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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 Reviewed

5 páginas, 4 figuras.-- El pdf del artículo es la versión pre-print.-- et al. ; We describe transmission-mode scattering-type near-field optical microscopy (s-SNOM) with interferometric detection. Using this technique, we map the near-field modes of infrared antennas in both amplitude and phase. The use of dielectric probing tips, higher-harmonic demodulation, and a complex-valued subtraction of residual background yields accurate near-field images of the antenna modes. We map metallic nanorods, disks, and triangles, designed for antenna resonance at mid-infrared frequencies, in good agreement with numerical calculations of the modal field distribution. Furthermore, we show that transmission-mode s-SNOM can map the z-component of the antenna near fields. Our results establish a basis for future near-field characterization of complex antenna structures for molecular sensing and spectroscopy. ; Supported by the Etortek program of the Department of Industry of the Basque Government and the European FP7 project "Nanoantenna" (Health-F5-2009- 241818). ; Peer reviewed

7 páginas, 4 figuras, 1 esquema.-- PACS numbers 73.20.Mf, 78.30.cd, 82.33.Hk. ; We report the synthesis, characterization and modellization of optical anion sensors based on gold nanoparticles (Au NPs) stabilized by amino-functional imidazolium ionic liquids (AFIL). The addition of different salts results in anion exchange of the imidazolium ionic liquid stabilizer leading to a change in the optical response of the original nanoparticle aqueous solution. In all cases except with dodecylbenzenesulfonic acid sodium salt, a sufficient amount of salt concentration (5 times larger than equimolar) leads to the appearance of an absorption band between 600 and 700 nm in the ultravioletvisible (UV-vis) spectrum. The presence of the salt produces aggregation of the particles that localise the optical response and produce a large spectral red shift. Transmission electron microscopy images demonstrated that this optical change was due to the aggregation of the nanoparticles. We simulate the optical response of both situations, before and after salt addition, and interpret the experimental observations in terms of the different response of metallic single nanoparticles and nanoparticle aggregates. Theoretical calculations for single nanoparticle and single nanoparticle dimers demonstrate that the colour change is not due to the enlargement or structural changes of the Au NPs, but due to the formation of NP aggregation. These results show the potential of nanoparticle plasmonics to perform effective chemical sensing. ; The authors acknowledge financial supports from the Department of Industry of the Government of the Basque Country (Eusko Jaurlaritza) and the Department of innovation of the Diputación Foral de Gipuzkoa through the ETORTEK project NANOTRON and inanoGUNE. ; Peer reviewed

We identify weak and strong coupling regimes between a near-field probing tip and a plasmonic sample by imaging plasmon-resonant gold nanodisks with scattering-type scanning near-field optical microscopy (s-SNOM). By means of rigorous electrodynamical calculations based on a model system, we find that in the weak coupling regime, s-SNOM can be applied for direct mapping of plasmonic nanoantenna modes, while in the strong coupling regime, the near-field probe allows for high-precision opto-mechanical control of the antenna response. © 2009 The American Physical Society. ; We acknowledge P. Hanarp and D. S. Sutherland for provision of the disk samples, and financial support from the project ETORTEK-2008 of the Department of Industry of the Basque Government and project FIS2007-66711-C01-01 of the Spanish Ministry of Education and Science. ; Peer Reviewed

4 pages, 4 figures.-- PACS nrs.: 78.67.Lt, 33.70.-w, 73.20.Mf, 78.30.-j. ; A novel resonant mechanism involving the interference of a broadband plasmon with the narrowband vibration from molecules is presented. With the use of this concept, we demonstrate experimentally the enormous enhancement of the vibrational signals from less than one attomol of molecules on individual gold nanowires, tailored to act as plasmonic nanoantennas in the infrared. By detuning the resonance via a change in the antenna length, a Fano-type behavior of the spectral signal is observed, which is clearly supported by full electrodynamical calculations. This resonant mechanism can be a new paradigm for sensitive infrared identification of molecular groups. ; Acknowledgements for financial support are for F. N. and A. P. from the DFG (DFG Pu 193/9), for S. K. from the Higher Education Commission (HEC), Pakistan, and for J. A. from the ETORTEK project of the Government of the Basque Country. ; Peer reviewed

This work is licensed under a Creative Commons Attribution-NonCommercial-Share Alike 3.0 Unported License.-- et al. ; Light scattering at nanoparticles and molecules can be dramatically enhanced in the 'hot spots' of optical antennas, where the incident light is highly concentrated. Although this effect is widely applied in surface-enhanced optical sensing, spectroscopy and microscopy, the underlying electromagnetic mechanism of the signal enhancement is challenging to trace experimentally. Here we study elastically scattered light from an individual object located in the well-defined hot spot of single antennas, as a new approach to resolve the role of the antenna in the scattering process. We provide experimental evidence that the intensity elastically scattered off the object scales with the fourth power of the local field enhancement provided by the antenna, and that the underlying electromagnetic mechanism is identical to the one commonly accepted in surface-enhanced Raman scattering. We also measure the phase shift of the scattered light, which provides a novel and unambiguous fingerprint of surface-enhanced light scattering. © 2012 Macmillan Publishers Limited. All rights reserved. ; This study was supported by the European FP7 project 'Nanoantenna' (FP7-HEALTH-F5-2009-241818-NANOANTENNA), the ERC Starting grant no. 258461 (TERATOMO), the National Projects MAT2009-08398 and FIS2010-19609-C02-C01 from the Spanish Ministerio de Ciencia e Innovacion and the Etortek-2011 project 'nanoiker' of the Department of Industry of the Basque Government. M.S. and L.A. acknowledges financial support from 'Programa de Formación de Personal Investigador' promoted by the Department of Education, Universities and Research of the Basque Government. A.G.-E. was supported by 'Programa de perfeccionamiento de doctores y doctoras en el extranjero del Departamento de Educación, Universidades e Investigación del Gobierno Vasco'. ; Peer Reviewed

