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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.

Electromagnetic field confinement on sub-wavelength scales is possible at the expense of increasing optical losses. Examples are surface plasmon polaritons and resonant structures, which suffer from absorption losses and radiative outcoupling, respectively. Here, the first experimental evidence of strong electromagnetic field confinement along the lateral (out-of-plane) direction in a near lossless metasurface that supports a symmetry-protected Bound State in the Continuum is shown, which fully suppresses the radiative losses of resonant scatterers. This experimental demonstration, supported by coupled dipole calculations, is measured by a double near-field probe technique to locally excite and detect the time evolution and confinement of the electric field onto the surface, achieving an amplitude decay length from the metasurface of λ/38 at 0.39 THz.

8 pags., 5 figs., 1 tab., Various optical phenomena can be induced in periodic arrays of nanoparticles by the radiative coupling of the local dipoles in each particle. Probably the most impressive example is bound states in the continuum (BICs), which are electromagnetic modes with a dispersion inside the light cone but infinite lifetime, that is, modes that cannot leak to the continuum. Symmetry-protected BICs appear at highly symmetric points in the dispersion of periodic systems. Although the addition of nonequivalent lattice points in a unit cell is an easy and straightforward way of tuning the symmetry, BICs in such particle lattice, that is, non-Bravais lattice, are less explored among periodic systems. Starting from a periodic square lattice of Si nanodisks, three non-Bravais lattices are prepared by detuning size and position of the second disk in the unit cell. Diffraction-induced coupling excites magnetic/electric dipoles in each nanodisk, producing two surface lattice resonances at the Γ point with a band gap in between. The high/low energy branch becomes a BIC for the size/position-detuned array, respectively, while both branches are bright (or leaky) when both size and position are detuned simultaneously. The unexplored role of the interplay between magnetic and electric dipoles in dielectric nanoparticles in connection with the change of BIC to bright character in the detuned arrays is discussed with the aid of a coupled electric and magnetic dipole model. This study gives a simple way of tuning BICs at telecom wavelengths in non-Bravais lattices, including both plasmonic and dielectric systems, thus scalable to a wide range of frequencies., This work was partly supported by the Nanotechnology Hub, Kyoto University (JPMXP09 F19NMC042) and Kitakyusyu FAIS in the “Nanotechnol-ogy Platform Project,” sponsored by MEXT, Japan. The authors gratefully acknowledge the financial support from Kakenhi (21H04619, 22H01776),JSPS Bilateral Program (JPJSBP120219920), and the Netherlands Organisation for Scientific Research (NWO) through Gravitation grant “ResearchCentre for Integrated Nanophotonics” and Innovational Research Activities Scheme (Vici project SCOPE no. 680-47-628). J.A.S.-G. and D.R.A. ac-knowledge support from the Spanish Ministerio de Ciencia e Innovaciónthrough grants MELODIA PGC2018-095777-B-C21 and FPU Ph.D. Fellow-ship FPU15/03566 (MCIU/AEI/FEDER, UE)

Bound states in the continuum (BICs) emerge throughout physics as leaky/resonant modes that remain, however, highly localized. They have attracted much attention in photonics, and especially in metasurfaces. One of their most outstanding features is their divergent Q-factors, indeed arbitrarily large upon approaching the BIC condition (quasi-BICs). Here, we investigate how to tune quasi-BICs in magneto-optic (MO) all-dielectric metasurfaces. The impact of the applied magnetic field in the BIC parameter space is revealed for a metasurface consisting of lossless semiconductor spheres with MO response. Through our coupled electric/magnetic dipole formulation, the MO activity is found to manifest itself through the interference of the out-of-plane electric/magnetic dipole resonances with the (MO-induced) in-plane magnetic/electric dipole, leading to a rich, magnetically tuned quasi-BIC phenomenology, resembling the behavior of Brewster quasi-BICs for tilted vertical-dipole resonant metasurfaces. Such resemblance underlies our proposed design for a fast MO switch of a Brewster quasi-BIC by simply reversing the driving magnetic field. This MO-active BIC behavior is further confirmed in the optical regime for a realistic Bi:YIG nanodisk metasurface through numerical calculations. Our results present various mechanisms to magneto-optically manipulate BICs and quasi-BICs, which could be exploited throughout the electromagnetic spectrum with applications in lasing, filtering, and sensing., Financial support is acknowledged from the Spanish Ministerio de Ciencia e Innovación (MICIU/AEI/FEDER,UE) through the grants PGC2018- 095777-B-C21 (MELODIA) and PID2019-109905GA-C22, and from the Ministerio de Educación, Cultura y Deporte through a PhD Fellowship (FPU15/03566). J.-M. Llorens acknowledges funding from MINECO RYC-2017-21995, PID2019-106088RB-C31 and support from CSIC Research Platform PTI-001.

