Showing papers by "David R. Smith published in 2014"

Probing the mechanisms of large Purcell enhancement in plasmonic nanoantennas

Abstract: Plasmonic nanostructures enable spontaneous emission enhancement factors of greater than 1,000 — the largest observed to date. The orientation of dipole emitters in nanogaps plays a vital role.

Terahertz compressive imaging with metamaterial spatial light modulators

Abstract: Active metamaterials have been used to realize terahertz imaging with a single-pixel detector Compressive techniques permit high-fidelity images to be acquired at high frame rates The technique involves no moving parts and yields improved signal-to-noise ratios over standard raster scanning techniques

Control of radiative processes using tunable plasmonic nanopatch antennas.

Abstract: The radiative processes associated with fluorophores and other radiating systems can be profoundly modified by their interaction with nanoplasmonic structures. Extreme electromagnetic environments can be created in plasmonic nanostructures or nanocavities, such as within the nanoscale gap region between two plasmonic nanoparticles, where the illuminating optical fields and the density of radiating modes are dramatically enhanced relative to vacuum. Unraveling the various mechanisms present in such coupled systems, and their impact on spontaneous emission and other radiative phenomena, however, requires a suitably reliable and precise means of tuning the plasmon resonance of the nanostructure while simultaneously preserving the electromagnetic characteristics of the enhancement region. Here, we achieve this control using a plasmonic platform consisting of colloidally synthesized nanocubes electromagnetically coupled to a metallic film. Each nanocube resembles a nanoscale patch antenna (or nanopatch) whose ...

Metamaterial microwave holographic imaging system

Abstract: We demonstrate a microwave imaging system that combines advances in metamaterial aperture design with emerging computational imaging techniques. The flexibility inherent to guided-wave, complementary metamaterials enables the design of a planar antenna that illuminates a scene with dramatically varying radiation patterns as a function of frequency. As frequency is swept over the K-band (17.5–26.5 GHz), a sequence of pseudorandom radiation patterns interrogates a scene. Measurements of the return signal versus frequency are then acquired and the scene is reconstructed using computational imaging methods. The low-cost, frequency-diverse static aperture allows three-dimensional images to be formed without mechanical scanning or dynamic beam-forming elements. The metamaterial aperture is complementary to a variety of computational imaging schemes, and can be used in conjunction with other sensors to form a multifunctional imaging platform. We illustrate the potential of multisensor fusion by integrating an infrared structured-light and optical image sensor to accelerate the microwave scene reconstruction and to provide a simultaneous visualization of the scene.

System wirelessly transferring power to a target device over a tested transmission pathway

Abstract: Described embodiments include a system, method, and apparatus A system includes an antenna comprising a sub-Nyquist holographic aperture configured to define selectable arbitrary complex radiofrequency electromagnetic fields on a surface of the antenna A path analysis engine tests power transmission pathways from the antenna to a target device located in an environment within a space radiateable by the antenna The environment includes a human being An optimization circuit selects responsive to the tested power transmission pathways a power transmission regime The regime includes an electromagnetic radiation pattern shaped to transfer radiofrequency electromagnetic power from the antenna to the target device without exceeding a radiation exposure limit for humans A gain definition circuit selects a complex radiofrequency electromagnetic field implementing the selected power transmission regime from the at least two selectable, complex radiofrequency electromagnetic fields An antenna controller defines the selected arbitrary complex radiofrequency electromagnetic field in the sub-Nyquist holographic aperture

Third-Harmonic Generation Enhancement by Film-Coupled Plasmonic Stripe Resonators

Abstract: Because of their ability to strongly localize and enhance optical fields, plasmonic nanostructures have the potential to dramatically amplify the inherent nonlinear response of materials. We illustrate the impact of this plasmonic interaction by investigating the third-harmonic generation (THG) from a system of film-coupled nanostripes operating at 1500 nm. Both the film and the stripes are gold, separated by a nanoscale layer of aluminum oxide (Al2O3) grown using atomic layer deposition. This nanoscale junction, with an ultrasmooth interface, forms a waveguide cavity resonator with a large and controllable electric field enhancement, whose plasmon resonance can be tuned independently by changing the stripe width. We study experimentally the dependence of THG on the field enhancement by varying the gap size between the stripe and the metal film while simultaneously maintaining a fixed plasmon resonance. The experiments are supported with numerical simulations in which nonlinear contributions of the dielec...

