44 research outputs found
A combination of concave/convex surfaces for field-enhancement optimization: the indented nanocone
We introduce a design strategy to maximize the Near Field (NF) enhancement near plasmonic antennas. We start by identifying and studying the basic electromagnetic effects that contribute to the electric near field enhancement. Next, we show how the concatenation of a convex and a concave surface allows merging all the effects on a single, continuous nanoantenna. As an example of this NF maximization strategy, we engineer a nanostructure, the indented nanocone. This structure, combines all the studied NF maximization effects with a synergistic boost provided by a Fano-like interference effect activated by the presence of the concave surface. As a result, the antenna exhibits a NF amplitude enhancement of ∼ 800, which transforms into ∼1600 when coupled to a perfect metallic surface. This strong enhancement makes the proposed structure a robust candidate to be used in field enhancement based technologies. Further elaborations of the concept may produce even larger and more effective enhancements.This work was supported by the Etortek-2011 project nanoiker of the Department of Industry of the Basque Government, project FIS2010-19609-C02-01 of the Spanish Ministry of Science and Innovation and the Swedish Foundation for Strategic Research through the project RMA08 Functional Electromagnetic Metamaterials.Peer Reviewe
Optical mirages from spinless beams
Spin-orbit interactions of light are ubiquitous in multiple branches of
nanophotonics, including optical wave localization. In that framework, it is
widely accepted that circularly polarized beams lead to spin-dependent apparent
shifts of dipolar targets commonly referred to as optical mirages. In contrast,
these optical mirages vanish when the illumination comes from a spinless beam
such as a linearly polarized wave. Here we show that optical localization
errors emerge for particles sustaining electric and magnetic dipolar response
under the illumination of spinless beams. As an example, we calculate the
optical mirage for the scattering by a high refractive index nanosphere under
the illumination of a linearly polarized plane wave carrying null spin,
orbital, and total angular momentum. Our results point to an overlooked
interference between the electric and magnetic dipoles rather than the
spin-orbit interactions of light as the origin for the tilted position of the
nanosphere
On the origin of the Kerker phenomena
We provide an insight into the origin of the phenomena reported 40 years ago
by Kerker, Wang and Giles (Journal of the Optical Society of America, 73, 6,
pp. 765-767, (1983)). We show that the impedance and refractive index matching
conditions, discussed in Sections II and IV of the seminal paper, are
intimately related with space-time symmetries. We derive our results starting
from the theory of representations of the Poincar\'e group, as it is the theory
on which one of the most elemental descriptions of electromagnetic waves is
based. We show that fundamental features of electromagnetic waves in material
environments can be derived from group theoretical arguments. In particular, we
identify the Casimir invariants of subgroup as
the magnitudes which describe the nature of monochromatic electromagnetic waves
propagating in matter. Finally, we show that the emergence of the Kerker
phenomena is associated with the conservation of such Casimir invariants in
piecewise homogeneous media
Transversality-Enforced Tight-Binding Model for 3D Photonic Crystals aided by Topological Quantum Chemistry
Tight-binding models can accurately predict the band structure and topology
of crystalline systems and they have been heavily used in solid-state physics
due to their versatility and low computational cost. It is quite
straightforward to build an accurate tight-binding model of any crystalline
system using the maximally localized Wannier functions of the crystal as a
basis. In 1D and 2D photonic crystals, it is possible to express the wave
equation as two decoupled scalar eigenproblems where finding a basis of
maximally localized Wannier functions is feasible using standard Wannierization
methods. Unfortunately, in 3D photonic crystals, the vectorial nature of the
electromagnetic solutions cannot be avoided. This precludes the construction of
a basis of maximally localized Wannier functions via usual techniques. In this
work, we show how to overcome this problem by using topological quantum
chemistry which will allow us to express the band structure of the photonic
crystal as a difference of elementary band representations. This can be
achieved by the introduction of a set of auxiliary modes, as recently proposed
by Solja\v{c}i\'c et. al., which regularize the -point obstruction
arising from transversality constraint of the Maxwell equations. The
decomposition into elementary band representations allows us to isolate a set
of pseudo-orbitals that permit us to construct an accurate
transversality-enforced tight-binding model (TETB) that matches the dispersion,
symmetry content, and topology of the 3D photonic crystal under study.
