Array Of Spheres Research Articles

Frustrated total internal reflection in organic light-emitting diodes employing sphere cavity embedded in polystyrene

The light extraction efficiency of top-emitting organic light-emitting diodes (OLEDs) is numerically investigated employing the finite-difference time-domain method. The periodic nanostructures formed by embedding the sphere arrays in polystyrene (PS) are placed on top of OLED to frustrate the total internal reflection at the interface between OLED and free space. These nanostructures serve as an intermediate medium to extract the light out of OLED devices. Efficiently coupling both evanescent waves and propagation waves into spheres and subsequently extracting these light waves out of the sphere is key to achieving high extraction efficiency. By tuning the thickness of PS layer, both of the in-coupling efficiency and out-coupling efficiency are optimized for achieving high light extraction efficiency. Thicker PS layer results in higher in-coupling efficiency in sphere while the thinner PS layer leads to higher out-coupling efficiency. Thus the maximum light extraction is a trade-off between the in-coupling efficiency and out-coupling efficiency. The study shows that light extraction efficiency of 89% can be achieved by embedding 0.90 μm TiO2 sphere in 0.30 μm PS layer with optimized in-coupling efficiency, out-coupling efficiency and cavity effect.

Extreme stiffness tunability through the excitation of nonlinear defect modes.

The incremental stiffness characterizes the variation of a material's force response to a small deformation change. In lattices with noninteracting vibrational modes, the excitation of localized states does not have any effect on material properties, such as the incremental stiffness. We report that, in nonlinear lattices, driving a defect mode introduces changes in the static force-displacement relation of the material. By varying the defect excitation frequency and amplitude, the incremental stiffness can be tuned continuously to arbitrarily large positive or negative values. Furthermore, the defect excitation parameters also determine the displacement region at which the force-displacement relation is being tuned. We demonstrate this phenomenon experimentally in a compressed array of spheres tuning its incremental stiffness from a finite positive value to zero and continuously down to negative infinity.

Phantom-based characterization of distortion on a magnetic resonance imaging simulator for radiation oncology

One of the major issues potentially limiting treatment planning with solely MR images is the possibility of geometric distortion inherent in MR images. We designed a large distortion phantom containing a 3D array of spheres and proposed a three-dimensional (3D) approach to determine the distortion of MR image volume. The approach to overcome partially filled spheres is also presented.The phantom was assembled with a 3D array of spheres filled with contrast and was scanned with a 3T MRI simulator. A 3D whole-sphere or half-sphere template is used to match the image pattern. The half-sphere template is used when the normalized cross-correlation value for the whole-sphere template is below a predetermined threshold. Procrustes method was applied to remove the shift induced by rotation and translation of the phantom. Then the distortion map was generated. Accuracy of the method was verified using CT images of a small phantom of the same design.The analysis of the small phantom showed that the method is accurate with an average offset of estimated sphere center 0.12 ± 0.04 mm. The Procrustes analysis estimated the rotation angle to be 1.95° and 0.01°, respectively, when the phantom was placed at 2° and 0° from the ceiling laser. The analysis showed that on the central plane through the magnet center, the average displacement is less than 1 mm for all radii. At distal planes, when the radius is less than 18 cm, the average displacement is less than 1 mm. However, the average displacement is over 1 mm but still less than 1.5 mm for larger radii.A large distortion phantom was assembled and analysis software was developed to characterize distortions in MRI scans. The use of two templates helps reduce the potential impact of residual air bubbles in some of the spheres.

2D-ordered dielectric sub-micron bowls on a metal surface: a useful hybrid plasmonic-photonic structure.

