Experimental observation of higher-order anapoles in individual silicon disks under in-plane illumination

[EN] Anapole states¿characterized by a strong suppression of far-field scattering¿naturally arise in high-index nanoparticles as a result of the interference between certain multipolar moments. Recently, the first-order electric anapole, resulting from the interference between the electric and toroidal dipoles, was characterized under in-plane illumination as required in on-chip photonics. Here, we go a step further and report on the observation of higher-order (magnetic and second-order electric) anapole states in individual silicon disks under in-plane illumination. To do so, we increase the disk dimensions (radius and thickness) so that such anapoles occur at telecom wavelengths. Experiments show dips in the far-field scattering perpendicular to the disk plane at the expected wavelengths and the selected polarizations, which we interpret as a signature of high-order anapoles. Some differences between normal and in-plane excitation are discussed, in particular, the non-cancelation of the sum of the Cartesian electric and toroidal moments for in-plane incidence. Our results pave the way toward the use of different anapole states in photonic integrated circuits either on silicon or other high-index dielectric materials.This work was supported by Generalitat Valenciana under Grant No. GRISOLIAP/2018/164, the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through the International Research Training Group (IRTG) 2675 "Meta-ACTIVE," Project No. 437527638, and Generalitat Valenciana (Grant Nos. PROMETEO/2019/123, IDIFEDER/2020/041, and IDIFEDER/2021/061).Díaz-Escobar, E.; Barreda-Gómez, ÁI.; Griol Barres, A.; Martínez, A. (2022). Experimental observation of higher-order anapoles in individual silicon disks under in-plane illumination. Applied Physics Letters. 121(20):1-8. https://doi.org/10.1063/5.0108438181212

Photonic Bandgap Closure and Metamaterial Behavior in 1D Periodic Chains of High-Index Nanobricks

It has been shown that the photonic bandgap of one-dimensional (1D) dielectric periodic thin films can vanish at the first Bragg condition for TM modes. Here, we address the case of 1D photonic crystal slabs formed by a chain of high-index dielectric particles with transversal confinement and show that the Bragg bandgap can vanish for both TE- and TM-like modes. Calculations using plane-wave expansion and finite-difference time-domain methods confirm that the PBG vanishes. PBG closure is explained as being a result of the interplay between the electric and magnetic dipole resonances of the isolated nanoparticle with Bragg resonance, as confirmed by calculating the electric and magnetic dipoles of the isolated nanobricks. This can be considered as a manifestation of the metamaterial behavior of the 1D system when using silicon as an underlying material. Our finding may have important consequences for the fields of photonic crystals and all-dielectric metamaterials

Spectral response of dielectric nano-antennas in the far- and near-field regimes

Recent studies show that the spectral behaviour of localized surface plasmon resonances (LPSRs) in metallic nanoparticles su er from both a redshift and a broadening in the transition from the far- to the near-field regimes. An interpretation of this efect was given in terms of the evanescent and propagating components of the angular spectrum representation of the radiated eld. Due to the increasing interest awakened by magnetodielectric materials as a both low-loss material option for nanotechnology applications, and also for their particular scattering properties, here we study the spectral response of a magnetodielectric nanoparticle as a basic element of a dielectric nano-antenna. This study is made by analyzing the changes su ered by the scattered electromagnetic field when propagating from the surface of this dielectric nanostructure to the far-zone in terms of propagating and evanescent plane wave components of the radiated fields.This research was supported by MICINN (Spanish Ministry of Science and Innovation) with project FIS2013- 45854-P

Analysis of directionality effects in magnetodielectric core-shell nanoparticles by means of polarimetric techniques

The influence of increasing the core size of a Ag-Si core-shell nanoparticle has been investigated by using the values of the linear polarization degree at right angle scattering configuration, PL(90º). Changes in dipolar resonances and Scattering Directionality Conditions as a function of the core radius (Rint) for a fixed shell size (Rext = 230 nm) have been analyzed. An empirical formula to obtain the ratio Rint/Rext by monitoring the influence of the magnetic dipolar resonance in PL(90º) has been found.This research was supported by MICINN (FIS2013-45854-P). Ángela I. Barreda and Y. Gutiérrez want to express their gratitude to the University of Cantabria for their FPU grant

