Showing papers by "David R. Smith published in 2004"

Metamaterials and negative refractive index.

Abstract: Recently, artificially constructed metamaterials have become of considerable interest, because these materials can exhibit electromagnetic characteristics unlike those of any conventional materials. Artificial magnetism and negative refractive index are two specific types of behavior that have been demonstrated over the past few years, illustrating the new physics and new applications possible when we expand our view as to what constitutes a material. In this review, we describe recent advances in metamaterials research and discuss the potential that these materials may hold for realizing new and seemingly exotic electromagnetic phenomena.

Terahertz magnetic response from artificial materials.

Abstract: We show that magnetic response at terahertz frequencies can be achieved in a planar structure composed of nonmagnetic conductive resonant elements. The effect is realized over a large bandwidth and can be tuned throughout the terahertz frequency regime by scaling the dimensions of the structure. We suggest that artificial magnetic structures, or hybrid structures that combine natural and artificial magnetic materials, can play a key role in terahertz devices.

Reversing Light With Negative Refraction

Abstract: Materials engineered to have negative permittivity and permeability demonstrate exotic behavior, from a negative refractive index to subwavelength focusing.

Negative refraction in indefinite media

Abstract: Initial experiments on wedge samples composed of isotropic metamaterials with simultaneously negative permittivity and permeability have indicated that electromagnetic radiation can be negatively refracted. In more recently reported experiments [Phys. Rev. Lett. 90, 1074011 (2003)], indefinite metamaterial samples, for which the permittivity and permeability tensors are negative along only certain of the principal axes of the metamaterial, have also been used to demonstrate negative refraction. We present here a detailed analysis of the refraction and reflection behavior of electromagnetic waves at an interface between an indefinite medium and vacuum. We conclude that certain classes of indefinite media have identical refractive properties as isotropic negative index materials. However, there are limits to this correspondence, and other complicating phenomena may occur when indefinite media are substituted for isotropic negative index materials. We illustrate the results of our analysis with finite-element-based numerical simulations on planar slabs and wedges of negative index and indefinite media.

Partial focusing of radiation by a slab of indefinite media

Abstract: Negative refraction can occur at the interface between vacuum and an indefinite medium—an anisotropic medium for which not all elements of the permittivity and permeability tensors have the same sign. We show experimentally and via simulations that a metamaterial composed of split ring resonators, designed to provide a permeability equal to −1 along the longitudinal axis, will redirect s-polarized electromagnetic waves from a nearby source to a partial focus. The dispersion characteristics of indefinite media prohibit the possibility of true aplanatic points for a planar slab; however, by contouring the surfaces aplanatic points may be realized, as well as other geometrical optical behavior.

Surface-Enhanced Raman Scattering from Silver-Plated Porous Silicon

Abstract: Silver micro- and nanocrystallites are prepared on porous Si substrates by immersion plating, and their activity toward SERS (surface-enhanced Raman scattering) is assessed. Scanning electron microscopy reveals a rough silver film containing randomly spaced dendritic structures. SERS spectra of Rhodamine 6G (R6G) and adenine are obtained using an 18-mW, 488-nm laser. The SERS signal from these analytes is dramatically improved by pretreatment of the silver-plated porous Si samples (Ag−PS) with a 1 mM mineral acid solution. Detection of R6G and adenine from 1 nM solutions is demonstrated, corresponding to (at most) 9 × 105 molecules in the experimental configuration used in the current study. Ag−PS samples that have been stored in air for 10 days still display high sensitivity. The presence of chloride either in the analyte solution or in the pretreatment solution is found to dramatically reduce the limit of detection for R6G.

EM-based design of large-scale dielectric-resonator filters and multiplexers by space mapping

Abstract: A novel design methodology for filter and multiplexer design is presented. For the first time, finite-element electromagnetic (EM)-based simulators and space-mapping optimization are combined to produce an accurate design for manifold-coupled output multiplexers with dielectric resonator (DR) loaded raters. Finite-element EM-based simulators are used as a fine model of each multiplexer channel, and a coupling matrix representation is used as a coarse model. Fine details such as tuning screws are included in the fine model. The DR filter and multiplexer design parameters are kept bounded during optimization. The sparsity of the mapping between the design parameters and the coupling elements has been exploited. Our approach has been used to design large-scale output multiplexers and it has significantly reduced the overall tuning time compared to traditional techniques. The technique is illustrated through design of a five-pole DR filter and a ten-channel output multiplexer.