Recently, it has been demonstrated that the combination of periodic dielectric structures with metallic structures provides an efficient means to yield a synergetic optical response or functionality in the resultant hybrid plasmonic-photonic systems. In this work, a new hybrid plasmonic-photonic structure of 2D-ordered dielectric sub-micron bowls on a flat gold surface was proposed, prepared, and theoretically and experimentally characterized. This hybrid structure supports two types of modes: surface plasmon polaritons bound at the metallic surface and waveguided mode of light confined in the cavity of bowls. Optical responses of this hybrid structure as well as the spatial electric field distribution of each mode are found to be strongly dependent on the structural parameters of this system, and thus could be widely modified on demand. Importantly, compared to the widely studied hybrid systems, namely the flat metallic surface coated with a monolayer array of latex spheres, the waveguided mode with strong field enhancement appearing in the cavities of bowls is more facilely accessible and thus suitable for practical use. For demonstration, a 2D-ordered silica sub-micron bowl array deposited on a flat gold surface was fabricated and used as a regenerable platform for fluorescence enhancement by simply accommodating emitters in bowls. All the simulation and experiment results indicate that the 2D-ordered dielectric sub-micron bowls on a metal surface should be a useful hybrid plasmonic-photonic system with great potential for applications such as sensors or tunable emitting devices if appropriate periods and materials are employed.

Monolayer titanium carbide hollow sphere arrays formed via an atomic layer deposition assisted method and their excellent high-temperature supercapacitor performance

High-quality titanium carbide (TiC) hollow sphere arrays on a conductive substrate are fabricated, and a new type of organic symmetric supercapacitor is assembled that shows impressive capacitance at high temperatures.

Comparison of Ag and Si nanoparticle arrays: mimicking subwavelength plasmonic field concentrations with dielectric components

We present a comparative theoretical study of the optical response of metallic Ag and dielectric Si sphere arrays on top of reflecting substrates. The interaction of particle modes with guided modes of the substrate leads to a rich optical spectrum. We design structures that sustain highly concentrated electromagnetic fields around periodic arrays of Si nanoparticles through the coupling of incident light to waveguide modes of a dielectric spacer. We discuss the origin of the different modes responsible for the reflectivity spectra of particle arrays and propose that all dielectric structures can support strong field enhancement which is similar to nanostructured metallic surfaces.

Sphere-on-cone microstructures on Teflon surface: Repulsive behavior against impacting water droplets

Teflon (polytetrafluoroethylene) surface has been modified with a single step oxygen-fed plasma process resulting in the formation of peculiar “sphere-on-cone” micro-scale relieves. Though presenting some irregularity and random distribution, surfaces with a steep variation of topography can be obtained by properly tuning plasma process parameters. We show that these surfaces exhibit similar superhydrophobic properties in terms of water contact angle but, the ability to withstand vertically impacting water droplets falling at medium/high impacting speeds can be sensitively different. In particular, high repulsive properties characterize surfaces with denser and smaller relieves. In the other cases the occurrence of pinning events results in longer time-of-contact and a shorter time-of-flight of the drop. Interestingly, the impact pressure values at which pinning transitions have been observed are consistent with critical pressures for penetration calculated by modeling the surface as an array of spheres. When critical pressure is exceeded, and water penetration is expected, reversible or non reversible pinning can be explained on the basis of the wetting/de-wetting cycle within the sphere-on-cone's layer.

Formation of Carbonized Polystyrene Sphere/hemisphere Shell Arrays by Ion Beam Irradiation and Subsequent Annealing or Chloroform Treatment.

Heat-resistant two-dimensional (2D) sphere/hemisphere shell array is significant for the fabrication of novel nanostructures. Here large-area, well-ordered arrays of carbonized polystyrene (PS) hollow sphere/hemisphere with controlled size and morphology are prepared by combining the nanosphere self-assembly, kV Ag ion beam modification, and subsequent annealing or chloroform treatment. Potential mechanisms for the formation and evolution of the heat-resistant carbonized PS spherical shell with increasing ion fluence and energy are discussed. Combined with noble metal or semiconductor, these modified PS sphere arrays should open up new possibilities for high-performance nanoscale optical sensors or photoelectric devices.

Aspect ratio engineering of microlens arrays in thin-film flip-chip light-emitting diodes.

