Roberto Righini

Localization of light in a disordered medium

Among the unusual transport properties predicted for disordered materials is the Anderson localization phenomenon. This is a disorder-induced phase transition in the electron-transport behaviour from the classical diffusion regime, in which the well-known Ohms law holds, to a localized state in which the material behaves as an insulator. The effect finds its origin in the interference of electrons that have undergone multiple scattering by defects in the solid. A similar phenomenon is anticipated for multiple scattering of electromagnetic waves, but with one important simplification: unlike electrons, photons do not interact with one another. This makes transport of photons in disordered materials an ideal model system in which to study Anderson localization. Here we report direct experimental evidence for Anderson localization of light in optical experiments performed on very strongly scattering semiconductor powders.

Traveling-wave parametric generation of widely tunable, highly coherent femtosecond light pulses

We report on the generation of ultrashort tunable pulses with a cavityless traveling-wave scheme consisting of a parametric superfluorescence seed source and a parametric amplifier. We show that the traveling-wave approach, with its advantages of simplicity and direct generation of tunable energetic single pulses, can be used in the femtosecond regime, and to this end we discuss the performances that were obtained with pump pulses of ≈1-ps and 200-fs duration at wavelengths of 0.53 and 0.6 μm, respectively. Of particular interest is the β-barium borate-based traveling-wave parametric generator (type-II phase matching), since it offers the possibility of generating nearly transform-limited pulses that are continuously tunable within a wide spectral range to as high as 3 μm in the IR. With a diffraction-limited pump at 0.53 μm. we obtained tunable pulses in a 1.2× diffraction-limited beam, which could be focused, with an f/20 optics lens, to an intensity of 1013 GW/cm2. A temperature-tuned lithium triborate-based femtosecond parametric generator, with its smaller group-velocity dispersion and absence of walk-off, can operate at a pump energy of as low as 30 μJ in a 200-fs pulse.

Structural relaxation in supercooled water by time-resolved spectroscopy

Water has many kinetic and thermodynamic properties that exhibit an anomalous dependence on temperature, in particular in the supercooled phase. These anomalies have long been interpreted in terms of underlying structural causes, and their experimental characterization points to the existence of a singularity at a temperature of about 225 K. Further insights into the nature and origin of this singularity might be gained by completely characterizing the structural relaxation in supercooled water. But until now, such a characterization has only been realized in simulations that agree with the predictions of simple mode-coupling theory; unambiguous experimental support for this surprising conclusion is, however, not yet available. Here we report time-resolved optical Kerr effect measurements that unambiguously demonstrate that the structural relaxation of liquid and weakly supercooled water follows the behaviour predicted by simple mode-coupling theory. Our findings thus support the interpretation of the singularity as a purely dynamical transition. That is, the anomalous behaviour of weakly supercooled water can be explained using a fully dynamic model and without needing to invoke a thermodynamic origin. In this regard, water behaves like many other, normal molecular liquids that are fragile glass-formers.

Ultrafast Optical Kerr Effect in Liquids and Solids

In the optical Kerr effect, the electric field of light incident on a transparent sample induces an anisotropic refractive index, which is measured by its effect on the passage of a second light beam. The advent of lasers powerful enough to generate a measurable effect, and which can be pulsed on femtosecond time scales, has made the optical Kerr effect into a practical technology for investigating the molecular structure and interactions of condensed systems such as pure liquids, liquid solutions, and plastic crystals.

Hydrogen bond dynamics in liquid methanol

A Car–Parrinello molecular dynamics simulation has been performed on fully deuterated liquid methanol. The results are compared with the latest available experimental and theoretical data. It is shown that the liquid is aggregated in chains of hydrogen bonded molecules. The structure of the aggregates is characterized and it is found that the dynamics includes a fast and a slow regime. The weak H bond formed by the methyl group hydrogens and oxygen atom of surrounding molecules has been characterized. The importance of inductive effects is shown and discussed in terms of maximally localized Wannier function centers. Special attention is devoted to clarify how the molecular dipole moment depends on the number of H bonds formed by each molecule. The IR spectrum is computed and analyzed in terms of H-bond interactions. Insights on the short time dynamics and on the H-bond network are illustrated.

