Surface inspection by monitoring spectral shifts of localized plasmon resonances

We present a numerical study of the spectral variations of localized surface plasmon resonances (LSPR) in a 3D-probe metallic nanoparticle scanned over an inhomoegeneous dielectric surface. The possibilities for both, index monitoring and lateral resolution at nanoscale level are explored, with special attention paid to the shape of the probe and the profile of the near field underneath

Spectral behavior of the linear polarization degree at right-angle scattering configuration for nanoparticle systems

We present a numerical study of the spectral evolution of the linear polarization degree at right-angle scattering configuration (PL(90º)) for two different particle systems: an isolated nanosphere and a nanodimer composed of two finite size spherical particles separated by a gap distance d. We shall focus on the influence of charge oscillation modes other than the dipolar on the linear polarization degree of the scattered light. The possibility of using this alternative parameter for characterizing nanoparticle systems and particle interaction is analyzed.We acknowledge financial support from USAITCA (US Army International Technology Center—Atlantic) under the project R&D1390-PH-01 and from the Ministry of Education of Spain under the project FIS2007-60158

CDDA: extension and analysis of the discrete dipole approximation for chiral systems

Discrete dipole approximation (DDA) is a computational method broadly used to solve light scattering problems. In this work, we propose an extension of DDA that we call Chiral-DDA (CDDA), to study light-chiral matter interactions with the capability of describing the underlying physics behind. Here, CDDA is used to solve and analyze the interaction of a nanoantenna (either metallic or dielectric) with a chiral molecule located in its near field at different positions. Our method allowed to relate near field interactions with far field spectral response of the system, elucidating the role that the nanoantenna electric and magnetic polarizabilities play in the coupling with a chiral molecule. In general, this is not straightforward with other methods. We believe that CDDA has the potential to help researchers revealing some of the still unclear mechanisms responsible for the chiral signal enhancements induced by nanoantennas.Ramon y Cajal Fellowship (RYC-2016- 20831); Ministerio de Educación, Cultura y Deporte (PGC2018-096649-B-I); Horizon 2020 Framework Programme (899598)

Polar decomposition of Mueller matrices for 2D-structured surfaces

In this research, the Polar Decomposition (PD) has been applied to the Mueller matrices (MMs) of the light scattered by linear ribs of rectangular profile on a flat substrate. Although photo-lithographic technique produces a silicon surface, metallic character is acquired by sputtering with gold some of the samples. With a dual rotating compensator polarimeter the MMs are obtained by Fourier Transform analysis. The samples have been numerically modeled by using both FDTD and Extinction Theorem (ET) and MMs have been computed from the results. The scattering depends strongly on the geometry and composition of the ribs, and this sensitivity is noticed for instance in M11 element. But information offered by PD parameters is shown to be more apprehensible, like the substrate-induced depolarization or the retardance associated to the rib width.This research has been supported by the Ministry of Education of Spain under project FIS2007-60158 and by USAITCA through R&D 1390-PH-01

On the performance of a tunable grating-based high sensitivity unidirectional plasmonic sensor

Optical biosensing is currently an intensively active research area, with an increasing demand of highly selective, sensitivity-enhanced and low-cost devices where different plasmonic approaches have been developed. In this work we propose a tunable optimized grating-based gold metasurface that can act both as a high sensitivity sensor device (up to 1500 nm/RIU) and as an unidirectional plasmon source. The theory behind surface plasmon polariton generation is recalled to thoroughly understand the influence that every parameter of the grating source has on the performance of the proposed device. The results and conclusions discussed here offer a key step toward the design of biosensors based on excitation of surface plasmons polaritons by grating-based structures or in the process of creating new nanophotonic circuit devices.We gratefully acknowledge financial support from Spanish national project INMUNOTERMO (No. PGC2018-096649-B-I). J. G-C. thanks the Ministry of science of Spain for his FPI grant. G. S. thanks the Ministry of education for his collaboration grant and P.A. acknowledges funding for a Ramon y Cajal Fellowship (Grant No. RYC-2016-20831)

Electromagnetic Study of Behaviour of Plasmonic Units

SUMMARY: For any memory or computing device, fast switching speed and low switching energy are most attractive attributes, and approaches by which speed and energy efficiency can be improved are always desirable. Plasmonics offers a way to achieve those attributes of fast switching and low energy consumption: plasmonic resonant structures are inherently capable of harnessing and focusing optical energy on sub-wavelength scales, far beyond the capabilities of conventional optical and photonic elements. Plasmonics can provide us with access to both of these scenarios. Indeed, plasmonics offers additional light manipulation tools, otherwise inaccessible with conventional photonics. The collective oscillation of conduction electrons in a suitably shaped metallic nanoparticle (the so-called localized surface plasmon, LSP) can couple with impinging radiation, which in turn squeezes light into much reduced volumes, and greatly magnifies the local electric field, usually leading to a much reduced (non-diffraction limited) device footprint. This deliverable presents an analysis of the electromagnetic interaction of plasmonic units with phase-change materials (PCMs) as selected in the project PHEMTRONICS. As plasmonic units, we start by considering the common plasmonic metals of gold and silver, analyzing their possibilities and limits. Based on those, we consider the use of metallic nanoantennas made of Ga nanoparticle dimers. Ga has been selected due to its good plasmonic performance, physical and chemical properties and to its polymorphism. We have analyzed the coupling of plasmonic nanoantennas with the PCMs under consideration at the moment in the project, namely, GaS and Sb2S3 in their amorphous and crystalline phases. These two PCMs have been combined with Ga NPs and some gold configurations to make the nanoantenna reconfigurabilty wider and improve its tunability and performance. Further, plasmon coupling to PCM waveguides made of Sb2S3, has been analyzed through metallic grating couplers. Two basic configurations have been selected which could be the base to design a plasmonic enhanced PCM photodetector in collaboration with the PHEMTRONICS partners. Finally, conclusions have been drawn together with the identification of the practical solutions to couple plasmonics with novel PCMs

