Publications by authors named "Giovanni Mattei"

Metasurfaces tailor electromagnetic confinement at the nanoscale and can be appropriately designed for polarization-dependent light-matter interactions. Adding the asymmetry degree to the desing allows for circular polarizations of opposite handedness to be differently absorbed or emitted, which is of interest in fields spanning from chiral sensing to flat optics. Here, we show that simple, low-cost asymmetric metasurfaces can control Stokes parameters in the transmitted far-field.

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Developing intense, coherent and ultra-fast light sources with nanoscale dimensions is a crucial issue for many applications in nanophotonics. To date, plasmonic nanolasers represent one of the most promising nanophotonic devices capable of this remarkable feature. In the present work we report on the emission properties of two-dimensional Au hexagonal nanodome arrays, fabricated by nanosphere lithography, coupled with a dye liquid solution used as the gain medium.

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Scattering scanning near-field optical microscopes (s-SNOMs) based on pseudoheterodyne detection and operating at ambient conditions typically suffer from instabilities related to the variable optical path length of the interferometer arms. These cause strong oscillations in the measured optical amplitude and phase comparable with those of the signal and, thus, resulting in dramatic artifacts. Besides hampering the comparison between the topography and the optical measurements, such oscillations may lead to misinterpretations of the physical phenomena occurring at the sample surface, especially for nanostructured materials.

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In the quest for new and increasingly efficient photon sources, the engineering of the photonic environment at the subwavelength scale is fundamental for controlling the properties of quantum emitters. A high refractive index particle can be exploited to enhance the optical properties of nearby emitters without decreasing their quantum efficiency, but the relatively modest -factors ( ∼ 5-10) limit the local density of optical states (LDOS) amplification achievable. On the other hand, ultrahigh -factors (up to ∼ 10) have been reported for quasi-BIC modes in all-dielectric nanostructures.

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In recent years the quest for novel materials possessing peculiar abilities of manipulating light at the nanoscale has been significantly boosted due to the strict demands of advanced nanophotonics and quantum technologies. In this framework radiative decay engineering of quantum emitters is of paramount importance for developing efficient single-photon sources or nanolasers. Hyperbolic metamaterials stand out among the best cutting-edge candidates for photoluminescence control owing to their potentially unlimited photonic density of states and their ability to sustain high-k modes that allow us to strongly enhance the radiative decay rate of quantum light emitters.

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This paper reports on the fabrication and characterization of a plasmonic/sol-gel sensor for the detection of aromatic molecules. The sol-gel film was engineered using polysilsesquioxanes groups to capture the analyte, through π-π interaction, and to concentrate it close to the plasmonic surface, where Raman amplification occurs. Xylene was chosen as an analyte to test the sensor.

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Combining finite elements method electrodynamic simulations and cost-effective and scalable nanofabrication techniques, we carried out a systematic investigation and optimization of the sensing properties of non-interacting gold nanodisk arrays. Such plasmonic nanoarchitectures offer a very effective platform for fast and simple, label-free, optical bio- and chemical-sensing. We varied their main geometrical parameters (diameter and height) to monitor the plasmonic resonance position and to find the configurations that maximize the sensitivity to small layers of an analyte (local sensitivity) or to the variation of the refractive index of an embedding medium (bulk sensitivity).

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Honeycomb plasmonic lattices are paradigmatic examples of non-Bravais lattices. We experimentally measure surface lattice resonances in effectively free-standing honeycomb lattices composed of silver nanospheres. By combining numerical simulations with analytical methods, we analyze the dispersion relation and the near-field properties of these modes along high symmetry trajectories.

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A detailed structural investigation of the semiconductor-to-metal transition (SMT) in vanadium dioxide thin films deposited on sapphire substrates by pulsed laser deposition was performed by in situ temperature-dependent X-ray diffraction (XRD) measurements. The structural results are correlated with those of infrared radiometry measurements in the SWIR (2.5-5 μm) and LWIR (8-10.