Surface scattering antennas with lumped elements

Abstract: Surface scattering antennas with lumped elements provide adjustable radiation fields by adjustably coupling scattering elements along a wave-propagating structure. In some approaches, the surface scattering antenna is a multi-layer printed circuit board assembly, and the lumped elements are surface-mount components placed on an upper surface of the printed circuit board assembly. In some approaches, the scattering elements are adjusted by adjusting bias voltages for the lumped elements. In some approaches, the lumped elements include diodes or transistors.

Multi-sensor compressive imaging

Abstract: Multi-sensor compressive imaging systems can include an imaging component (such an an RF, microwave, or mmW metamaterial surface antenna) and an auxialiary sensing component (such as an EO/IR sensor). In some approaches, the auxiliary sensing component includes a structured light sensor configured to identify the location or posture of an imaging target within a field of view of the imaging component. In some approaches, a reconstructed RF, microwave, or mmW image may be combined with a visual image of a region of interest to provide a multi-spectral representation of the region of interest.

Voltage switching of a VO2 memory metasurface using ionic gel

Abstract: We demonstrate an electrolyte-based voltage tunable vanadium dioxide (VO2) memory metasurface. Large spatial scale, low voltage, non-volatile switching of terahertz (THz) metasurface resonances is achieved through voltage application using an ionic gel to drive the insulator-to-metal transition in an underlying VO2 layer. Positive and negative voltage application can selectively tune the metasurface resonance into the “off” or “on” state by pushing the VO2 into a more conductive or insulating regime respectively. Compared to graphene based control devices, the relatively long saturation time of resonance modification in VO2 based devices suggests that this voltage-induced switching originates primarily from electrochemical effects related to oxygen migration across the electrolyte–VO2 interface.

Enhanced optical bistability with film-coupled plasmonic nanocubes

Abstract: Colloidally synthesized nanocubes strongly coupled to conducting films hold great promise for enhancing different nonlinear optical processes. They exhibit a robust and sensitive scattering response that can be easily controlled by their geometrical and material parameters. Strong local field enhancement is generated at the gap regions between the nanocubes and the metallic film. We show that strong optical bistability and all-optical switching behavior can be obtained by loading these nanogaps with Kerr nonlinear materials. Relatively low input intensities are required to obtain these nonlinear effects. The proposed design can lead to efficient, low-power, and ultrafast all-optical memories and scattering nanoswitches.

Film-coupled nanoparticles by atomic layer deposition: Comparison with organic spacing layers

Abstract: Film-coupled nanoparticle systems have proven a reliable platform for exploring the field enhancement associated with sub-nanometer sized gaps between plasmonic nanostructures. In this Letter, we present a side-by-side comparison of the spectral properties of film-coupled plasmon-resonant, gold nanoparticles, with dielectric spacer layers fabricated either using atomic layer deposition or using organic layers (polyelectrolytes or self-assembled monolayers of molecules). In either case, large area, uniform spacer layers with sub-nanometer thicknesses can be accurately deposited, allowing extreme coupling regimes to be probed. The observed spectral shifts of the nanoparticles as a function of spacer layer thickness are similar for the organic and inorganic films and are consistent with numerical calculations taking into account the nonlocal response of the metal.

Microwave imaging using indirect holographic techniques

Abstract: This work describes how indirect holographic techniques, previously applied to the determination of antenna radiation patterns, can be adapted for the imaging of passive objects. It provides details of how complex scattered field values can be obtained in a simple and inexpensive manner from sampled scalar intensity measurements taken over a single scanning aperture. This work provides a brief outline of the basic theory of indirect microwave holography, and how the transformation of the holographic intensity pattern into the Fourier domain enables the isolation of the terms required for complex field reconstruction to be isolated from the remaining terms. The work is supported by a range of experimental results, illustrating the reconstructed complex fields for a number of simple test objects. Back-propagation techniques have also been included to reconstruct complex fields at the position of the scattering objects.

