The thesis reports the results of an experimental research on rare earth ion-doping effects on the structural, chemical, optical and near-infra-red photoluminescence properties of wide band gap oxide films. The aim of the work was to develop materials with good photoluminescence properties, which can be applied to increase the photovoltaic conversion efficiency of crystalline Si-based solar cells, through the increase of the most efficient and useful fraction of the solar spectrum which hits the cells, thanks to a photon frequency down-shifting process. Neodymium trivalent ion (Nd3+) was used as dopant of TiO2 and ZnO thin films. The films, with different Nd concentrations were grown onto quartz by RF plasma co-sputtering and annealed at different temperatures (400°-800°C). Different film architectures were investigated for their photoluminescence properties. Structural changes such as phase transformation from anatase to rutile, internal strain building and lattice distortion due to Nd3+ incorporation in titania, correlated with optical changes, were evidenced. Exciting titania and zinc oxide matrices with optimal Nd concentrations, with ultra-violet (UV) light energies equal to or above their gap values resulted in an efficient frequency down-shifting from UV to near-infra-red emission. The joint study of the vibrational, chemical and structural properties of the doped films allows the understanding of the excitation energy transfer process between the matrix to the ion, where self-trapped excitons can be involved. To conclude this study, the doped films were tested as down-shifter layers onto a Si-solar cell where they gave promising results. They were also tested for their photoactivity with methylene blue, showing their inhibitor effect on the photo-degradation of this organic dye molecule. Keywords: Co-Sputtering, rare earth doping, Neodymium, Titanium dioxide, Zinc oxide thin films, Photoluminescence

The propagation of light through an ultracold atomic gas is the main topic of the present work. The thesis consists of two parts. In Part I (Chapters 1,2,3), we give a complete description of the 1D photonic bands of a MI of two-level atoms paying attention to both band diagrams and reflectivity spectra. The role of regular periodicity of the system is addressed within a polariton formalism. The scattering on defects inside lattices of three-level atoms is also studied in view of optical detection of impurities in such structures. The light is used as a probe of systems engineered by the use of other laser beams. Part II (Chapters 4,5) is devoted to the development of a general framework for the time-dependent processing of a propagating slow Dark Polariton in a spatially inhomogeneous system. The coherently tunable atomic gas acts as a Dynamic Photonic Structure. Applications of this concept concerning wavelength conversion and reshaping of the pulse are also discussed for realistic experimental situations.

Graphitic materials, thanks to the lamellar structure and chemical stability, are of particular interest to realize barriers against the degradation of surface properties induced by water. Many studies showed that water could be a source of degradation of surface properties. To develop a method to overcome the problem related to the deterioration of the surface it is fundamental to study the water- material interaction. For this reason, in this thesis, the water-surface interaction of graphitic- materials and the use of graphitic materials as impermeable barriers against water were explored. Different experimental set up were realized to study the liquid-gas-solid interaction, such as time evolution of the sessile water drop contact angle, captive bubble contact angle and contact angle measurements in a controlled atmosphere. Moreover, a method of deposition of protective graphene-based films using a Meyer rod to apply graphene-inks onto a surface was developed. To understand the intrinsic wettability of graphitic materials a detailed study of the gas-liquid-solid interactions of graphite was conducted in a wide range of experimental conditions. The surface chemical properties and morphology were studied by X-ray photoelectron spectroscopy (XPS), profilometry and atomic force microscopy(AFM), sessile drop contact angle, captive bubble and secondary emission microscopy (SEM). The results of the gas-liquid–surface interaction study indicated that HOPG surface was sensitive to experimental conditions like airborne contamination and the presence of gases. Similarly, a detailed study of the interaction of water with PDMS surface in various experimental conditions (in the air and immersed in water) were conducted. The findings showed that when PDMS was immersed in water, its surface changed. In fact, the volume of air bubbles in contact with the surface of PDMS increased by increasing immersion time in the water. The experimental results indicated that such dynamic evolution of the air bubbles was related to the rearrangement of surface polymer chains via the migration of the polar groups. This phenomenon induced a degradation of the surface properties of PDMS when it is immersed in water. When graphene monolayer was added to PDMS surface, it acted as a barrier against water, suppressing the dynamic evolution of the bubble. We studied the protective properties also of graphene-based films deposited on lead (Pb). We observed that Pb surface degradation occurred when Pb was in contact with a drop of water. The results showed that degradation of Pb surface in contact with water happened very rapidly but graphene-based films, in particular, graphene oxides films, were able to reduce degradation of the surface significantly.

