Tailoring accidental double bound states in the continuum in all-dielectric metasurfaces

Bound states in the continuum (BICs) have been thoroughly investigated due to their formally divergent Q-factor, especially those emerging in all-dielectric, nanostructured metasurfaces from symmetry protection at the $\Gamma$ point (in-plane wavevector $k_{||}=0$). Less attention has been paid to accidental BICs that may appear at any other $k_{||}\not =0$ in the band structure of supported modes, being in turn difficult to predict. Here we make use of a coupled electric/magnetic dipole model to determine analytical conditions for the emergence of accidental BICs, valid for any planar array of meta-atoms that can be described by dipolar resonances, which is the case of many nanostructures in the optical domain. This is explored for all-dielectric nanospheres through explicit analytical conditions that allow us in turn to predict accidental BIC positions in the parameter space $(\omega,\bf{k_{||}}$). Finally, such conditions are exploited to determine not only single, but also double (for both linear polarizations) accidental BICs occurring at the same position in the dispersion relation $\omega-\bf{k_{||}}$ for realistic semiconductor nanodisk meta-atoms. This might pave the way to a variety of BIC-enhanced light-matter interaction phenomena at the nanoscale such as lasing or non-linear conversion, that benefit from emerging at wavevectors away from the $\Gamma$ point (off-normal incidence) overlapping for both linear polarizations.Comment: 18 pages, 7 figure

Resonant metal-semiconductor nanostructures as building blocks of low-loss negative- and zero-index metamaterials

5th International Conference on Metamaterials, Photonic Crystals and Plasmonics; Conferencia invitada.Here we propose a 2D isotropic metamaterial with negative electric and magnetic responses in the optical regime, based on hybrid metallo-dielectric core-shell nanowires. The magnetic response stems from the lowest magnetic resonance of the dielectric shell with high refractive index (i.e., lossless semiconductor), and can be tuned to coincide with the plasmon resonance of the metal core, responsible for the electric response. Also, the same metamaterial design is shown to yield zero refractive index for a different spectral regime (in connection with overlapping resonances), exhibiting in turn an impedance close to that of vacuum.The authors acknowledge financial support from the Spanish “Ministerio de Economía y Competitividad” (CSD2008-00066 and FIS2012-31070), and European Social Fund and CSIC (JAE-Pre and JAE-Doc grants).Peer Reviewe

Brewster quasi bound states in the continuum in all-dielectric metasurfaces from single magnetic-dipole resonance meta-atoms

Bound states in the continuum (BICs) are ubiquitous in many areas of physics, attracting especial 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 normalincidence(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.Comment: 15 pages, 7 figures; typos corrected, Figs. 3 & 5 modified, new Fig. 7 & references adde

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

Strong coupling between weakly guided semiconductor nanowire modes and an organic dye

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 (MDM-2014-0377); 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

Directional emission from leaky and guided modes in GaAs nanowires measured by cathodoluminescence

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 SocietyThis 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).Peer Reviewe

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

Engineering bound states in the continuum at telecom wavelengths with non-Bravais lattices

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, i.e., 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, i.e., non-Bravais lattice, are less explored among periodic systems. Starting from a periodic square lattice of Si nanodisks, we have prepared three non-Bravais lattices 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 $\Gamma$ point with a band gap in between. %of $\sim$ 41 meV. 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 role of magnetic and electric resonances in dielectric nanoparticles and the change of BIC to bright character of the modes is explained by the two different origins of BICs in the detuned arrays, which is further 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.Comment: 26 pages, 5 figure

Room Temperature Exciton-Polariton Condensation in Silicon Metasurfaces Emerging from Bound States in the Continuum

We show the first experimental demonstration of room-temperature exciton-polariton (EP) condensation from a bound state in the continuum (BIC). This demonstration is achieved by strongly coupling stable excitons in an organic perylene dye with the extremely long-lived BIC in a dielectric metasurface of silicon nanoparticles. The long lifetime of the BIC, mainly due to the suppression of radiation leakage, allows for EP thermalization to the ground state before decaying. This property results in a condensation threshold of less than 5 \mu J cm^{-2}, one order of magnitude lower that the lasing threshold reported in similar systems in the weak coupling limit