Showing papers by "David R. Smith published in 2011"
TL;DR: In this article, a power relay system based on a near-field metamaterial superlens is proposed and a thorough theoretical analysis of this system is presented. But the authors do not consider the nonradiative coupling between conductive coils.
Abstract: Nonradiative coupling between conductive coils is a candidate mechanism for wireless energy transfer applications. In this paper we propose a power relay system based on a near-field metamaterial superlens and present a thorough theoretical analysis of this system. We use time-harmonic circuit formalism to describe all interactions between two coils attached to external circuits and a slab of anisotropic medium with homogeneous permittivity and permeability. The fields of the coils are found in the point-dipole approximation using Sommerfeld integrals which are reduced to standard special functions in the long-wavelength limit. We show that, even with a realistic magnetic loss tangent of order 0.1, the power transfer efficiency with the slab can be an order of magnitude greater than free-space efficiency when the load resistance exceeds a certain threshold value. We also find that the volume occupied by the metamaterial between the coils can be greatly compressed by employing magnetic permeability with a large anisotropy ratio.
192 citations
TL;DR: This Letter experimentally demonstrates the nonlinear-optical mirror effect in a bulk negative-index nonlinear metamaterial, along with two other novel phase-matching configurations, utilizing periodic poling to switch between the three phase- matching domains.
Abstract: Nonlinear metamaterials have been predicted to support new and exciting domains in the manipulation of light, including novel phase-matching schemes for wave mixing. Most notable is the so-called nonlinear-optical mirror, in which a nonlinear negative-index medium emits the generated frequency towards the source of the pump. In this Letter, we experimentally demonstrate the nonlinear-optical mirror effect in a bulk negative-index nonlinear metamaterial, along with two other novel phase-matching configurations, utilizing periodic poling to switch between the three phase-matching domains.
120 citations
01 Oct 2011
TL;DR: The basic concepts associated with transformation optics are reviewed and several examples to illustrate its application are provided, including the use of artificially structured metamaterials.
Abstract: Transformation optics is an emerging technique for the design of advanced electromagnetic media. Transformation optical devices exploit the form invariance of Maxwell's equations, allowing geometry to play the dominant role in the design process rather than traditional wave or ray optics. The use of coordinate transformations vastly eases the burden of design for a large class of devices, though at the expense of increasing the complexity of the underlying materials used. Although the required constitutive parameters of a transformation optical structure can be challenging-inherently anisotropic and spatially varying, with both magnetic and electric response-nevertheless the parameter requirements can often be met or approximated through the use of artificially structured metamaterials. Here, we review the basic concepts associated with transformation optics and provide several examples to illustrate its application.
118 citations
TL;DR: It is shown that an impermeable object placed either in a free-flowing fluid or in a fluid-filled porous medium can be coated with an inhomogeneous, anisotropic permeable medium to preserve the flow that would have existed in the absence of the object.
Abstract: We introduce a new concept for the manipulation of fluid flow around three-dimensional bodies. Inspired by transformation optics, the concept is based on a mathematical idea of coordinate transformations and physically implemented with anisotropic porous media permeable to the flow of fluids. In two situations—for an impermeable object placed either in a free-flowing fluid or in a fluid-filled porous medium—we show that the object can be coated with an inhomogeneous, anisotropic permeable medium, such as to preserve the flow that would have existed in the absence of the object. The proposed fluid flow cloak eliminates downstream wake and compensates viscous drag, hinting at the possibility of novel propulsion techniques.
91 citations
TL;DR: In this article, the authors demonstrate tuning of a metamaterial device that incorporates a form of spatial gradient control, achieved through a vanadium dioxide layer which interacts with an array of split ring resonators.
Abstract: We demonstrate tuning of a metamaterial device that incorporates a form of spatial gradient control. Electrical tuning of the metamaterial is achieved through a vanadium dioxide layer which interacts with an array of split ring resonators. We achieved a spatial gradient in the magnitude of permittivity, writeable using a single transient electrical pulse. This induced gradient in our device is observed on spatial scales on the order of one wavelength at 1 THz. Thus, we show the viability of elements for use in future devices with potential applications in beamforming and communications.