Coupled-mode theory for film-coupled plasmonic nanocubes

Abstract: Planar metallic nanoparticles separated by nanoscale distances from a metal film support unique plasmonic resonances useful for controlling a wide range of photodynamic processes. The fundamental resonance of a film-coupled planar nanoparticle arises from a transmission-line mode localized between nanoparticle and film, whose properties can be roughly approximated by closed form expressions similar to those used in patch antenna theory. The insight provided by the analytical expressions, and the potential of achieving similar closed-form expressions for a range of plasmonic phenomenon such as spasing, fluorescence enhancement, and perfect absorbers, motivates a more detailed study of the film-coupled patch. Here, we present an expanded analytical analysis of the plasmonic patch geometry, applying an eigenmode expansion method to arrive at a more accurate description of the field distribution underneath a film-coupled plasmonic nanocube. The fields corresponding to the inhomogeneous Maxwell's equations are expanded in a set of lossless waveguide eigenmodes. Radiation damping and Ohmic losses are then perturbatively taken into account by considering an equivalent surface impedance. We find that radiative loss couples the lossless eigenmodes, leading to discernible features in the scattering spectra of the nanocubes. The method presented can be further applied to the case of point source excitations, in which accounting for all potential eigenmodes becomes essential.

Light sterile neutrino sensitivity at the nuSTORM facility

Abstract: A facility that can deliver beams of electron and muon neutrinos from the decay of a stored muon beam has the potential to unambiguously resolve the issue of the evidence for light sterile neutrinos that arises in short-baseline neutrino oscillation experiments and from estimates of the effective number of neutrino flavors from fits to cosmological data. In this paper, we show that the nuSTORM facility, with stored muons of 3.8GeV/c±10%, will be able to carry out a conclusive muon neutrino appearance search for sterile neutrinos and test the LSND and MiniBooNE experimental signals with 10σ sensitivity, even assuming conservative estimates for the systematic uncertainties. This experiment would add greatly to our knowledge of the contribution of light sterile neutrinos to the number of effective neutrino flavors from the abundance of primordial helium production and from constraints on neutrino energy density from the cosmic microwave background. The appearance search is complemented by a simultaneous muon neutrino disappearance analysis that will facilitate tests of various sterile neutrino models. © 2014 American Physical Society.

Surface scattering reflector antenna

Abstract: A surface scattering reflector antenna includes a plurality of adjustable scattering elements and is configured to produce a reflected beam pattern according to the configuration of the adjustable scattering elements.

Epigenetic Alterations in the Brain Associated with HIV-1 Infection and Methamphetamine Dependence

Abstract: HIV involvement of the CNS continues to be a significant problem despite successful use of combination antiretroviral therapy (cART). Drugs of abuse can act in concert with HIV proteins to damage glia and neurons, worsening the neurotoxicity caused by HIV alone. Methamphetamine (METH) is a highly addictive psychostimulant drug, abuse of which has reached epidemic proportions and is associated with high-risk sexual behavior, increased HIV transmission, and development of drug resistance. HIV infection and METH dependence can have synergistic pathological effects, with preferential involvement of frontostriatal circuits. At the molecular level, epigenetic alterations have been reported for both HIV-1 infection and drug abuse, but the neuropathological pathways triggered by their combined effects are less known. We investigated epigenetic changes in the brain associated with HIV and METH. We analyzed postmortem frontal cortex tissue from 27 HIV seropositive individuals, 13 of which had a history of METH dependence, in comparison to 14 cases who never used METH. We detected changes in the expression of DNMT1, at mRNA and protein levels, that resulted in the increase of global DNA methylation. Genome-wide profiling of DNA methylation in a subset of cases, showed differential methylation on genes related to neurodegeneration; dopamine metabolism and transport; and oxidative phosphorylation. We provide evidence for the synergy of HIV and METH dependence on the patterns of DNA methylation on the host brain, which results in a distinctive landscape for the comorbid condition. Importantly, we identified new epigenetic targets that might aid in understanding the aggravated neurodegenerative, cognitive, motor and behavioral symptoms observed in persons living with HIV and addictions.

Numerical studies of the modification of photodynamic processes by film-coupled plasmonic nanoparticles

Abstract: The local plasmon resonances of metallic nanostructures are commonly associated with massive local field enhancements, capable of increasing the photoexcitation of nearby quantum emitters by orders of magnitude. However, these same plasmonic structures support high densities of bound and dissipative states, often quenching the nearby emitter or at least offering competitive nonradiative channels. Thus, finding a plasmonic platform that supports massive field enhancements and a high proportion of radiating to nonradiating states remains an active and promising area of research. In this paper, we outline a simple method for numerically studying plasmonic enhancements in fluorescence and apply it to several variants of the film-coupled nanoparticle platform. Film-coupled nanoparticles make excellent candidates for these investigations since the gap dimension between nanoparticle and film—key to the enhancement mechanism—can be precisely controlled in experimental realizations. By correlating the properties of embedded fluorophores with the resonances of the film-coupled nanoparticles, we show quantum yield engineering that is nearly independent of the fluorophore’s intrinsic quantum yield, yielding overall fluorescence enhancements of over four orders of magnitude.