The properties of a broad range of materials are due to processes which occur at the nanoscale. Recently, an increasing interest has been devoted to nanostructured materials, in which the basic components are nanoscopic, and low-dimensional nanomaterials such as nanoparticles, nanowires and layered materials, in which one or more dimensions are confined. This thesis deals with nanostructured materials, in particular based on graphene, such as Graphene Nanofoams and Pillared Graphene Frameworks, and low dimensional nanomaterials such as SiC/SiO2 core/shell nanowires and graphene layers. The work is divided in four parts treating four different topics with the underlying theme of material modeling, the first two parts deal with mechanical properties and gas treatment applications, for which a description at the atomistic level is adequate, while the third and the forth focus on X-ray spectra and electron holography simulations for which electronic structure calculations are needed. The present thesis gives a general overview on various computational approaches that are useful in modeling novel low dimensional and nanostructured materials, using these approaches in dealing with specific systems.

The aims of this dissertation are twofold. Firstly, the study of superfluidity, presented in the first part, has more a fundational character, since we investigate some well known problems, already raised in the context of superfluid helium, in order to gain a deeper understanding, by exploiting the cleannes of dilute BECs systems, as well as analyzing the new features emerging from the unique dilute BECs properties. We study the BEC flow through weak-links, first analyzing the various regimes of transport by the current-phase relation and the Josephson plasma oscillations, and then turning to the superfluid instability, determining critical velocities and examining the dissipation dynamics in different geometries and dimensionalities. Secondly, the analysis of quantum interferometry, given in the second part, has instead a more technological character, since we propose two possible implementation of interferometric protocols in double-well traps, with application to the measurement of weak-forces, and study their sensitivity in detail, especially in relation to its possible quantum enhancement.

Although protein folding has been studied for decades many open issues still resist, and we yet lack a clear and general description of the mechanisms leading from the unfolded to the folded state. In particular, it is still under debate whether proteins fold through few well-defined pathways or trough a large multitude of independent ways. Answering these questions is made difficult by the fact that standard molecular dynamics (MD) simulations are very computationally expensive and often impracticable. Moreover, often even experimental techniques lack the necessary resolution to give a definitive answer. We will introduce and develope the Dominant Reaction Pathway (DRP), which is an approach that permits to efficiently study the thermally activated conformational dynamics of bio-molecules in atomistic detail. In particular, it can be used to characterize and portray the folding pathways of a protein once the unfolded and folded configurations are given. We firstly applied the DRP to a realistic protein studying the folding pathways of the Fip35 WW Domain, a 35 amino-acids long protein. Performing all atom simulations we were able to show that this small protein folds following only two pathways, defined by the order of formation of secondary structures. Notably, our results are compatible with ultra long MD simulations and consistent with the analysis of the experimental available data on the folding kinetics of the same system. Exploiting the efficiency of the DRP formalism, computing a folding trajectory of this protein only required about one hour on 48 CPU’s. We applied then our simulation scheme to a much more challenging task: performing an all-atom folding simulation of a 82 amino-acids long protein displaying a topological knot in its native conformation. We were able to portray the folding mechanism and to identify the essential key contacts leading to the proper formation of this knot. Interestingly, we showed that non native contacts, i.e., transient contacts formed during the folding of the protein but absent in its native state, can sensibly enhance the probability of correctly forming the knot.

The subject of this thesis is the use of BECs for atom interferometry. The standard way atom interferometry is today performed is by interrogating free falling samples of atoms. The employed samples are cold (but not condensed) to have high coherence, and dilute, not to interact significantly with each other. This technique represents nowadays an almost mature field of research in which the achievable interferometric sensitivity is bounded by the atomic shot noise. Until a few years ago the employment of BECs in such devices was strongly limited by the effect of the interactions between the condensed atoms. This obstacle is today removable exploiting interaction tuning techniques. The use of BECs would be advantageous for atom interferometry inasmuch they represents the matter analogue of the optical laser providing the maximum coherence allowed by quantum mechanics. Moreover, non-linear dynamic can be exploited in order to prepare entangled states of the system. The realization of entangled samples can lead to sub-shot noise sensitivity of the interferometers. At today very nice proof-of-principle experiments have been realized in this direction but a competitive device is still missing. This thesis work is inserted in a long term project whose goal is the realization of such a device. The basic operational idea of the project starts with the preparation of a BEC in a double well potential. By the effect of strong interactions the atomic system can be driven into an entangled state. Once the entangled state is prepared, interactions can be †switched off†and the interferometric sequence performed. This thesis begins with the description of the apparatus for the production of tunable BECs to be used in the interferometer. We chose to work with 39K atoms because this atomic species presents many convenient Feshabch resonances at easily accessible magnetic field values. The cooling of this particular atomic species presents many difficulties, both for the laser and evaporative cooling processes. For this reason, this was the last alkaline atom to be condensed. Its condensation up to now was only possible by employing sympathetic cooling with another species. In this thesis our solutions to the various cooling issues is reported. In particular we realized sub-Doppler cooling for the first time for this species and we achieved condensation via evaporation in an optical dipole trap taking advantage of a Feshbach resonance. In the last part of this work, are presented original calculations for the effects of thermal fluctuations on the coherence of a BEC in a double well, discussing the interplay between thermal fluctuations and interactions in this system. Estimations and feasibility studies regarding the double well trap to be realized are also reported.

