Showing papers by "David R. Smith published in 2009"

Broadband Ground-Plane Cloak

Abstract: The possibility of cloaking an object from detection by electromagnetic waves has recently become a topic of considerable interest. The design of a cloak uses transformation optics, in which a conformal coordinate transformation is applied to Maxwell's equations to obtain a spatially distributed set of constitutive parameters that define the cloak. Here, we present an experimental realization of a cloak design that conceals a perturbation on a flat conducting plane, under which an object can be hidden. To match the complex spatial distribution of the required constitutive parameters, we constructed a metamaterial consisting of thousands of elements, the geometry of each element determined by an automated design process. The ground-plane cloak can be realized with the use of nonresonant metamaterial elements, resulting in a structure having a broad operational bandwidth (covering the range of 13 to 16 gigahertz in our experiment) and exhibiting extremely low loss. Our experimental results indicate that this type of cloak should scale well toward optical wavelengths.

Design, theory, and measurement of a polarization-insensitive absorber for terahertz imaging

Abstract: We present the theory, design, and realization of a polarization-insensitive metamaterial absorber for terahertz frequencies. Effective-medium theory is used to describe the absorptive properties of the metamaterial in terms of optical constants---a description that has been thus far lacking. From our theoretical approach, we construct a device that yields over 95% absorption in simulation. Our fabricated design consists of a planar single unit-cell layer of metamaterial and reaches an absorptivity of 77% at 1.145 THz.

Metamaterials: Theory, Design, and Applications

Abstract: Metamaterials:Theory, Design, and Applications goes beyond left-handed materials (LHM) or negative index materials (NIM) and focuses on recent research activity. Included here is an introduction to optical transformation theory, revealing invisible cloaks, EM concentrators, beam splitters, and new-type antennas, a presentation of general theory on artificial metamaterials composed of periodic structures, coverage of a new rapid design method for inhomogeneous metamaterials, which makes it easier to design a cloak, and new developments including but not limited to experimental verification of invisible cloaks, FDTD simulations of invisible cloaks, the microwave and RF applications of metamaterials, sub-wavelength imaging using anisotropic metamaterials, dynamical metamaterial systems, photonic metamaterials, and magnetic plasmon effects of metamaterials.

Optical lens compression via transformation optics

Abstract: Transformation optics is widely associated with the design of unconventional electromagnetic devices, such as electromagnetic cloaks or concentrators. However, a wide range of conventional optical devices with potentially advantageous properties can be designed by the transformation optical approach. For example, a coordinate transformation can be introduced that compresses a region of space, resulting in an overall decrease in the thickness of an optical instrument such as a lens. The optical properties of a transformed lens, such as Fresnel reflection or aberration profile, are equivalent to those of the original lens, while the transformed lens and the bounding transformation optical material are thinner than the original lens. This approach to flattening the profile of a lens represents an advantage over the use of a higher dielectric material because it does not introduce greater Fresnel reflections or require a redesign of the basic optic. Though transformation optical media are generally anisotropic, with both electric and magnetic response, it is possible to arrive at a dielectric-only transformation optical distribution for a lens interacting with transverse-magnetic (TM) polarized light. The dielectric-only distribution can be implemented using broad-band, low-loss metamaterials. Lens designs for both a full transformation and a dielectric-only implementation are discussed and confirmed via finite-element simulations.

Metamaterials for surfaces and waveguides

Abstract: Complementary metamaterial elements provide an effective permittivity and/or permeability for surface structures and/or waveguide structures. The complementary metamaterial resonant elements may include Babinet complements of "split ring resonator" (SRR) and "electric LC" (ELC) metamaterial elements. In some approaches, the complementary metamaterial elements are embedded in the bounding surfaces of planar waveguides, e.g. to implement waveguide based gradient index lenses for beam steering/focusing devices, antenna array feed structures, etc..

