118 research outputs found
Development of a high brightness ultrafast Transmission Electron Microscope based on a laser-driven cold field emission source
We report on the development of an ultrafast Transmission Electron Microscope
based on a cold field emission source which can operate in either DC or
ultrafast mode. Electron emission from a tungsten nanotip is triggered by
femtosecond laser pulses which are tightly focused by optical components
integrated inside a cold field emission source close to the cathode. The
properties of the electron probe (brightness, angular current density,
stability) are quantitatively determined. The measured brightness is the
largest reported so far for UTEMs. Examples of imaging, diffraction and
spectroscopy using ultrashort electron pulses are given. Finally, the potential
of this instrument is illustrated by performing electron holography in the
off-axis configuration using ultrashort electron pulses.Comment: 23 pages, 9 figure
Origin of Second Harmonic Generation from individual Silicon Nanowires
We investigate Second Harmonic Generation from individual silicon nanowires
and study the influence of resonant optical modes on the far-field nonlinear
emission. We find that the polarization of the Second Harmonic has a
size-dependent behavior and explain this phenomenon by a combination of
different surface and bulk nonlinear susceptibility contributions. We show that
the Second Harmonic Generation has an entirely different origin, depending on
whether the incident illumination is polarized parallel or perpendicularly to
the nanowire axis. The results open perspectives for further geometry-based
studies on the origin of Second Harmonic Generation in nanostructures of
high-index centrosymmetric semiconductors.Comment: 7 Pages, 4 Figures + 3 Pages, 6 Figures in Appendi
Engineered Near and Far Field Optical Response of Dielectric Nanostuctures using Focused Cylindrical Vector Beams
Near- and far-field optical properties of silicon nanostructures under linear
polarization (Gaussian beam), and azimuthally or radially focused cylindrical
vector beams are investigated by finite-difference time-domain method (FDTD) in
Meep open-source software. A python toolkit allowing FDTD simulations in Meep
for using those excitation sources is provided. In addition to the preferential
excitation of specific electric or magnetic resonance modes as function of the
excitation beam polarization, it is shown in the case of spheroids that shape
anisotropy affects the resonance wavelength and the dipole orientation of the
magnetic or electric dipole mode. For radial or linear polarization, the
electric dipole resonance is split by an anapole mode depending on the spheroid
symmetry axis with respect to the electric field orientation. Finally, the
optical properties in both far-field (scattering pattern) and near-field
(electric and magnetic field hot spots) can be tuned by changing the excitation
polarization at a fixed wavelength and selecting properly the spheroid shape
and dimensions. These numerical simulations can be extended to more complex
shapes, or fabrication-friendly nanostructures such as nanocylinders with
circular or elliptic sections
Manipulating and squeezing the photon local density of states with plasmonic nanoparticle networks
International audienceIn this Brief Report, we show that when interconnected networks of gold particles are deposited onto a clean planar surface, they strongly modify the photonic local density of states LDOS in the immediate proximity of the self-assembled nanoparticles. They represent unique architectures for the subwavelength patterning of initially ïŹat photonic LDOS. Moreover, we show that their local spectral signatures are well suited for the generation of sites able to enhance molecular ïŹuorescence intensity
Optimal polarization conversion in coupled dimer plasmonic nanoantennas for metasurfaces
We demonstrate that polarization conversion in coupled dimer antennas, used in phase discontinuity metasurfaces, can be tuned by careful design. By controlling the gap width, a strong variation of the coupling strength and polarization conversion is found between capacitively and conductively coupled antennas. A theoretical two-oscillator model is proposed, which shows a universal scaling of the degree of polarization conversion with the energy splitting of the symmetric and antisymmetric modes supported by the antennas. Using single antenna spectroscopy, we find good agreement for the scaling of mode splitting and polarization conversion with gap width over the range from capacitive to conductive coupling. Next to linear polarization conversion, we demonstrate single-antenna linear to circular polarization conversion. Our results provide strategies for phase-discontinuity metasurfaces and ultracompact polarization optics
Directional silicon nano-antennas for quantum emitter control designed by evolutionary optimization
We optimize silicon nano-antennas to enhance and steer the emission of local
quantum sources. We combine global evolutionary optimization (EO) with
frequency domain electrodynamical simulations, and compare design strategies
based on resonant and non-resonant building blocks. Specifically, we
investigate the performance of models with different degrees of freedom but