Fabrication and characterization of a negative-refractive-index composite metamaterial

Abstract: We have designed, produced, and experimentally characterized $2.7\phantom{\rule{0.3em}{0ex}}\mathrm{mm}$ thick composite panels having negative refractive index between 8.4 and $9.2\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$. The composite metamaterial is fabricated using conventional commercial multilayer circuit-board lithography; three-dimensional physical (as opposed to electromagnetic) structure is introduced by the use of vias to form sections of the scattering elements in the direction perpendicular to the circuit board surfaces. From scattering parameter measurements, we show that the complex permittivity, permeability, index, and impedance of the composite can be unambiguously determined. The measurements enable the quantitative determination of the negative index band and associated losses. The extracted material parameters are shown to be in excellent agreement with simulation results.

Negative index lens aberrations.

Abstract: We examine the Seidel aberrations of thin spherical lenses composed of media with refractive index not restricted to be positive. We find that consideration of this expanded parameter space allows for the reduction or elimination of more aberrations than is possible with only positive index media. In particular, we find that spherical lenses possessing real aplanatic focal points are possible only with a negative index. We perform ray tracing, using a custom code that relies only on Maxwell's equations and conservation of energy, that confirms the results of the aberration calculations.

Reversing Light: Negative Refraction

Abstract: ictor Veselago, in a paper 1 published in 1967, pondered the consequences for electromagnetic waves interacting with a hypothetical material for which both the electric permittivity, e , and the magnetic permeability, µ, were simultaneously negative. As no naturally occurring material or compound has ever been demonstrated with negative e and µ, Veselago wondered whether this apparent asymmetry in material properties was just happenstance, or perhaps had a more fundamental origin. Veselago concluded that not only should such materials be possible but, if ever found, would exhibit remarkable properties unlike those of any known materials, giving a twist to virtually all electromagnetic phenomena. So why are there no materials with negative e and µ? We first need to understand what it means to have a negative e or µ, and how it occurs in materials. The Drude-Lorentz model of a material is a good starting point, as it conceptually replaces the atoms and molecules of a real material by a set of harmonically bound electron oscillators, resonant at some frequency 0 ω . At

Enhanced diffraction from a grating on the surface of a negative-index metamaterial

Abstract: We show by numerical simulation as well as by measurements on negative-index metamaterial wedge samples, that the unavoidable stepping of the refraction interface-due to the finite unit-cell size inherent to metamaterials-can give rise to a well-defined diffracted beam in addition to the negatively refracted beam. The direction of the diffracted beam is consistent with elementary diffraction theory; however, the coupling to this higher order beam is much larger than would be the case for a positive index material. The results confirm recent theoretical predictions of enhanced diffraction for negative-index grating surfaces.

Reply to Comments on "Resonant and antiresonant frequency dependence of the effective parameters of metamaterials"

Abstract: We argue, in agreement with our previous work, that periodicity of the metamaterial is responsible for antiresonant behavior of the effective permittivity as well as for the negative sign of the imaginary part of an effective permittivity or permeability. This agrees with the Comment of Efros.

Random telegraph signals in charge coupled devices

Abstract: An investigation of fluctuating pixels resulting from proton irradiation of an E2V Technologies CCD47-20 device is presented. The device structure, experimental set up and irradiation methodology are described, followed by a detailed analysis of radiation induced random telegraph signals, RTS. The characteristics of the observed flickering pixels are discussed in detail and the proposed models explaining the mechanism behind the phenomena are viewed in light of the collected data.

Microwave scattering system design for ρe scale turbulence measurements on NSTX

Abstract: Despite suppression of ρi scale turbulent fluctuations, electron thermal transport remains anomalous in NSTX. For this reason, a microwave scattering system will be deployed to directly observe the ω and k spectra of ρe scale turbulent fluctuations and characterize the effect on electron thermal transport. The scattering system will employ a Gaussian probe beam produced by a high power 280 GHz microwave source. A five-channel heterodyne detection system will measure radial turbulent spectra in the range |kr|=0–20 cm−1. Inboard and outboard launch configurations cover most of the normalized minor radius. Improved spatial localization of measurements is achieved with low aspect ratio and high magnetic shear configurations. This article will address the global design of the scattering system, such as choice of frequency, size, launching system, and detection system.

Comment on "Electrodynamics of metallic photonic crystals and the problem of left-handed materials".

Abstract: A Comment on the Letter by A. L. Pokrovsky and A. L. Efros, Phys. Rev. Lett. 89, 093901 (2002).

Existence and properties of microwave surface plasmons at the interface between a right-handed and a left-handed media

Abstract: Surface plasmons, (SPs) are interface waves very similar to Zenneck waves but existing only at optical frequencies where metals exhibit a negative permittivity. The availability of novel structured metamaterials with tailorable positive/negative constitutive parameters allows the generation of SPs in the microwave range, which may lead to novel components and antennas applications. This paper describes SPs existing at the interface between right-handed (RH) and left-handed (LH) materials. The dispersion relation is established, the properties of these SPs are discussed, a transmission line (TL) LC implementation of the RH/LH interface is proposed, and the phenomena are demonstrated by full-wave simulation in an effective medium approach.