Spin-orbit interaction of light can lead to the so-called optical mirages, i.e., a perceived displacement in the position of a particle due to the spiraling structure of the scattered light. In electric dipoles, the maximum displacement is subwavelength and does not depend on the optical properties of the scatterer. Here we will show that the optical mirage in high refractive index dielectric nanoparticles depends strongly on the ratio between electric and magnetic dipolar responses. When the dual symmetry is satisfied (at the first Kerker condition), there is a considerable enhancement (far above the wavelength) of the spin-orbit optical mirage which can be related to the emergence of an optical vortex in the backscattering direction. ; This research was supported by the Spanish Ministerio de Economía y Competitividad (MICINN) and European Regional Development Fund (ERDF) Projects No. FIS2014-55987-P, No. FIS2015-69295-C3-3-P, and No. FIS2017-82804-P, by the Basque Dep. de Educación Project No. PI-2016-1-0041 and by the Basque Government ELKARTEK program (KK-2016/00030, KK-2017/00089). A.G.-E. received funding from the Fellows Gipuzkoa fellowship of the Gipuzkoako Foru Aldundia through FEDER "Una Manera de hacer Europa."

We study the non-trivial phase of the two-dimensional breathing kagome lattice, displaying both edge and corner modes. The corner localized modes of a two-dimensional flake were initially identified as a signature of a higher-order topological phase but later shown to be trivial for perturbations that were thought to protect them. Using various theoretical and simulation techniques, we confirm that it does not display higher-order topology the corner modes are of trivial nature. Nevertheless, they might be protected. First, we show a set of perturbations within a tight-binding model that can move the corner modes away from zero energy, also repeat some perturbations that were used to show that the modes are trivial. In addition, we analyze the protection of the corner modes in more detail and find that only perturbations respecting the sublattice or generalized chiral and crystalline symmetries, and the lattice connectivity, pin the corner modes to zero energy robustly. A destructive interference model corroborates the results. Finally, we analyze a muffin-tin model for the bulk breathing kagome lattice. Using topological and symmetry markers, such as Wilson loops and Topological Quantum Chemistry, we identify the two breathing phases as adiabatically disconnected different obstructed atomic limits. ; The work of M.A.J.H. and D.B. is supported by the Ministerio de Ciencia e Innovación (MICINN) through Project No. PID2020-120614GB-I00, and by the Transnational Common Laboratory QuantumChemPhys (D.B.). A.G.E. and M.B.P. acknowledge support from the Spanish Ministerio de Ciencia e Innovación (Project No. PID2019-109905GA-C2) and from Eusko Jaurlaritza (Grants No. IT1164-19 and No. KK-2021/00082). A.G.-E. and D.B. acknowledge Programa Red Guipuzcoana de Ciencia, Tecnología e Innovación 2021, Grant No. 2021-CIEN-000070-01, Gipuzkoa Next. A.G.-E., M.B.P., and D.B. acknowledge funding from the Basque Government's IKUR initiative on Quantum technologies (Department of Education). I.S. gratefully acknowledges financial ...

Time Reversal Symmetry (TRS) broken topological phases provide gapless surface states protected by topology, regardless of additional internal symmetries, spin or valley degrees of freedom. Despite the numerous demonstrations of 2D topological phases, few examples of 3D topological systems with TRS breaking exist. In this article, we devise a general strategy to design 3D Chern insulating (3D CI) cubic photonic crystals in a weakly TRS broken environment with orientable and arbitrarily large Chern vectors. The designs display topologically protected chiral and unidirectional surface states with disjoint equifrequency loops. The resulting crystals present the following characteristics: First, by increasing the Chern number, multiple surface states channels can be supported. Second, the Chern vector can be oriented along any direction simply changing the magnetization axis, opening up larger 3D CI/3D CI interfacing possibilities as compared to 2D. Third, by lowering the TRS breaking requirements, the system is ideal for realistic photonic applications where the magnetic response is weak. ; A.G.E., C.D. and M.B.P. acknowledge support from the Spanish Ministerio de Ciencia e Innovación (PID2019-109905GA-C2) and from Eusko Jaurlaritza (IT1164-19, KK-2019/00101 and KK-2021/00082). M.G.D., I.R. and M.G.V. acknowledge the Spanish Ministerio de Ciencia e Innovacion (grant PID2019-109905GB-C21). J.L.A. acknowledges support from the Spanish Ministerio de Ciencia e Innovación (PID2019-109905GA-C2). The work of B.B. is supported by the Air Force Office of Scientific Research under award number FA9550-21-1-0131. C.D. acknowledges financial support from the MICIU through the FPI PhD Fellowship CEX2018-000867-S-19-1. The work of J.L.M. has been supported by Spanish Science Ministry grant PGC2018-094626-BC21 (MCIU/AEI/FEDER, EU) and Basque Government grant IT979-16. A.G.E. and M. G. V. acknowledge funding from Programa Red Guipuzcoana de Ciencia, Tecnología e Innovación 2021 (Grant Nr. 2021-CIEN-000070-01. Gipuzkoa Next). ; Peer reviewed