Confining electromagnetic fields as tightly as possible has driven research in optics and photonics. Conventionally, extreme sub-wavelength confinement is possible at the cost of absorption losses in resonant structures. Here, we experimentally show that it is possible to get an extreme confinement of the electromagnetic field in a resonant system without losses. We exploit symmetry protected bound states in the continuum which fully suppress any radiative losses. Using our unique local excitation and detection terahertz microscope we can measure the confinement of the electric field at the surface of our array, revealing an amplitude decay of λ/38 at 0.39 THz.

12 pags., 6 figs., Spin and orbital angular momenta of light have been a subject of fundamental interest since long ago, classically associated with circular polarization and wave vector. In recent years, extraordinary spin angular momenta in structured electromagnetic waves have been investigated, mostly in subwavelength evanescent fields at the nanoscale. Here we present an in-depth theoretical analysis of the transverse spin density and related momentum induced by mode confinement inside waveguides, with alternating spin layers governed by guided mode spatial symmetry, different from and indeed richer than that in the evanescent region outside. Furthermore, hybrid guided modes with intrinsic helicity exhibit in addition longitudinal spin density. Such fundamental features are manifested through fascinating phenomenology relevant to spin-orbit coupling in nanophotonic waveguides. Thus, guided light intrinsically carrying a wealth of spin momenta holds promise of superb devices to control spin-orbit interaction within confined geometries throughout the electromagnetic spectra., This work has been supported by the Spanish Ministerio de Ciencia, Innovacion y Universidades (MCIU/AEI/FEDER, ́ UE) through Grant Nos. MELODIA PGC2018-095777-B-C21 and NANOTOPO FIS2017-91413-EXP (MCIU/AEI/ FEDER, UE), and through the FPU PhD Fellowship FPU15/03566.

7 pags., 7 figs., Bound states in the continuum (BICs) represent a new paradigm in photonics due to the full suppression of radiation losses. However, this suppression has also hampered the direct observation of them. By using a double terahertz (THz) near-field technique that allows the local excitation and detection of the THz amplitude, we are able to map for the first time the electromagnetic field amplitude and phase of BICs over extended areas, unveiling the field-symmetry protection that suppresses the far-field radiation. This investigation, done for metasurfaces of dimer scatterers, reveals the in-plane extension and formation of BICs with antisymmetric phases, in agreement with coupled-dipole calculations. By displacing the scatterers, we show experimentally that a mirror symmetry is not a necessary condition for a BIC formation. Only π-rotation symmetry is required, making BICs exceptionally robust to structural changes. This work makes the local field of BICs experimentally accessible, which is crucial for the engineering of cavities with infinite lifetimes., The authors thank K. de Mare for help in the development of the setup and Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) (Vici 680-47-628); Spanish Ministerio de Ciencia e Innovación (MICIU/AEI/FEDER, UE) through the grants MELODIA (PGC2018-095777−B-C21) and NANOTOPO (FIS2017-91413-EXP), and Ministerio de Educación, Cultura y Deporte through a PhD Fellowship (FPU15/03566) for funding.

11 pags., 4 figs., In recent years, optical forces and torques have been investigated in sub-wavelength evanescent fields yielding a rich phenomenology of fundamental and applied interest. Here we demonstrate analytically that guided modes carrying transverse spin density induce optical torques depending on the character, either electric or magnetic, of the dipolar particles. The existence of a nonzero longitudinal extraordinary linear spin momentum suitable to manipulate optical forces and torques modifies optical forces either enhancing or inhibiting radiation pressure. Hybrid modes supported by cylindrical waveguides also exhibit intrinsic helicity that leads to a rich distribution of longitudinal optical torques. Finally, we show that chiral dipolar particles also undergo lateral forces induced by transverse spin density, amenable to chiral particle sorting. These properties are revealed in configurations on achiral and chiral dipolar particles within confined geometries throughout the electromagnetic spectra., Ministerio de Ciencia, Innovación y Universidades (CEX2018-000805-M, FPU15/03566, PGC2018-095777- B-C21, PGC2018-095777-B-C22, UAM-CAM project SI1/PJI/2019-00052).

Resumen del trabajo presentado a la Conferencia Española de Nanofotónica (CEN), celebrada en Vigo del 20 al 22 de septiembre de 2021.

10 pags., 4 figs., 2 apps.-- The authors dedicate this work to the memory of their beloved colleague and friend, Prof. Juan J. Sáenz, who passed away on March 22, 2020., The optical properties of infinite planar array of scattering particles, metasurfaces and metagratings, are attracting special attention lately for their rich phenomenology, including both plasmonic and high-refractive-index dielectric meta-atoms with a variety of electric and magnetic resonant responses. Herein we derive a coupled electric and magnetic dipole analytical formulation to describe the reflection and transmission of such periodic arrays, including specular and diffractive orders, valid in the spectral regimes where only dipolar multipoles are needed. The two-dimensional lattice Green function is rewritten in terms of a one-dimensional (chain) version that fully converges in the complex frequency plane and can be easily calculated. Modes emerging as poles of such lattice Green function can be extracted, as evidenced by calculating resonances and bound states in the continuum for an array of Si spheres. This formulation can be applied to investigate a wealth of plasmonic, all-dielectric, and hybrid metasurfaces and metagratings of interest throughout the electromagnetic spectrum., This work has been supported by the Spanish Ministerio de Ciencia e Innovación (MICIU/AEI/FEDER, UE) through the grants MELODIA (Grant No. PGC2018-095777-B-C21) and NANOTOPO (Grant No. FIS2017-91413-EXP), and from the Ministerio de Educación, Cultura y Deporte through a Ph.D. Fellowship (FPU15/03566).