Nanoparticle–Film Plasmon Ruler Interrogated with Transmission Visible Spectroscopy

Abstract: The widespread use of plasmonic nanorulers (PNRs) in sensing platforms has been plagued by technical challenges associated with the development of methods to fabricate precisely controlled nanostructures with high yield and characterize them with high throughput. We have previously shown that creating PNRs in a nanoparticle–film (NP–film) format enables the fabrication of an extremely large population of uniform PNRs with 100% yield using a self-assembly approach, which facilitates high-throughput PNR characterization using ensemble spectroscopic measurements and eliminates the need for expensive microscopy systems required by many other PNR platforms. We expand upon this prior work herein, showing that the NP–film PNR can be made compatible with aqueous sensing studies by adapting it for use in a transmission localized surface plasmon resonance spectroscopy format, where the coupled NP–film resonance responsible for the PNR signal is directly probed using an extinction measurement from a standard spectro...

Effect of a deuterium gas puff on the edge plasma in NSTX

Abstract: This paper describes a detailed examination of the effects of a relatively small pulsed deuterium gas puff on the edge plasma and edge turbulence in NSTX. This gas puff caused little or no change in the line-averaged plasma density or total stored energy, or in the edge density and electron temperature up to the time of the peak of the gas puff. The radial profile of the Dα light emission and the edge turbulence within this gas puff did not vary significantly over its rise and fall, implying that these gas puffs did not significantly perturb the local edge plasma or edge turbulence. These measurements are compared with modeling by DEGAS 2, UEDGE, and with simplified estimates for the expected effects of this gas puff.

Surface scattering antenna array

Abstract: An array of scattering and/or reflector antennas are configured to produce a series of beam patterns, where in some embodiments the scattering antenna and/or the reflector antenna includes complementary metamaterial elements. In some embodiments control circuitry is operably connected to the array to produce an image of an object in the beam pattern.

Sub-nyquist complex-holographic aperture antenna configured to define selectable, arbitrary complex electromagnetic fields

Abstract: Described embodiments include an antenna, method, and an apparatus. The antenna includes a sub-Nyquist complex-holographic aperture configured to define at least two selectable, arbitrary complex radiofrequency electromagnetic fields on a surface with tangential wavenumbers up to 2π over the aperture element spacing (k_apt=2π/a).

Progress in understanding the enhanced pedestal H-mode in NSTX

Abstract: This paper describes the enhanced pedestal (EP) H-mode observed in the National Spherical Torus Experiment (NSTX). The defining characteristics of EP H-mode are given, namely (i) transition after the L- to H-mode transition, (ii) region of very steep ion temperature gradient, and (iii) associated region of strong rotational shear. A newly observed long-pulse EP H-mode example shows quiescent behaviour for as long as the heating and current drive sources are maintained. Cases are shown where the region of steep ion temperature gradient is located at the very edge, and cases where it is shifted up to 10 cm inward from the plasma edge; these cases are united by a common dependence of the ion temperature gradient on the toroidal rotation frequency shear. EP H-mode examples have been observed across a wide range of q95 and pedestal collisionality. No strong changes in the fluctuation amplitudes have been observed following the EP H-mode transition, and transport analysis indicates that the ion thermal transport is comparable to or less than anticipated from a simple neoclassical transport model. Cases are shown where EP H-modes were reliably generated, though these low-q95 examples were difficult to sustain. A case where an externally triggered edge localized mode (ELM) precipitates the transition to EP H-mode is also shown, though an initial experiment designed to trigger EP H-modes in this fashion was unsuccessful.

Methods and apparatus for controlling a surface scattering antenna array

Abstract: An array of scattering and/or reflector antennas are configured to produce a series of beam patterns, where in some embodiments the scattering antenna and/or the reflector antenna includes complementary metamaterial elements. In some embodiments circuitry may be configured to set a series of conditions corresponding to the array to produce the series of beam patterns, and to produce an image of an object that is illuminated by the series of beam patterns.