Light extraction efficiency of thin-film flip-chip InGaN-based light-emitting diodes (LEDs) with a TiO2 microlens arrays was calculated by employing the finite-difference time-domain method. The microlens arrays, formed by embedding hexagonal close-packed TiO2 sphere arrays in a polystyrene (PS) layer, were placed on top of the InGaN LED to serve as an intermediate medium for light extraction. By tuning the thickness of the PS layer, in-coupling and out-coupling efficiencies were optimized to achieve maximum light extraction efficiency. A thicker PS layer resulted in higher in-coupling efficiency, while a thinner PS layer led to higher out-coupling efficiency. Thus, the maximum light extraction efficiency becomes a trade-off between in-coupling and out-coupling efficiency. In addition, the cavity formed by the PS layer also affects light extraction from the LED. Our study reveals that a maximum light extraction efficiency of 86% was achievable by tuning PS thickness to 75 nm with maximized in-coupling and out-coupling efficiency accompanied by the optimized resonant cavity condition.

Surface area characteristics of furfuryl-alcohol-derived inverse opal carbons produced from silica inverse opal templates

High specific surface area three dimensionally ordered carbons are produced from furfuryl alcohol in a double templating procedure utilizing a silica inverse opal. Unexpectedly, the carbons produced here do not form the expected array of carbon spheres; rather, a faithful reproduction of the underlying inverse opal is attained. It is found that the properties of the silica template strongly influence the surface area characteristics of the carbons. The surface areas of the carbons are proportional to that of the template, and are also found to increase with decreased macropore size, consistent with findings for furfuryl-alcohol-derived carbon inverse opals based on silica colloidal crystal templates. However, template calcination temperature has a very influential role on the surface area, playing a unique role in our materials when compared to the closest reported analogues.

Scalable Fabrication of Quasi-Three-Dimensional Chiral Plasmonic Oligomers Based on Stepwise Colloid Sphere Lithography Technology.

We report a simple and scalable method for the fabrication of spiral-type chiral plasmonic oligomers based on the stepwise colloid sphere lithography technology. Through carefully adjusting the azimuthal angle Φ of polystyrene (PS) sphere array monolayer and the deposition thickness kn, the chiral plasmonic oligomers composed of four achiral particles can be successfully fabricated on a desired substrate. And their chiral sign, i.e., left-hand or right-hand, is dependent on the anticlockwise or clockwise deposition sequence of the achiral particles. The measured results show a large chiroptical resonance in the visible region, and this resonance can be easily adjusted by using different sizes of PS spheres. Our in-depth theoretical and experimental researches further reveal that the obtained chiral plasmonic oligomers are indeed a kind of quasi-three-dimensional chiral nanostructures, which own a three-dimensional geometrical morphology, but with nonreciprocity chiroptical effect. The ease and scalability (>1 cm2) of the fabrication method make chiral plasmonic oligomers promising candidates for many applications, such as chiral biosensor and catalysis.

Investigating the optical AND gate using plasmonic nano-spheres

In this paper, a coherent perfect absorption (CPA)-type AND gate based on plasmonic nano particles is proposed. It consists of two gold nano sphere arrays on top of two serial arms with quartz substrate. The operation principle is based on the absorbable formation of a conductive path in the dielectric layer of a plasmonic nano-particle waveguide. Since the CPA efficiency depends strongly on the number of gold nano-sphere and the locations of nano-spheres, an efficient binary optimization method based the Particle Swarm Optimization (PSO) algorithm is used to design an optimized array of the plasmonic nano-sphere in order to achieve the maximum absorption coefficient in the `off' state and the minimum absorption coefficient in the `on' state. In Binary PSO, a group of birds consists a matrix with binary entries, control the presence (`1') or the absence (`0') of nano sphere in the array.

Segmented beryllium target for a 2 MW super beam facility

Building a proton-beam target from an array of spheres improves shock resistance and cooling compared with a conventional solid cylindrical target.

