditions for the AGQD layers were not optimized. A slight increase in growth temperature resulted in the merging of the double-peak feature to a single peak with a relatively narrow PL linewidth. [ABSTRACT FROM AUTHOR]

Shape-engineered InAs quantum dots (QDs) were grown by using thin (In)0.15(Ga)0.85(As and a)monolayer (ML) of InAs with different periods on 3 ML InAs QDs and their structural and optical properties were investigated by transmission electron microscopy (TEM), photoluminescence (PL), and photoreflectance (PR) spectroscopy. Cross-sectional TEM images of the QD samples showed that the shape, particularly the height of the QD, could be effectively controlled without any significant degradation in QD quality, such as the generation of dislocations in the QD, thus changing the optical properties. PL and PR spectra indicated that all the layers required for the formation of shaped-engineered QD were grown without any degradation in QD properties. PL spectra of shape-engineered QDs that had been subjected to rapid thermal annealing showed an abnormal behavior compared to those of conventionally grown InAs QDs. [ABSTRACT FROM AUTHOR]

Effects of a thin AlAs layer (1 nm) with different position on InAs quantum dots (QDs) and wetting layer have been investigated by transmission electron microscopy (TEM), photoluminescence (PL), and photoreflectance (PR). The PL peak position of InAs QDs directly grown on the thin AlAs is blueshifted from that of InAs QDs grown on the GaAs layer by 171 meV mainly due to the high potential barrier and reduced dot size shown in the TEM image. As the additional GaAs layer (1 and 2 nm) is inserted on top of the AlAs layer, the PL peak position is systematically shifted toward longer wavelength with increase in the thickness. Temperature dependent PL of QD samples shows that a thin AlAs layer significantly influences the thermal activation energy. The wetting layer related peak in PR spectra is changed to lower energy with increase in the thickness of an additional GaAs layer, which is mainly caused by the reduction in the effects of the AlAs layer. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Lattice defect structures embedded into perfect photonic crystal slabs have been fabricated onto the GaN surfaces of InGaN/GaN multi-quantum-well light-emitting diodes. The photonic crystal slab with a triangular lattice constant of 230 nm showed suppressed light extraction for the electroluminescence at a peak of 464 nm, but the light radiated through lattice defects. The perfect photonic crystal slab shows the cooperative phenomenon of a light scattering similar to Wood’s anomaly in a metallic grating, and lattice defects result in the leaky modes of light extractions by breaking the symmetry of cooperative light scattering. [ABSTRACT FROM AUTHOR]

The carrier lifetime of an InAs/InGaAsP quantum dot (QD) on an InP substrate is measured to be twice that of an InAs/InAlGaAs QD on the same substrate, although the ground-state energy levels and barrier heights of these QDs are comparable. These differences are interpreted in terms of the smaller conduction band-offset in InAs/InGaAsP QDs compared to InAs/InAlGaAs QDs. [ABSTRACT FROM AUTHOR]

Beam capture of injected ions and charge breeding in electron cyclotron resonance (ECR) charge breeder ion source plasmas has been investigated utilizing an ECR plasma modeling code, the generalized ECR ion source model, and a Monte Carlo beam capture code. Beam capturing dynamics, charge breeding in the plasma, and the extracted charged ion states are described. Optimization of ion beam energy is performed for (1) high beam capture efficiency and (2) high charge state ion beam extractions. A sample case study for ANL-ECR has been performed. Ions entering ECR ion source plasma are slowed down mostly by the background ions. Assuming Maxwellian plasma ions, maximum beam energy loss occurs when the beam velocity is around the background thermal velocity in magnitude. It is also found that beam capture location affects charge state distribution. For instance, with a majority of beam ions captured near the middle of the device higher currents for higher charge states are obtained. The beam ions captured near the entry have a higher probability of backstreaming after they are captured. For this reason, the optimum beam energy of the injected Ar+ beam ions for charge breeding is generally higher than the optimum input beam energy for maximum beam energy loss. [ABSTRACT FROM AUTHOR]