Two-dimensional metamaterial device design in the discrete dipole approximation

Abstract: We extend the discrete dipole method to enable the analysis and design of two-dimensional magnetoelectric metamaterial devices based on transformation optics. Key to this method is the evaluation of the dipole moments of the metamaterial elements, which can be accomplished within the framework of a rigorous Bloch wave model based on lattice sums. Corrections to the polarizabilities for spatial dispersion and magnetoelectric coupling are included in the formulation of a generalized Clausius-Mossotti relationship. We demonstrate the utility of the extended approach by designing a cloaking structure that shows considerably improved performance over that designed by assuming the standard Clausius-Mossotti relationship between constitutive parameter and polarizability.

A cloaking coating for murky media

Abstract: For centuries, magicians have created the illusion of invisibility with the assistance of smoke and mirrors: Mirrors redirect light and fool our eyes; smoke conceals the trickery. The concept of redirecting light has become routine in the ongoing development of “invisibility cloaks”—constructs of specially designed materials that can route light around an object, making it appear as if both the object and cloak are empty space. On page 427 of this issue, Schittny et al. ( 1 ) show how to use the other half of the magicians' tool kit, showing that objects in a smoky or “diffusive” environment can be perfectly hidden from sight.

One-way glass for microwaves using nonreciprocal metamaterials.

Abstract: We introduce a class of nonreciprocal metamaterials based on composite assemblies of metallic and biased ferrimagnetic elements. We show that such structures act as ultrathin one-way glasses due to the competition between two modes at the surface of the ferrimagnetic elements---a low-loss surface wave that transmits the signal on the other side of the structure and a surface spin-wave resonance that produces strong isolation levels. These findings can be adapted to existing metamaterial geometries, offering a blueprint to achieve unidirectional propagation in a variety of artificial media at radio, microwave, and millimeter wave frequencies.

Comparison of different combining methods for space-diversity FSO systems

Abstract: This paper gives an overview of different signal combining schemes for free space optical (FSO) communication systems employing spatial diversity (SD) for the case of lognormal weak turbulence atmospheric channels. The receiver configurations are explained and categorized into two domains, namely optical and electrical. Moreover, a new combining scheme in the electrical domain, called “logical” domain combining, is introduced. We present closed-form expressions of bit-error-rate for the combining schemes, considering the typical case of a single-input multiple-output (SIMO) system. Through some numerical analyses, we show that depending on the number of receivers and the turbulence strength, the proposed logical combining method outperforms the other ones.

A transparent meshed solar monopole antenna for UWB applications

Abstract: This paper demonstrates the integration of a transparent meshed circular monopole antenna with a polycrystalline silicon (poly-Si) solar cell operating as an RF ground plane in addition to its photovoltaic function. While the meshed circular monopole with the feed line printed on a clear acrylic substrate placed upon the poly-Si solar cell offers an optical transparency of 91%, it is demonstrated that the meshed transparent monopole proposes similar radiation characteristics in comparison with its conventional solid counterpart and operates across a measured frequency band of 2.33-10.8 GHz covering UWB applications with an average broadband gain of 4.1 dBi. The proposed meshed solar monopole antenna generates a DC power output of 65.5 mW and operates with a measured adequate solar efficiency of 13.1%.

A Novel RF Excited Plasma Cathode Electron Beam Gun Design

Abstract: This paper presents a new radio frequency (RF) excited plasma cathode electron beam (EB) gun design and experimental results at a frequency of 84 MHz. The design offers the following benefits over thermionic cathode triode EB guns: much longer cathode lifetime and as a result improved reliability and reduced maintenance costs; no requirement for a grid electrode, avoiding beam aberration, and rapid beam pulsing. The construction of the diode gun was completed and the results of this paper demonstrate that the EB can be switched on in 200 ns and off within 800 ns. Electrons were extracted from a plasma chamber and then accelerated by an electric field applied in a vacuum chamber at a pressure of 10 $^{-5}$ to $10^{-6}$ mbar, producing a collimated EB. The ionized gas used was air at some 0.5 mbar pressure. The EB gun has been operated at ${-}{\rm 60}~{\rm kV}$ accelerating potential and has produced beams of up to 3.2 kW power continuous wave. Modulation of the RF signal was used to control the beam power. Details of the design features of the plasma device are given and evidence of the advantages over conventional EB guns is provided through empirical results.