7 pags., 4 figs., Solid-state room-temperature lasing with tunability in a wide range of wavelengths is desirable for many applications. To achieve this, besides an efficient gain material with a tunable emission wavelength, a high quality-factor optical cavity is essential. Here, we combine a film of colloidal CdSe/CdZnS core-shell nanoplatelets with square arrays of nanocylinders made of titanium dioxide to achieve optically pumped lasing at visible wavelengths and room temperature. The all-dielectric arrays support bound states in the continuum (BICs), which result from lattice-mediated Mie resonances and boast infinite quality factors in theory. In particular, we demonstrate lasing from a BIC that originates from out-of-plane magnetic dipoles oscillating in phase. By adjusting the diameter of the cylinders, we tune the lasing wavelength across the gain bandwidth of the nanoplatelets. The spectral tunability of both the cavity resonance and nanoplatelet gain, together with efficient light confinement in BICs, promises low-threshold lasing with wide selectivity in wavelengths., This work was supported by the A*STAR SERC Pharos programme (grant number 152 73 00025; Singapore). D.R.A. and J.A.S.-G. acknowledge support from the Spanish Ministerio de Ciencia e Innovacion (NANOTOPO FIS2017-91413-EXP, ́ MELODIA PGC2018-095777-B-C21, and a FPU Ph.D. Fellowship FPU15/03566, MCIU/AEI/FEDER, UE). H.V.D. gratefully acknowledges support from TUBA. The authors acknowledge Vytautas Valuckas (IMRE, A*STAR) for SEM characterization.

9 pags., 4 figs., Increasing the sensitivity of chiral spectroscopic techniques such as circular dichroism (CD) spectroscopy is a current aspiration in the research field of nanophotonics. Enhancing CD spectroscopy depends on two complementary requirements: the enhancement of the electromagnetic fields perceived by the molecules under study and the conservation of the helicity of those fields, guaranteed by duality symmetry. In this work, we introduce a systematic method to design nanostructured dual periodic photonic systems capable of enhancing molecular CD spectroscopy resonantly. As an illustration, we engineer a dual 1D silicon nanoparticle array and show that its collective optical modes can be efficiently employed to resonantly enhance by 2 orders of magnitude the local density of optical chirality and, thus, the CD signal obtained from a given molecular sample on its vicinity., J.L.A., J.J.S., and A.G.E. acknowledge the PID2019-109905GAC22 Project of the Spanish Ministerio de Ciencia, Innovacion y Universidades (MICIU). D.R.A. acknowledges partial financial support from Grant No. MELODIA PGC2018- 095777-B-C21 of MICIU and from the Ministerio de Educación, Cultura y Deporte through the FPU Ph.D. Fellowship FPU15/03566. A.N. acknowledges finacial support from the MICIU through the FPI Ph.D. Fellowship BES-2017- 080073. G.M.T. acknowledges the FIS2017-87363-P Project of the Spanish MINECO. A.G.E. and J.A. acknowledge the FIS2016-80174-P Project of the Spanish MINECO. A.G.E. received funding from the Gipuzkoako Foru Aldundia OF23/ 2019 (ES) Project, “Fellows Gipuzkoa” Fellowship through FEDER “Una Manera de hacer Europa”, and by Eusko Jaurlaritza Grant Numbers KK-2017/00089, IT1164-19, PI2016-41, and KK-2019/00101.

6 pags., 4 figs., The near-zero forward intensity condition for light scattering by a dielectric dipolar sphere is usually associated with the generalized second Kerker condition, at which equal amplitude electric and magnetic dipolar responses are phase-shifted by π. As we show, this condition does lead to optimal backward light scattering for a given scattering cross section. However, it is clearly insufficient to give rise to the nearly zero optical forward scattering, in striking contrast to the actual view of the problem. In fact, we show that the generalized second Kerker condition leads to an energy radiation pattern that ranges all possible optical scattering diagrams depending on the total scattering cross section. Interestingly, we demonstrate that optimization of backward intensity, near the electric and magnetic dipolar resonances, leads to the counterintuitive result of a far-field energy radiation pattern with nearly zero backscattering., This research was supported by the Basque Government (Project PI-2016-1-0041 and PhD Fellowship PRE-2018- 2-0252) and by the Spanish MINECO and MICINN and European Regional Development Fund (ERDF) Projects: FIS2015-69295-C3-2-P, FIS2015-69295-C3-3-P, FIS2017- 91413-EXP, FIS2017-82804-P, PGC2018-095777-B-C21, and PhD Fellowship FPU15/03566.

