70 research outputs found
Far-field analysis of axially symmetric three-dimensional directional cloaks.
Axisymmetric radiating and scattering structures whose rotational invariance is broken by non-axisymmetric excitations present an important class of problems in electromagnetics. For such problems, a cylindrical wave decomposition formalism can be used to efficiently obtain numerical solutions to the full-wave frequency-domain problem. Often, the far-field, or Fraunhofer region is of particular interest in scattering cross-section and radiation pattern calculations; yet, it is usually impractical to compute full-wave solutions for this region. Here, we propose a generalization of the Stratton-Chu far-field integral adapted for 2.5D formalism. The integration over a closed, axially symmetric surface is analytically reduced to a line integral on a meridional plane. We benchmark this computational technique by comparing it with analytical Mie solutions for a plasmonic nanoparticle, and apply it to the design of a three-dimensional polarization-insensitive cloak.This work was supported by the Air Force Office of Scientific Research (AFOSR, Grant No.
FA9550-09-1-0562) and by the Army Research Office through a Multidisciplinary University
Research Initiative (Grant No. W911NF-09-1-0539).
Construction of invisibility cloaks of arbitrary shape and size using planar layers of metamaterials
Transformation optics (TO) is a powerful tool for the design of electromagnetic and optical devices with novel functionality derived from the unusual properties of the transformation media. In general, the fabrication of TO media is challenging, requiring spatially varying material properties with both anisotropic electric and magnetic responses. Though metamaterials have been proposed as a path for achieving such complex media, the required properties arising from the most general transformations remain elusive, and cannot implemented by state-of-the-art fabrication techniques. Here, we propose faceted approximations of TO media of arbitrary shape in which the volume of the TO device is divided into flat metamaterial layers. These layers can be readily implemented by standard fabrication and stacking techniques. We illustrate our approximation approach for the specific example of a two-dimensional, omnidirectional "invisibility cloak", and quantify its performance using the total scattering cross section as a practical figure of merit. © 2012 American Institute of Physics.U.S. Army Research Office (Contract No. W911NF-09-1-0539)
Engineering Electromagnetic Properties of Periodic Nanostructures Using Electrostatic Resonances
Electromagnetic properties of periodic two-dimensional sub-wavelength
structures consisting of closely-packed inclusions of materials with negative
dielectric permittivity in a dielectric host with positive
can be engineered using the concept of multiple electrostatic
resonances. Fully electromagnetic solutions of Maxwell's equations reveal
multiple wave propagation bands, with the wavelengths much longer than the
nanostructure period. It is shown that some of these bands are described using
the quasi-static theory of the effective dielectric permittivity
, and are independent of the nanostructure period. Those bands
exhibit multiple cutoffs and resonances which are found to be related to each
other through a duality condition. An additional propagation band characterized
by a negative magnetic permeability develops when a magnetic moment is induced
in a given nano-particle by its neighbors. Imaging with sub-wavelength
resolution in that band is demonstrated
Broadband electromagnetic cloaking with smart metamaterials.
The ability to render objects invisible with a cloak that fits all objects and sizes is a long-standing goal for optical devices. Invisibility devices demonstrated so far typically comprise a rigid structure wrapped around an object to which it is fitted. Here we demonstrate smart metamaterial cloaking, wherein the metamaterial device not only transforms electromagnetic fields to make an object invisible, but also acquires its properties automatically from its own elastic deformation. The demonstrated device is a ground-plane microwave cloak composed of an elastic metamaterial with a broad operational band (10-12 GHz) and nearly lossless electromagnetic properties. The metamaterial is uniform, or perfectly periodic, in its undeformed state and acquires the necessary gradient-index profile, mimicking a quasi-conformal transformation, naturally from a boundary load. This easy-to-fabricate hybrid elasto-electromagnetic metamaterial opens the door to implementations of a variety of transformation optics devices based on quasi-conformal maps.This work was supported by the Low Observable Technology Research
Centre programme of the Defence Acquisition Program Administration and Agency for
Defense Development and the National Research Foundation of Korea grants funded by
the Ministry of Education, Science and Technology (2012R1A1B3003933, 2009-
0093428). Y.U. and D.R.S. acknowledge support from the U.S. Army Research Office
(grant number W911NF-09-1-0539)
Active Negative Index Metamaterial Powered by an Electron Beam
A novel active negative index metamaterial that derives its gain from an
electron beam is intro- duced. The metamaterial consists of a stack of
equidistant parallel metal plates perforated by a periodic array of holes
shaped as complementary split-ring resonators. It is shown that this structure
supports a negative-index transverse magnetic electromagnetic mode that can
resonantly interact with a relativistic electron beam. Such metamaterial can be
used as a coherent radiation source or a particle accelerator.Comment: 5 pages, 4 figure
Probing the ultimate limits of plasmonic enhancement.