Moreover, we show how to introduce the effects of a gyrotropic bias in the
framework, modeled via non-minimal coupling to a static magnetic field. Our
work provides the first systematic method to analytically model the photonic
bands of the lowest transverse modes over the entire BZ via a TETB model.Comment: 3 figure
Kerker Conditions Upon Lossless, Absorption, and Optical Gain Regimes
The directionality and polarization of light show peculiar properties when
the scattering by a dielectric sphere can be described exclusively by electric
and magnetic dipolar modes. Particularly, when these modes oscillate in-phase
with equal amplitude, at the so-called first Kerker condition, the zero optical
backscattering condition emerges for non-dissipating spheres. However, the role
of absorption and optical gain in the first Kerker condition remains
unexplored. In this work, we demonstrate that either absorption or optical gain
precludes the first Kerker condition and, hence, the absence of backscattered
radiation light, regardless of the size of the particle, incident wavelength,
and incoming polarization. Finally, we derive the necessary prerequisites of
the second Kerker condition of the zero forward light scattering, finding that
optical gain is a compulsory requirement
Optical polarization Möbius strips on all-dielectric optical scatterers
In this article, we study the emergence of polarization singularities in the scattering of optical resonators excited by linearly polarized light. First, we prove analytically that spherical all-dielectric nanoparticles described by combinations of electric and magnetic isotropic polarizabilities can sustain L surfaces and C lines that propagate from the near-field to the far field. Based on these analytical results, we are able to derive anomalous scattering Kerker conditions using singular optics arguments. Next, through full-field calculations, we demonstrate that high refractive index spherical resonators present such topologically protected features. We calculate the polarization structure of light around the generated C lines, unveiling a Möbius strip structure in the main axis of the polarization ellipse when calculated on a closed path around the C line. These results prove that high-index nanoparticles are excellent candidates for the generation of polarization singularities and that they may lead to new platforms for the experimental study of the topology of light fields around optical antennas.A.G.-E. received funding from the Fellows Gipuzkoa fellowship of the Gipuzkoako Foru Aldundia through FEDER “Una Manera de hacer Europa”.Peer Reviewe
Modelization of plasmonic nanoantennas for optical microscopy and surface enhanced spectroscopy
122 páginas, 48 figuras.This thesis presents a set of contributions to the field of nanophotonics which are based
on the potential of plasmonic nanoantennas to enhance spectroscopy and microscopy
techniques.
The introductory chapter presents historical approach to electromagnetic antennas
and the differences and new physical phenomena that arise when trying to adapt those
resonant structures to the visible and infrared frequency range of the spectrum. The
potential of these structures and several emerging applications are also introduced.
On Chapter 2 we demonstrate how infrared vibration signals of a small number of
molecules can be enormously enhanced by 5 orders of magnitude through the resonant
interaction of the vibration dipoles of the molecules with the broadband IR plasmonic
antenna resonance of a gold nanowire. This interaction can be interpreted as classical
analogy to the antiresonances produced in the Fano effect. This enhancement
mechanism enables a new powerful technique for surface enhanced IR scattering with
general importance for a variety of fields. By exploiting the resonant enhancement
in the vibrational spectra, as shown in this work, it has been possible to detect and
to study extremely small quantities of molecules, thus lowering the detection limit of direct IR vibration spectroscopy considerably, establishing a new paradigm in SEIRA
spectroscopy.
Chapter 3 summarizes the electromagnetic effects that can be used to optimize the
near-field enhancement of metallic nanostructures. Furthermore, we show how it is
possible to combine all these effects on a single nanostructure to design a single geometry
where the field enhancement reaches the largest values reported in the literature.
The designed structure presents a field enhancement of 3 orders of magnitude in amplitude
at the near IR part of the spectrum. To obtain these factors realistic values for
the dielectric response and damping of the metals have been used, therefore this type
of factors could be reached experimentally in principle.
On Chapter 4, we introduce a theoretical approach to the scattering type scanning
near field optical microscopy technique (s-SNOM) and we address quantitatively the
influence of the tip in the near-field imaging of plasmonic structures, establishing the
weak and strong near-field coupling regimes between probe and antennas. Based on
rigorous numerical calculations and on a simple interaction scheme, we have interpreted
correctly different near-field images of similar plasmonic disks.
Finally, Chapter 5 presents two methods to control the antenna response of plasmonic
nanostructures. First, we show how the strong tip sample interaction regime
presented on the previous chapter introduces the capability for a near-field probe to
control the far field and near field response of a plasmonic antenna with a precision in
the sub-nanometer range, which is otherwise not yet accessible with current fabrication
technology. Moreover, we show how modifying the gap impedance of gap antennas
allows to establish a control over their near field and far field response.Peer reviewe