Paolo Bartolini

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.

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.

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.

Phase-locking to a free-space terahertz comb for metrological-grade terahertz lasers

Optical frequency comb synthesizers have represented a revolutionary approach to frequency metrology, providing a grid of frequency references for any laser emitting within their spectral coverage. Extending the metrological features of optical frequency comb synthesizers to the terahertz domain would be a major breakthrough, due to the widespread range of accessible strategic applications and the availability of stable, high-power and widely tunable sources such as quantum cascade lasers. Here we demonstrate phase-locking of a 2.5 THz quantum cascade laser to a free-space comb, generated in a LiNbO(3) waveguide and covering the 0.1-6 THz frequency range. We show that even a small fraction (<100 nW) of the radiation emitted from the quantum cascade laser is sufficient to generate a beat note suitable for phase-locking to the comb, paving the way to novel metrological-grade terahertz applications, including high-resolution spectroscopy, manipulation of cold molecules, astronomy and telecommunications.

Diffusive and oscillatory dynamics of liquid iodobenzene measured by femtosecond optical Kerr effect

The dynamics of liquid iodobenzene are studied by means of time resolved heterodyne detected optical Kerr effect in a wide temperature range (263–371 K). In the picosecond regime the relaxation is characterized by a biexponential decay, attributed to the rotational diffusion of an anisotropic rotator. The temperature dependence of the two relaxation times agrees only in part with the prediction of the hydrodynamic theory. The subpicosecond dynamics is essentially oscillatory in nature; the Raman spectra obtained by Fourier transform of the time domain data show the contribution of two intramolecular low-frequency vibrations, and that of the intermolecular dynamics. The intermolecular spectra at different temperatures are interpreted on the basis of the Brownian oscillator model, and consist of the superposition of overdamped and underdamped modes. The intermolecular spectrum of the liquid shows a close resemblance with the low-frequency Raman spectrum of crystalline iodobenzene, and suggests an interpreta...

Optical Kerr-effect investigation of the reorientational dynamics of CS2 in CCl4 solutions

The reorientational relaxation of CS2 in solution in CCl4 and its dependence on both concentration and temperature is investigated with the use of the optical Kerr effect (OKE). The comparison of the single-particle reorientational relaxation time extracted from OKE data and obtained by Raman spectroscopy supports the hypothesis that the vibrational and reorientational relaxation processes for CS2 are not correlated. It is shown that the reorientational dynamics of CS2 is influenced by the solvent configuration around the solute molecule (cage effect), and that it is also influenced by the pair orientational correlation between solute molecules, even in dilute solutions.

Reply: Localization or classical diffusion of light?

Wiersma et al. reply — Scheffold et al. have compared different data sets from our group and suggest that our evidence for localization is not conclusive and that the role of absorption should be characterized further. We believe that their analysis is misleading, and that our conclusions about localization are still valid.

Time resolved optical Kerr effect analysis of urea–water system

The nuclear dynamics of urea aqueous solution was analyzed by time resolved optical Kerr effect (OKE). The data analysis was achieved in time and in frequency domains. Three relaxation times characterize the time decay of the OKE signal at high mole fractions of urea, while only two relaxation times characterize this decay for the low mole fractions. The observed slowest relaxation time increases with increasing the mole fraction of urea. The comparison between this relaxation time and the ones determined by Raman and nuclear magnetic resonance spectroscopies suggests that the slow relaxation time is related to the reorientation of an axis lying in the plane of the urea molecule. At high mole fractions, the power spectra derived from the Fourier transform of the OKE signal are characterized by one broad peak at around 70 cm−1 and by a shoulder at around 160 cm−1 in the high frequency part of the former peak. This shoulder is related to the hydrogen bond interactions which involve urea molecules. Molecular...

The fast dynamics of benzene in the liquid phase. Part I. Optical Kerr effect experimental investigation

Femtosecond heterodyne detected optical Kerr effect experiments are performed on liquid benzene in a wide temperature range. Besides the diffusive orientational relaxation, at short times an oscillatory behaviour is observed, superimposed to a faster quasi-exponential decay. The spectral densities obtained by Fourier transform are characterised by a broad band in the region 0–150 cm−1, and their profiles show a marked temperature dependence. Similar behaviour is observed in a 1:6 molar solution of benzene in carbon tetrachloride. The experimental observations can be interpreted by assuming that the basic microscopic system, able to account for the main dynamical properties of the liquid at short times, consists of a benzene molecule librating and oscillating in a “ solvent” cage; consequently, the width of the observed intermolecular vibrational band has an essentially inhomogeneous origin. With the assumption of a bi-modal structure of the frequency distribution, a Kubo treatment is able to reproduce the main features of the experimental spectra at different temperatures. In particular, it is confirmed that the peculiar low frequency shape of the spectrum, corresponding to the fast quasi-exponential decay observed in the time-domain experiments, can be attributed to the motional narrowing effect. The value of the Kubo correlation time τc thus estimated ranges between 0.70 and 0.25 ps on going from temperatures close to the benzene melting point to near the boiling temperature.

Time-resolved optical Kerr effect experiments on supercooled benzene and test of mode-coupling theory

We have performed time-resolved optical Kerr effect experiments with heterodyne detection on benzene. We succeeded in supercooling benzene by 21 K below the melting point T m = 279 K, and we investigated the full range of existence from the supercooled phase up to the boiling point T b = 353 K. Our time-resolved data show clearly the complex relaxation pattern of benzene that is characterized by different time scales. These dynamic features, common to many other molecular liquids, to date have not been addressed and there is not a unique theoretical model able to explain them, even in a simple molecular liquid such as benzene. We compare our data with the predictions of a schematic mode-coupling model, fitting the experimental relaxations with a numerical solution of the time-dependent correlation functions. Although the temperature range investigated is clearly outside the asymptotic scaling regime, we found the mode-coupling model able to describe properly the measured dynamics in large time and temperature ranges.