7 pags., 6 figs., Bound states in the continuum (BICs) are electromagnetic modes with a dispersion inside the light cone and infinite lifetimes. This exceptional property has led to intensive research and the demonstration of BICs in the gigahertz, teraherz, and near-infrared, up to the visible region. In this study, we design and experimentally demonstrate optical BICs using a subdiffraction lattice of Si nanodisks. The out-of-plane magnetic dipole resonance in the dielectric nanodisks couples with the subdiffraction lattice and defines a symmetry-protected BIC at normal incidence. This mode becomes a quasi-BIC as the angle of incidence is increased. The spectral position of the BIC can be controlled with the diameter of the nanodisks, which governs the out-of-plane magnetic dipole. The investigated BIC is robust to shape irregularities of the individual nanodisks. This robustness makes the design strategy of the BICs presented here very attractive for applications in which high fabrication precision can not be achieved., We thank K. Agata and M. Higashino (Kyoto U., Japan) for the RTA and optical measurements. This work was partly supported by the Nanotechnology Hub, Kyoto University and Kitakyusyu FAIS in the “Nanotechnology Platform Project,” sponsored by MEXT, Japan. We gratefully acknowledge the financial support from Kakenhi (17KK0133, 19H02434) and The Netherlands Organisation for Scientific Research (NWO) through Gravitation Grant “Research Centre for Integrated Nanophotonics” and Innovational Research Activities Scheme (Vici Project SCOPE No. 680-47-628). J.A.S.-G. and D.R.A. acknowledge support from the Spanish Ministerio de Ciencia e Innovacion through Grants NANOTOPO FIS2017-91413- ́ EXP, MELODIA PGC2018-095777-B-C21, and FPU Ph.D. Fellowship FPU15/03566 (MCIU/AEI/FEDER, UE)

6 pags., 4 figs., The ability to tailor waveguide cavities and couple them with quantum emitters has developed a realm of nanophotonics encompassing, for example, highly efficient single photon generation or the control of giant photon nonlinearities. Opening new grounds by pushing the interaction of the waveguide cavity and integrated emitters further into the deep subwavelength regime, however, has been complicated by nonradiative losses due to the increasing importance of surface defects when decreasing cavity dimensions. Here, we show efficient suppression of nonradiative recombination for thin waveguide cavities using core-shell semiconductor nanowires. We experimentally reveal the advantages of such nanowires, which host mobile emitters, that is, free excitons, in a one-dimensional (1D) waveguide, highlighting the resulting potential for tunable, active, nanophotonic devices. In our experiment, controlling the nanowire waveguide diameter tunes the luminescence lifetime of excitons in the nanowires across 2 orders of magnitude up to 80 ns. At the smallest wire diameters, we show that this luminescence lifetime can be manipulated by engineering the dielectric environment of the nanowires. Exploiting this unique handle on the spontaneous emission of mobile emitters, we demonstrate an all-dielectric spatial control of the mobile emitters along the axis of the 1D nanowire waveguide., We gratefully acknowledge financial support by the German Research Foundation (DFG) via SFB 689 and 1277 Project A01. J.A.S.-G. and D.R.A. acknowledge partial financial support from the Spanish Ministerio de Ciencia, Innovación y Universidades through Grants NANOTOPO (FIS201791413-EXP) and MELODIA (PGC2018-095777-B-C21, MCIU/AEI/FEDER, UE), and through the FPU Ph.D.Fellowship FPU15/03566.

Metasurfaces consisting of two metallic rods per unit-cell in a lattice can support resonances at THz frequencies with infinitely long lifetime. These resonances are known as bound states in the continuum (BICs). We investigate theoretically and experimentally the conditions leading to the formation of these resonances in gold particle arrays.

6 pags., 5 figs., Photonic bound states in the continuum (BICs) are protected eigenstates in optical systems with infinite lifetimes.This unique property, which translates in infinite Q-factor resonances, makes BICs extremely interesting not only from a fundamental perspective but also for various applications such as lasing and sensing. General means to achieve robust BICs are, however, elusive. Here we demonstrate analytically that BICs emerge in metasurfaces formed by arrays of detuned resonant dipolar dimers as a universal behavior occurring regardless of both dipole position within the unit cell and lattice constant in the nondiffracting regime. These resonances evolve continuously from a Fano resonance into a symmetry-protected BIC as the dipole detuning vanishes. We have experimentally verified this very robust response at terahertz frequencies through dimer rod arrays with different rod sizes by simultaneously measuring the reduction of linewidth and the increase of lifetime before the BIC is formed, as it is impossible to couple to it from the continuum. Similar configurations can be straightforwardly envisioned throughout the electromagnetic spectrum, enabling a simple geometry that is easy to fabricate with resonances of arbitrarily high Q factors., Ministerio de Economía, Industria y Competitividad, Gobierno de España (FIS2015-69295-C3-2-P, FIS2017-91413- EXP); Ministerio de Educación, Cultura y Deporte (MECD) (FPU15/03566); Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) (680-47-628); Ministerio de Ciencia, Innovación y Universidades (PGC2018-095777-B-C21).