A contrast enhancing technique (CET) for soft x-ray (SXR) measurements has been developed and tested for the early identification of the low amplitude resistive wall mode (RWM) on the DIII-D tokamak. The technique is simple and fast. It utilizes the chord-by-chord difference of low-pass digitally filtered time derivatives of the signals from the twelve-chord fan-shape soft x-ray arrays located at toroidal angles of 195° and 45°. The two arrays allow a demonstration of the CET method principle, although they cannot completely resolve the RWM structure. The time derivative of the x-ray signal amplifies the effect of the temperature perturbation convected by the RWM, while naturally incorporating the equilibrium evolution effect. The correlation with the parameters measured by other diagnostics, such as the radial magnetic field δBr from the magnetic probes, the radial profiles of plasma current density j, pressure p, and safety factor q from the motional Stark effect, the radial profile of the temperature perturbation ΔTe from electron cyclotron emission, the normalized beta parameter βN=βT(I/aB)-1, and the Dα light emission, confirm the early detection of the low-amplitude RWMs by the soft x-ray CET. Upon validation the SXR CET could be implemented into a multisensor scheme using other diagnostics for the real-time identification of RWM. [ABSTRACT FROM AUTHOR]

A matched filter analysis has been developed to identify the amplitude and phase of magnetohydrodynamic modes in DIII-D tokamak plasmas using magnetic probe signals (δB[sub p]). As opposed to conventional Fourier spatial analysis of toroidally spaced probes, this analysis includes data from both toroidally and poloidally spaced magnetic probe arrays. Using additional probes both improves the statistics of the analysis and more importantly incorporates poloidal information into the mode analysis. The matched filter is a numeric filter that matches signals from the magnetic probes with numerically predicted signals for the mode. The numerical predictions are developed using EFIT equilibrium reconstruction data as input to the stability code GATO and the vacuum field code VACUUM. Changes is the plasma equilibrium that occur on the same time scale as the mode are taken into account by modeling simple matched filter vectors corresponding to changes in total plasma current, plus vertical and horizontal plasma shifts. The matched filter method works well when there is good understanding of a mode and good modeling of its structure. Matched filter analysis results for a fast growing ideal kink mode, where equilibrium change effects are minimal, show the effectiveness of this method. A slow growing resistive-wall mode (RWM) is also analyzed using the matched filter method. The method gives good results for identifying the amplitude and phase of the RWM but the simple equilibrium vectors are insufficient for complete elimination of equilibrium changes on this time scale. An analysis of the computational requirements of the scheme indicates that real-time application of the matched filter for RWM identification will be possible. [ABSTRACT FROM AUTHOR]

Electron cyclotron resonance (ECR) heating influences two of the main parameters (electron temperature and, indirectly, density) that determine the charge state of the ions produced in an ECR ion source (ECRIS). Therefore, various schemes to optimize ECR heating in the ECRIS have been pursued such as multiple-frequency heating, the radio-frequency tuning effect, volume heating, or wide-band heating. We investigate two-frequency ECR heating of electrons in a simple magnetic mirror field by right handed circularly polarized waves with infinite phase velocity. The study shows a heating barrier different from the well-know adiabatic barrier. Study also revealed a mechanism whereby multiple frequencies give improved heating. A preliminary interpretation of the study is presented. [ABSTRACT FROM AUTHOR]

We observed cathode-interfaced electroreduction and a variation of oxygen content in an irreversible low resistance state of highly oriented cuprous oxide (Cu2O) films. These local microstructural and stoichiometric changes in hard breakdown of film allow speculation on the unipolar resistive switching mechanism: formation of metallic filaments originating from generation/migration of oxygen ions (O2-) from the cathode and release of oxygen gas through the anode. Based on the as-proposed switching model, endurance switching properties could be modulated from tens up to over 104 switching cycles by controlling ambient gas or the interface between Cu2O and anode. [ABSTRACT FROM AUTHOR]