Half-wave dielectric rod resonator filter

Abstract: A new half-wave dielectric rod resonator and its filter realization are proposed for satellite input multiplexer applications. Single resonator and a 10-pole filter with 8 real and complex transmission zeros are modeled and designed to desired specification by using efficient optimization technique and a finite element method solver. Excellent filter responses are obtained.

Low noise charge injection in the CCD22

Abstract: The inclusion of a charge injection structure on a charge coupled device (CCD) allows for the mitigation of charge transfer loss which can be caused by radiation induced charge trapping defects. Any traps present in the pixels of the CCD are filled by the injected charge as it is swept through the device and consequently, the charge transfer efficiency is improved in subsequently acquired images. To date, a number of different types of CCD have been manufactured featuring a variety of charge injection techniques. The e2v Technologies CCD22, used in the EPIC MOS focal plane instruments of XMM-Newton, is one such device and is the subject of this paper. A detailed understanding of charge injection operation and the use of charge injection to mitigate charge transfer losses resulting from radiation damage to CCDs will benefit a number of space projects planned for the future, including the ESA GAIA and X-ray Evolving Universe Spectrometry (XEUS) missions. The charge injection structure and mode of operation of the CCD22 are presented, followed by a detailed analysis of the uniformity and repeatability of the charge injection amplitude across the columns of the device. The effects of proton irradiation on the charge injection characteristics are also presented, in particular the effect of radiation induced bright pixels on the injected charge level.

Wind-induced structural response of a large telescope

Abstract: In May of 2000, the construction progress of the Gemini South 8m telescope at Cerro Pachon in Chile was such that the telescope and dome were installed and able to move, but the primary mirror had not been installed. This provided a unique opportunity to make extensive tests of the structure in its nearly-completed state, including a modal impact test and simultaneous measurements of wind pressure and structural response. The testing was even more comprehensive because the Gemini dome design allows for a wide range of wind flow configurations, from nearly enclosed to almost fully exposed. In these tests, the operating response of 24 surface pressures on the primary mirror cell, 5 wind velocity channels (each with direction vector information), and more than 70 channels of accelerometers on the telescope structure were measured. The data were taken in a variety of wind loading configurations. While previous analysis efforts have focused on the wind velocity and pressure measurement, this paper investigates the dynamic behavior of the telescope structure itself. Specifically, the discussion includes the participation of the modes measured in the modal impact test as a function of wind loading configuration. Data that indicate the most important frequency ranges in the operating response of the telescope are also presented. Finally, the importance of the response of the enclosure on the structural vibration of the telescope structure is discussed.

Beating the diffraction limit

Abstract: In 1968 the Russian physicist Victor Vese-lago predicted the existence of a material with a negative index of refraction, which he termed "left-handed."He concluded that in the presence of such a negative-index material nearly all known wave propagation and optical phenomena would be substantially altered, although negative-index materials were not known to exist at the time.

Design of metamaterials with negative refractive index

Abstract: Due to the recent experimental validations of left-handed metamaterials, negative refractive index has now become recognized as a new parameter space for the electromagnetic response of materials. Because materials with negative index behave quite differently than materials with positive index, many familiar electromagnetic phenomena must be reconsidered. Having established now the scientific basis of negative index, the effort of the community is turning toward the practical realization of both the predicted scientific phenomena and associated applications. In both of these pursuits, the ability to design, characterize and fabricate negative index materials is critical; we can consider the current status of negative refraction in some sense a materials issue, as our ability to demonstrate the predicted phenomena is linked to the quality of metamaterials we can produce. In this paper we consider several issues associated with the design and simulation of negative index metamaterials. Keywords: Negative index, left-handed materials, metamaterials

Angle resolved microwave spectrometer for metamaterial studies

Abstract: We describe an angle resolved microwave spectrometer (ARMS) based on a planar waveguide scattering chamber, capable of acquiring the angular distribution of TE polarized microwaves scattered from samples centered within the chamber The spacing between the upper and lower conducting circular plates is 04 in (∼1 cm), which, with the associated X-band waveguide adapters, fixes the frequency of operation of the ARMS to be optimally within the X-band frequency range (8–12 GHz) Microwave energy can be injected either as an apertured beam via an extended arm connected to the chamber, or via an antenna located in the center of the chamber Power is detected at a waveguide adapter located on the periphery of the chamber, attached to a rotating arm that has an angular range of 180° A computer controlled stepper motor attached to the rotating arm facilitates angular scanning with the data acquired at every angle in an automated fashion The ARMS has excellent reproducibility and signal-to-noise characteristics, making it ideal for characterizing the refraction properties of metamaterial samples, or as a probe of the interaction between antennas and metamaterial substrates

Imaging concealed objects from scalar microwave holograms

Abstract: A new technique for the imaging of concealed objects using indirect holographic imaging principles is outlined. A brief examination of the underlying theory followed by a description of the novel experimental system that has been developed to overcome some of the previous issues regarding this technique, is presented along with some initial results obtained from the imaging of passive objects concealed by material and plastic.