6 pags., 6 figs.-- This paper is part of the Special Topic on Dielectric Nanoresonators and Metamaterials., All dielectric metasurfaces are attracting great attention for their potential to control light reflection and transmission at the nanoscale. Here, we make use of a coupled electric/magnetic dipole formulation for a periodic array of high-refractive-index cylinders to analytically explore the emergence of Fano resonances for both s- and p-polarized waves. Strong Fano resonances with large Q-factors are found in Si nanocylinder metasurfaces as a result of the lattice-induced interplay between electric and magnetic dipole resonances, where the narrow resonances are connected to bound states in the continuum supported by the lattice. The impact on the refractive index of the surrounding medium is analyzed, revealing that such simple all dielectric metasurfaces are suitable for refractive-index sensing devices with figures of merit of two orders of magnitude., Financial support from the Spanish Ministerio de Economía, Industria y Competitividad through the grants LENSBEAM (Nos. FIS2015-69295-C3-2-P and FIS2015-69295-C3-3-P), NANOTOPO (No. FIS2017-91413-EXP), and FPU PhD Fellowship (No. FPU15/03566) is acknowledged.

11 pags., 7 figs. -- Open Access funded by Creative Commons Atribution Licence 4.0, Bound states in the continuum (BICs) are ubiquitous in many areas of physics, attracting special interest for their ability to confine waves with infinite lifetimes. Metasurfaces provide a suitable platform to realize them in photonics; such BICs are remarkably robust, being however complex to tune in frequency-wavevector space. Here we propose a scheme to engineer BICs and quasi-BICs with single magnetic-dipole resonance meta-atoms. Upon changing the orientation of the magnetic-dipole resonances, we show that the resulting quasi-BICs, emerging from the symmetry-protected BIC at normal incidence, become transparent for plane-wave illumination exactly at the magnetic-dipole angle, due to a Brewster-like effect. While yielding infinite Q-factors at normal incidence (canonical BIC), these are termed Brewster quasi-BICs since a transmission channel is always allowed that slightly widens resonances at oblique incidences. This is demonstrated experimentally through reflectance measurements in the microwave regime with high-refractive-index mm-disk metasurfaces. Such Brewster-inspired configuration is a plausible scenario to achieve quasi-BICs throughout the electromagnetic spectrum inaccessible through plane-wave illumination at given angles, which could be extrapolated to other kind of waves., J.A.S.G., J.J.S. and D.R.A. acknowledge partial financial support from the Spanish Ministerio de Ciencia, Innovación y Universidades (MCIU/AEI/FEDER, UE) through the grants LENSBEAM (FIS2015-69295-C3-2-P, FIS2015-69295-C3-3-P), MELODIA (PGC2018-095777-B-C21), and NANOTOPO (FIS2017-91413-EXP), and from the Ministerio de Educación, Cultura y Deporte through a PhD Fellowship (FPU15/03566). A.B. wants to express her gratitude to the University of Cantabria for her PhD grant.

9 pags., 6 figs., The light-matter coupling between electromagnetic modes guided by a semiconductor nanowire and excitonic states of molecules localized in its surrounding media is studied from both classical and quantum perspectives, with the aim of describing the strong-coupling regime. Weakly guided modes (bare photonic modes) are found through a classical analysis, identifying those lowest-order modes presenting large electromagnetic fields spreading outside the nanowire while preserving their robust guided behavior. Experimental fits of the dielectric permittivity of an organic dye that exhibits excitonic states are used for realistic scenarios. A quantum model properly confirms through an avoided mode crossing that the strong-coupling regime can be achieved for this configuration, leading to Rabi splitting values above 100 meV. In addition, it is shown that the coupling strength depends on the fraction of energy spread outside the nanowire, rather than on the mode field localization. These results open up a new avenue towards strong-coupling phenomenology involving propagating modes in nonabsorbing media., The authors acknowledge the Spanish Ministerio de Economía, Industria y Competitividad for financial support through Grants No. MAT2014-53432-C5-5-R, No. FIS2015- 69295-C3-2-P, and No. FIS2017-91413-EXP; the María de Maeztu program for Units of Excellence in R&D (MDM2014-0377); and an FPU Fellowship (D.R.A.) and a Ramón y Cajal grant (J.F.). We also acknowledge funding from the European Research Council (ERC-2016-STG-714870).