Light extraction efficiency enhancement of top-emitting organic light-emitting diodes employing low-Q whispering gallery modes in spheres

Reducing Fresnel reflection and enlarging light-escape cones are instrumental in enhancing light-extraction efficiency of top-emitting organic light-emitting diodes (OLEDs). The periodic sphere arrays designed on top of the OLED serve as intermediate media to enlarge the light-escape cone as well as to reduce reflection at the interface. Both the evanescent waves and propagation waves were coupled into spheres with suitable refractive index. The coupled light waves circled around the sphere cavity and, by tuning the diameter of the sphere, they returned to the same point after one round trip, which led to the constructive interference and the light intensity was significantly enhanced. The light waves inside the spheres slowly leaked out due to the curved interface, which resulted in the enhancement in light-extraction efficiency. Both maximizing in-coupling efficiency and increasing the amount of light extracted from the spheres are key to improving light extraction, which was achieved by choosing the spheres with suitable refractive index and diameter. Significant enhancement in light extraction was observed by designing hexagonal close-packed spheres on top of the OLED.

The raspberry model for hydrodynamic interactions revisited. I. Periodic arrays of spheres and dumbbells.

The so-called "raspberry" model refers to the hybrid lattice-Boltzmann and Langevin molecular dynamics scheme for simulating the dynamics of suspensions of colloidal particles, originally developed by Lobaskin and Dünweg [New J. Phys. 6, 54 (2004)], wherein discrete surface points are used to achieve fluid-particle coupling. This technique has been used in many simulation studies on the behavior of colloids. However, there are fundamental questions with regards to the use of this model. In this paper, we examine the accuracy with which the raspberry method is able to reproduce Stokes-level hydrodynamic interactions when compared to analytic expressions for solid spheres in simple-cubic crystals. To this end, we consider the quality of numerical experiments that are traditionally used to establish these properties and we discuss their shortcomings. We show that there is a discrepancy between the translational and rotational mobility reproduced by the simple raspberry model and present a way to numerically remedy this problem by adding internal coupling points. Finally, we examine a non-convex shape, namely, a colloidal dumbbell, and show that the filled raspberry model replicates the desired hydrodynamic behavior in bulk for this more complicated shape. Our investigation is continued in de Graaf et al. [J. Chem. Phys. 143, 084108 (2015)], wherein we consider the raspberry model in the confining geometry of two parallel plates.

Light trapping above the light cone in a one-dimensional array of dielectric spheres

We demonstrate bound states in the first TE and TM diffraction continua (BSC) in a linear periodic array of dielectric spheres in air above the light cone. We classify the BSCs according to the symmetry specified by the azimuthal number $m$, the Bloch wave vector $\beta$ directed along the array, and polarization. The most simple symmetry protected TE and TM polarized BSCs have $m=0$ and $\beta=0$ and occur in a wide range of the radius of the spheres and dielectric constant. More complicated BSCs with $m\neq 0$ and $\beta=0$ exist only for a selected radius of spheres at a fixed dielectric constant. We also find robust Bloch BSCs with $\beta\neq 0, m=0$. We present also the BSCs embedded into two and three diffraction continua. We show that the BSCs can be easily detected by the collapse of Fano resonance for scattering of electromagnetic plane waves by the array.

Direct numerical simulation of moderate-Reynolds-number flow past arrays of rotating spheres

Direct numerical simulations with an immersed boundary-lattice Boltzmann method are used to investigate the effects of particle rotation on flows past random arrays of mono-disperse spheres at moderate particle Reynolds numbers. This study is an extension of a previous study of the authors [Q. Zhou and L.-S. Fan, “Direct numerical simulation of low-Reynolds-number flow past arrays of rotating spheres,” J. Fluid Mech. 765, 396–423 (2015)] that explored the effects of particle rotation at low particle Reynolds numbers. The results of this study indicate that as the particle Reynolds number increases, the normalized Magnus lift force decreases rapidly when the particle Reynolds number is in the range lower than 50. For the particle Reynolds number greater than 50, the normalized Magnus lift force approaches a constant value that is invariant with solid volume fractions. The proportional dependence of the Magnus lift force on the rotational Reynolds number (based on the angular velocity and the diameter of the spheres) observed at low particle Reynolds numbers does not change in the present study, making the Magnus lift force another possible factor that can significantly affect the overall dynamics of fluid-particle flows other than the drag force. Moreover, it is found that both the normalized drag force and the normalized torque increase with the increase of the particle Reynolds number and the solid volume fraction. Finally, correlations for the drag force, the Magnus lift force, and the torque in random arrays of rotating spheres at arbitrary solids volume fractions, rotational Reynolds numbers, and particle Reynolds numbers are formulated.