Recent advances in high refractive index dielectric nanoantennas: basics and applications

Nanoparticles made of High Refractive Index dielectric materials have been proposed as an alternative to metals driven by their low-losses and magnetic response. The coherent effects between the electric and magnetic resonances are responsible for their exceptional directionality properties that make them attractive in applications where enhancing light-matter interaction and controlling the radiation direction is extremely relevant. These nanoparticles, when used as unit-cells of more complex systems, such as metasurfaces, result to be essential in the design of novel optical devices. Their low-losses, strong confinement of electromagnetic energy and the outstanding scattering efficiencies show these nanoantennas as promising candidates for Surface Enhanced Spectroscopies, non-linear phenomena or sensing. Here, we describe and discuss the origins and recent advances in this rapidly developing field of dielectric nanophotonics, paying special attention to the main significant contributions we have done since its startup to boost its progress. In particular, light directivity, steering and switching of light, spectroscopy, sensing and non-linear phenomena, third harmonic generation are some of the applications that motivated this brief overview.This research has been supported by the Army Research Laboratory under Cooperative Agreement Number W911NF-17-2-0023 and by SODERCAN (Sociedad para el Desarrollo de Cantabria) through the Research Vicerrectorate of the University of Cantabria. P.A acknowledges funding from the Ramon y Cajal Fellowship RYC-2016-20831 and A.I.B. wants to thank the University of Cantabria for her FPU grant

Exotic nanophotonics with subwavelength high-index disks

[EN] High-index dielectric disks with subwavelength dimensions seem relatively simple electromagnetic structures. However, they can give rise to exotic phenomena in nanophotonics. Here we show recent findings that show that high-index disks can be used to manipulate light in subwavelength dimensions as a result of Mie and Fabry-Perot resonances. We also propose that such disks can be used for new applications in photonic integrated circuits (PICs) using silicon technology.A. M. acknowledges funding from Generalitat Valenciana (BEST/2020/178, IDIFEDER/2018/033, PPC/2018/002, PROMETEO/2019/123, IDIFEDER/2020/041); Ministerio de Ciencia e Innovación (PGC2018-094490-B-C21, ICTS-2017- 28-UPV-9); European Commission, H2020 Future and Emerging Technologies (713450, 829067).Díaz-Escobar, E.; Pinilla-Cienfuegos, E.; Barreda, ÁI.; Mercadé-Morales, L.; Griol Barres, A.; Martínez, A. (2021). Exotic nanophotonics with subwavelength high-index disks. Íñigo Cuiñas Gómez. 1-4. http://hdl.handle.net/10251/1910791

Comportamiento electromagnético de sustratos nanoestructurados contaminados con objetos micrométricos

En este trabajo se estudia el comportamiento electromagnético de objetos dieléctricos micrométricos, situados sobre sustratos metálicos estructurados mediante nanoagujeros distribuidos periódicamente. Atendiendo a la resonancia (1,0) de Transmisión Óptica Extraordinaria, se ha investigado numéricamente la influencia de la geometría del objeto en los espectros de transmisión. Concretamente, el estudio se ha basado en el desplazamiento espectral del máximo de transmisión a medida que la superficie de contacto entre el objeto y el sustrato nanoestructurado cambia

Hybrid photonic-plasmonic cavities based on the nanoparticle-on-a-mirror configuration

