3,038 research outputs found
Time-of-flight imaging of invisibility cloaks
As invisibility cloaking has recently become experimental reality, it is
interesting to explore ways to reveal remaining imperfections. In essence, the
idea of most invisibility cloaks is to recover the optical path lengths without
an object (to be made invisible) by a suitable arrangement around that object.
Optical path length is proportional to the time of flight of a light ray or to
the optical phase accumulated by a light wave. Thus, time-of-flight images
provide a direct and intuitive tool for probing imperfections. Indeed, recent
phase-sensitive experiments on the carpet cloak have already made early steps
in this direction. In the macroscopic world, time-of-flight images could be
measured directly by light detection and ranging (LIDAR). Here, we show
calculated time-of-flight images of the conformal Gaussian carpet cloak, the
conformal grating cloak, the cylindrical free-space cloak, and of the invisible
sphere. All results are obtained by using a ray-velocity equation of motion
derived from Fermat's principle.Comment: 11 pages, 6 figures, journal pape
Luneburg lens in silicon photonics
The Luneburg lens is an aberration-free lens that focuses light from all directions equally well. We fabricated and tested a Luneburg lens in silicon photonics. Such fully-integrated lenses may become the building blocks of compact Fourier optics on chips. Furthermore, our fabrication technique is sufficiently versatile for making perfect imaging devices on silicon platforms. (C) 2011 Optical Society of AmericaPublisher PDFPeer reviewe
Design method for quasi-isotropic transformation materials based on inverse Laplace's equation with sliding boundaries
The deformation method of transformation optics has been demonstrated to be a
useful tool, especially in designing arbitrary and nonsingular transformation
materials. Recently, there are emerging demands for isotropic material
parameters, arising from the broadband requirement of the designed devices. In
this work, the deformation method is further developed to design
quasi-isotropic/isotropic transformation materials. The variational functional
of the inverse Laplace's equation is investigated and found to involve the
smooth and quasi-conformal nature of coordinate transformation. Together with
the sliding boundary conditions, the inverse Laplace's equation can be utilized
to give transformations which are conformal or quasi-conformal, depending on
functionalities of interest. Examples of designing an arbitrary carpet cloak
and a waveguide with arbitrary cross sections are given to validate the
proposed idea. Compared with other quasi-conformal methods based on grid
generation tools, the proposed method unifies the design and validation of
transformation devices, and thus is much convenient.Comment: 8 pages, 4 figure
Homogeneous optical cloak constructed with uniform layered structures
The prospect of rendering objects invisible has intrigued researchers for
centuries. Transformation optics based invisibility cloak design is now
bringing this goal from science fictions to reality and has already been
demonstrated experimentally in microwave and optical frequencies. However, the
majority of the invisibility cloaks reported so far have a spatially varying
refractive index which requires complicated design processes. Besides, the size
of the hidden object is usually small relative to that of the cloak device.
Here we report the experimental realization of a homogenous invisibility cloak
with a uniform silicon grating structure. The design strategy eliminates the
need for spatial variation of the material index, and in terms of size it
allows for a very large obstacle/cloak ratio. A broadband invisibility behavior
has been verified at near-infrared frequencies, opening up new oppotunities for
using uniform layered medium to realize invisibility at any frequency ranges,
where high-quality dielectrics are available
Transformation bending device emulated by graded-index waveguide
We demonstrate that a transformation device can be emulated using a
gradient-index waveguide. The effective index of the waveguide is spatially
varied by tailoring a gradient thickness dielectric waveguide. Based on this
technology, we demonstrate a transformation device guiding visible light around
a sharp corner, with low scattering loss and reflection loss. The experimental
results are in good agreement with the numerical results.Comment: This paper is published at Optics Express 20, 13006 (2012
Perfect imaging: they don't do it with mirrors
Imaging with a spherical mirror in empty space is compared with the case when
the mirror is filled with the medium of Maxwell's fish eye. Exact
time-dependent solutions of Maxwell's equations show that perfect imaging is
not achievable with an electrical ideal mirror on its own, but with Maxwell's
fish eye in the regime when it implements a curved geometry for full
electromagnetic waves
An invisibility cloak using silver nanowires
In this paper, we use the parameter retrieval method together with an
analytical effective medium approach to design a well-performed invisible