Broadband gradient index microwave quasi-optical elements based on non-resonant metamaterials

Abstract: Utilizing non-resonant metamaterial elements, we demonstrate that complex gradient index optics can be constructed exhibiting low material losses and large frequency bandwidth. Although the range of structures is limited to those having only electric response, with an electric permittivity always equal to or greater than unity, there are still numerous metamaterial design possibilities enabled by leveraging the non-resonant elements. For example, a gradient, impedance matching layer can be added that drastically reduces the return loss of the optical elements due to reflection. In microwave experiments, we demonstrate the broadband design concepts with a gradient index lens and a beam-steering element, both of which are confirmed to operate over the entire X-band (roughly 8-12 GHz) frequency spectrum.

An efficient broadband metamaterial wave retarder

Abstract: Metamaterials with anisotropic electromagnetic properties have the capability to manipulate the polarization states of electromagnetic waves. We describe a method to design a broadband, low-loss wave retarder with graded constitutive parameter distributions based on non-resonant metamaterial elements. A structured metamaterial half-wave retarder that converts one linear polarization to its cross polarization is designed and its performance is characterized experimentally.

Gradient index circuit by waveguided metamaterials

Abstract: Metamaterials are artificially structured materials that provide considerable flexibility for control of electromagnetic waves. The metamaterial concept can also be applied to the design of planar waveguiding structures. Here, we illustrate this design approach with the development of two-dimensional (2D) planar gradient index (GRIN) circuits. To form the structure, we make use of a 2D complementary split ring resonator, which exhibits an electric response to guided transverse-electric waves. To confirm the properties of the planar GRIN structure predicted from numerical simulations, we present experimental results for a beam-steering and a focusing GRIN circuit. These examples illustrate the versatility of the metamaterial approach in the design of complex waveguiding structures.

Directional coupling between dielectric and long-range plasmon waveguides

Abstract: We have designed, fabricated and characterized integrated directional couplers capable of converting the mode of an optical dielectric waveguide into a long-range plasmon propagating along a thin metal stripe. We demonstrate that the coupling between the two types of waveguides is generally very weak unless specific conditions are met. This sensitivity could be potentially exploited in sensing applications or for developing novel active photonic components.

Overview of results from the National Spherical Torus Experiment (NSTX)

Abstract: The mission of the National Spherical Torus Experiment (NSTX) is the demonstration of the physics basis required to extrapolate to the next steps for the spherical torus (ST), such as a plasma facing component test facility (NHTX) or an ST based component test facility (ST-CTF), and to support ITER. Key issues for the ST are transport, and steady state high β operation. To better understand electron transport, a new high-k scattering diagnostic was used extensively to investigate electron gyro-scale fluctuations with varying electron temperature gradient scale length. Results from n = 3 braking studies are consistent with the flow shear dependence of ion transport. New results from electron Bernstein wave emission measurements from plasmas with lithium wall coating applied indicate transmission efficiencies near 70% in H-mode as a result of reduced collisionality. Improved coupling of high harmonic fast-waves has been achieved by reducing the edge density relative to the critical density for surface wave coupling. In order to achieve high bootstrap current fraction, future ST designs envision running at very high elongation. Plasmas have been maintained on NSTX at very low internal inductance li ~ 0.4 with strong shaping (κ ~ 2.7, δ ~ 0.8) with βN approaching the with-wall β-limit for several energy confinement times. By operating at lower collisionality in this regime, NSTX has achieved record non-inductive current drive fraction fNI ~ 71%. Instabilities driven by super-Alfvenic ions will be an important issue for all burning plasmas, including ITER. Fast ions from NBI on NSTX are super-Alfvenic. Linear toroidal Alfven eigenmode thresholds and appreciable fast ion loss during multi-mode bursts are measured and these results are compared with theory. The impact of n > 1 error fields on stability is an important result for ITER. Resistive wall mode/resonant field amplification feedback combined with n = 3 error field control was used on NSTX to maintain plasma rotation with β above the no-wall limit. Other highlights are results of lithium coating experiments, momentum confinement studies, scrape-off layer width scaling, demonstration of divertor heat load mitigation in strongly shaped plasmas and coupling of coaxial helicity injection plasmas to ohmic heating ramp-up. These results advance the ST towards next step fusion energy devices such as NHTX and ST-CTF.