88 citations
TL;DR: The design, fabrication and characterization of an artificially structured, gradient index metamaterial with a linear index variation of Δn ~ 3.0 is demonstrated, which has the potential to enable compact infrared diffractive and gradient index optics, as well as more exotic transformation optical media.
Abstract: We demonstrate the design, fabrication and characterization of an artificially structured, gradient index metamaterial with a linear index variation of Δn ∼ 3.0. The linear gradient profile is repeated periodically to form the equivalent of a blazed grating, with the gradient occurring across a spatial distance of 61μm. The grating, which operates at a wavelength of 10.6μm, is composed of non-resonant, progressively modified “I-beam” metamaterial elements and approximates a linear phase shift gradient using 61 distinguishable phase levels. The grating structure consists of four layers of lithographically patterned metallic I-beam elements separated by dielectric layers of SiO2. The index gradient is confirmed by comparing the measured magnitudes of the −1, 0 and +1 diffracted orders to those obtained from full wave simulations incorporating all material properties of the metals and dielectrics of the structures. The large index gradient has the potential to enable compact infrared diffractive and gradient index optics, as well as more exotic transformation optical media.
60 citations
TL;DR: In this article, the authors present an overview of phase matching solutions for wave-mixing processes in nonlinear metamaterials and divide them into conventional techniques (anomalous dispersion, birefringence, and quasi-phase matching) and metamatter-inspired techniques (negative-index and index-near-zero phase matching).
Abstract: Nonlinear metamaterials have potentially interesting applications in highly efficient wave-mixing and parametric processes, owing to their ability to combine enhanced nonlinearities with exotic and configurable linear properties. However, the strong dispersion and unconventional configurations typically associated with metamaterials place strong demands on phase matching in such structures. In this paper, we present an overview of potential phase matching solutions for wave-mixing processes in nonlinear metamaterials. Broadly speaking, we divide the phase matching solutions into conventional techniques (anomalous dispersion, birefringence, and quasi-phase matching) and metamaterial-inspired techniques (negative-index and index-near-zero phase matching), offering numerical and experimental examples where possible. We find that not only is phase matching feasible in metamaterials, but metamaterials can support a wide range of phase matching configurations that are otherwise impossible in natural materials. These configurations have their most compelling applications in those devices where at least one of the interacting waves is counter-propagating, such as the mirror-less optical parametric oscillator and the nonlinear optical mirror.
59 citations
TL;DR: It is observed that longer wavelength modes, k(⊥)ρ(s) ≲ 10, are most stabilized by density gradient, and the stabilization is accompanied by about a factor of 2 decrease in the plasma effective thermal diffusivity.
Abstract: In this letter we report the first clear experimental observation of density gradient stabilization of electron temperature gradient driven turbulence in a fusion plasma. It is observed that longer wavelength modes, k⊥ρs ≤10, are most stabilized by density gradient, and the stabilization is accompanied by about a factor of two decrease in the plasma effective thermal diffusivity.
50 citations
TL;DR: In this article, a new concept for modulating guided and lossless transmission of electromagnetic energy in the gigahertz and tera-hertz frequency range is proposed, where metamaterials and surface electromagnetic waves join force in an experimentally demonstrated new concept.
Abstract: Metamaterials and surface electromagnetic waves join force in an experimentally demonstrated new concept for modulating guided and lossless transmission of electromagnetic energy in the gigahertz and terahertz frequency range important to modern telecommunication.
49 citations
TL;DR: This work discusses the merits of various TO strategies proposed for the long-sought ‘invisibility cloak’—a structure that renders opaque objects invisible and evaluates the cloaking capabilities of structures designed by the related CM approach, which makes use of conformal mapping to achieve index-only material distributions.
