16 research outputs found
Fluid flow control with transformation media
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 us at the possibility of novel propulsion techniques.Comment: 4 pages, 7 figure
Flow stabilization with active hydrodynamic cloaks
We demonstrate that fluid flow cloaking solutions based on active
hydrodynamic metamaterials exist for two-dimensional flows past a cylinder in a
wide range of Reynolds numbers, up to approximately 200. Within the framework
of the classical Brinkman equation for homogenized porous flow, we demonstrate
using two different methods that such cloaked flows can be dynamically stable
for in the range 5-119. The first, highly efficient, method is based on a
linearization of the Brinkman-Navier-Stokes equation and finding the
eigenfrequencies of the least stable eigen-perturbations; the second method is
a direct, numerical integration in the time domain. We show that, by
suppressing the Karman vortex street in the weekly turbulent wake, porous flow
cloaks can raise the critical Reynolds number up to about 120, or five times
greater than for a bare, uncloaked cylinder.Comment: 5 pages, 3 figure
Transformation Optics with Photonic Band Gap Media
We introduce a class of optical media based on adiabatically modulated,
dielectric-only, and potentially extremely low-loss, photonic crystals. The
media we describe represent a generalization of the eikonal limit of
transformation optics (TO). The foundation of the concept is the possibility to
fit frequency isosurfaces in the k-space of photonic crystals with elliptic
surfaces, allowing them to mimic the dispersion relation of light in
anisotropic effective media. Photonic crystal cloaks and other TO devices
operating at visible wavelengths can be constructed from optically transparent
substances like glasses, whose attenuation coefficient can be as small as 10
dB/km, suggesting the TO design methodology can be applied to the development
of optical devices not limited by the losses inherent to metal-based, passive
metamaterials.Comment: 4 pages, 4 figure
Wide-angle infrared absorber based on negative index plasmonic metamaterial
A metamaterials-based approach to making a wide-angle absorber of infrared
radiation is described. The technique is based on an anisotropic Perfectly
Impedance Matched Negative Index Material (PIMNIM). It is shown analytically
that a sub-wavelength in all three dimensions PIMNIM enables absorption of
close to 100% for incidence angles up to to the normal. A specific
implementation of such frequency-tunable PIMNIM based on plasmonic
metamaterials is presented. Applications to infrared imaging and coherent
thermal sources are described.Comment: To be published in Phys. Rev.
Magnetic superlens-enhanced inductive coupling for wireless power transfer
We investigate numerically the use of a negative-permeability "perfect lens"
for enhancing wireless power transfer between two current carrying coils. The
negative permeability slab serves to focus the flux generated in the source
coil to the receiver coil, thereby increasing the mutual inductive coupling
between the coils. The numerical model is compared with an analytical theory
that treats the coils as point dipoles separated by an infinite planar layer of
magnetic material [Urzhumov et al., Phys. Rev. B, 19, 8312 (2011)]. In the
limit of vanishingly small radius of the coils, and large width of the
metamaterial slab, the numerical simulations are in excellent agreement with
the analytical model. Both the idealized analytical and realistic numerical
models predict similar trends with respect to metamaterial loss and anisotropy.
Applying the numerical models, we further analyze the impact of finite coil
size and finite width of the slab. We find that, even for these less idealized
geometries, the presence of the magnetic slab greatly enhances the coupling
between the two coils, including cases where significant loss is present in the
slab. We therefore conclude that the integration of a metamaterial slab into a
wireless power transfer system holds promise for increasing the overall system
performance
Structurally Rigid Elastic Composites for Acoustic Imaging Countermeasures
We explore the possibilities coming from transformation acoustics and beyond for creating rigid elastic composite shells capable of suppressing the total scattering cross-section of acoustically large objects. The reported design methodology is based on generalized shape and topology optimization, and the outcomes are suitable for rapid prototyping techniques.U.S. Office of Naval Researc
Nanophotonics: Optical time reversal with graphene
Would you ever guess that a microscopic flake of graphite could reverse the diffraction of light? An experiment that demonstrates just such an effect highlights the exciting optical applications of graphene — an atomic layer of carbon with a two-dimensional honeycomb lattice
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
Thin low-loss dielectric coatings for free-space cloaking
We report stereolithographic polymer-based fabrication and experimental operation of a microwave X-band cloaking
device. The device is a relatively thin (about one wavelength thick) shell of an air-dielectric composite, in which
the dielectric component has negligible loss and dispersion. In a finite band (9.7–10.1 GHz), the shell eliminates the
shadow and strongly suppresses scattering from a conducting cylinder of six-wavelength diameter for TE-polarized
free-space plane waves. The device does not require an immersion liquid or conducting ground planes for its operation.
The dielectric constant of the polymer is low enough (ϵ 2.45) to suggest that this cloaking technique would
be suitable for higher frequency radiation, including visible light.U.S. Army Research Office; Multidisciplinary University Research Initiative (Grant No. W911NF-09-1-0539)
Going beyond Axisymmetry: 2.5D Vector Electromagnetics
Linear wave propagation through inhomogeneous structures of size R≫λ (Fig.1) is a computationally challenging problem, in particular when using finite element methods, due to the steep increase of the number of degrees of freedom as a function of R/λ. Fortunately, when the geometry of the problem possesses symmetries, one may choose an appropriate basis in which the stiffness matrix of the discretized problem is block-diagonal. A particular scenario is the case of a cylindrically-symmetric geometry, where an appropriate basis is the set of cylindrical waves with all possible azimuthal numbers (m). Each of the excited cylindrical harmonics propagate through the structure independently of all other harmonics, and therefore the fields associated with that harmonic can be found by solving an essentially two-dimensional PDE problem in the ρ-z (half)-plane. The cylindrical waves have a prescribed dependence on the azimuthal angle variable (φ), hence the name – 2.5D electromagnetics. This novel approach is applied to the problem of cloaking and wave scattering off a spherical nanoparticle on metallic and/or dielectric substrates.COMSOL, Inc