13 research outputs found
Evolutionary optimization of all-dielectric magnetic nanoantennas
Magnetic light and matter interactions are generally too weak to be detected,
studied and applied technologically. However, if one can increase the magnetic
power density of light by several orders of magnitude, the coupling between
magnetic light and matter could become of the same order of magnitude as the
coupling with its electric counterpart. For that purpose, photonic nanoantennas
have been proposed, and in particular dielectric nanostructures, to engineer
strong local magnetic field and therefore increase the probability of magnetic
interactions. Unfortunately, dielectric designs suffer from physical
limitations that confine the magnetic hot spot in the core of the material
itself, preventing experimental and technological implementations. Here, we
demonstrate that evolutionary algorithms can overcome such limitations by
designing new dielectric photonic nanoantennas, able to increase and extract
the optical magnetic field from high refractive index materials. We also
demonstrate that the magnetic power density in an evolutionary optimized
dielectric nanostructure can be increased by a factor 5 compared to state of
the art dielectric nanoantennas. In addition, we show that the fine details of
the nanostructure are not critical in reaching these aforementioned features,
as long as the general shape of the motif is maintained. This advocates for the
feasibility of nanofabricating the optimized antennas experimentally and their
subsequent application. By designing all dielectric magnetic antennas that
feature local magnetic hot-spots outside of high refractive index materials,
this work highlights the potential of evolutionary methods to fill the gap
between electric and magnetic light-matter interactions, opening up new
possibilities in many research fields.Comment: 13 pages, 4 figure
Ultracompact and unidirectional metallic antennas
International audienceWe investigate the angular redistribution of light radiated by a single emitter located in the vicinity of dipolar silver nanoparticles. We point out the fundamental role of the phase differences introduced by the optical path difference between the emitter and the particle and demonstrate that the polarizability of the metallic nanoparticle alone cannot predict the emission directionality. In particular, we show that collective or reflective properties of single nanoparticles can be controlled by tuning the distance of a single emitter at a λ/30 scale. These results enable us to design unidirectional and ultracompact nanoantennas composed of just two coupled nanoparticles separated by a distance achievable with biological linkers
Fabrication of poly-crystalline Si-based Mie resonators via amorphous Si on SiO2 dewetting (vol 8, pg 2844, 2016)
International audienc
Fabrication of poly-crystalline Si-based Mie resonators via amorphous Si on SiO2 dewetting
International audienceWe report the fabrication of Si-based dielectric Mie resonators via a low cost process based on solid-state dewetting of ultra-thin amorphous Si on SiO2. We investigate the dewetting dynamics of a few nanometer sized layers annealed at high temperature to form submicrometric Si-particles. Morphological and structural characterization reveal the polycrystalline nature of the semiconductor matrix as well as rather irregular morphologies of the dewetted islands. Optical dark field imaging and spectroscopy measurements of the single islands reveal pronounced resonant scattering at visible frequencies. The linewidth of the low-order modes can be ∼20 nm in full width at half maximum, leading to a quality factor Q exceeding 25. These values reach the state-of-the-art ones obtained for monocrystalline Mie resonators. The simplicity of the dewetting process and its cost-effectiveness opens the route to exploiting it over large scales for applications in silicon-based photonics
Correction: Fabrication of poly-crystalline Si-based Mie resonators via amorphous Si on SiO2 dewetting
International audienceCorrection for ‘Fabrication of poly-crystalline Si-based Mie resonators via amorphous Si on SiO2 dewetting’ by Meher Naffouti, et al., Nanoscale, 2016, 8, 2844–2849
Laboratory-Produced X-Ray Photoionized Plasmas for Astrophysics Exploration
X-ray photoionized plasmas are rare in the laboratory, but of broad importance in astrophysical objects such as active galactic nuclei, x-ray binaries. Indeed, existing models are not yet able to accurately describe these plasmas where ionization is driven by radiation rather than electron collisions. Here, we describe an experiment on the LULI2000 facility whose versatility allows for measuring the X-ray absorption of the plasma while independently probing its electron density and temperature. The bright X-ray source is created by the two main beams focused inside a gold hohlraum and is used to photoionise a Neon gas jet. Then, a thin gold foil serves as a source of backlit photons for absorption spectroscopy. The transmitted spectrum through the plasma is collected by a crystal spectrometer. We will present the experimental setup used to characterize both plasma conditions and X-ray emission. Then we will show the transmitted spectra through the plasma to observe the transition from collision dominated to radiation dominated ionization and compare it to model predictions. This work was performed under the auspices of the U.S.Department of Energy by Lawrence Livermore Natl Lab under Contract No. DE-AC52-07NA27344
Influence of magnetic field strength on magnetic resonance imaging radiomics features in brain imaging, an in vitro and in vivo study
International audienceBackground: The development and clinical adoption of quantitative imaging biomarkers (radiomics) has established the need for the identification of parameters altering radiomics reproducibility. The aim of this study was to assess the impact of magnetic field strength on magnetic resonance imaging (MRI) radiomics features in neuroradiology clinical practice.Methods: T1 3D SPGR sequence was acquired on two phantoms and 10 healthy volunteers with two clinical MR devices from the same manufacturer using two different magnetic fields (1.5 and 3T). Phantoms varied in terms of gadolinium concentrations and textural heterogeneity. 27 regions of interest were segmented (phantom: 21, volunteers: 6) using the LIFEX software. 34 features were analyzed.Results: In the phantom dataset, 10 (67%) out of 15 radiomics features were significantly different when measured at 1.5T or 3T (student’s t-test, p < 0.05). Gray levels resampling, and pixel size also influence part of texture features. These findings were validated in healthy volunteers.Conclusions: According to daily used protocols for clinical examinations, radiomic features extracted on 1.5T should not be used interchangeably with 3T when evaluating texture features. Such confounding factor should be adjusted when adapting the results of a study to a different platform, or when designing a multicentric trial
Large scale fabrication of silicon Mie nanoresonators: an alternative to gold?
High index dielectric nanoparticles have been proposed for many different applications. The multiples optical properties of such called Mie resonators allowed us to question there use as an alternative to gold. Actually, the optical resonnances, coupled with a strong nearfield can open perspective in plasmonic uses. Widespread utilization in practice also requires largescale production methods for crystalline silicon nanoparticles. We demonstrate a lowcost, and largescale fabrication method of crystalline spherical silicon colloidal Mie resonators in water, using a blender