236 research outputs found
Helicity Density Maximization in a Planar Array of Achiral High-Density Dielectric Nanoparticles
We investigate how a periodic array composed of achiral isotropic
high-refractive index dielectric nanospheres generates nearfield over the array
surface reaching helicity density very close to its upper bound. The required
condition for an array of nanospheres to generate optimally chiral nearfield,
which represents the upper bound of helicity density, is derived in terms of
array effective electric and magnetic polarizabilities that almost satisfy the
effective Kerker condition for arrays. The discussed concepts find applications
in improving chirality detection based on circular dichroism (CD) at surface
level instead of in the bulk. Importantly the array would not contribute to the
generated CD signal when used as a substrate for detecting chirality of a thin
layer of chiral molecules. This eliminates the need to separate the CD signal
generated by the array from that of the chiral sample
Helicity Maximization of Structured Light to Empower Nanoscale Chiral Matter Interaction
Structured light enables the characterization of chirality of optically small
nanoparticles by taking advantage of the helicity maximization concept recently
introduced in[1]. By referring to fields with nonzero helicity density as
chiral fields, we first investigate the properties of two chiral optical beams
in obtaining helicity density localization and maximization requirements. The
investigated beams include circularly polarized Gaussian beams and also an
optical beam properly composed by a combination of a radially and an
azi-muthally polarized beam. To acquire further enhancement and localization of
helicity density beyond the diffraction limit, we also study chiral fields at
the vicinity of a spherical dielectric nanoantenna and demon-strate that the
helicity density around such a nanoantenna is a superposition of helicity
density of the illu-minating field, scattered field, and an interference
helicity term. Moreover, we illustrate when the nanoan-tenna is illuminated by
a proper combination of azimuthal and radially polarized beams, the scattered
nearfields satisfy the helicity maximization conditions beyond the diffraction
limit. The application of the concept of helicity maximization to nanoantennas
and generating optimally chiral nearfield result in helici-ty enhancement which
is of great advantage in areas like detection of nanoscale chiral samples,
microsco-py, and optical manipulation of chiral nanoparticles
Focused Azimuthally Polarized Vector Beam and Spatial Magnetic Resolution below the Diffraction Limit
An azimuthally electric-polarized vector beam (APB), with a polarization
vortex, has a salient feature that it contains a magnetic-dominant region
within which electric field ideally has a null while longitudinal magnetic
field is maximum. Fresnel diffraction theory and plane-wave spectral (PWS)
calculations are applied to quantify field features of such a beam upon
focusing through a lens. The diffraction-limited full width at half maximum
(FWHM) of the beam's longitudinal magnetic field intensity profile and
complementary FWHM (CFWHM) of the beam's annular-shaped total electric field
intensity profile are examined at the lens's focal plane as a function of the
lens's paraxial focal distance. Then, we place a subwavelength dense dielectric
Mie scatterer in the minimum-waist plane of a self-standing converging APB and
demonstrate for the first time that a very high resolution magnetic field at
optical frequency is achieved with total magnetic field FWHM of 0.23{\lambda}
(i.e., magnetic field spot area of 0.04{\lambda}^2) within a magnetic-dominant
region. The theory shown here is valuable for development of optical microscopy
and spectroscopy systems based on magnetic dipolar transitions which are in
general much weaker than their electric counterparts
Exceptional Point of Degeneracy in Linear-Beam Tubes for High Power Backward-Wave Oscillators
Abstract An exceptional point of degeneracy (EPD) is induced in a system made
of an electron beam interacting with an electromagnetic (EM) guided mode. This
enables a degenerate synchronous regime in backward wave oscillators (BWOs)
where the electron beams provides distributed gain to the EM mode with
distributed power extraction. Current particle-in-cell simulation results
demonstrate that BWOs operating at an EPD have a starting-oscillation current
that scales quadratically to a non-vanishing value for long interaction lengths
and therefore have higher power conversion efficiency at arbitrarily higher
level of power generation compared to standard BWOs
Theory and New Amplification Regime in Periodic Multi Modal Slow Wave Structures with Degeneracy Interacting with an Electron Beam
We present the theory of a new amplification regime in Travelling Wave Tubes
(TWTs) composed of a slow-wave periodic structure that supports multiple
electromagnetic modes that can all be synchronized with the electron beam. The