Microwave Scattering System Design for {rho}{sub i}e-Scale Turbulence Measurements on NSTX

Abstract: Despite suppression of {rho}{sub i}-scale turbulent fluctuations, electron thermal transport remains anomalous in NSTX. For this reason, a microwave scattering system will be deployed to directly observe the w and k spectra of {rho}{sub e}-scale turbulent fluctuations and characterize the effect on electron thermal transport. The scattering system will employ a Gaussian probe beam produced by a high power 280 GHz microwave source. A five-channel heterodyne detection system will measure radial turbulent spectra in the range |k{sub r}| = 0-20 cm{sup -1}. Inboard and outboard launch configurations cover most of the normalized minor radius. Improved spatial localization of measurements is achieved with low aspect ratio and high magnetic shear configurations. This paper will address the global design of the scattering system, such as choice of frequency, size, launching system, and detection system.

The role of Cr substitution on the ferromagnetic properties of Ga1-xCrxN

Abstract: Angular-dependent channeling Rutherford Backscattering Spectroscopy (c-RBS) has been used to quantify the fraction of Cr atoms on substitutional, interstitial, and random sites in epitaxial Ga1-xCrxN films grown by reactive molecular-beam epitaxy. The morphology of these films and correlation with their magnetic properties has been investigated. Films grown at temperatures below ~ 750oC have up to 90% of Cr occupying substitutional sites. Post-growth annealing at 825oC results in a systematic drop in the fraction of substitutional Cr as well as a fall off in the ferromagnetic signal. The roles of non-substitutional Cr in transferring charge from the Cr t2 band and segregation of substitutional Cr in the loss of magnetism are discussed. Overall, these results provide strong microscopic evidence that Cr-doped III-N systems are dilute magnetic semiconductors.

Improving the alignment of a large telescope structure using thermographic measurements

Abstract: While many telescopes employ some form of thermal monitoring and control to reach their surface accuracy and pointing specifications, such thermal systems are generally not available during field erection of the telescope structure. This presents a problem for large structures, because their size results in structural members with long time constants and substantial total deformations. These characteristics can potentially result in the development of significant thermal gradients across the structure or even across the width of a member. The resulting temperature variations complicate the alignment of critical features, including the main axes. As a result, it would be advantageous to monitor the thermal behavior of a large telescope structure during its construction. In the past, such monitoring was made difficult because of the size of the structure, continuing construction work, or rugged field conditions. However, with the advent of affordable field-ready thermal imaging systems, it is now possible to perform such monitoring. In this paper, we present thermal images of the alidade structure of the Large Millimeter-wave Telescope (LMT/GTM) at its site at 4600m on the top of Volcan Sierra Negra in central Mexico. We present images of typical thermal distributions for different times of day, and compare them with basic analytical models. Finally, we use the thermal imaging results to predict the effects of the temperature distribution on the location of the azimuth bogie connections and the elevation axis.

Using an active primary surface to correct for low-order manufacturing errors in secondary mirrors of large reflector antennas

Abstract: In the fabrication of high-performance, low-cost secondary reflectors for radio telescopes, it is a significant challenge to avoid introduction of low-order surface errors such as astigmatism or coma. This arises primarily because low-order surface errors are easily induced by support structure placement or simple thermal variations in the manufacturing process. It is, of course, possible to bring these errors to within the required tolerance, but if an active primary reflector is present, it may be possible to relax the requirements on the secondary and perhaps lower its cost. In this paper, we take the Large Millimeter-wave Telescope (LMT/GTM) as an example system. We model the effects of correcting a deformed sub-reflector by using the existing segmented active primary. The sub-reflector deformation patterns employed are low-order (e.g., astigmatism or coma), but are allowed significant excursions from the nominal surface figure. For each case, we demonstrate the best theoretical performance, using the active primary to correct for the errors. Additionally, to determine whether such an approach would be practical, we also demonstrate the likely performance improvement that could be achieved using brief measurements on an astronomical source. In this approach, we introduce varying amounts of known low-order deformation patterns into the active primary and seek the combination that results in the maximum signal. Finally, we compare this result to the theoretical maximum and make recommendations on the practical utility of the approach.