32 research outputs found
Low-loss directional cloaks without superluminal velocity or magnetic response
The possibility of making an optically large (many wavelengths in diameter)
object appear invisible has been a subject of many recent studies. Exact
invisibility scenarios for large (relative to the wavelength) objects involve
(meta)materials with superluminal phase velocity (refractive index less than
unity) and/or magnetic response. We introduce a new approximation applicable to
certain device geometries in the eikonal limit: piecewise-uniform scaling of
the refractive index. This transformation preserves the ray trajectories, but
leads to a uniform phase delay. We show how to take advantage of phase delays
to achieve a limited (directional and wavelength-dependent) form of
invisibility that does not require loss-ridden (meta)materials with
superluminal phase velocities.Comment: 3 pages, 2 figure
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
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