interaction between the multimodal slow-wave structure and the electron beam is
quantified using a Multi Transmission Line approach (MTL) based on a
generalized Pierce model and transfer matrix analysis leading to the
identification of modes with complex Bloch wavenumber. In particular, we
address a new possible operation condition for TWTs based on the super
synchronism between an electron beam and four modes exhibiting a degeneracy
condition near a band edge of the periodic slowwave MTL. We show a
phenomenological change in the band structure of periodic MTL where we observe
at least two growing modal cooperating solutions as opposed to a uniform MTL
interacting with an electron beam where there is rigorously only one growing
modal solution. We discuss the advantage of using such a degeneracy condition
in TWTs that leads to larger gain conditions in amplifier regimes and also to
very lowstarting beam current in high power oscillators.Comment: Version 2. 33 pages, 16 figure
Novel concept for pulse compression via structured spatial energy distribution
We present a novel concept for pulse compression scheme applicable at RF,
microwave and possibly to optical frequencies based on structured energy
distribution in cavities supporting degenerate band-edge (DBE) modes. For such
modes a significant fraction of energy resides in a small fraction of the
cavity length. Such energy concentration provides a basis for superior
performance for applications in microwave pulse compression devices (MPC) when
compared to conventional cavities. The novel design features: larger loaded
quality factor of the cavity and stored energy compared to conventional
designs, robustness to variations of cavity loading, energy feeding and
extraction at the cavity center, substantial reduction of the cavity size by
use of equivalent lumped circuits for low energy sections of the cavity,
controlled pulse shaping via engineered extraction techniques. The presented
concepts are general, in terms of equivalent transmission lines, and can be
applied to a variety of realistic guiding structures.Comment: 18 pages, 10 figure
Exceptional Point of Degeneracy in Backward-Wave Oscillator with Distributed Power Extraction
We show how an exceptional point of degeneracy (EPD) is formed in a system
composed of an electron beam interacting with an electromagnetic mode guided in
a slow wave structure (SWS) with distributed power extraction from the
interaction zone. Based on this kind of EPD, a new regime of operation is
devised for backward wave oscillators (BWOs) as a synchronous and degenerate
regime between a backward electromagnetic mode and the charge wave modulating
the electron beam. Degenerate synchronization under this EPD condition means
that two complex modes of the interactive system do not share just the
wavenumber, but they rather coalesce in both their wavenumbers and eigenvectors
(polarization states). In principle this new condition guarantees full
synchronization between the electromagnetic wave and the beam's charge wave for
any amount of output power extracted from the beam, setting the threshold of
this EPD-BWO to any arbitrary, desired, value. Indeed, we show that the
presence of distributed radiation in the SWS results in having high-threshold
electron-beam current to start oscillations which implies higher power
generation. These findings have the potential to lead to highly efficient BWOs
with very high output power and excellent spectral purity
Graphene-Dielectric Composite Metamaterials: Evolution from Elliptic to Hyperbolic Wavevector Dispersion and The Transverse Epsilon-Near-Zero Condition
We investigated a multilayer graphene-dielectric composite material,
comprising graphene sheets separated by subwavelength-thick dielectric spacer,
and found it to exhibit hyperbolic isofrequency wavevector dispersion at far-
and mid-infrared frequencies allowing propagation of waves that would be
otherwise evanescent in a dielectric. Electrostatic biasing was considered for
tunable and controllable transition from hyperbolic to elliptic dispersion. We
explored the validity and limitation of the effective medium approximation
(EMA) for modeling wave propagation and cutoff of the propagating spatial
spectrum due to the Brillouin zone edge. We found that EMA is capable of
predicting the transition of the isofrequency dispersion diagram under certain
conditions. The graphene-based composite material allows propagation of
backward waves under the hyperbolic dispersion regime and of forward waves
under the elliptic regime. Transition from hyperbolic to elliptic dispersion
regimes is governed by the transverse epsilon-near-zero (TENZ) condition, which
implies a flatter and wider propagating spectrum with higher attenuation, when
compared to the hyperbolic regime. We also investigate the tunable transparency
of the multilayer at that condition in contrast to other materials exhibiting
ENZ phenomena.Comment: to be published in Journal of Nanophotonic
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