95 research outputs found
On the use of bianisotropic huygens' metasurfaces to build leaky-wave antennas
The Electromagnetics AcademyHuygens' metasurfaces are considered a powerful tool to achieve anomalous electromagnetic field transformations. They consist of an artifcial surface built of pairs of collocated electric and magetic dipoles that force the boundary conditions for the desired transformation to be ful lled [1]. Despite their possibilities, the achievable transformations must ful l some conditions. In [2] it was
shown that Huygens' metasurfaces with passive and lossless particles can achieve an arbitrary field transformation provided that the power is conserved at each point of the metasurface and there is wave impedance matching. However, it was shown in [3], that by introducing bianisotropy of the omega-type, the matching condition can be suppressed, which allows the control of both the transmission and rejection coe cients on the metasurface.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech
A spatially shifted beam approach to subwavelength focusing
Although negative-refractive-index metamaterials have successfully achieved
subwavelength focusing, image resolution is limited by the presence of losses.
In this Letter, a metal transmission screen with subwavelength spaced slots is
proposed that focuses the near-field beyond the diffraction limit and
furthermore, is easily scaled from microwave frequencies to the optical regime.
An analytical model based on the superposition of shifted beam patterns is
developed that agrees very well with full-wave simulations and is corroborated
by experimental results at microwave frequencies.Comment: 5 pages, 7 figures. Content updated following reviewer comments to
match final published pape
Free-Space Imaging Beyond the Diffraction Limit Using a Veselago-Pendry Transmission-Line Superlens
Focusing using conventional lenses relies on the collection and interference
of propagating waves, but discounts the evanescent waves that decay rapidly
from the source. Since these evanescent waves contain the finest details of the
source, the image suffers a loss of resolution and is referred to as
'diffraction-limited'. Superlensing is the ability to create an image with fine
features beyond the diffraction limit, and can be achieved with a
'Veselago-Pendry' lens made from a metamaterial. Such a Veselago-Pendry
superlens for imaging in free space must be stringently designed to restore
both propagating and evanescent waves, but meeting these design conditions
(isotropic n = epsilon_r = mu_r = -1) has proven difficult and has made its
realization elusive. We demonstrate free-space imaging with a resolution over
three times better than the diffraction limit at microwave frequencies using a
Veselago-Pendry metamaterial superlens based on the negative-refractive-index
transmission-line (NRI-TL) approach, which affords precise control over its
electromagnetic properties and is also less susceptible to losses than other
approaches. A microwave superlens can be particularly useful for illumination
and discrimination of closely spaced buried objects over practical distances by
way of back-scattering, e.g. in tumour or landmine detection, or for targeted
irradiation/hyperthermia.Comment: 19 pages, 7 figures, submitted to IEEE Transactions on Antennas and
Propagatio
Plus-minus construction leads to perfect invisibility
Recent theoretical advances applied to metamaterials have opened new avenues
to design a coating that hides objects from electromagnetic radiation and even
the sight. Here, we propose a new design of cloaking devices that creates
perfect invisibility in isotropic media. A combination of positive and negative
refractive indices, called plus-minus construction, is essential to achieve
perfect invisibility (i.e., no time delay and total absence of reflection).
Contrary to the common understanding that between two isotropic materials
having different refractive indices the electromagnetic reflection is
unavoidable, our method shows that surprisingly the reflection phenomena can be
completely eliminated. The invented method, different from the classical
impedance matching, may also find electromagnetic applications outside of
cloaking devices, wherever distortions are present arising from reflections.Comment: 24 pages, 10 figure
Experimental Demonstration of A Dual-Input/Dual-Output Reflective Impedance Metasurface
This paper presents the experimental demonstration of a
dual-input/dual-output reflective impedance metasurface. The design of the
metasurface relies on the Method of Moments and leverages auxiliary surface
waves to achieve anomalous reflection of two impinging plane waves with
controlled sidelobe levels. The two beams are chosen independently compared to
those in a conventional phase-gradient metasurface where the design presents a
single slope to achieve a certain reflection and all other incident beams would
depend on that slope. A prototype that ensures maximum directivity at two
prescribed reflection angles for the two input waves is then fabricated on a
Rogers RO3003 printed-circuit board using 42 metawires loaded with printed
capacitors. The proposed metasurface is capable of reflecting an incident beam
from to and a second from to at
9.93 GHz. The metasurface is experimentally characterized and an illumination
efficiency of at least 89% is calculated for each of the reflected waves,
indicating a high multiplexing efficacy
Unidirectional Invisibility and PT-Symmetry with Graphene
We investigate the reflectionlessness and invisibility properties in the
transverse electric (TE) mode solution of a linear homogeneous optical system
which comprises the -symmetric structures covered by graphene
sheets. We derive analytic expressions, indicate roles of each parameter
governing optical system with graphene and justify that optimal conditions of
these parameters give rise to broadband and wide angle invisibility. Presence
of graphene turns out to shift the invisible wavelength range and to reduce the
required gain amount considerably, based on its chemical potential and
temperature. We substantiate that our results yield broadband reflectionless
and invisible configurations for realistic materials of small refractive
indices, usually around , and of small thickness sizes with graphene
sheets of rather small temperatures and chemical potentials. Finally, we
demonstrate that pure -symmetric graphene yields invisibility at
small temperatures and chemical potentials.Comment: 20 pages, 1 table 17 figure
A Novel Design of Dielectric Perfect Invisibility Devices
The aim of an invisibility device is to guide light around any object put
inside, being able to hide objects from sight. In this work, we propose a novel
design of dielectric invisibility media based on negative refraction and
optical conformal mapping that seems to create perfect invisibility. This
design has some advantages and more relaxed constraints compared with already
proposed schemes. In particular, it represents an example where the time delay
in a dielectric invisibility device is zero. Furthermore, due to impedance
matching of negatively refracting materials, the reflection should be close to
zero. These findings strongly indicate that perfect invisibility with optically
isotropic materials is possible. Finally, the area of the invisible space is
also discussed
Three-Dimensional Nanotransmission Lines at Optical Frequencies: A Recipe for Broadband Negative-Refraction Optical Metamaterials
Here we apply the optical nanocircuit concepts to design and analyze in
detail a three-dimensional (3-D) plasmonic nanotransmission line network that
may act as a negative-refraction broadband metamaterial at infrared and optical
frequencies. After discussing the heuristic concepts at the basis of our
theory, we show full-wave analytical results of the expected behavior of such
materials, which show increased bandwidth and relative robustness to losses.
The possibility and constraints of getting a 3-D fully isotropic response is
also explored and conditions for minimal losses and increased bandwidth are
discussed. Full-wave analytical results for some design examples employing
realistic plasmonic materials at infrared and optical frequencies are also
presented, and a case of sub-wavelength imaging system using a slab of this
material is numerically investigated.Comment: 70 pages, 15 figure
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