Abstract: We review several approaches to optical invisibility designed using transformation optics (TO) and optical conformal mapping (CM) techniques TO is a general framework for solving inverse scattering problems based on mimicking spatial coordinate transformations with distributions of material properties There are two essential steps in the design of TO media: first, a coordinate transformation that achieves some desired functionality, resulting in a continuous spatial distribution of constitutive parameters that are generally anisotropic; and, second, the reduction of the derived continuous constitutive parameters to a metamaterial that serves as a stepwise approximation We focus here on the first step, discussing the merits of various TO strategies proposed for the long-sought ‘invisibility cloak’—a structure that renders opaque objects invisible We also evaluate the cloaking capabilities of structures designed by the related CM approach, which makes use of conformal mapping to achieve index-only material distributions The performance of the various cloaks is evaluated and compared using a universal measure—the total (all-angle) scattering cross section
48 citations
TL;DR: In this article, the authors present experimental measurements of three and four-wave mixing phenomena in an artificially structured nonlinear magnetic metacrystal at microwave frequencies, and the sum frequency generation signal for the varactor-loaded split-ring resonator (VLSRR) metamaterial agrees quantitatively with that predicted using an analytical effective medium model describing the VLSRR medium.
Abstract: We present experimental measurements of three- and four-wave mixing phenomena in an artificially structured nonlinear magnetic metacrystal at microwave frequencies. The sum frequency generation signal for the varactor-loaded split-ring resonator (VLSRR) metamaterial agrees quantitatively with that predicted using an analytical effective medium model describing the VLSRR medium. A resonant enhancement of the nonlinear response is observed near the metamaterial resonance.
TL;DR: The recently developed technique of transformation optics is applied to a lesser-known refractive Luneburg lens and the design with a metamaterial composed of a semi-crystalline distribution of holes drilled in a dielectric is implemented.
Abstract: The Luneburg lens is a powerful imaging device, exhibiting aberration free focusing for parallel rays incident from any direction. However, its advantages are offset by a focal surface that is spherical and thus difficult to integrate with standard planar detector and emitter arrays. Using the recently developed technique of transformation optics, it is possible to transform the curved focal surface to a flat plane while maintaining the perfect focusing behavior of the Luneburg over a wide field of view. Here we apply these techniques to a lesser-known refractive Luneburg lens and implement the design with a metamaterial composed of a semi-crystalline distribution of holes drilled in a dielectric. In addition, we investigate the aberrations introduced by various approximations made in the implementation of the lens. The resulting design approach has improved mechanical strength with small aberrations and is ideally suited to implementation at infrared and visible wavelengths.
TL;DR: Density fluctuation measurements from high-k microwave scattering are verified to be the electron temperature gradient (ETG) mode by matching measured rest frequency and linear growth rate to gyrokinetic calculations.
Abstract: Negative magnetic shear is found to suppress electron turbulence and improve electron thermal transport for plasmas in the National Spherical Torus Experiment (NSTX). Sufficiently negative magnetic shear results in a transition out of a stiff profile regime. Density fluctuation measurements from high-$k$ microwave scattering are verified to be the electron temperature gradient (ETG) mode by matching measured rest frequency and linear growth rate to gyrokinetic calculations. Fluctuation suppression under negligible $\mathbf{E}\ifmmode\times\else\texttimes\fi{}\mathbf{B}$ shear conditions confirm that negative magnetic shear alone is sufficient for ETG suppression. Measured electron temperature gradients can significantly exceed ETG critical gradients with ETG mode activity reduced to intermittent bursts, while electron thermal diffusivity improves to below 0.1 electron gyro-Bohms.
TL;DR: In this paper, the authors analyzed the strain-dependent effective medium properties for a metamaterial electric-LC (ELC) resonator, commonly used in metammaterial designs to provide a tailored electric response to electromagnetic waves.
Abstract: In this paper, we analytically describe the strain-dependent effective medium properties for a metamaterial electric-LC (ELC) resonator, commonly used in metamaterial designs to provide a tailored electric response to electromagnetic waves. Combining an equivalent circuit model of the ELC resonator with existing analytic expressions for the capacitive and inductive regions comprising the structure, we obtain strain-dependent permittivity and permeability curves for the metamaterial. The derived expressions account for the effects of spatial dispersion and losses.
TL;DR: In this article, a morphology-correlated surface-enhanced Raman scattering (SERS) from molecules on the surface of individual clusters of gold nanoparticles of two types was reported.