Adsorption of Polyether Block Copolymers at Silica-Water and Silica-Ethylammonium Nitrate Interfaces.

Atomic force microscope (AFM) force curves and images are used to characterize the adsorbed layer structure formed by a series of diblock copolymers with solvophilic poly(ethylene oxide) (PEO) and solvophobic poly(ethyl glycidyl ether) (PEGE) blocks at silica-water and silica-ethylammoniun nitrate (EAN, a room temperature ionic liquid (IL)) interfaces. The diblock polyethers examined are EGE109EO54, EGE113EO115, and EGE104EO178. These experiments reveal how adsorbed layer structure varies as the length of the EO block varies while the EGE block length is kept approximately constant; water is a better solvent for PEO than EAN, so higher curvature structures are found at the interface of silica with water than with EAN. At silica-water interfaces, EGE109EO54 forms a bilayer and EGE113EO115 forms elongated aggregates, while a well-ordered array of spheres is present for EGE104EO178. EGE109EO54 does not adsorb at the silica-EAN interface because the EO chain is too short to compete with the ethylammonium cation for surface adsorption sites. However, EGE113EO115 and EGE104EO178 do adsorb and form a bilayer and elongated aggregates, respectively.

Electrostriction enhancement in metamaterials

In this proof-of-concept analytical study (supported by numerical examples) the authors are able to achieve controllable enhancement of the electrostriction in a metamaterial medium consisting of an array of spheres in a host medium. Accounting for material dispersion and losses for a few test cases, they show a sizable enhancement (or suppression) and tunability of the electrostrictive properties. We anticipate that this study will open interesting directions previously unexplored in metamaterials research.

SU-D-303-04: A Survey of MRI Distortion Measurements

Purpose: Magnetic Resonance Imaging (MRI) is hampered by geometrical distortions that arise from inhomogeneity in the B0 field, gradient non-linearity, and patient-induced susceptibility artifacts. We provide a survey of precise and accurate distortion measurements made over eight years on a number of different scanner/protocol arrangements. Methods: A phantom was originally developed for the Alzheimer’s Disease Neuroimaging Initiative (ADNI Phantom) to provide detailed mapping of MRI image distortion. The phantom contains an array of polycarbonate spheres each filled with a copper sulfate solution to provide image contrast. Included are 158 1.0 cm diameter and 2 1.5 cm diameter fiducial spheres enclosed in a 20 cm diameter spherical urethane shell. The center of gravity of the spheres can be determined to sub-voxel precision through segmentation of the phantom images. A software analysis determines the actual locations of the spheres independent of the position and orientation of the phantom. The imaged locations of spheres are compared to the known locations yielding up to fourth-order measurements of scanner distortion. We have surveyed MR distortion measurements acquired by groups doing routine QA and longitudinal studies over an eight year period. Results: Distortion fields vary in magnitude and form from between scanners and protocols. Corrections applied by the scanner typically reduce distortion to 1 mm or less throughout a 20 cm diameter volume centered on isocenter. Corrections are typically less successful in one direction depending on the protocol. Distortion increases with distance from the isocenter and can be highly non-linear. Conclusion: Distortion of MR images is inherent to current scanners. Distortion is a function of many factors and although often less than 1mm can be much greater depending on protocol, distance from isocenter, and other factors. Scanners are typically stable with respect to their distortion profile but variations are not uncommon. Research supported by Image Owl, Inc., The Phantom Laboratory, Inc. and Raforninn ehf. Authors Mallozzi and Healy employed by The Phantom Laboratory, Inc. Author Goodenough is a consultant to The Phantom Laboratory, Inc. Authors Fredriksson, Kristbjornsson, and Olafsdottir are consultants to Image Owl, Inc.