Tiziana Cesca

Local-field enhancement effect on the nonlinear optical response of gold-silver nanoplanets

We report on the nonlinear optical properties of Au-Ag nanoplanets produced by ion implantation and irradiation in silica, experimentally investigated by means of the single beam z-scan technique. The measurements provided experimental evidence of the intense local-field enhancement effect theoretically demonstrated for these plasmonic nanosystems. In particular, this has a dramatic impact on their nonlinear absorption behavior and results in a tunable changeover from reverse saturable absorption to saturable absorption by slightly varying the pump intensity and in the possibility to activate and observe nonlinear phenomena of the electron dynamics otherwise unaccessible in the intensity range that can be employed to study these materials. Finally, for the nanoplanet configuration we found a dramatic decrease of the intensity-dependent absorption coefficient, which could be very promising for obtaining optical gain materials.

Nanocluster formation in silicate glasses by sequential ion implantation procedures

Abstract Cluster formation is studied after sequential double implantation (Cu, Ni; Ag, S) in silica and soda–lime glass. The structure and properties of nanocluster composites are investigated by optical absorption spectroscopy and transmission electron microscopy, evidencing the formation of core-shell structures. The presence of metal alloy clusters is also investigated by means of synchrotron-radiation-based techniques.

SiOC thin films: an efficient light source and an ideal host matrix for Eu 2+ 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. Eu(2+) ions benefit also of the occurrence of an energy transfer mechanism involving the matrix, which increases the efficiency of photon absorption for exciting wavelengths shorter than 300 nm. We evaluate that Eu doping of SiOC produces an enhancement of the luminescence intensity at 440 nm accounting for about a factor of 15. These properties open the way to new promising perspectives for the application of Eu-doped materials in photonic and lighting technologies.

Implantation damage effects on the Er 3+ luminescence in silica

The possibility to control the room temperature Er3+ photoluminescence efficiency in silica is investigated in terms of the damage produced in Er-doped silica by implantations at different fluences with Xe or Au ions. These implantations are tailored to reproduce the same level of damage in Er-doped silica. The remarkable differences in terms of the photoluminescence intensity between Xe- and Au-irradiated samples allowed to decouple the detrimental effect of the implantation damage on the photoluminescence from the beneficial broad-band energy transfer process provided by molecule-like Au clusters formed upon thermal annealing. The evolution of the implantation damage is followed by photoluminescence and correlated to the local Er-site by x-ray absorption spectroscopy.

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. 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.

Eu 3+ reduction and efficient light emission in Eu 2 O 3 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.

Oxidation effects on the SERS response of silver nanoprism arrays

Silver nanostructures are widely employed for Surface Enhanced Raman Scattering (SERS) characterizations owing to their excellent properties of field confinement in plasmonic resonances. However, the strong tendency to oxidation at room temperature of these substrates may represent a major limitation to their performances. In the present work, we investigated in detail the effects of oxidation on the SERS response of a peculiar kind of Ag nanostructured substrates, i.e., bi-dimensional ordered arrangements of Ag nanoprisms synthesized by nanosphere lithography. Particularly, wavelength-scanned SERS measurements were performed on Ag nanoprism arrays with a different level of oxidation to determine the SERS enhancement curves as a function of the excitation wavelength around the dipolar plasmonic resonance of the arrays. The experimental results were compared with those obtained by finite elements method simulations. With this approach, we were able to decouple the effects of spectral shift and decrease of the maximum value of the SERS enhancement observed for the different oxidation conditions. The results could be interpreted taking into account the inhomogeneities of the electromagnetic field distribution around the Ag nanostructures, as demonstrated by the simulations.

Mechanisms for the activation of ion-implanted Fe in InP

In this paper we present structural and electrical investigations on high temperature Fe-implanted InP. The aim of the work is to relate the lattice position of the implanted atoms after annealing treatments (from 300to600°C) with their electrical activation as compensating deep traps and to draw a comprehensive picture of the activation mechanisms. The overall results demonstrate that the electrical behavior and the Fe2+ deep trap activation properties are strictly connected to the annealing evolution of the implant-induced damage and to the escape process of the Fe atoms from substitutional sites, which in turn is controlled by the background doping density in the substrates.

Deep levels controlling the electrical properties of Fe-implanted GaInP/GaAs

The authors investigated the electrical compensation induced by deep levels introduced in metal organic vapor phase epitaxy grown n+-InGaP∕GaAs epitaxial layers by high temperature Fe implantation. The activation of the Fe2+-related deep levels has been assessed by current-voltage analyses performed at different temperatures. In the framework of the space charge limited current model, they determined the energy location in the gap of the deep levels that control the electrical properties of the semi-insulating epilayers. A donor level which acts as an electron trap located at EC−0.5eV and a Fe-related acceptor level which is responsible for the stable increase of resistivity located at EV+0.72eV were identified.

Electrical activation of the Fe2+/3+ trap in Fe-implanted InP

We have studied the electrical activation of the Fe2+∕3+ trap in Fe-implanted InP by means of capacitance-voltage and deep level transient spectroscopy analyses. Five deep traps have been identified and we have characterized the concentration and depth distribution of the Fe2+∕3+ deep trap, located at EC–0.66eV. The InP substrate background doping, i.e., the Fermi-level position, plays a crucial role in the Fe activation process by setting an upper limit to the amount of Fe centers electrically activated as deep acceptor traps.