Resistance change random access memory devices using NiOx films with resistance switching properties have immense potential for high-density nonvolatile memory exceeding currently used flash memory. The only critical failure of a NiOx film is to write wrong information due to large fluctuations of switching voltages during successive resistance switching operations. The authors show that failure-free NiOx film can be grown directly on Pt electrode just by process control. Intensive analyses show that the superior resistance switching behaviors of their simple Pt/NiOx/Pt structure may result from a very thin Ni–Pt layer self-formed at the bottom interface during deposition of NiOx. [ABSTRACT FROM AUTHOR]

Triangular hole arrays with nanoscaled lattice constants of 230 and 460 nm were fabricated on a p-type GaN epitaxial layer grown on an InGaN/GaN multi-quantum-well light emitting diode structure by metal-organic chemical vapor deposition. The hole geometries of dry-etched thin slabs for triangular lattice constants of 230 and 460 nm possessed diameters of 223 and 218 nm at the surface, and 108 and 76 nm at the bottom, with depths of 31 and 27 nm, respectively. The hole array with a lattice constant of 230 nm enhances photoluminescence intensity at wavelengths of 364 and 406 nm, but reduces light extraction at a wavelength of 450 nm, which indicates destructive surface diffraction correlated with light scattering in the photonic crystal structure. [ABSTRACT FROM AUTHOR]

The formation characteristics on the vertical stacks of shape-engineered InAs/InAlGaAs quantum dots (QDs), which were formed by the alternate growth method (AGQDs), were studied in terms of the modulation in strain field and phase separation. The threshold current of the broad-area laser diodes (LDs) with five stacks of the AGQDs (AGQD-LDs) was 4.5 times smaller than that of the LDs with seven stacks of the conventionally grown QDs (CQD-LDs). The slope efficiency for the AGQD-LDs was 0.16 W/A, which was higher than that of the CQD-LDs of 0.9 W/A. These results can be attributed to better confinement of the electron wave function in QDs. [ABSTRACT FROM AUTHOR]

Seven stacks of self-assembled InAs quantum dots (QDs) separated by 28 nm thick InAlGaAs barriers were grown on InP (001) substrate by a solid-source molecular-beam epitaxy and were investigated by cross-sectional transmission electron microscopy and photoluminescence spectroscopy. Gain guided broad-area lasers with a stripe width of 75 μm were fabricated by using the seven-stacked InAs QD layers with the InAlGaAs–InAlAs material system on InP (001). The lasing operation from InAs QDs was observed up to 260 K and the characteristic temperature of the uncoated QD laser calculated from the temperature dependence of threshold current density was 377 K for temperatures up to 200 K, and 138 K above 200 K. The drastic decrease in the characteristic temperature above 200 K was mainly related to the thermal behavior of carriers in QDs, and possibly the thermal coupling of the QDs to the wetting layer and the waveguide region. [ABSTRACT FROM AUTHOR]

Dielectric SrMoO4 thin films were deposited from Sr2FeMoO6 target in oxygen atmosphere, showing obvious M–H hysteresis loops at room temperature. It was revealed by transmission electron microscopy that SrFeO3 nanoparticles are dispersed in the SrMoO4 grains, to which the hystersis loops of the thin films are ascribed. This SrMoO4 thin film can be useful as a barrier material for Sr2FeMoO6-based devices, owing to easy fabrication process and compatibility with Sr2FeMoO6. Magnetic SrFeO3 nanoparticles are expected to enhance tunneling magnetoresistance. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The structural and optical properties of height-controlled InAs quantum dots (QDs) have been investigated by transmission electron microscopy (TEM) and photoluminescence (PL). By depositing 1.4 nm In[sub 0.15]Ga[sub 0.85]As and a 1 monolayer (ML) InAs layer with different periods on 3 ML InAs QDs, the height of InAs QDs was systematically controlled with similar lateral size. In TEM images, the indication of dislocations due to the large strain, which can be easily seen in large QDs, is not observed even for the QD sample with the highest aspect ratio (height/width). The PL peak position is shifted toward the longer wavelength with an increase in the aspect ratio of QDs. As the aspect ratio is increased, the full width at half maximum in PL measured at 10 K is decreased from 71 to 34 meV indicating that the inhomogeneous broadening caused by the fluctuation in QD size, especially the height, is significantly reduced. [ABSTRACT FROM AUTHOR]