The conservation of the photon total angular momentum in the incident direction in an axially symmetric scattering process is a very well known fact. Nonetheless, the redistribution of this conserved magnitude into its spin and orbital components, an effect known as the spin-orbit interaction (SOI) of light, is still a matter of active research. Here, we discuss the effect of the absorption on the SOI in the scattering of a subwavelength silicon particle. Describing the scattering process of an electric and a magnetic dipole, we show via the asymmetry parameter that the SOI of light in the scattering of high refractive index nanoparticles endures in the presence of optical losses. This effect results in optical mirages whose maximum values surpass those of an electric dipolar scatterer., This research was supported by the Spanish Ministerio de Economía y Competitividad (MICINN) and European Regional Development Fund (ERDF) Projects (FIS2015- 69295-C3, FIS2016-80174-P, FIS2017-82804-P, FIS2017- 91413-EXP) and FPU PhD Fellowship (FPU15/03566), and by the Basque Government Projects (PI-2016-1- 0041, KK- 2017/00089) and PhD Fellowship (PRE2018-2-0252). AGE received funding from the Fellows Gipuzkoa fellowship of the Gipuzkoako Foru Aldundia through FEDER “Una Manera de hacer Europa”.

8 págs.; 4 figs., We measure the polarization-resolved angular emission distribution from thin and thick GaAs nanowires (diameters ∼110 and ∼180 nm) with cathodoluminescence polarimetry. The nanowires, which horizontally rest on a thin carbon film, are excited by a 5 keV electron beam and emit band gap luminescence at a central wavelength of 870 nm. The emission can couple to different waveguide modes that propagate along the wire, are dependent on the wire diameter, and determine the directionality and polarization of the emission. Although each measured nanowire can support different modes, the polarized emission is dominated by the TM01 waveguide mode in all cases, independently of wire diameter. When exciting the nanowires close to the end facets, the thin and thick wires exhibit opposite directional emission. The emission from thin nanowires is dominated by a leaky TM01 mode that leads to emission toward the opposite end facet (emission to the right when exciting the left-side edge). For the thick wires, however, the TM01 mode is guided but also lossy due to absorption in the substrate. In such a case, the wires emit toward the excited end facet (to the left when exciting the left-side edge). The emission directionality switches for nanowire diameters in the range of 145-170 nm. We show that the measurements agree well with both a simple 1D current model and numerical simulations. The high spatial resolution of angle- and polarization-resolved cathodoluminescence spectroscopy provides detailed insight into the nanoscale emission and propagation of light in semiconductor nanowires. Copyright © 2016 American Chemical Society, This work is part of the Stichting voor Fundamenteel Onderzoek der Materie (FOM) as well as the Dutch Technology Foundation STW, which are financially supported by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) and the Dutch Ministry of Economic Affairs. It is also part of NanoNextNL, a nanotechnology program funded by the Dutch Ministry of Economic Affairs, part of an industrial partnership program between Philips and FOM, and is supported by the European Research Council (ERC). The Spanish Ministerio de Economıía y Competitividad is also acknowledged for financial support through the grants NANOPLAS+ (FIS2012-31070) and LENSBEAM (FIS2015- 69295-C3-2-P).

Artificial optical metamaterial with a zero index of refraction holds promise for many diverse phenomena and applications, which can be achieved with vacuum (or related) surface impedance and materials in the optical domain. Here, we propose simple metal-oxide nanorods as meta-atoms on the basis of an effective medium approach, based on their weak overlapping (electric/magnetic) resonances. We thus studied the optical properties of TiO 2 nanowire arrays with a high-filling fraction through their photonic band structure, which exhibits a double-degeneracy point without a band gap at the center of the Brillouin zone. Various configurations are considered that reveal their performance over a reasonable range of incident wave vectors as impedance-matched, double-zero, bulk (low-loss) metamaterials., This work was supported in part by the Spanish Ministerio de Economía, Industria y Competitividad (projects Consolider-Ingenio EMET CSD2008-000666 and LENSBEAM FIS2015-69295-C3-2-P). We acknowledge helpful discussions with L. S. Froufe-Pérez, J. J. Sáenz, R. Marqués, and M. Nieto-Vesperinas.

The tremendous impact that cardiovascular diseases have in modern society is motivating the research of novel imaging techniques that would make possible early diagnosis and, therefore, efficient treatments. Cardiovascular optical coherence tomography (CV-OCT) emerged as a result of such a demand, and it has already been used at the clinical level. Full utilization of CV-OCT requires the development of novel contrast molecular agents characterized by a large scattering efficiency in the infrared (800-1400 nm). Gold nanoparticles (GNPs) seem to be the best candidates, but their scattering properties in the infrared are hardly known. In most of the cases, scattering properties are extracted from numerical simulations. This knowledge gap here is covered by providing an experimental evaluation of the infrared scattering properties of different GNPs (nanoshells, nanostars, and nanorods). These GNPs display remarkable extinction coefficients in the first and second biological windows, including the particular CV-OCT wavelength. We use a unique combination of techniques (thermal loading experiments, infrared optical coherence tomography, infrared dark field microscopy, and optoacoustic spectroscopy) to experimentally determine the scattering efficiency at three different near-infrared wavelengths (808 nm, 980 nm, and 1280 nm), lying in the first and second biological windows. Consequently, this work determines experimentally the influence of particle morphology on the infrared scattering efficiency of GNPs and evidences the existence of remarkable discrepancies between experimental data and numerical simulations, This work was supported by the Spanish Ministerio de Economía y Competitividad under Project No. MAT2016-75362-C3-1-R. J.A.S.-G. and D.R.A. acknowledge the partial financial support from the Spanish Ministerio de Economía y Competitividad through the grant LENSBEAM (No. FIS2015-69295-C3-2-P), Ministerio de Educación, Cultura y Deporte through a Ph.D. Fellowship (No. FPU15/03566), and also Consejo Superior de Investigaciones Cientficas (i-COOP LIGHT 2015CD0011). J.H. acknowledges the scholarship from the China Scholarship Council (CSC, No. 201506650003). D.T. and P.R.G. acknowledge the Mineco and MdM Excellence Intiative (Nos. FJCI-2014-22398, RYC-2012-10059, CTQ2013-45433-P[FEDER], and MDM-2014-0370-04)

