52 research outputs found
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
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
Theory of a Directive Optical Leaky Wave Antenna Integrated into a Resonator and Enhancement of Radiation Control
We provide for the first time the detailed study of the radiation performance
of an optical leaky wave antenna (OLWA) integrated into a Fabry-P\'erot
resonator. We show that the radiation pattern can be expressed as the one
generated by the interference of two leaky waves counter-propagating in the
resonator leading to a design procedure for achieving optimized broadside
radiation, i.e., normal to the waveguide axis. We thus report a realizable
implementation of the OLWA made of semiconductor and dielectric regions. The
theoretical modeling is supported by full-wave simulation results, which are
found to be in good agreement. We aim to control the radiation intensity in the
broadside direction via excess carrier generation in the semiconductor regions.
We show that the presence of the resonator can provide an effective way of
enhancing the radiation level modulation, which reaches values as high as 13.5
dB, paving the way for novel promising control capabilities that might allow
the generation of very fast optical switches, as an example.Comment: 10 pages, 14 figure
Silicon Nitride Waveguides for Plasmon Optical Trapping and Sensing Applications
We demonstrate a silicon nitride trench waveguide deposited with bowtie
antennas for plasmonic enhanced optical trapping. The sub-micron silicon
nitride trench waveguides were fabricated with conventional optical lithography
in a low cost manner. The waveguides embrace not only low propagation loss and
high nonlinearity, but also the inborn merits of combining micro-fluidic
channel and waveguide together. Analyte contained in the trapezoidal trench
channel can interact with the evanescent field from the waveguide beneath. The
evanescent field can be further enhanced by plasmonic nanostructures. With the
help of gold nano bowtie antennas, the studied waveguide shows outstanding
trapping capability on 10 nm polystyrene nanoparticles. We show that the bowtie
antennas can lead to 60-fold enhancement of electric field in the antenna gap.
The optical trapping force on a nanoparticle is boosted by three orders of
magnitude. A strong tendency shows the nanoparticle is likely to move to the
high field strength region, exhibiting the trapping capability of the antenna.
Gradient force in vertical direction is calculation by using a point-like
dipole assumption, and the analytical solution matches the full-wave simulation
well. The investigation indicates that nanostructure patterned silicon nitride
trench waveguide is suitable for optical trapping and nanoparticle sensing
applications
Electric field enhancement with plasmonic colloidal nanoantennas excited by a silicon nitride waveguide
We investigate the feasibility of CMOS-compatible optical structures to
develop novel integrated spectroscopy systems. We show that local field
enhancement is achievable utilizing dimers of plasmonic nanospheres that can be
assembled from colloidal solutions on top of a CMOS-compatible optical
waveguide. The resonant dimer nanoantennas are excited by modes guided in the
integrated silicon nitride waveguide. Simulations show that 100 fold electric
field enhancement builds up in the dimer gap as compared to the waveguide
evanescent field amplitude at the same location. We investigate how the field
enhancement depends on dimer location, orientation, distance and excited
waveguide modes
Array of dipoles near a hyperbolic metamaterial: Evanescent-to-propagating Floquet wave transformation
We investigate the capabilities of hyperbolic metamaterials (HMs) to couple
near-fields (i.e., evanescent waves) emitted by a two-dimensional periodic
array of electric dipoles to propagating waves. In particular, large order
Floquet harmonics with transverse magnetic (TM) polarization, that would be
evanescent in free space and therefore confined near the array surface, are
transformed into propagating spectrum inside the HM, and thus carry power away.
Because of this property, independent of the finite or infinite extent of the
HM, the power generated by an array of elementary electric dipoles is strongly
enhanced when the array is located near a HM surface and is mostly directed
into the HM. In particular, the power coupled to the HM exhibits narrow
frequency features that can be employed in detection applications. The results
shown in this paper provide a clear signature on wave dynamics in HMs. A link
between the results pertaining to the case of an isolated dipole on top of HM
and the planar array is found convenient to explain both wave dynamics and
spectral power distribution. The narrow frequency emission features appear in
the array case only; they depend on its spatial periodicity and remarkable on
the HM thickness.Comment: 13 pages, 12 figure
Optical Leaky-Wave Antenna Integrated in Ring Resonator
A leaky-wave antenna at optical frequencies is designed and integrated with a
ring resonator at 1550 nm wavelength. The leaky wave is generated by using
periodic perturbations in the integrated dielectric waveguide that excite the
-1 spatial harmonic. The antenna consists of a dielectric waveguides with
semiconductor corrugations, and it is compatible with CMOS fabrication
technology. We show that integrating the leaky wave antenna in an optical ring
resonator that is fed by directional couplers, we can improve the electronic
control of the radiation through carrier injection into the semiconductor
corrugations.Comment: 2 pages, 3 figures, conferenc
Photo-induced Magnetic Force Between Nanostructures
Photo-induced magnetic force between nanostructures, at optical frequencies,
is investigated theoretically. Till now optical magnetic effects are not used
in scanning probe microscopy because of the vanishing natural magnetism with
increasing frequency. On the other hand, artificial magnetism in engineered
nanostructures led to the development of measurable optical magnetism. Here,
two examples of nanoprobes that are able to generate strong magnetic dipolar
fields at optical frequency are investigated: first an ideal magnetically
polarizable nanosphere and then a circular cluster of silver nanospheres that
has a loop-like collective plasmonic resonance equivalent to a magnetic dipole.
Magnetic forces are evaluated based on nanostructure polarizabilities, i.e.
induced magnetic dipoles, and magnetic-near field evaluations. As an initial
assessment on the possibility of a magnetic nanoprobe to detect magnetic
forces, we consider two identical magnetically polarizable nanoprobes and
observe magnetic forces in the order of piconewtons thereby bringing it within
detection limits of conventional atomic force microscopes at ambient pressure
and temperature. The detection of magnetic force is a promising method in
studying optical magnetic transitions that can be the basis of innovative
spectroscopy applications.Comment: 9 pages, 10 figure
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