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Article Synopsis
  • Researchers explored how a nanoantenna made of gold (Au) and a vanadium dioxide (VO) layer, which can switch from a semiconductor to a metallic state, can control emission properties at the nanoscale.
  • When emitters are placed between the nanoantenna and the VO layer, the unique phase change of the VO boosts or reduces the emission efficiency, resulting in a high contrast ratio.
  • The study optimized this control by adjusting the orientation and distribution of dipoles and suggested potential applications for enhanced light sources, particularly for an Er doped SiO emission layer.
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Implantable devices need specific tailored surface morphologies and chemistries to interact with the living systems or to actively induce a biological response also by the release of drugs or proteins. These customized requirements foster technologies that can be implemented in additive manufacturing systems. Here, we present a novel approach based on spraying processes that allow to control separately topographic features in the submicron range (∼60 nm to 2 μm), ammine or carboxylic chemistry, and fluorophore release even on temperature-sensitive biodegradable polymers such as polycaprolactone (PCL).

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The laser-induced plasmon heating of an ordered array of silver nanoparticles, under continuous illumination with an Ar laser, was probed by rare-earth fluorescence thermometry. The rise in temperature in the samples was monitored by measuring the temperature-sensitive photoluminescent emission of a europium complex (EuTTA) embedded in PMMA thin-films, deposited onto the nanoparticles array. A maximum temperature increase of 19 °C was determined upon resonant illumination with the surface plasmon resonance of the nanoarray at the highest pump Ar laser power (173 mW).

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In this work we present the study of the ultra-fast dynamics of the nonlinear optical response of a honeycomb array of silver triangular nanoprisms, performed using a femtosecond pulsed laser tuned with the dipolar surface plasmon resonance of the nanoarray. Nonlinear absorption and refraction, and their time-dependence, were explored using the z-scan and time-resolved excite-probe techniques. Nonlinear absorption is shown to change sign with the input irradiance and the behavior was explained on the basis of a three-level model.

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One fundamental requirement in the search for novel magnetic materials is the possibility of predicting and controlling their magnetic anisotropy and hence the overall hysteretic behavior. We have studied the magnetism of Au:Co films (~30 nm thick) with concentration ratios of 2:1, 1:1, and 1:2, grown by magnetron sputtering co-deposition on natively oxidized Si substrates. They consist of a AuCo ferromagnetic alloy in which segregated ultrafine Co particles are dispersed (the fractions of Co in the AuCo alloy and of segregated Co increase with decreasing the Au:Co ratio).

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Ordered metallic nanoprism arrays have been proposed as novel and versatile systems for the observation of nonlinear effects such as nonlinear absorption. The study of the effect of the local field reinforcement on the fast optical third order nonlinear response around the Surface Plasmon Resonance is of great interest for many plasmonic applications. In this work, silver nanoprism arrays have been synthesized by the nanosphere lithography method.

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Nanosphere lithography coupled with reactive ion etching has been used to synthesize hexagonal ordered arrays of Au-Ag bimetallic semi-nanoshells to be used as plasmonic biosensors. The degree of lateral interaction between adjacent semi-nanoshells can be controlled by tailoring the reactive ion etching time in order to boost the global plasmonic properties through the formation of near-field hot-spots, which in turn can improve the sensitivity of the biosensors. To test the efficiency of the proposed system as a biosensor, we used an established protocol for the detection of biomolecules (local sensitivity), based on the receptor-ligand approach and using the biotin-streptavidin model system.

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The nonlinear absorption properties of bidimensional arrays of Au-Ag bilayered nanoprisms have been investigated by z-scan measurements as a function of the bimetallic nanoprism composition. A tunable ps laser system was used to excite the ultrafast, electronic nonlinear response matching the laser wavelength with the quadrupolar surface plasmon resonances, in the visible range, of each nanoprism array. Due to the strong electromagnetic field confinement effects at the nanoprism tips, demonstrated by finite element method simulations, these nanosystems proved to have enhanced nonlinear optical properties.

