Editorial for the Special Issue on Nanomaterials Photonics.

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Extrinsic chirality tailors Stokes parameters in simple asymmetric metasurfaces.

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.

Polarized coherent emission outside high-symmetry points of dye-coupled plasmonic lattices.

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.

Active stabilization of a pseudoheterodyne scattering scanning near field optical microscope.

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.

Quasi-BIC Modes in All-Dielectric Slotted Nanoantennas for Enhanced Er Emission.

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.

Selective Control of Eu Radiative Emission by Hyperbolic Metamaterials.

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.

Optimal geometry for plasmonic sensing with non-interacting Au nanodisk arrays.

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).

Diffractive dipolar coupling in non-Bravais plasmonic lattices.

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.

Correlation between in situ structural and optical characterization of the semiconductor-to-metal phase transition of VO thin films on sapphire.

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.

Control of Au nanoantenna emission enhancement of magnetic dipolar emitters by means of VO phase change layers.

Active, ultra-fast external control of the emission properties at the nanoscale is of great interest for chip-scale, tunable and efficient nanophotonics. Here we investigated the emission control of dipolar emitters coupled to a nanostructure made of an Au nanoantenna, and a thin vanadium dioxide (VO) layer that changes from semiconductor to metallic state. If the emitters are sandwiched between the nanoantenna and the VO layer, the enhancement and/or suppression of the nanostructure's magnetic dipole resonance enabled by the phase change behavior of the VO layer can provide a high contrast ratio of the emission efficiency.

Rare-earth fluorescence thermometry of laser-induced plasmon heating in silver nanoparticles arrays.

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).

Ultra-fast dynamics in the nonlinear optical response of silver nanoprism ordered arrays.

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.

GaN-Based Laser Wireless Power Transfer System.

The aim of this work is to present a potential application of gallium nitride-based optoelectronic devices. By using a laser diode and a photodetector, we designed and demonstrated a free-space compact and lightweight wireless power transfer system, whose efficiency is limited by the efficiency of the receiver. We analyzed the effect of the electrical load, temperature, partial absorption and optical excitation distribution on the efficiency, by identifying heating and band-filling as the most impactful processes.

Spectral dependence of nonlinear absorption in ordered silver metallic nanoprism arrays.

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.

Gold-silver alloy semi-nanoshell arrays for label-free plasmonic biosensors.

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.

Nonlinear absorption tuning by composition control in bimetallic plasmonic nanoprism arrays.

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.

Core-shell-like Au sub-nanometer clusters in Er-implanted silica.

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.

Au-Ag nanoalloy molecule-like clusters for enhanced quantum efficiency emission of Er³⁺ ions in silica.

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.

Optimal geometric parameters of ordered arrays of nanoprisms for enhanced sensitivity in localized plasmon based sensors.

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.

Energy-transfer from ultra-small Au nanoclusters to Er³⁺ ions: a short-range mechanism.

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.

Near-infrared room temperature luminescence of few-atom Au aggregates in silica: a path for the energy-transfer to Er³⁺ ions.

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.

SiOC thin films: an efficient light source and an ideal host matrix for Eu2+ ions.

The intense luminescence of SiOC layers is studied and its dependence on the parameters of the thermal annealing process elucidated. Although the emission of SiOC is bright enough to be interesting for practical applications, this material is even more promising as a host matrix for optically active Eu ions. Indeed, when incorporated in a SiOC matrix, Eu(3+) ions are efficiently reduced to Eu(2+), producing a very strong visible luminescence peaked at 440 nm.

Eu3+ reduction and efficient light emission in Eu2O3 films deposited on Si substrates.

A stable Eu3+ → Eu2+ reduction is accomplished by thermal annealing in N2 ambient of Eu2O3 films deposited by magnetron sputtering on Si substrates. Transmission electron microscopy and x-ray diffraction measurements demonstrate the occurrence of a complex reactivity at the Eu2O3/Si interface, leading to the formation of Eu2+ silicates, characterized by a very strong (the measured external quantum efficiency is about 10%) and broad room temperature photoluminescence (PL) peak centered at 590 nm. This signal is much more efficient than the Eu3+ emission, mainly consisting of a sharp PL peak at 622 nm, observed in O2-annealed films, where the presence of a SiO2 layer at the Eu2O3/Si interface prevents Eu2+ formation.