Evidence of two distinct local structures of water from ambient to supercooled conditions

The liquid and supercooled states of water show a series of anomalies whose nature is debated. A key role is attributed to the formation of structural aggregates induced by critical phenomena occurring deep in the supercooled region; the nature of the water anomalies and of the hidden critical processes remains elusive. Here we report a time-resolved optical Kerr effect investigation of the vibrational dynamics and relaxation processes in supercooled bulk water. The experiment measures the water intermolecular vibrations and the structural relaxation process in an extended temperature range, and with unprecedented data quality. A mode-coupling analysis of the experimental data enables to characterize the intermolecular vibrational modes and their interplay with the structural relaxation process. The results bring evidence of the coexistence of two local configurations, which are interpreted as high-density and low-density water forms, with an increasing weight of the latter at low temperatures.

Electrical response in chemical potential equalization schemes

In this paper we compare the polarization response given by two different chemical potential equalization schemes to be applied to molecular dynamics simulations: the standard fluctuating point charge model (FQ) and the atom–atom charge transfer model (AACT). We have tested the transferability of FQ and AACT parameters, fitted to the polarizability of small size alkanes and polyenes, to large size homologues. We show that the FQ scheme is not adequate for the n-alkanes as it strongly overestimates the polarizability tensor components as the number of carbon atoms increases. The FQ approach has been found more predictive for highly conjugated systems like polyenes, although still unsatisfactory. The AACT parameters tuned on ethane are instead perfectly transferable to alkanes of any length and conformation. The AACT scheme satisfactorily reproduces the polarization response also for highly conjugated systems.

Picosecond transient grating measurements of singlet exciton transport in anthracene single crystals

Abstract Picosecond transient grating experiments are used to measure the rate of exciton transport in anthracene single crystals. The method permits a direct measurement of the transport by introducing an accurate distance scale into the sample. Results from transient grating experiments at 10 K are presented, and the diffusion constant along the â crystal axis for the anthracene singlet exciton is reported. Preliminary studies of the temperature dependence of the exciton diffusion constant are also discussed.

Intermolecular potentials for ammonia based on SCF–MO calculations

The results of SCF molecular orbital calculations on the ammonia dimer have been used in part to parameterize a set of atom–atom potentials. When combined with a charge distribution which reproduces the experimental dipole and quadrupole moments of the monomer, and with independent estimates of the dispersion energy, the resulting intermolecular potential yields a fair description of certain properties of the condensed phases of ammonia. Liquid ammonia is predicted to have a weakly associated character.

Anharmonic processes in molecular crystals. Calculation of the anharmonic shifts, bandwidths and energy decay processes in crystalline naphthalene

Abstract The anharmonic frequencies and the bandwidths of the lattice phonons of naphthalene at 4 K have been calculated using an intermolecular potential which includes atom-atom and quadrupole-quadrupole contributions. The calculated bandwidths agree well with the available experimental data. The contribution of the cubic and quartic terms of the crystal hamiltonian to the anharmonic shifts is discussed in terms of phonon diagrams. The calculated quartic shifts are found to be positive whereas the cubic shifts are smaller and negative. The total anharmonic shifts are then positive and it is shown that this is in agreement with the observed dependence of the phonon frequencies with temperature. The mechanism of energy transfer between the optical lattice phonons and the two-phonon manifold of the crystal is discussed in terms of phonon-phonon coupling processes. Up and down conversions are analyzed and their relative efficiency is evaluated. The temperature dependence of both the anharmonic shifts and bandwidths is studied in the range 4–68 K at constant volume. Comparison with the available experimental data at constant pressure is discussed.