Fano resonances in plasmonics have received widespread attention for their distinctly narrow asymmetric line shapes. A variety of configurations have been considered, either requiring complex metallic nanostructures or being extremely faint if originated in simple single nanoparticles. Here we report on the emergence of high-contrast, strongly asymmetric Fano line shapes in the light scattered from semiconductor nanorods. Numerical calculations are carried out for the scattering cross sections of finite semiconducting nanorods, with dimensions such that the lowest-order (transverse) Mie resonances coexist with the lowest-order guided modes. Such intense narrow Fano resonances are strongly/weakly asymmetric in TE/TM polarization and overlap with the Mie-like background. A physical interpretation is presented stemming from the (strong or weak) interference of the far-field angular patterns of Mie resonances (indeed, of both magnetic and dielectric dipole character) with narrow Fabry-Perot (guided mode) resonances, the latter calculated through a 1D line current model. A quasi-analytical expression is developed for the scattering cross sections that reproduce fairly well the Fano numerical line shapes, along with a generalized Fano formula, with fitting factors confirming their high asymmetry and contrast. These high-contrast, strongly asymmetric Fano resonances herein obtained could be potentially exploited in nanophotonics and sensing in the visible and near-IR, eased by simplified fabrication requirements of shape (nanorod) and material (semiconductor)., This work has been supported by the Spanish “Ministerio de Economía y Competitividad” (projects LENSBEAM FIS2015-69295-C3-2-P and RedMETA TEC2015- 69195-REDC, and FPU Fellowship) and CSIC (project NANOPHOTOPLAS, i-COOP LIGHT 2015CD0011).

4 pags., 5 figs., The emission from nanowires can couple to waveguide modes supported by the nanowire geometry, thus governing the far-field angular pattern. To investigate the geometry-induced coupling of the emission to waveguide modes, we acquire Fourier microscopy images of the photoluminescence of nanowires with diameters ranging from 143 to 208 nm. From the investigated diameter range, we conclude that a few nanometers difference in diameter can abruptly change the coupling of the emission to a specific mode. Moreover, we observe a diameter-dependent width of the Gaussian-shaped angular pattern in the far-field emission. This dependence is understood in terms of interference of the guided modes, which emit at the end facets of the nanowire. Our results are important for the design of quantum emitters, solid state lighting, and photovoltaic devices based on nanowires. VC 2016 AIP Publishing LLC, This research was supported by the Dutch technology foundation STW, which is part of the “Netherlands Organisation for Scientific Research (NWO),” and partially funded by the Dutch Ministry of Economic Affairs. This work was also part of the research program of the “Foundation for Fundamental Research on Matter (FOM),” which is financially supported by NWO. J.A.S.G. acknowledges the Spanish Ministerio de Economía y Competitividad for financial support through the Grant Nos. NANOPLASþ (FIS2012-31070) and LENSBEAM (FIS2015-69295-C3-2-P).

The interference between electric and magnetic dipolar fields is known to lead to asymmetric angular distributions of the scattered intensity from small high refractive index (HRI) particles. Properly designed all-dielectric metasurfaces based on HRI spheres have been shown to exhibit zero reflectivity, a generalized Brewster's effect, potentially for any angle, wavelength and polarization of choice. At normal incidence, the effect is related to the absence of backscattering from small dielectric spheres or disks at the, so-called, first Kerker condition. In contrast, homogeneous HRI cylinders do not fulfil the first Kerker condition due to the mismatch between the local electric and magnetic density of states. In this work, we show that although a zero back-scattering condition can never be achieved for individual cylinders, when they are arranged in a periodic array their mutual interaction leads to an anomalous Kerker condition, leading to a generalized Brewster's effect in a nanorod-based metasurface. We derive a coupled electric and magnetic dipole (CEMD) analytical formulation to describe the properties of a periodic array of HRI nanorods in full agreement with exact numerical calculations.