Abstract: We report a morphology-correlated surface-enhanced Raman scattering (SERS) from molecules on the surface of individual clusters of gold nanoparticles of two types and compare the signal from clusters of two, three, four, and five nanoparticles with the signal from single particles. Cluster geometry and particle morphology are determined from transmission electron microscopy for both clusters of 78- to 133-nm nanospheres and clusters of ~250-nm-etched cylindrical particles with crevices and sharp edges, formed in templates. Scattering from molecules on etched cylinders, but not spheres, is sufficiently strong to allow spectra to be collected from single particles illuminated at 632.8 nm. SERS intensities from clusters of cylinders are found to scale linearly with particle number, whereas, for nanospheres, the scaling is non-linear. The linear scaling of SERS from cylinders is a reflection of the high enhancement provided by the sharp features of the individual particles; whereas, the non-linear scaling of SERS from clusters of spheres is found to be consistent with the near-field enhancement from inter-particle coupling simulated for clusters of spheres arranged in representative-observed geometries.
TL;DR: Numerically and experimentally the accuracy of an analytical model used to derive the effective nonlinear susceptibilities of a varactor-loaded split ring resonator (VLSRR) magnetic medium is verified, illustrating that the effective medium techniques associated with metamaterials can accurately be transitioned to nonlinear systems.
Abstract: We verify numerically and experimentally the accuracy of an analytical model used to derive the effective nonlinear susceptibilities of a varactor-loaded split ring resonator (VLSRR) magnetic medium. For the numerical validation, a nonlinear oscillator model for the effective magnetization of the metamaterial is applied in conjunction with Maxwell equations and the two sets of equations solved numerically in the time-domain. The computed second harmonic generation (SHG) from a slab of a nonlinear material is then compared with the analytical model. The computed SHG is in excellent agreement with that predicted by the analytical model, both in terms of magnitude and spectral characteristics. Moreover, experimental measurements of the power transmitted through a fabricated VLSRR metamaterial at several power levels are also in agreement with the model, illustrating that the effective medium techniques associated with metamaterials can accurately be transitioned to nonlinear systems.
TL;DR: In this paper, the role of changing incidence (and phase) angle geometry and sample grain-size on the intensity of XRF from regolith-like samples was investigated as an analogue to planetary X-ray fluorescence missions.
TL;DR: In this paper, a nonlinear MM for second-order harmonic generation at terahertz frequencies was proposed, which is predicted to have a tunable bandwidth of over 100%.
Abstract: The ability to tune the quasi-phase-matching (QPM) frequency is a highly desirable though lacking feature of many nonlinear devices. To this end, we consider QPM in a special class of active nonlinear metamaterials (MMs), whose properties can be controlled postfabrication. By application of a tunable, periodic perturbation in the linear susceptibility (magnetic or electric) of a MM, a single nonlinear device can be constructed to operate over an exceedingly broad bandwidth. We propose a nonlinear MM for QPM second-order harmonic generation at terahertz frequencies, predicted to have a tunable bandwidth of over 100%.
TL;DR: In this article, it was shown that microtearing modes below the ion gyroscale are robustly unstable with respect to simulation parameters, radial location and discharge time, and electron temperature gradient modes are the most unstable modes in the outer plasma region.
Abstract: Gyrokinetic calculations indicate microtearing modes below the ion gyroscale arelinearlyunstableinaNationalSphericalTorusExperiment(NSTX)plasma. The modes are robustly unstable with respect to simulation parameters, radial location and discharge time. The modes exist at higher wavenumbers and exhibit narrower electric potential mode structures than conventional microtearing modes, but both modes extend to similar normalized radial wavenumbers. Mode growth rates increase with higher electron temperature gradients and higher collisionality. Finally, microtearing modes below the ion gyroscale are the most unstable modes near the magnetic axis, but electron temperature gradient modes are the most unstable modes in the outer plasma region. (Some figures in this article are in colour only in the electronic version)
TL;DR: In this article, the authors investigate the nonlinear enhancement effect in metamaterials through a numerical study of four nonlinear metammaterial designs comprising arrays of metallic structures embedded in nonlinear dielectrics and operating around 10 THz.