In this work we will face the problem of the vibrational properties of glasses focusing on the origin and nature of the boson peak (BP). This feature is an universal characteristic of glasses and a fingerprint of the presence of disorder. Two samples have been chosen for this study. The first is permanently densified vitreous SiO$_2$. Permanent densification has been exploited to tune the glassy properties focusing on their evolution. The second sample is a silicon-selenium glass whose low sound velocity allows a detailed study of its dynamics by means of neutron inelastic scattering.

In this thesis we address relevant problems of the physics of quantum disordered systems from a numerical and theoretical point of view, with specific attention to the connection of our findings with ultracold atomic gases experiments. We concentrate on two main issues: the interplay between localization and interaction in disordered systems and the problem of localization in correlated random potentials. The first problem is investigated considering the expansion of a weakly interacting Bose gas in a bichromtic optical lattice. We observe that interaction has a destructive effect on the disorder-induced localization and leads to a subdiffusive expansion of the atomic gas. By comparing three characteristic energy scales of the system one can identify three different spreading regimes: weak chaos, strong chaos and self-trapping. The spreading behaviour in these regimes is predicted theoretically and verified numerically. We also interpreted existing experimental data on the basis of our findings and showed that there is a qualitative agreement between our numerical simulations and experiments. The second problem is investigated proposing a new model of correlated disorder that can be implemented experimentally using ultracold dipolar gases. We show that this model is characterized by the presence of both short and long range correlations. We study the localization properties of the model and highlight the role played by short and long range correlations in the determination of those properties. In particular we show that when short-range correlations are dominant, extended states can appear in the spectrum. The effect of long-range correlations is instead to restore localization over the whole spectrum and lead to counterintuitive behaviours of the localization length. More precisely, depending on the localization regime they can enhance or reduce the localization length at the centre of the band.

In this Thesis we will theoretically address some issues concerning the Physics of ultracold Fermi gases, all of them with experimental relevance. The field of ultracold gases, and more recently of ultracold Fermi gases, is gathering a lot of experimental and theoretical interest for two main reasons: the great control on the relevant parameters of the problem and the relative simplicity of the minimal theory which is able to correctly describe the system. Building upon this solid background, ultracold gases provided an ideal laboratory for testing more refined theories and also for addressing fundamental issues of quantum mechanics as well as for simulating more complex physical systems such as those encountered in condensed matter. Even if the effective hamiltonian of ultracold gases can be simple, due to the diluteness of the system and the low temperature, which imply low energy physics, the solution of the quantum mechanic equations governing the state of these systems is not always simple. For the static properties usually mean-field solutions exist or perturbative expansions can be produced in some regimes. However Quantum Monte Carlo (QMC) techniques provide more accurate results especially in the strongly interacting regimes. For confined systems it is possible to use QMC only for a few particles, so that, for large number of particles, a fruitful combined use of Density Functional Theory (DFT) and QMC is necessary. The study of the dynamics of ultracold gases has received little attention with QMC techniques, due to the intrinsic computational difficulty of the many-body problem, so that general hydrodynamic equations are often used for studying the propagation of smoothly varying perturbations. In this Thesis we use QMC techniques for studying the problem of ferromagnetism in repulsive or effectively repulsive ultracold gases without a lattice and the problem of the Bardeen-Cooper-Schrieffer to Bose-Einstein-Condensation (BCS-BEC) crossover in two dimensions. We use DFT in the Local Density Approximation (LDA) for calculating the density profiles of ultracold Fermi gases in harmonic magneto-optical traps, starting from QMC equations of state. We study the propagation of first and second sound in ultracold Fermi gases in cylindrical geometry, using the hydrodynamic equations of superfluids.