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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. With only 50 nm of asymmetric plasmonic shells on self-assembled polystyrene nanospheres, our metasurfaces allow for great spectral and incident angle tunability. We first investigated broadband extrinsic chirality in metasurfaces with asymmetric plasmonic semishells; we found high extinction circular dichroism (CD) in the near-infrared range. We then excited it with linear polarization and performed hyperspectral Stokes polarimetry on the transmitted field. We showed that the S3 parameter follows the behavior of CD in extinction, and that the output field position on the Poincaré sphere can be widely controlled by using the incidence angle and wavelength. Furthermore, simulations agreed well with the experiments and showed how the near-field chiro-optical response influences the extrinsic chiral behavior in absorption and the polarization state of the transmitted field.

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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 (8-10.6 µm) spectral ranges. The main results indicate a good agreement between XRD and optical analysis, therefore demonstrating that the structural transition from monoclinic to tetragonal phases is the dominating mechanism for controlling the global properties of the SMT transition. The picture that emerges is a SMT transition in which the two phases (monoclinic and tetragonal) coexist during the transition. Finally, the thermal hysteresis, measured for thin films with different thickness, showed a clear dependence of the transition temperature and the width of the hysteresis loop on the film thickness and on the size of the crystallites.

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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. We also tested the sensitivity to a homogeneous change in the refractive index of the buffer over the sensor (bulk sensitivity). Comparing the obtained results to those of an array of nanoprisms, chosen as a benchmark, significantly higher performances both in local and bulk sensitivities have been found, in agreement with electrodynamics simulations based on finite-element methods.

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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. Moreover, a tunable changeover from reverse saturable absorption (RSA) to saturable absorption (SA) can be obtained by properly controlling the bimetallic composition of the nanoprisms, without modifying the overall morphology of the nanosystems. This capability makes these nanosystems extremely interesting for the realization of solid-state nanophotonic devices with enhanced ultrafast nonlinear optical properties.

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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. For comparison, AuN nanoclusters obtained by the same approach and with the same size and numerical density showed an enhancement by only a factor of 2 with respect to pure Er emission, demonstrating the beneficial effect of using nanoalloyed clusters. The temperature evolution of the energy transfer process is investigated by photoluminescence and exhibits a maximum efficiency at about 600 °C, where the clusters reach the optimal size and the silica matrix completely recovers the implantation damage. The nanoalloy cluster composition and size have been studied by EXAFS analysis, which indicated a stronger Ag-O interaction with respect to the Au-O one and a preferential location of the Ag atoms at the nanoalloy cluster surface.

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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. The estimated upper limit of the Gibbs free energy for the associated heterogeneous nucleation is 0.06 eV per atom, suggesting that the Au nucleation is assisted by matrix defects, most likely non-bridging oxygen atoms. The experimental results indicate that the formed subnanometer Au clusters can be applied as effective core-shell systems in which the Au atoms of the 'core' develop a metallic character, whereas the Au atoms in the 'shell' can retain a partially covalent bond with O atoms of the silica matrix. High structural disorder at the Au site is found upon neutral annealing at a moderate temperature (600 °C), likely driven by the configurational disorder of the defective silica matrix. A suitable choice of the Au concentration and annealing temperature allows tailoring of the Au cluster size in the sub-nanometer range. The interaction of the Au cluster surface with the surrounding silica matrix is likely responsible for the infrared luminescence previously reported on the same systems.

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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. The two figures of merit used for the optimization are the bulk and the surface sensitivity: the first is important for optimizing the sensing to large volume analytes whereas the latter is more important when dealing with small bio-molecules immobilized in close proximity to the nanoparticle surface. A comparison is made between experimentally obtained nanoprisms arrays and simulated ones by using Finite Elements Methods (FEM) techniques.

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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. Moreover, an upper limit to the interaction length is also set by the Au-Au intercluster semi-distance which is smaller than 2.4 nm in the present case.

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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. Ultra-high-vacuum temperature-programmed desorption measurements demonstrated an interaction energy between the sensitive film and analytes in the range of 55-71 kJ/mol. The far-field optical properties and the detection sensitivity of the sensors, modeled using a finite element method, were correlated to experimental data from gas sensing tests. An absorbance variation of 1.2% could be observed and associated with a theoretical increase in the functional film refractive index of ∼0.001, as a consequence to the interaction with 30 ppm xylene. The possibility of detecting such a small variation in the refractive index suggests the highly promising sensing capabilities of the presented technique.

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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. Indeed, energy maxima of bands A and C scale with N(-1/3) as expected for quantum confined systems. Conversely, the energy maximum of band B appears to be almost independent of size, suggesting a contribution of electronic surface states. A clear correlation between the formation of band B in the samples and Er-related photoemission is demonstrated: the band at 980 nm related to AuN nanoclusters resonant with the corresponding Er(3+) absorption level, is suggested as an effective de-excitation channel through which the Au-related photon energy may be transferred from Au nanoclusters to Er ions (either directly or mediated by photon absorption), eventually producing the Er-related infrared emission at 1540 nm.