211 research outputs found
Effective material parameter retrieval for thin sheets: theory and application to graphene, thin silver films, and single-layer metamaterials
An important tool in the field of metamaterials is the extraction of
effective material parameters from simulated or measured scattering parameters
of a sample. Here we discuss a retrieval method for thin-film structures that
can be approximated by a two-dimensional scattering sheet. We determine the
effective sheet conductivity from the scattering parameters and we point out
the importance of the magnetic sheet current to avoid an overdetermined
inversion problem. Subsequently, we present two applications of the sheet
retrieval method. First, we determine the effective sheet conductivity of thin
silver films and we compare the resulting conductivities with the sheet
conductivity of graphene. Second, we apply the method to a cut-wire
metamaterial with an electric dipole resonance. The method is valid for
thin-film structures such as two-dimensional metamaterials and
frequency-selective surfaces and can be easily generalized for anisotropic or
chiral media.Comment: 5 pages, 5 figure
Enhancing Optical Gradient Forces with Metamaterials
We demonstrate how the optical gradient force between two waveguides can be
enhanced using transformation optics. A thin layer of double-negative or
single-negative metamaterial can shrink the interwaveguide distance perceived
by light, resulting in a more than tenfold enhancement of the optical force.
This process is remarkably robust to the dissipative loss normally observed in
metamaterials. Our results provide an alternative way to boost optical gradient
forces in nanophotonic actuation systems and may be combined with existing
resonator-based enhancement methods to produce optical forces with an
unprecedented amplitude.Comment: 5 pages, 4 figures; supplemental information available from AP
Strong group velocity dispersion compensation with phase-engineered sheet metamaterials
Resonant metamaterials usually exhibit substantial dispersion, which is
considered a shortcoming for many applications. Here we take advantage of the
ability to tailor the dispersive response of a metamaterial introducing a new
method of group-velocity dispersion compensation in telecommunication systems.
The method consists of stacking a number of highly dispersive sheet
metamaterials and is capable of compensating the dispersion of optical fibers
with either negative or positive group-velocity dispersion coefficients. We
demonstrate that the phase-engineered metamaterial can provide strong
group-velocity dispersion management without being adversely affected by large
transmission loss, while at the same time offering high customizability and
small footprint.Comment: 10 pages, 4 figure
Optical signatures of nonlocal plasmons in graphene
We theoretically investigate under which conditions nonlocal plasmon response
in monolayer graphene can be detected. To this purpose, we study optical
scattering off graphene plasmon resonances coupled using a subwavelength
dielectric grating. We compute the graphene conductivity using the Random Phase
Approximation (RPA) obtaining a nonlocal conductivity and we calculate the
optical scattering of the graphene-grating structure. We then compare this with
the scattering amplitudes obtained if graphene is modeled by the local RPA
conductivity commonly used in the literature. We find that the graphene plasmon
wavelength calculated from the local model may deviate up to from the
more accurate nonlocal model in the small-wavelength (large-) regime. We
also find substantial differences in the scattering amplitudes obtained from
the two models. However, these differences in response are pronounced only for
small grating periods and low temperatures compared to the Fermi temperature.Comment: Accepted for publication in Physical Review B. 15 pages, 9 figure
Current-controlled light scattering and asymmetric plasmon propagation in graphene
We demonstrate that plasmons in graphene can be manipulated using a DC
current. A source-drain current lifts the forward/backward degeneracy of the
plasmons, creating two modes with different propagation properties parallel and
antiparallel to the current. We show that the propagation length of the plasmon
propagating parallel to the drift current is enhanced, while the propagation
length for the antiparallel plasmon is suppressed. We also investigate the
scattering of light off graphene due to the plasmons in a periodic dielectric
environment and we find that the plasmon resonance separates in two peaks
corresponding to the forward and backward plasmon modes. The narrower linewidth
of the forward propagating plasmon may be of interest for refractive index
sensing and the DC current control could be used for the modulation of
mid-infrared electromagnetic radiation.Comment: 5 pages, 5 figure
Comparison of gold- and graphene-based resonant nano-structures for terahertz metamaterials and an ultra-thin graphene-based modulator
Graphene exhibits unique material properties and in electromagnetic wave
technology, it raises the prospect of devices miniaturized down to the atomic
length scale. Here we study split-ring resonator metamaterials made from
graphene and we compare them to gold-based metamaterials. We find that
graphene's huge reactive response derived from its large kinetic inductance
allows for deeply subwavelength resonances, although its resonance strength is
reduced due to higher dissipative loss damping and smaller dipole coupling.
Nevertheless, tightly stacked graphene rings may provide for negative
permeability and the electric dipole resonance of graphene meta-atoms turns out
to be surprisingly strong. Based on these findings, we present a terahertz
modulator based on a metamaterial with a multi-layer stack of alternating
patterned graphene sheets separated by dielectric spacers. Neighbouring
graphene flakes are biased against each other, resulting in modulation depths
of over 75% at a transmission level of around 90%.Comment: 16 pages, 5 figure
Analytical solution for wave propagation through a graded index interface between a right-handed and a left-handed material
We have investigated the transmission and reflection properties of structures
incorporating left-handed materials with graded index of refraction. We present
an exact analytical solution to Helmholtz' equation for a graded index profile
changing according to a hyperbolic tangent function along the propagation
direction. We derive expressions for the field intensity along the graded index
structure, and we show excellent agreement between the analytical solution and
the corresponding results obtained by accurate numerical simulations. Our model
straightforwardly allows for arbitrary spectral dispersion.Comment: 7 pages, 3 figure
High-sensitivity plasmonic refractive index sensing using graphene
We theoretically demonstrate a high-sensitivity, graphene-plasmon based
refractive index sensor working in the mid-infrared at room temperature. The
bulk figure of merit of our sensor reaches values above , but the key
aspect of our proposed plasmonic sensor is its surface sensitivity which we
examine in detail. We have used realistic values regarding doping level and
electron relaxation time, which is the limiting factor for the sensor
performance. Our results show quantitatively the high performance of
graphene-plasmon based refractive index sensors working in the mid-infrared.Comment: This is an author-created, un-copyedited version of an article
accepted for publication/published in 2DMaterials. IOP Publishing Ltd is not
responsible for any errors or omissions in this version of the manuscript or
any version derived from it. The Version of Record is available online at
https://doi.org/10.1088/2053-1583/aa70f
Validation of the Anxiety Scale for Pregnancy in a Sample of Iranian Women
We investigated propagation of electromagnetic waves through composite structures with negative refractive index, the popular ”left-handed metamaterials”, for the case when there is a gradient of refractive index. We obtained the exact analytical solutions to the Helmholtz equation valid for arbitrary steepness of the graded interface between the positive and the negative index part. We analyzed the special case of matched impedances of the two constituent materials within the metamaterial composite. We derived analytical expressions for the field intensity, transmission and reflection coefficients and compared them with the results obtained by the numerical simulations using the Finite Element Method. The model allows for arbitrary spectral dispersion and lossy media.QC 20120126</p
Miniaturization of photonic waveguides by the use of left-handed materials
We propose the use of a left-handed material in an optical waveguide
structure to reduce its thickness well below the wavelength of light. We
demonstrate that a layer of left-handed material, added to the cladding of a
planar waveguide rather than to its core, allows for good light confinement in
a subwavelength thin waveguide. We attribute the observed behavior to the
change in phase evolution of electromagnetic waves in the guide. This technique
can be used for the miniaturization of photonic integrated circuits.Comment: 4 pages, 4 figure
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