7 pags.; 5 figs., © 2015 American Chemical Society. Lighting applications require directional and polarization control of the emitted light, which is currently achieved by bulky optical components such as lenses, parabolic mirrors, and polarizers. Ideally, this control would be achieved without any external optics, but at the nanoscale, during the generation of light. Semiconductor nanowires are promising candidates for lighting devices due to their efficient light outcoupling and synthesis flexibility. In this work, we demonstrate a precise control of both the directionality and the polarization of the nanowire array emission by changing the nanowire diameter. We change the angular emission pattern from a large-angle doughnut shape to a narrow-angle beaming along the nanowire axis. In addition, we tune the polarization from unpolarized to either p- or s-polarized. Both the far-field emission pattern and its polarization are controlled by the number and type of guided or leaky modes supported by the nanowire, which are determined by the nanowire diameter., The authors acknowledge the Spanish “Ministerio de Economia y Competitividad ́ ” for financial support through the Consolider-Ingenio project EMET (CSD2008-00066), and NANOPLAS+ (FIS2012- 31070).

© 2015 American Chemical Society. Light absorption from finite semiconductor nanowires is investigated through a unified theoretical picture and numerical simulations. We first show theoretically from the electromagnetic mode dispersion relation and the Mie extinction cross sections for infinite nanowires that formally equivalent (Janus) formulas yield both leaky/guided modes and Mie resonances as, respectively, complex wave vector or complex frequency solutions that, in turn, overlap only at grazing angles with respect to the nanowire axis. For finite semiconductor nanowires, the angle of incidence becomes critical: absorption is dominated by Mie resonances at incident angles perpendicular (and oblique) to the nanowire axis; by contrast, guided mode excitation (if spectrally available) governs and enhances absorption at grazing incidence. Such theoretical prediction can be exploited to optimize nanowire designs for broad band and broad angle light harvesting applications.

Metallic nanowires (NW) coated with a high permittivity dielectric are proposed as means to strongly reduce the light scattering of the conducting NW, rendering them transparent at infrared wavelengths of interest in telecommunications. Based on a simple, universal law derived from electrostatics arguments, we find appropriate parameters to reduce the scattering efficiency of hybrid metal-dielectric NW by up to three orders of magnitude as compared with the scattering efficiency of the homogeneous metallic NW. We show that metal@dielectric structures are much more robust against fabrication imperfections than analogous dielectric@metal ones. The bandwidth of the transparent region entirely covers the near IR telecommunications range. Although this effect is optimum at normal incidence and for a given polarization, rigorous theoretical and numerical calculations reveal that transparency is robust against changes in polarization and angle of incidence, and also holds for relatively dense periodic or random arrangements. A wealth of applications based on metal-NWs may benefit from such invisibility. © 2013 Optical Society of America., The authors acknowledge the Spain Ministerio de Economía y Competitividad, through the Consolider-Ingenio project EMET (CSD2008-00066) and NanoLight (CSD2007-00046), and NANOPLAS+ (FIS2012-31070) and FIS2012- 3611, and the Comunidad de Madrid (grant MICROSERES P2009/TIC-1476) for support. R. P.-D. and L.S F-P, also acknowledge support from the European Social Fund and CSIC through a JAE-Pre and JAE-Doc grants, respectively.

7 pags., 5 figs., Recently, many fascinating properties predicted for metamaterials (negative refraction, superlensing, electromagnetic cloaking,...) were experimentally demonstrated. Unfortunately, the best achievements have no direct translation to the optical domain, without being burdened by technological and conceptual difficulties. Of particular importance within the realm of optical negative-index metamaterials (NIM), is the issue of simultaneously achieving strong electric and magnetic responses and low associated losses. Here, hybrid metal-semiconductor nanowires are proposed as building blocks of optical NIMs. The metamaterial thus obtained, highly isotropic in the plane normal to the nanowires, presents a negative index of refraction in the near-infrared, with values of the real part well below-1, and extremely low losses (an order of magnitude better than present optical NIMs). Tunability of the system allows to select the operating range in the whole telecom spectrum. The design is proven in configurations such as prisms and slabs, directly observing negative refraction., They also acknowledge the Spain Ministerio de Economıa y Competitividad, through the Consolider-Ingenio project EMET (CSD2008-00066) and NANOPLAS1 (FIS2012-31070), and the Comunidad de Madrid (grant MICROSERES P2009/TIC-1476) for support. R. P.-D. also acknowledges support from The European Social Fund and CSIC through a JAE-Pre grant.

We investigate theoretically and numerically the photonic band structure in the optical domain of an array of core-shell metal-semiconductor nanowires. Corresponding negative-index photonic bands are calculated, showing isotropic equifrequency surfaces. The effective (negative) electric permittivity and magnetic permeability, retrieved from S parameters, are used to compare the performance of such nanowire arrays with homogeneous media in canonical examples, such as refraction through a prism and flat-lens focusing. Very good agreement is found, indeed confirming the effective medium behavior of the nanowire array as a low-loss, isotropic (2D) and bulk, optical negative index metamaterial. Indeed, disorder is introduced to further stress its robustness, Comment: 11 pages, 13 figures