[EN] Hybrid photonic-plasmonic cavities have emerged as a new platform to increase light-matter interaction capable to enhance the Purcell factor in a singular way not attainable with either photonic or plasmonic cavities separately. In the hybrid cavities proposed so far, the plasmonic element is usually a metallic bow-tie antenna, so the plasmonic gap-defined by lithography-is limited to minimum values of several nanometers. Nanoparticle-on-a-mirror (NPoM) cavities are far superior to achieve the smallest possible mode volumes, as plasmonic gaps smaller than 1 nm can be created. Here, we design a hybrid cavity that combines an NPoM plasmonic cavity and a dielectric-nanobeam photonic crystal cavity operating at transverse-magnetic polarization. The metallic nanoparticle can be placed very close (<1 nm) to the upper surface of the dielectric cavity, which acts as a low-reflectivity mirror. We demonstrate through numerical calculations of the local density of states that this hybrid plasmonic-photonic cavity exhibits quality factors Q above 10(3) and normalized mode volumes V down to 10(-3), thus resulting in high Purcell factors (F-P approximate to 10(5)), while being experimentally feasible with current technology. Our results suggest that hybrid cavities with sub-nanometer gaps should open new avenues for boosting light -matter interaction in nanophotonic systems.Horizon 2020 Framework Programme (829067 THOR); Generalitat Valenciana (PPC/2018/002, PROMETEO/2019/123); Ministerio de Ciencia, Innovacion y Universidades (PGC2018-094490-B, PRX18/00126); Alexander von Humboldt-Stiftung.Barreda, ÁI.; Zapata-Herrera, M.; Palstra, IM.; Mercadé-Morales, L.; Aizpurua, J.; Koenderink, AF.; Martínez Abietar, AJ. (2021). Hybrid photonic-plasmonic cavities based on the nanoparticle-on-a-mirror configuration. Photonics Research. 9(12):2398-2419. https://doi.org/10.1364/PRJ.433761S2398241991

Polarimetric techniques for determining morphology and optical features of High Refractive Index dielectric nanoparticles size

The spectral evolution of the degree of linear polarization (PL) at a scattering angle of 90º is studied numerically for high refractive index (HRI) dielectric spherical nanoparticles. The behaviour of PL(90º) is analyzed as a function of the refractive index of the surrounding medium and the particle radius. We focus on the spectral region where both electric and magnetic resonances of order not higher than two are located for various semiconductor materials with low absorption. The spectral behavior of PL(90º) has only a small, linear dependence on nanoparticle size R. This weak dependence makes it experimentally feasible to perform real-time retrievals of both the refractive index of the external medium and the NP size R. From an industrial point of view, pure materials are nonrealistic, since they can only be provided under certain conditions. For this reason, we also study the effect of contaminants on the resonances of silicon NPs by considering the spectral evolution of PL(90º).This research was supported by MICINN (Spanish Ministry of Science and Innovation, project FIS2013-45854-P)

Electromagnetic behavior of dielectric objects on metallic periodically nanostructured substrates

In this research, we investigate the electromagnetic behavior of a metallic thin-film with a periodic array of subwavelength apertures when dielectric objects are located on it. The influence of size, geometry and optical properties of the objects on the transmission spectra is numerically analyzed. We study the sensitivity of this system to changes in the refractive index of the illuminated volume induced by the presence of objects with sizes from hundreds of nanometers (submicron-sized objects) to a few microns (micron-sized objects). Parameters such as the object volume within the penetration depth of the surface plasmon in the buffer medium or the contact surface between the object and the nanostructured substrate strongly affect the sensitivity. The proposed system models the presence of objects and their detection through the spectral shifts undergone by the transmission spectra. Also, we demonstrate that these can be used for obtaining information about the refractive index of a micron-sized object immersed in a buffer and located on the nanostructured sensitive surface. We believe that results found in this study can help biomedical researchers and experimentalists in the process of detecting and monitoring biological organisms of large sizes (notably, cells).Ministerio de Economía y Competitividad (MINECO) (Secretaría de Estado de Investigación, Desarrollo e Innovación) (FIS2013-45854-P); Fundación IBERDROLA-ESPAÑA Research on Energy and the Environment Program. A.I.B. wants to thank the University of Cantabria for her FPU grant