J. J. Sáenz

Strong magnetic response of submicron Silicon particles in the infrared

High-permittivity dielectric particles with resonant magnetic properties are being explored as constitutive elements of new metamaterials and devices. Magnetic properties of low-loss dielectric nanoparticles in the visible or infrared are not expected due to intrinsic low refractive index of optical media in these regimes. Here we analyze the dipolar electric and magnetic response of lossless dielectric spheres made of moderate permittivity materials. For low material refractive index (<∼3) there are no sharp resonances due to strong overlapping between different multipole contributions. However, we find that Silicon particles with index of refraction∼3.5 and radius∼200 nm present strong electric and magnetic dipolar resonances in telecom and near-infrared frequencies, (i.e. at wavelengths≈1.2-2 mm) without spectral overlap with quadrupolar and higher order resonances. The light scattered by these Si particles can then be perfectly described by dipolar electric and magnetic fields.

Dielectric antennas--a suitable platform for controlling magnetic dipolar emission.

Plasmonic nanoparticles are commonly used to tune and direct the radiation from electric dipolar emitters. Less progress has been made towards understanding complementary systems of magnetic nature. However, it has been recently shown that high-index dielectric spheres can act as effective magnetic antennas. Here we explore the concept of coupling dielectric magnetic antennas with either an electric or magnetic dipolar emitter in a similar fashion to the purely electric systems reported previously. We investigate the enhancement of radiation from systems comprising admixtures of these electric and magnetic elements and perform a full study of its dependence on the distance and polarization of the emitter with respect to the antenna. A comparison to the plasmon antennas reveals remarkable symmetries between electric and magnetic systems, which might lead to novel paradigms in the design of nanophotonic devices that involve magnetic activity.

Intensity Distribution of Modes in Surface Corrugated Waveguides

Exact calculations of transmission and reflection coefficients in surface randomly corrugated optical waveguides are presented. As the length of the corrugated part of the waveguide increases, there is a strong preference to forward coupling through the lowest mode. An oscillating behavior of the enhanced backscattering as a function of the wavelength is predicted. Although the transport is strongly nonisotropic, the analysis of the probability distributions of the transmitted waves confirms in this configuration distributions predicted by random matrix theory for volume disorder. [S0031-9007(98)06066-9]

Magneto-Optical Activity in High Index Dielectric Nanoantennas.

The magneto-optical activity, namely the polarization conversion capabilities of high-index, non-absorbing, core-shell dielectric nanospheres is theoretically analyzed. We show that, in analogy with their plasmonic counterparts, the polarization conversion in resonant dielectric particles is linked to the amount of electromagnetic field probing the magneto-optical material in the system. However, in strong contrast with plasmon nanoparticles, due to the peculiar distribution of the internal fields in resonant dielectric spheres, the magneto-optical response is fully governed by the magnetic (dipolar and quadrupolar) resonances with little effect of the electric ones.

Density of states and extinction mean free path of waves in random media: dispersion relations and sum rules.

We establish a fundamental relationship between the averaged local density of states and the extinction mean free path of waves propagating in random media. From the principle of causality and the Kramers-Kronig relations, we show that both quantities are connected by dispersion relations and are constrained by a frequency sum rule. The results should be helpful in the analysis of wave transport through complex media and in the design of materials with specific transport properties.

STATISTICAL DISTRIBUTION OF INTENSITIES REFLECTED FROM DISORDERED MEDIA

A theoretical analysis of the statistical distributions of the reflected intensities from random media is presented. We use random matrix theory to analytically deduce the probability densities in the localization regime. Numerical calculations of the coupling to backward modes in surface corrugated waveguides are also put forward for comparison. Interestingly, the speckle distributions are found to be independent of the transport regime. Despite the scattering being highly non-isotropic, the predicted probability densities reproduce accurately the numerical results.

Speckle fluctuations resolve the interdistance between incoherent point sources in complex media

We acknowledge helpful discussions with M. Fink and nF. Scheffold. This work was supported by MINECO (Spain) nthrough Grant No. FIS2012-36113, by the Comunidad de nMadrid through Grant No. S2009/TIC-1476 (Microseres nProject), by LABEX WIFI (Laboratory of Excellence within nthe French Program “Investments for the Future”) under references nANR-10-LABX-24 and ANR-10-IDEX-0001-02 PSL*. nJ.J.S. acknowledges an IKERBASQUE Visiting Fellowship nand R.C. and G.C. acknowledge the hospitality of theDonostia nInternational Physics Center (DIPC) (Spain) where part of this nwork was done

Localized magnetic plasmons in all-dielectric μ < metastructures

This work was supported by the Spanish MINECO (FIS2012-31070 and FIS2012-36113) and Consolider-Ingenio EMET (CSD2008-00066)

Fluctuations of the electromagnetic local density of states as a probe for structural phase switching

This research was supported by the Spanish Ministry of nEconomy and Competitiveness through Grants No. FIS2012- n36113-C03, No. FIS2015-69295-C3-3-P, and No. MAT2014- n58860-P, and by the Comunidad de Madrid (Contract No. nS2013/MIT-2740). F.S. and L.S.F.-P. acknowledge funding nfrom the Swiss National Science Foundation through the nNational Centre of Competence in Research Bio-Inspired nMaterials.

Analysis of the dynamics of electric dipoles in fluctuating electromagnetic fields

Fluctuating isotropic electromagnetic fields are obtained by considering a large group of plane waves with wave vectors, polarizations and phases randomly distributed and fluctuating on time. Due to the isotropic character of this electromagnetic field, the optical force induced on an electric dipole is, in average, equal to zero. However, the dynamics of electric dipoles on these kind of systems are far from being trivial. In this work we analyze the dynamics of two dipoles using molecular dynamics simulations. In particular, we consider two silver nanoparticles of 5nm radius at Fr¨ohlich resonance. Under these conditions a gravity-like interaction among the two particles is induced. The molecular dynamics numerical simulations show how Keplerian-like trajectories are obtained under these particular conditions