cloak, which is based on an empirical revised version of the reduced cloak. The
designed cloak can be implemented by silver nanowires with elliptical
cross-sections embedded in a polymethyl methacrylate host. This cloak is
numerically proved to be robust for both the inner hidden object as well as
incoming detecting waves, and is much simpler thus easier to manufacture when
compared with the earlier proposed one [Nat. Photon. 1, 224 (2007)].Comment: 7 pages, 4 figures, 2 table
Accurate estimators of power spectra in N-body simulations
abridged] A method to rapidly estimate the Fourier power spectrum of a point
distribution is presented. This method relies on a Taylor expansion of the
trigonometric functions. It yields the Fourier modes from a number of FFTs,
which is controlled by the order N of the expansion and by the dimension D of
the system. In three dimensions, for the practical value N=3, the number of
FFTs required is 20. We apply the method to the measurement of the power
spectrum of a periodic point distribution that is a local Poisson realization
of an underlying stationary field. We derive explicit analytic expression for
the spectrum, which allows us to quantify--and correct for--the biases induced
by discreteness and by the truncation of the Taylor expansion, and to bound the
unknown effects of aliasing of the power spectrum. We show that these aliasing
effects decrease rapidly with the order N. The only remaining significant
source of errors is reduced to the unavoidable cosmic/sample variance due to
the finite size of the sample. The analytical calculations are successfully
checked against a cosmological N-body experiment. We also consider the initial
conditions of this simulation, which correspond to a perturbed grid. This
allows us to test a case where the local Poisson assumption is incorrect. Even
in that extreme situation, the third-order Fourier-Taylor estimator behaves
well. We also show how to reach arbitrarily large dynamic range in Fourier
space (i.e., high wavenumber), while keeping statistical errors in control, by
appropriately "folding" the particle distribution.Comment: 18 Pages, 9 Figures. Accepted for publication in MNRAS. The
Fourier-Taylor module as well as the associated power spectrum estimator tool
we propose is available as an F90 package, POWMES, at
http://www.projet-horizon.fr or on request from the author
Gravitational collapse in an expanding background and the role of substructure II: Excess power at small scales and its effect of collapse of structures at larger scales
We study the interplay of clumping at small scales with the collapse and
relaxation of perturbations at larger scales using N-Body simulations. We
quantify the effect of collapsed haloes on perturbations at larger scales using
two point correlation function, moments of counts in cells and mass function.
The purpose of the study is twofold and the primary aim is to quantify the role
played by collapsed low mass haloes in the evolution of perturbations at large
scales, this is in view of the strong effect seen when the large scale
perturbation is highly symmetric. Another reason for this study is to ask
whether features or a cutoff in the initial power spectrum can be detected
using measures of clustering at scales that are already non-linear. The final
aim is to understand the effect of ignoring perturbations at scales smaller
than the resolution of N-Body simulations. We find that these effects are
ignorable if the scale of non-linearity is larger than the average
inter-particle separation in simulations. Features in in the initial power
spectrum can be detected easily if the scale of these features is in the linear
regime, detecting such features becomes difficult as the relevant scales become
non-linear. We find no effect of features in initial power spectra at small
scales on the evolved power spectra at large scales. We may conclude that in
general, the effect on evolution of perturbations at large scales of clumping
on small scales is very small and may be ignored in most situations.Comment: Accepted for publication in MNRA
Macroscopic invisibility cloaking of visible light
Invisibility cloaks, which used to be confined to the realm of fiction, have now been turned into a scientific reality thanks to the enabling theoretical tools of transformation optics and conformal mapping. Inspired by those theoretical works, the experimental realization of electromagnetic invisibility cloaks has been reported at various electromagnetic frequencies. All the invisibility cloaks demonstrated thus far, however, have relied on nano- or micro-fabricated artificial composite materials with spatially varying electromagnetic properties, which limit the size of the cloaked region to a few wavelengths. Here, we report the first realization of a macroscopic volumetric invisibility cloak constructed from natural birefringent crystals. The cloak operates at visible frequencies and is capable of hiding, for a specific light polarization, three-dimensional objects of the scale of centimetres and millimetres. Our work opens avenues for future applications with macroscopic cloaking devices
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