A $Ku$ -Band High- $Q$ Tunable Filter With Stable Tuning Response

Abstract: A Ku-band tunable filter with a very stable notch level and in-band RF performance is presented in this paper. A new coupling iris configuration for stable RF performance over the tuning range is realized. The filter employs two dual-mode TE113 tunable cavity resonators with a set of flexible bellows. A tuning range of more than 500 MHz at 12 GHz while maintaining a high Q is demonstrated using the introduced resonator. Design techniques, including an RF-mechanical tradeoff analysis, are detailed and addressed.

Electromagnetic source transformations using superellipse equations

Abstract: Transformation optics can be used to design media with unique properties that alter the behavior of electromagnetic waves in passive space and recently in space containing source distributions. We present source transformations where current from a linear radiator is spread over a cylindrical shell with various cross sections. The semianalytic transformations are based on superellipse equations. Finite-element full-wave simulations of transformations from a dipole to a cylinder, diamond-shaped cylinder, and flattened cylinder are presented. The radiation pattern of the dipole seen by an outside observer is replicated in all cases demonstrating the potential applicability of source transformations to conformal antenna design.

Fabrication of metamaterials

Abstract: An example method of fabricating a metamaterial comprises providing a first metamaterial layer, the first metamaterial layer including a first plurality of conducting patterns, such as electrically coupled resonators. A second metamaterial layer is then formed, including a second plurality of conducting patterns, to form a multilayer metamaterial. Positional alignment of the first and second plurality of conducting patterns can be achieved relative to the same fiducial mark, which may be associated with the first metamaterial layer, for example supported by a first substrate or on an alignment layer that is attached to the first substrate.

Internal transport barriers in the National Spherical Torus Experiment

Abstract: In the National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 41, 1435 (2001)], internal transport barriers (ITBs) are observed in reversed (negative) shear discharges where diffusivities for electron and ion thermal channels and momentum are reduced. While neutral beam heating can produce ITBs in both electron and ion channels, high harmonic fast wave heating can also produce electron ITBs (e-ITBs) under reversed magnetic shear conditions without momentum input. Interestingly, the location of the e-ITB does not necessarily match that of the ion ITB (i-ITB). The e-ITB location correlates best with the magnetic shear minima location determined by motional Stark effect constrained equilibria, whereas the i-ITB location better correlates with the location of maximum E×B shearing rate. Measured electron temperature gradients in the e-ITB can exceed critical gradients for the onset of electron thermal gradient microinstabilities calculated by linear gyrokinetic codes. A high-k microwave scattering di...

Study of turbulent fluctuations driven by the electron temperature gradient in the National Spherical Torus Experiment

Abstract: Various theories and numerical simulations support the conjecture that the ubiquitous problem of anomalous electron transport in tokamaks may arise from a short-scale turbulence driven by the electron temperature gradient. To check whether this turbulence is present in plasmas of the National Spherical Torus Experiment (NSTX), measurements of turbulent fluctuations were performed with coherent scattering of electromagnetic waves. Results from plasmas heated by high harmonic fast waves (HHFW) show the existence of density fluctuations in the range of wave numbers k⊥ρe=0.1-0.4, corresponding to a turbulence scale length of the order of the collisionless skin depth. Experimental observations and agreement with numerical results from the linear gyro-kinetic GS2 code indicate that the observed turbulence is driven by the electron temperature gradient. These turbulent fluctuations were not observed at the location of an internal transport barrier driven by a negative magnetic shear.

Observations of Reduced Electron Gyroscale Fluctuations in National Spherical Torus Experiment H-Mode Plasmas with Large E × B Flow Shear

Abstract: Electron gyroscale fluctuation measurements in National Spherical Torus Experiment H-mode plasmas with large toroidal rotation reveal fluctuations consistent with electron temperature gradient (ETG) turbulence. Large toroidal rotation in National Spherical Torus Experiment plasmas with neutral beam injection generates ExB flow shear rates comparable to ETG linear growth rates. Enhanced fluctuations occur when the electron temperature gradient is marginally stable with respect to the ETG linear critical gradient. Fluctuation amplitudes decrease when the ExB flow shear rate exceeds ETG linear growth rates. The observations indicate that ExB flow shear can be an effective suppression mechanism for ETG turbulence.

