CESCA TIZIANA

Control of the quantum efficiency of rare-earth emitters Erbium has an important role in the optical telecommunication field for the realization of devices as light amplifiers, since its characteristic emission at 1.54 μm corresponds to the window of minimum absorbance of silica-based optical fibers. The research activity in this field is focused on investigating the mechanisms to control and enhance the quantum efficiency of rare-earth (RE) emitters in dielectric matrices to obtain novel photon sources in the visible and NIR (nanolasers, single photon sources). The control of the emission efficiency is investigated by coupling with sub-nanometric Au or Ag molecular nanostructures (to increase the excitation cross-section) and coupling with plasmonic metasurfaces with engineered local density of states (nanohole arrays, nanoprism arrays, hyperbolic metamaterials) to enhance the emission decay rate. The photoluminescence analyses (emission intensity and lifetime) are related to advanced structural characterizations as synchrotron radiation techniques (EXAFS, XANES) and transmission electron microscopy in order to determine the best synthesis and treatment conditions for the optimization of the optical performances of these systems.

Electrical and optical properties of III-V and wide bandgap semiconductors The research activity is performed both on InP and In-based ternary alloys as GaInP grown lattice matched on GaAs, and on wide band-gap semiconductors as GaN or nanostructured thin films of ZnO. The study is aimed at the understanding of the basic physical mechanisms that control the interaction among impurities, defects and atoms of the crystalline matrix. The impurities are incorporated in the crystalline matrices by high temperature ion implantation. For its nonequilibrium character this technique allows to overcome the thermodynamic limitations of the equilibrium growth techniques, which are related to the low solid solubility of the impurities, and therefore to the realization of layers containing a high concentration of impurities. Moreover, the high implantation temperature allows to strongly reduce the lattice damage of the matrix and to increase of several orders of magnitude the concentration of active impurities owing to dynamical annealing processes of the atoms of the semiconductor crystalline host. From a structural point of view, the samples are investigated with ion beam spectrometric techniques (SIMS, RBS-PIXE-channeling), X-ray diffraction and X-ray absorption spectroscopy (XAS) in order to study the modifications induced on the crystalline structure and on the lattice location of the implanted impurities. Besides, electrical characterizations combined with simulations of the transport properties are performed and allow to investigate the electronic properties of the incorporated impurities and the electrical activation/deactivation processes of the material. The optical properties of these materials are also studied by photoluminescence and electroluminescence measurements.

Metasurfaces obtained with 2D arrays of plasmonic nanostructures The possibility of realizing 2D ordered arrays of metallic nanoparticles, in particular of noble metals (Au, Ag, Cu), has interesting applications in different fields, as plasmonics and optical sensing of organic and inorganic molecules. Most of these applications are related to the capability of these structures to sustain effectively L-SP (localized surface plasmon) or E-SP (extended surface plasmon) modes at optical frequencies, with consequently strong effects of enhancement of the local electromagnetic field in proximity of these nanostructures. The research activity in this field is aimed to the realization of 2D arrays of nanotriangles and of nanohole arrays (NHA) obtained combining nanosphere lithography (NSL), where the metallic species (mainly Au and Ag) is deposited by sputtering or evaporation through a monolayer of polystyrene nanospheres self-assembled on silica or silicon substrates, and reactive ion etching (RIE) processes. By controlling the dimensions of the nanospheres and properly tuning the deposition parameters it is possible to obtain nanostructures with hexagonal pattern ordered over areas of the order of cm2 formed by nanotriangles with a wide range of sizes (from 50 to 1000 nm), whose surface plasmon resonances can be shifted in the visible and near-infrared spectrum continuously, obtaining optically active nanoarchitectures with interesting properties as extraordinary optical transmission, dichroism or extrinsic chirality.

Study of the nonlinear optical response of plasmonic nanostructures The research is devoted to the study of the linear and nonlinear optical response of plasmonic nanostructures. In particular, the nonlinear optical properties (nonlinear refractive index and nonlinear absorption coefficient) of nanostructures as nanoplanets (systems obtained by ion irradiation of nanocluster containing glasses and made of a central core cluster surrounded of small satellite clusters) and ordered arrays of nanotriangles obtained by nanosphere lithography are investigated by the single laser beam technique known as “z-scan”. The measurements are performed with laser excitation in the picoseconds regimes in order to measure the fast electronic component of the optical nonlinearity avoiding the occurrence of thermal effects. The results have shown how the strong local-field enhancement effects demonstrated to occur in these systems (in particular in the nanoplanets) affect their nonlinear optical response activating the concurrent presence of saturable absorption and reverse saturable absorption phenomena. The mechanisms have been effectively interpreted by finite elements electrodynamic simulations.

Synthesis and characterization of phase-change materials thin films The phase-change materials (PCM) are active materials characterized by a semiconductor-to-metal phase transition, and consequently a change of their electrical and optical properties, which can be induced thermally but also electrically and optically. Among the PCMs, vanadium dioxide is of great interest and it finds application in many different fields, in nanophotonics, sensoristics, spintronics, to name a few. The research activity in this field is focused on the synthesis of thin films of PCMs and the characterization of their structural and phase-transition properties. It is also investigated the coupling of these materials with quantum emitters to get an active control of the emission properties.