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The very early steps of Au metal cluster formation in Er-doped silica have been investigated by high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS). A combined analysis of the near-edge and extended part of the experimental spectra shows that Au cluster nucleation starts from a few Au and O atoms covalently interconnected, likely in the presence of embryonic Au-Au correlation. The first Au clusters, characterized by a well defined Au-Au coordination distance, form upon 400 °C inert annealing.

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The occurrence of a very efficient non-resonant energy transfer process forming ultrasmall Au-Ag nanoalloy clusters and Er(3+) ions is investigated in silica. The enhancement of the room temperature Er(3+) emission efficiency by an order of magnitude is achieved by coupling rare-earth ions to molecule-like (Au(x)Ag(1-x))N alloy nanoclusters with N = 10-15 atoms and x = 0.6 obtained by optimized sequential ion implantation on Er-implanted silica.

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Plasmonic sensors based on ordered arrays of nanoprisms are optimized in terms of their geometric parameters like size, height, aspect ratio for Au, Ag or Au0.5-Ag0.5 alloy to be used in the visible or near IR spectral range.

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Sub-nanometric Au nanoclusters are known to act as very efficient sensitizers for the luminescent emission of Er(3+) ions in silica through a non-resonant broad-band energy-transfer mechanism. In the present work the energy-transfer process is investigated in detail by room temperature photoluminescence characterization of Er and Au co-implanted silica systems in which a different degree of coupling between Er(3+) ions and Au nanoclusters is obtained. The results allow us to definitely demonstrate the short-range nature of the interaction in agreement with non-radiative energy-transfer mechanisms.

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We report the achievement of sensitive gas detection using periodic silver nanoprisms fabricated by a simple and low-cost lithographic technique. The presence of sharp tips combined with the periodic arrangement of the nanoprisms allowed the excitement of isolated and interacting localized surface plasmon resonances. Specific sensing capabilities with respect to aromatic hydrocarbons were achieved when the metal nanoprism arrays were coupled in the near field with functional hybrid films, providing a real-time, label-free, and reversible methodology.

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Ultra-small molecule-like AuN nanoclusters made by a number of atoms N less than 30 were produced by ion implantation in silica substrates. Their room temperature photoluminescence properties in the visible and near-infrared range have been investigated and correlated with the Er sensitization effects observed in Er-Au co-implanted samples. The intense photoluminescence emission under 488 nm laser excitation occurs in three different spectral regions around 750 nm (band A), 980 nm (band B) and 1150 nm (band C) as a consequence of the formation of discrete energy levels in the electronic structure of the molecule-like AuN nanoclusters.

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The preparation of palladium alloy nanoparticles is of great interest for many applications, especially in catalysis. Starting from presynthesized nanoparticles of a less noble metal, a transmetallation reaction involving a redox process at the nanoparticle surface can be exploited to modify the nanoparticle composition and crystalline phase. As an example, monodispersed ε-cobalt and face-centered cubic copper nanoparticles were synthesized in organic solvents at high temperature and the as-formed nanoparticles were reacted with palladium(ii) hexafluoroacetylacetonate resulting in the formation of alloyed nanoparticles whose composition closely follows the reactant ratio.

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The quest for efficient ways of modulating localized surface plasmon resonance is one of the frontiers in current research in plasmonics; the use of a magnetic field as a source of modulation is among the most promising candidates for active plasmonics. Here we report the observation of magnetoplasmonic modes on colloidal gold nanoparticles detected by means of magnetic circular dichroism (MCD) spectroscopy and provide a model that is able to rationalize and reproduce the experiment with unprecedented qualitative and quantitative accuracy. We believe that the steep slope observed at the plasmon resonance in the MCD spectrum can be very efficient in detecting changes in the refractive index of the surrounding medium, and we give a simple proof of principle of its possible implementation for magnetoplasmonic refractometric sensing.

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