Impurity transport studies in NSTX neutral beam heated H-mode plasmas

Abstract: The first experimental assessment of low-Z impurity transport in a neutral beam heated, high-confinement H-mode plasma sustained in a low-field, low-aspect ratio spherical tokamak, was performed at the National Spherical Torus Experiment (NSTX). The injected impurities penetrate to the core on a hundred millisecond time scale, indicating a low core particle diffusivity (1 m2 s−1) in good agreement with the values predicted by neoclassical transport theory. In addition, a fixed q-profile magnetic field scan that showed reduced impurity penetration at high fields is also reported. This result suggests that anomalous ion particle transport associated with turbulent long-wavelength electrostatic instabilities must be largely suppressed in the NSTX core.

Hybrid resonant phenomena in a SRR/YIG metamaterial structure

Abstract: We consider the hybridization of the resonance of a SRR metamaterial with the gyromagnetic material resonance of yittrium iron garnet (YIG) inclusions. The combination of an artificial structural resonance and natural material resonance generates a unique hybrid resonance that can be harnessed to make tunable metamaterials and further extend the range of achievable electromagnetic materials. A predictive analytic model is applied that accurately describes the characteristics of this SRR/YIG hybridization. We suggest that this hybridization has been observed in experimental data presented by Kang et al. [Opt. Express, 16, 8825 (2008)] and present numerical simulations to support this assertion. In addition, we investigate a design for optimizing the SRR/YIG structure that shows strong hybridization with a minimum amount of YIG material.

Design and Analysis of Three-Dimensionalized ELC Metamaterial Unit Cell

Abstract: The use of a three-dimensional electric-LC (3D-ELC) resonator to obtain a minimally refractive and strongly transmissive composite metamaterial is explored. The 3D-ELC repeated unit cell consists of two ELC elements that, when put together, may be used to generate responding electric dipole moments in the x?, y?, and z? directions. When embedded inside a host material whose permittivity is significantly greater than unity, the repeated 3D-ELC metamaterial can be used to depress the effective dielectric constant of the resulting composite material to a near-unity value in all Cartesian directions. The concept of designing a unity-index metamaterial-with the properties of free space-may prove useful in creating mechanically durable materials for use as radomes or other enclosures for radiating structures.

Sub-Tenon's space abscess after strabismus surgery.

Abstract: Inflammatory orbital complications of strabismus surgery are a rare occurrence. They include cellulitis, subconjunctival and sub-Tenon's abscesses, myositis, and endophthalmitis. The incidence of periocular infection is assumed to be one case per 1,100 surgeries. In this report, we describe a case of sub-Tenon's abscess after strabismus surgery.

Plasmonic multi-mode interference couplers.

Abstract: Plasmonic multi-mode interference (MMI) couplers have been investigated both numerically and experimentally at the telecommunication wavelength of 1.55 μm. In this study, the couplers are implemented using thin Au stripes that support long-range surface plasmons. We first detail the operation principle of these devices with numerical simulations and show that useful effects can be obtained despite the high material losses inherent to metallic structures. A series of MMI couplers is subsequently fabricated and experimentally characterized, showing a quantitative agreement with our numerical predictions. We conclude by discussing some of the possible applications for these structures.

Metamaterial microwave lens

Abstract: A metamaterial microwave lens having an array of electronic inductive capacitive cells in which each cell has an electrically conductive pattern which corresponds to incident electromagnetic radiation as a resonator. At least one cell has a first and second electrical sections insulated from each other and each which section has at least two legs. A static capacitor is electrically connected between one leg of the first section of the cell and one leg of the second section of the cell. A MEMS device is electrically disposed between the other legs of the first and second sections of the cell. The MEMS device is movable between at least two positions in response to an electrical bias between the first and second sections of the cell to vary the index of refraction and resonant frequency of the cell.

Evaluation of commercial programmable floating gate devices as radiation dosimeters

Abstract: Programmable floating gate MOSFET transistors were tested with gamma radiation with doses up to approximately 100Gy (air equivalent), to evaluate their suitability as dosimeters in radiotherapy. After characterization and programming at different threshold voltages, the devices were irradiated and their Vgs shift with dose monitored in real time. Post-irradiation analysis was carried out to evaluate sensitivity, linearity, reproducibility and voltage threshold annealing. A subsequent re-programming phase followed by characterization was performed to asses their post-irradiation charge restoring capabilities. It was found that up to 73% of the initial maximum threshold voltage could be recovered. A sensitivity of up to 9 mV/Gy with an uncertainty of less than 1%, an excellent linearity up to the maximum programmable threshold voltage and low noise suggest the use of this technology for in vivo dosimetry applications.