Abstract: Artificially structured metamaterials offer a means to enhance the weak optical nonlinearities of natural materials. The enhancement results from the inhomogeneous nature of the metamaterial unit cell, over which the local field distribution can likewise be strongly inhomogeneous, with highly localized and concentrated field regions. We investigate the nonlinear enhancement effect in metamaterials through a numerical study of four nonlinear metamaterial designs comprising arrays of metallic structures embedded in nonlinear dielectrics and operating around 10 THz. Through full-wave simulations and by employing an extended version of the transfer-matrix-based nonlinear parameter retrieval method, we confirm and quantify the enhanced nonlinearities, showing bulk quadratic nonlinear properties that are up to two orders of magnitude larger, and cubic nonlinear properties that are up to four orders of magnitude larger than the bulk nonlinear dielectric alone. Furthermore, the proposed nonlinear metamaterials support a variety of configurable nonlinear properties and regimes, including electric, magnetic, broadband, and low loss, depending on the particular geometry chosen. Finally, we use the retrieved parameters in a coupled-mode theory to predict the optimal crystal lengths and conversion efficiencies of these structures, displaying the possibility of efficient and subwavelength nonlinear devices based on metamaterials.
TL;DR: In this article, the authors detect passive fast ion Dα (FIDA) light from fast ions that charge exchange in the high neutral density region at the edge of the plasma make appreciable contributions to the beam emission spectroscopy (BES) signals.
Abstract: Beam-emission spectroscopy (BES) diagnostics normally detect fluctuations in the light emitted by an injected neutral beam. Under some circumstances, however, light from fast ions that charge exchange in the high neutral-density region at the edge of the plasma make appreciable contributions to the BES signals. This 'passive' fast-ion Dα (FIDA) light appears in BES signals from both the DIII-D tokamak and the National Spherical Torus Experiment (NSTX). One type of passive FIDA light is associated with classical orbits that traverse the edge. Another type is caused by instabilities that expel fast ions from the core; this light can complicate measurement of the instability eigenfunction.
TL;DR: In this article, a multilayer metamaterial diffraction grating designed for operation at 10.6 μm is analyzed, and the impact of material losses and impedance mismatch on the diffraction efficiency is explored.
Abstract: The equivalent of a blazed diffraction grating can be formed from an array of metamaterial elements arranged so as to produce a linear gradient in the effective refractive index. By spreading the gradient over a multiwavelength distance, and repeating the pattern many times, a gradient index (GRIN) diffraction grating is formed. Using lithographically patterned, metallic metamaterial elements, dozens of distinguishable phase levels can be implemented by slightly modifying the design of each successive metamaterial element. We analyze here a multilayer metamaterial diffraction grating designed for operation at 10.6 μm, exploring the impact of material losses and impedance mismatch on the diffraction efficiency.
TL;DR: In this paper, the design and implementation of a two-dimensional metamaterial relay lens, conceptually formed by flattening a Maxwell fisheye lens through the use of a coordinate transformation, is considered.
Abstract: We consider the design and implementation of a two-dimensional metamaterial relay lens, conceptually formed by flattening a Maxwell fisheye lens—a perfect imaging device—through the use of a coordinate transformation. Because Maxwell’s equations are form-invariant under coordinate transformations, the specifications for the constitutive parameters of the device are obtained immediately in a procedure that has now become known as transformation optics. To obtain a more favorable implementation of the lens, we seek a quasi-conformal transformation optics transformation that minimizes the required anisotropy, such that the resulting lens can be formed using isotropic, dielectric-only media. We demonstrate a flattened Maxwell lens at microwave frequencies using a nonresonant metamaterial and confirm its focusing and broad bandwidth behavior. Such planar, dielectric-only structures can be readily implemented in infrared and optical waveguides.
TL;DR: Using a finite-element, full-wave modeling approach, a flexible method of analyzing and simulating dielectric and plasmonic waveguide structures as well as their mode coupling is presented.
Abstract: Using a finite-element, full-wave modeling approach, we present a flexible method of analyzing and simulating dielectric and plasmonic waveguide structures as well as their mode coupling. This method is applied to an integrated plasmonic circuit where a straight dielectric waveguide couples through a straight hybrid long-range plasmon waveguide to a uniformly bent hybrid one. The hybrid waveguide comprises a thin metal core embedded in a two–dimensional dielectric waveguide. The performance of such plasmonic circuits in terms of insertion losses is discussed.