Metals support surface plasmons at optical wavelengths and have the ability to localize light to subwavelength regions. The field enhancements that occur in these regions set the ultimate limitations on a wide range of nonlinear and quantum optical phenomena. We found that the dominant limiting factor is not the resistive loss of the metal, but rather the intrinsic nonlocality of its dielectric response. A semiclassical model of the electronic response of a metal places strict bounds on the ultimate field enhancement. To demonstrate the accuracy of this model, we studied optical scattering from gold nanoparticles spaced a few angstroms from a gold film. The bounds derived from the models and experiments impose limitations on all nanophotonic systems.Supported by Air
Force Office of Scientific Research grant FA9550-09-1-0562
and by the Army Research Office through Multidisciplinary
University Research Initiative grant W911NF-09-1-0539.
Also supported by the Leverhulme Trust and the Marie
Curie Actions (J.B.P., S.A.M., and A.I.F.-D.), NIH grant
R21EB009862 (A.C.), and NIH F32 award F32EB009299
(R.T.H.)
Magnetic metamaterial superlens for increased range wireless power transfer.
The ability to wirelessly power electrical devices is becoming of greater urgency as a component of energy conservation and sustainability efforts. Due to health and safety concerns, most wireless power transfer (WPT) schemes utilize very low frequency, quasi-static, magnetic fields; power transfer occurs via magneto-inductive (MI) coupling between conducting loops serving as transmitter and receiver. At the "long range" regime - referring to distances larger than the diameter of the largest loop - WPT efficiency in free space falls off as (1/d)(6); power loss quickly approaches 100% and limits practical implementations of WPT to relatively tight distances between power source and device. A "superlens", however, can concentrate the magnetic near fields of a source. Here, we demonstrate the impact of a magnetic metamaterial (MM) superlens on long-range near-field WPT, quantitatively confirming in simulation and measurement at 13-16 MHz the conditions under which the superlens can enhance power transfer efficiency compared to the lens-less free-space system
Magnetic levitation of metamaterial bodies enhanced with magnetostatic surface resonances
We propose that macroscopic objects built from negative-permeability
metamaterials may experience resonantly enhanced magnetic force in
low-frequency magnetic fields. Resonant enhancement of the time-averaged force
originates from magnetostatic surface resonances (MSR) which are analogous to
the electrostatic resonances of negative-permittivity particles, well known as
surface plasmon resonances in optics. We generalize the classical problem of
MSR of a homogeneous object to include anisotropic metamaterials, and consider
the most extreme case of anisotropy where the permeability is negative in one
direction but positive in the others. It is shown that deeply subwavelength
objects made of such indefinite (hyperbolic) media exhibit a pronounced
magnetic dipole resonance that couples strongly to uniform or weakly
inhomogeneous magnetic field and provides strong enhancement of the magnetic
force, enabling applications such as enhanced magnetic levitation.Comment: 19 pages, 5 figure
DNA-based Self-Assembly of Chiral Plasmonic Nanostructures with Tailored Optical Response
Surface plasmon resonances generated in metallic nanostructures can be
utilized to tailor electromagnetic fields. The precise spatial arrangement of
such structures can result in surprising optical properties that are not found
in any naturally occurring material. Here, the designed activity emerges from
collective effects of singular components equipped with limited individual
functionality. Top-down fabrication of plasmonic materials with a predesigned
optical response in the visible range by conventional lithographic methods has
remained challenging due to their limited resolution, the complexity of
scaling, and the difficulty to extend these techniques to three-dimensional
architectures. Molecular self-assembly provides an alternative route to create
such materials which is not bound by the above limitations. We demonstrate how
the DNA origami method can be used to produce plasmonic materials with a
tailored optical response at visible wavelengths. Harnessing the assembly power
of 3D DNA origami, we arranged metal nanoparticles with a spatial accuracy of 2
nm into nanoscale helices. The helical structures assemble in solution in a
massively parallel fashion and with near quantitative yields. As a designed
optical response, we generated giant circular dichroism and optical rotary
dispersion in the visible range that originates from the collective
plasmon-plasmon interactions within the nanohelices. We also show that the
optical response can be tuned through the visible spectrum by changing the
composition of the metal nanoparticles. The observed effects are independent of
the direction of the incident light and can be switched by design between left-
and right-handed orientation. Our work demonstrates the production of complex
bulk materials from precisely designed nanoscopic assemblies and highlights the
potential of DNA self-assembly for the fabrication of plasmonic nanostructures.Comment: 5 pages, 4 figure
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