Bi-excess SrBi[sub 2]Ta[sub 2]O[sub 9] (SBT) thin films on Pt/Ti/SiO[sub 2]/Si substrate were prepared by pulsed laser deposition technique. The SBT structure was characterized by x-ray diffraction studies. The ferroelectric properties were confirmed by P-E hysteresis loops at different applied electric fields. The dielectric constant and the ac conductivity of the Pt/SBT/Pt capacitor were investigated in the frequency range from 0.01 Hz to 100 kHz and in the temperature range from 25 to 400 °C. The thermal activation energy of 0.90 eV is observed in the frequency dependent dielectric constant. The activation energy for conduction process is calculated as 0.91 eV from the slope of ac conductivity at the lowest frequency. The low-frequency dielectric relaxation and the ac conductivity of Bi-excess SBT thin film are discussed in relation to the electrical conduction of SBT/Pt junction. [ABSTRACT FROM AUTHOR]

A one-dimensional (ID) fluid computer model for multiple ion species in an electron cyclotron resonance ion source (ECRIS) plasma has been developed. The ions species are assumed to be highly collisionally coupled and are treated using ID fluid equations. The non-Maxwellian anisotropic electron distribution function is modeled by a ID bounce-averaged Fokker-Planck code. ECR heating is included in the model as a quasilinear rf-diffusion term including relativistic detuning, rf pitch-angle scattering, and multiple resonance frequencies/locations. In a typical ECRIS, the electrons are very noncollisional and confined magnetically. The ions follow this electron confinement via the electrostatic potential. The iD axial electrostatic potential profile predicted by the model shows an ion confining core electrostatic well as expected in ECRIS plasmas. Modeling results for the Argonne National Laboratory ECR-I ECRIS configuration are presented along with a discussion of the difficulties in benchmarking the model with Faraday cup measurements. [ABSTRACT FROM AUTHOR]

A multi-purpose soft x-ray array diagnostic system was developed for measuring two-dimensional x-ray emissivity profile, electron temperature, Ar impurity transport, and total radiation power. A remotely controlled filter wheel was designed with three different choices of filters. The electron temperature profile can be determined from the ratio of two channels having different thickness of Be layer, and the Ar impurity concentration transport can be determined from a pair of Ar Ross filters (CaF2 and NaCl thin films). Without any filters, this diagnostic system can also be used as a bolometer. [ABSTRACT FROM AUTHOR]

Ripples in graphene monolayers deposited on SiO2/Si wafer substrates were recently shown to give rise to friction anisotropy. High friction appears when the AFM tip slides in a direction perpendicular to the ripple crests and low friction when parallel. The direction of the ripple crest is, however, hard to determine as it is not visible in topographic images and requires elaborate measurements of friction as a function of angle. Here we report a simple method to characterize ripple crests by measuring the cantilever torsion signal while scanning in the non-conventional longitudinal direction (i.e., along the cantilever axis, as opposed to the usual friction measurement). The longitudinal torsion signal provides a much clearer ripple domain contrast than the conventional friction signal, while both signals show respective rotation angle dependences that can be explained using the torsion component of the normal reaction force exerted by the graphene ripples. We can also determine the ripple direction by comparing the contrast in torsion images obtained in longitudinal and lateral scans without sample rotation or complicated normalization. [ABSTRACT FROM AUTHOR]

This article presents a correction of a previously published article regarding gallium nitride quantum dots.

The Electron Beam Ion Source particle-in-cell code (EBIS-PIC) tracks ions in an EBIS electron beam while updating electric potential self-consistently and atomic processes by the Monte Carlo method. Recent improvements to the code are reported in this paper. The ionization module has been improved by using experimental ionization energies and shell effects. The acceptance of injected ions and the emittance of extracted ion beam are calculated by extending EBIS-PIC to the beam line transport region. An EBIS-PIC simulation is performed for a Cs charge-breeding experiment at BNL. The charge state distribution agrees well with experiments, and additional simulation results of radial profiles and velocity space distributions of the trapped ions are presented.