23 research outputs found
Small dielectric spheres with high Refractive index as new multifunctional elements for optical devices
The future of ultra-fast optical communication systems is inevitably connected with progress in optical circuits and nanoantennas. One of the key points of this progress is the creation of elementary components of optical devices with scattering diagrams tailored for redirecting the incident light in a desired manner. Here we demonstrate theoretically and experimentally that a small, simple, spatially homogeneous dielectric subwavelength sphere with a high refractive index and low losses (as some semiconductors in the visible or near infrared region) exhibits properties allowing to utilize it as a new multifunctional element for the mentioned devices. This can be achieved by taking advantage of the coherent effects between dipolar and multipolar modes, which produce anomalous scattering effects. The effects open a new way to control the directionality of the scattered light. The directional tuning can be obtained in a practical way just by a change in the frequency of the incident wave, and/or by a well-chosen diameter of the sphere. Dielectric nanoparticles with the required optical properties in the VIS-NIR may be now readily fabricated. These particles could be an efficient alternative to the widely discussed scattering units with a more complicated design.This research was partly supported by MICINN (Spanish Ministry of Science and Innovation) through
project FIS2013-45854-P and by the Ministry of Education and Science of Russian Federation through
grant 14.Z50.31.0034
Surface electrons at plasma walls
In this chapter we introduce a microscopic modelling of the surplus electrons
on the plasma wall which complements the classical description of the plasma
sheath. First we introduce a model for the electron surface layer to study the
quasistationary electron distribution and the potential at an unbiased plasma
wall. Then we calculate sticking coefficients and desorption times for electron
trapping in the image states. Finally we study how surplus electrons affect
light scattering and how charge signatures offer the possibility of a novel
charge measurement for dust grains.Comment: To appear in Complex Plasmas: Scientific Challenges and Technological
Opportunities, Editors: M. Bonitz, K. Becker, J. Lopez and H. Thomse
Plasmonically Enhanced Reflectance of Heat Radiation from Low-Bandgap Semiconductor Microinclusions
Increased reflectance from the inclusion of highly scattering particles at
low volume fractions in an insulating dielectric offers a promising way to
reduce radiative thermal losses at high temperatures. Here, we investigate
plasmonic resonance driven enhanced scattering from microinclusions of
low-bandgap semiconductors (InP, Si, Ge, PbS, InAs and Te) in an insulating
composite to tailor its infrared reflectance for minimizing thermal losses from
radiative transfer. To this end, we compute the spectral properties of the
microcomposites using Monte Carlo modeling and compare them with results from
Fresnel equations. The role of particle size-dependent Mie scattering and
absorption efficiencies, and, scattering anisotropy are studied to identify the
optimal microinclusion size and material parameters for maximizing the
reflectance of the thermal radiation. For composites with Si and Ge
microinclusions we obtain reflectance efficiencies of 57 - 65% for the incident
blackbody radiation from sources at temperatures in the range 400 - 1600
{\deg}C. Furthermore, we observe a broadbanding of the reflectance spectra from
the plasmonic resonances due to charge carriers generated from defect states
within the semiconductor bandgap. Our results thus open up the possibility of
developing efficient high-temperature thermal insulators through use of the
low-bandgap semiconductor microinclusions in insulating dielectrics.Comment: Main article (8 Figures and 2 Tables) + Supporting Information (8
Figures
Fano resonances in plasmonic core-shell particles and the Purcell effect
Despite a long history, light scattering by particles with size comparable
with the light wavelength still unveils surprising optical phenomena, and many
of them are related to the Fano effect. Originally described in the context of
atomic physics, the Fano resonance in light scattering arises from the
interference between a narrow subradiant mode and a spectrally broad radiation
line. Here, we present an overview of Fano resonances in coated spherical
scatterers within the framework of the Lorenz-Mie theory. We briefly introduce
the concept of conventional and unconventional Fano resonances in light
scattering. These resonances are associated with the interference between
electromagnetic modes excited in the particle with different or the same
multipole moment, respectively. In addition, we investigate the modification of
the spontaneous-emission rate of an optical emitter at the presence of a
plasmonic nanoshell. This modification of decay rate due to electromagnetic
environment is referred to as the Purcell effect. We analytically show that the
Purcell factor related to a dipole emitter oriented orthogonal or tangential to
the spherical surface can exhibit Fano or Lorentzian line shapes in the near
field, respectively.Comment: 28 pages, 10 figures; invited book chapter to appear in "Fano
Resonances in Optics and Microwaves: Physics and Application", Springer
Series in Optical Sciences (2018), edited by E. O. Kamenetskii, A. Sadreev,
and A. Miroshnichenk
In situ size sorting in CVD synthesis of Si microspheres
