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 $P_{\scriptscriptstyle{{3,1}}}$ 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 $\Gamma$-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