Gold nanodoughnut as an outstanding nanoheater for photothermal applications

Photoinduced hyperthermia is a cancer therapy technique that induces death to cancerous cells via heat generated by plasmonic nanoparticles. While previous studies have shown that some nanoparticles can be effective at killing cancer cells under certain conditions, there is still a necessity (or the need) to improve its heating efficiency. In this work, we perform a detailed theoretical study comparing the thermoplasmonic response of the most effective nanoparticle geometries up to now with a doughnut-shaped nanoparticle. We numerically demonstrate that the latter exhibits a superior tunable photothermal response in practical illumination conditions (unpolarized light). Furthermore, we show that nanoparticle heating in fluidic environments, i.e., nanoparticles undergoing Brownian rotations, strongly depends on the particle orientation with respect to the illumination source. We conclude that nanodoughnuts are the best nanoheaters in our set of structures, with an average temperature increment 40% higher than the second best nanoheater (nanodisk). Furthermore, nanodoughnuts feature a weak dependence on orientation, being therefore ideal candidates for photothermal therapy applications. Finally, we present a designing guide, covering a wide range of toroid designs, which can help on its experimental implementation.Acknowledgments. Authors would like to thank A. Franco and C. R. Crick for the interesting discussions. We gratefully acknowledge financial support from Spanish national project (No. PGC2018-096649-B-I), the UK Leverhulme Turst (Grant No. RPG-2018-384), UK-EPSRC (EP/J003859/1) and Imperial College Europeans Partner Fund grant. J. G-C. thanks the Ministry of science of Spain for his FPI grant. G. S. thanks the Ministry of education for his collaboration grant and P.A. acknowledges funding for a Ramon y Cajal Fellowship (Grant No. RYC-2016-20831)

Polarization control of high transmission/reflection switching by all-dielectric metasurfaces

Metasurfaces built of high refractive dielectric nanostructures could play a key role in controlling the electromagnetic wave propagation, due to their low energy losses and their ability to excite not only electric but also magnetic resonances. In this study, we theoretically and experimentally demonstrate that an array of high-index dielectric nanodimers can perform as tuneable metasurfaces that can be switched from a high transmitter to a high reflector, by just changing the linear polarization of excitation. The incident polarization alters the hybridization mode of the excited electric and magnetic dipoles in the dimer, and this leads to either spectral overlap or separation of the two dipoles. The hybridization of the electric and magnetic modes modifies the effective permittivity and permeability of the tuneable dielectric metasurface, exhibiting the high transmission and reflection that can be easily switched by simply changing the linear polarization.The authors acknowledge funding provided by Grants from the EPSRC Reactive Plasmonics Programme EP/MO13812/1, the Leverhulme Trust (UK), ONR Global, and the U.S. Army Transatlantic Research Office. S.A.M. further acknowledges the Lee-Lucas Chair. P.A. acknowledges Programa “Viera y Clavijo” de la Agencia Canaria de Investigación, Innovación y Sociedad de la Información (ACIISI) y la Universidad de las Palmas de Gran Canaria (ULPGC)

Broadband unidirectional forward scattering with high refractive index nanostructures: Application in solar cells

High refractive index dielectric (HRID) nanoparticles are a clear alternative to metals in nanophotonic applications due to their low losses and directional scattering properties. It has been demonstrated that HRID dimers are more efficient scattering units than single nanoparticles in redirecting the incident radiation towards the forward direction. This effect was recently reported and is known as the ?near zero-backward? scattering condition, attained when nanoparticles forming dimers strongly interact with each other. Here, we analyzed the electromagnetic response of HRID isolated nanoparticles and aggregates when deposited on monolayer and graded-index multilayer dielectric substrates. In particular, we studied the fraction of radiation that is scattered towards a substrate with known optical properties when the nanoparticles are located on its surface. We demonstrated that HRID dimers can increase the radiation emitted towards the substrate compared to that of isolated nanoparticles. However, this effect was only present for low values of the substrate refractive index. With the aim of observing the same effect for silicon substrates, we show that it is necessary to use a multilayer antireflection coating. We conclude that dimers of HRID nanoparticles on a graded-index multilayer substrate can increase the radiation scattered into a silicon photovoltaic wafer. The results in this work can be applied to the design of novel solar cells