[EN] Silicon microspheres produced in gas-phase by hot-wall CVD offer unique quality in terms of sphericity, surface smoothness, and size. However, the spheres produced are polydisperse in size, which typically range from 0.5 mu m to 5 mu m. In this work we show through experiments and calculations that thermophoretic forces arising from strong temperature gradients inside the reactor volume effectively sort the particles in size along the reactor. These temperature gradients are shown to be produced by a convective gas flow. The results prove that it is possible to select the particle size by collecting them in a particular reactor region, opening new possibilities towards the production by CVD of size-controlled high-quality silicon microspheres.The authors acknowledge financial support from the following projects: ENE2013-49984-EXP, MAT2012-35040, MAT2015-69669-P and ESP2014-54256-C4-2-R of the Spanish Ministry of Economy and Competitiveness (MINECO), and PROMETEOII/2014/026 of the Regional Valencian Government.GarĂn EscrivĂĄ, M.; Fenollosa Esteve, R.; Kowalski, L. (2016). In situ size sorting in CVD synthesis of Si microspheres. Scientific Reports. 6:1-10. https://doi.org/10.1038/srep38719S110
Light guiding and switching using eccentric core-shell geometries
High Refractive Index (HRI) dielectric nanoparticles have been proposed as an alternative to metallic ones due to their low absorption and magnetodielectric response in the VIS and NIR ranges. For the latter, important scattering directionality effects can be obtained. Also, systems constituted by dimers of HRI dielectric nanoparticles have shown to produce switching effects by playing with the polarization, frequency or intensity of the incident radiation. Here, we show that scattering directionality effects can be achieved with a single eccentric metallo-HRI dielectric core-shell nanoparticle. As an example, the effect of the metallic core displacements for a single Ag-Si core-shell nanoparticle has been analyzed. We report rotation of the main scattering lobe either clockwise or counterclockwise depending on the polarization of the incident radiation leading to new scattering configurations for switching purposes. Also, the efficiency of the scattering directionality can be enhanced. Finally, chains of these scattering units have shown good radiation guiding effects, and for 1D periodic arrays, redirection of diffracted intensity can be observed as a consequence of blazing effects. The proposed scattering units constitute new blocks for building systems for optical communications, solar energy harvesting devices and light guiding at the nanoscale level.This research was supported by MICINN (Spanish Ministry of Science and Innovation, project FIS2013-45854-P) and FundaciĂłn Iberdrola Espan~a, Call for Research on Energy and the Environment Grants. Ă.I.B. and Y.G. want to express her gratitude to the University of Cantabria for their PhD grants
Dynamics of destructive Fano resonances
We discuss the effect of a sharp variation in the input signal temporal envelope upon the output at destructive interference in Fano resonances. The interference takes place between the resonant and off-resonant partitions canceling each other. For high-Q resonances, the characteristic relaxation time for the former may be much larger than that for the latter. Then, if the variation of the input is faster than the relaxation of the resonant partition, it destroys the balance between the partitions, i.e., the conditions for the destructive interference. Therefore, the variation (whether an increase or a decrease) must give rise to a sharp spike in the uncompensated output. To illustrate these general arguments, we study numerically the scattering of a linearly polarized laser pulse of a rectangular envelope by a dielectric cylinder. The simulation reveals pronounced spikes in the scattered light, situated just behind the leading edge of the incident pulse and, counterintuitively, behind its trailing edge, when the incident pulse is already complete. The second spike is explained by the rapid release of the electromagnetic energy confined in the cylinder in the steady-state scattering owing to the destructive Fano interference. The spikes have narrow scattering diagrams so that the effect may be employed to design new nanodevices, e.g., a single-nanoparticle laser generating ultrashort pulses
Ultimate Absorption in Light Scattering by a Finite Obstacle
Based on fundamental properties of the light scattering by a particle under a plane, linearly polarized wave illumination, we rigorously prove the existence of the ultimate upper limit for the light absorption by any partial mode and calculate this limit explicitly. The limit is a certain simple universal function of the incident light wave number, and the multipolarity of the corresponding partial mode solely. It does not depend on the optical constants of the scatterer, its size, or even its shape. First, we obtain this result for the scattering by a spherical particle. Then, we generalize it to an arbitrary finite obstacle. The results are valid for any polarization of the incident wave, any angle of its incidence, and any type of the scatterer (homogeneous, stratified, or with smoothly variable refractive index). We also prove that the maximal partial absorption cross section for any finite scatterer cannot exceed the corresponding value for a homogeneous sphere in 3D and circular cylinder in 2D. As an example, the results are applied to maximize the absorption cross section of a spherical core-shell structure