comparable amount of available material. We find that simpler geometric models
allow significantly faster convergence of the optimizer, which, expectedly,
comes at the cost of a reduced optical performance. We finally analyze the
physical mechanisms underlying the directional emission that also comes with an
emission rate enhancement, and find a surprising robustness against
perturbations of the source emitter location. This makes the structures highly
interesting for actual nano-fabrication. We believe that optimized,
all-dielectric silicon nano-antennas have high potential for genuine
breakthroughs in a multitude of applications in nanophotonics and quantum
technologies.Comment: 8 pages, 6 figure
Structured ZnO-based contacts deposited by non-reactive rf magnetron sputtering on ultra-thin SiO2/Si through a stencil mask
In this paper, we study the localized deposition of ZnO micro and nanostructures deposited by non-reactive rf-magnetron sputtering through a stencil mask on ultra-thin (10 nm) SiO2 layers containing a single plane of silicon nanocrystals (NCs), synthetized by ultra-low energy ion implantation followed by thermal annealing. The localized ZnO-deposited areas are reproducing the exact stencil mask patterns. A resistivity of around 5Ă10â3 Ω cm is measured on ZnO layer. The as-deposited ZnO material is 97% transparent above the wavelength at 400 nm. ZnO nanostructures can thus be used as transparent electrodes for Si NCs embedded in the gate-oxide of MOS devices
Ultrafast optical response of metallic nano-objects: size effects
Ce travail a porté sur l'étude expérimentale de la dynamique électronique et vibrationnelle dans les nanoparticules de métaux nobles par des techniques pompe/sonde femtosecondes basées sur un oscillateur titane-saphir. Dans un premier temps nous nous sommes intéressés à la thermalisation électrons-réseaux. Une étude en fonction de l'énergie des impulsions de pompe montre la transition entre un régime de forte perturbation et un régime de faible perturbation. Dans ce dernier cas, les temps caractéristiques mesurés sont indépendants de la puissance de pompe et diminuent avec la taille de la nanoparticule pour des diamÚtres inférieurs à 10 nm. Cette accélération du transfert d'énergie électrons-réseau a été attribuée à une diminution de l'écrantage de l'interaction coulombienne au voisinage des surfaces. Des études préliminaires sur des nanoparticules dont la surface a été modi fiée ont été réalisées. Nous avons ensuite étudié les modes de vibrations acoustiques de nanoparticules métalliques. La contribution au signal dans le domaine temporel de modes radiaux d'ordres supérieurs a été mise en évidence. En réalisant le contrÎle optique de ces oscillations, nous avons pu observer sélectivement le mode harmonique radial d'ordre 1 et extraire ses caractéristiques. ParallÚlement, une méthode optique de détection et de mesure directe de l'extinction d'une nanoparticule individuelle a été développée. Elle consiste à moduler périodiquement la position d'un échantillon formé par des nanoparticules déposées sur un substrat transparent à trÚs faible densité. L'absorption par une nanoparticule d'un faisceau laser fortement focalisé se traduit par une modulation de l'énergie transmise, qui est détectée. Cette méthode a été utilisée pour mesurer la section e fficace d'extinction de nanoparticules d'or isolées jusqu'à un diamÚtre de 5 nm. Les valeurs obtenues sont en bon accord avec les prédictions de la théorie de Mie réalisées en prenant une constante diélectrique effective.Electronic and vibrational dynamics have been studied in noble metal nanoparticles with high sensitivity femtosecond pump-probe techniques using a Ti :Sapphire oscillator. The electron-lattice thermalization has fi rst been studied. The evolution from a weak excitation to a strong excitation regime has been analyzed performing measurement as a function of the pump pulse energy. In the former case, the measured characteristic times are independent of the pump power and decrease with the particle size for diameters smaller than 10 nm. This acceleration of the energy transfer from the electron gas to the lattice has been ascribed to the reduction of the screening of the coulomb interaction close to the surface. The acoustic vibrations of metal nanoparticles have then been studied. Contribution of higher order modes to the time-domain signal has been demonstrated. Optical control of the acoustic motion of the particles has permitted selective investigation of the n = 1 radial mode and direct determination of its characteristics, period and damping time. Finally, a new optical method to detect individual nanoparticles and quantitatively measure their extinction cross section has been developed. This method consists in periodically modulating the position of a sample made of nanoparticles deposited on a glass substrate with low density. Absorption by a single nanoparticle of a tightly focused laser beam leads to a modulation of the transmitted energy, that is detected by a lockin ampli fier. The extinction cross section of single gold nanoparticles of diameters down to 5 nm has thus been determined. The obtained values are in good agreement with those calculated from the Mie theory using an e ffective dielectric constant approach
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