TL;DR: In this article, the authors demonstrate tuning of a metamaterial device that incorporates a form of spatial gradient control, achieved through a vanadium dioxide layer which interacts with an array of split ring resonators.
Abstract: We demonstrate tuning of a metamaterial device that incorporates a form of spatial gradient control. Electrical tuning of the metamaterial is achieved through a vanadium dioxide layer which interacts with an array of split ring resonators. We achieved a spatial gradient in the magnitude of permittivity, writeable using a single transient electrical pulse. This induced gradient in our device is observed on spatial sc ales on the order of one wavelength at 1 THz. Thus, we show the viability of elements for use in future devices with potential applications in beamforming and communications
TL;DR: In this article, the authors used transformation optics to compress a transmission line based Rotman lens by 27 percent along the optical axis while maintaining the beam steering range, gain and side lobe amplitudes over the full frequency range of the original lens.
Abstract: The solutions to the Rotman lens design equations constrain the minimum size of the device. Here we use
Transformation Optics to compress a transmission line based Rotman lens by 27 percent along the optical axis
while maintaining the beam steering range, gain and side lobe amplitudes over the full frequency range of the
original lens. The transformation applied requires an anisotropic magnetic response, which is achieved in the
transmission line context using complementary electric dipole structures patterned into the top conductor of
the lens. The non-resonant complementary metamaterial elements provide an anisotropic, eective magnetic
permeability with values that can be varied across a spatial region by varying the geometry of each element.
01 Oct 2011
TL;DR: In this paper, measured effects on the fluorescent emission spectra of commercially produced core-shell (CdSe/ZnS) quantum dots (QDs) of 25 nm, 33 nm and 63 nm size in toluene, following exposure to -1 MeV gamma irradiation in the range 1-110 Gy.
Abstract: In this work, we report measured effects on the fluorescent emission spectra of commercially produced core-shell (CdSe/ZnS) quantum dots (QDs) of 25 nm, 33 nm and 63 nm size in toluene, following exposure to -1 MeV gamma irradiation in the range 01-110 Gy We show that damage depends on the size of the QDs, an effect seen in solutions of a single type of QD as well as in a mixture, and that increasing the concentration of QDs in the toluene decreases the effect Measurements have recently been made to investigate and shift in peak wavelength following irradiation and these are reported here Recent work on the production of a prototype 2D imaging dosimeter, by absorbing a solution of green emitting QD in toluene into a sample of porous "Vycor" glass, has shown that QDs absorbed in the Vycor fluoresce under several hours of continual illumination and that the system continues to show fluorescence for several days after the initial preparation Initial results of experiments to dynamically image the Vycor during electron irradiation are presented as is progress on the development of a second prototype device for 2D radiation dosimetry
01 May 2011
TL;DR: In this paper, a nano-antenna composed of a particle and a polarizable surface provides control of the spatial distribution and high enhancement of Raman scattering, which may serve as a stable platform for single molecule detection.
Abstract: A nano-antenna composed of a particle and a polarizable surface provides control of the spatial distribution and high enhancement of Raman scattering. This structure may serve as a stable platform for single molecule detection.
TL;DR: In this article, the authors present results of theoretical analysis as well as full wave finite element simulations, proving that the complex power and impedance associated with an electromagnetic source are not changed by source transformation methods.
Abstract: Transformation optics is a methodology that has proven useful in the design of complex media used to alter and control electromagnetic waves in either passive space or regions that contain source distributions. Recently it has been shown that the application of source transformations can reshape field radiation patterns corresponding to arbitrarily shaped sources. In the accompanying numerical confirmations, the effect of source transformations on fundamental quantities that are critical to the design and evaluation of antenna based systems--such as complex power and impedance, remain unexplored. In this paper, we present results of theoretical analysis as well as full wave finite element simulations, proving that the complex power and impedance associated with an electromagnetic source are not changed by source transformation methods. These results open the door to the application of source transformations for the design of practical antenna based systems, such as conformal and phased array antenna design. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26077