The Chandrayaan-1 x-ray spectrometer

Abstract: The Chandrayaan-1 X-ray Spectrometer (C1XS) is a compact X-ray spectrometer for the Chandrayaan-1 lunar mission. It exploits heritage from the D-C1XS instrument on ESA’s SMART-1 mission. C1XS is designed to measure absolute and relative abundances of major rock-forming elements (principally Mg, Al, Si, Ti, Ca and Fe) over the lunar surface. The baseline design consists of 24 nadir pointing Swept Charge Device detectors, which provide high detection efficiency in the 1–7 keV range, which contains the X-ray fluorescence lines of the above elements of interest. Micromachined collimators provide a 14 degree FWHM FOV, equivalent to 25 km from 100 km altitude. A deployable door protects the instrument during launch and cruise, and also provides a 55Fe calibration X-ray source for detector calibration. Additional refinements compared to D-C1XS will result in a significantly improved energy resolution. To record the incident solar X-ray flux at the Moon, C1XS carries an X-ray Solar Monitor (XSM). C1XS will arrive at the Moon in the run up to the maximum of the solar cycle 24, and the expected high incident X-ray flux coupled to a 100 km circular polar orbit, will provide composition data accurate to better than 10% of major elemental abundances over the lunar surface.

The use of automatic scale selection to improve the spatial and spectral resolution of a scintillator-coupled EMCCD

Abstract: The technology behind the Electron-Multiplying Charge Coupled Device (EMCCD) was successfully exploited by e2v technologies in the late 1990s. Since then, many uses have been found for these low light level (L3) devices including surveillance and many scientific applications. The EMCCD increases or 'multiplies' the charge signal by the phenomenon of impact ionisation (or avalanche multiplication) allowing the detection of low signal events of only a few photons. When coupled with a scintillator, this low light capability can be used to image photon flashes from individual X-ray interaction events. The combination of depth of interaction effects in the scintillator, shot noise on the signal and the multiplication noise factor lead to large variations in the profile of the detected signal from a constant energy X-ray source. This variation leads to reduced spectral performance and can have adverse effects on the centering techniques used in photon-counting imagers. The concept of scale-space is similar in many ways to the Fourier or wavelet transforms. Automatic scale selection can be implemented through the scale-space transform as a method of fitting a known profile to the observed photon flash. The process is examined here in the context of the photon-counting EMCCD detector and the results obtained in both simulated and experimental data compared. Through the analysis of the fitting process and the results achieved, the implications on imaging performance and spectral resolution are discussed.

Evanescent electromagnetic wave conversion lenses III

Abstract: Apparatus, methods, and systems provide conversion of evanescent electromagnetic waves to non-evanescent electromagnetic waves and/or conversion of non-evanescent electromagnetic waves to evanescent electromagnetic waves. In some approaches the conversion includes propagation of electromagnetic waves within an indefinite electromagnetic medium, and the indefinite medium may include an artificially-structured material such as a layered structure or other metamaterial.

Life testing of EMCCD gain characteristics

Abstract: Two electron-multiplication, charge-coupled devices (e2v CCD97) were operated continuously for 52 days to assess their avalanche gain stability. This work was part of an evaluation study into their suitability for the radial velocity spectrometer (RVS) instrument on the European Space Agency's Gaia cornerstone mission. After one day the device gain was stable to within 10%, and at the end of the test both devices reached the required multiplication gain of 8 with an avalanche electrode voltage of less than 36 V.

Millimeter-wave artificial dielectric gradient index lenses

Abstract: We present an artificial dielectric gradient index lens designed to operate at millimeter-wave frequencies. Finite element simulations are used in conjunction with material parameter retrieval techniques to design an artificial dielectric unit cell with an optimum refractive index range. This refractive index range is then used with ray-tracing software to arrive at an optimized index profile. A lens with this index profile is realized using multilayer printed wiring boards (PWBs) and tested.