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Accurately measuring mechanical displacements is essential for a vast portion of current technologies. Several optical techniques accomplish this task, allowing for non-contact sensing even below the diffraction limit. Here we introduce an optical encoding technique, dubbed “linear photonic gears”, that enables ultra-sensitive measurements of a transverse displacement by mapping it into the polarization rotation of a laser beam. In ordinary ambient conditions, we measure the relative shift between two objects with a resolution of 400 pm. We argue that a resolution of 50 pm should be achievable with existing state-of-the-art technologies. Our single-optical-path scheme is intrinsically stable and it could be implemented as a compact sensor, using cost effective integrated optics. We anticipate it may have a strong impact on both research and industry.

Self-patterning processes originated by physical stimuli have been extensively documented in thin films, whereas spontaneous wrinkling phenomena due to chemical transformation processes are, to the best of our knowledge, unprecedented. Herein we report a case of spontaneous polymerization-driven surface nano-patterning (B500 nm) that develops in smooth thin solid films of 5,6-dihydroxyindole (DHI), a major precursor of eumelanin polymers, over a time scale of 30 to 60 days in air at room temperature. The phenomenon can be observed only above a critical film thickness of B250 nm and it is affected by exposure to ammonia vapors causing acceleration of the oxidation process. The thickness-dependent onset of wrinkling can be attributed to non-homogeneous rates of oxidation through the film causing slow swelling/expansion of the inner layers followed by fast stiffening and cross-linking in the outer layer exposed to higher oxygen levels. Self-patterning processes originated by physical stimuli have been extensively documented in thin films, whereas spontaneous wrinkling phenomena due to chemical transformation processes are, to the best of our knowledge, unprecedented. Herein we report a case of spontaneous polymerization-driven surface nano-patterning (∼500 nm) that develops in smooth thin solid films of 5,6-dihydroxyindole (DHI), a major precursor of eumelanin polymers, over a time scale of 30 to 60 days in air at room temperature. The phenomenon can be observed only above a critical film thickness of ∼250 nm and it is affected by exposure to ammonia vapors causing acceleration of the oxidation process. The thickness-dependent onset of wrinkling can be attributed to non-homogeneous rates of oxidation through the film causing slow swelling/expansion of the inner layers followed by fast stiffening and cross-linking in the outer layer exposed to higher oxygen levels.

Entanglement in high-dimensional quantum systems, where one or more degrees of freedom of light are involved, offers increased information capacities and enables new quantum protocols. Here, we demonstrate a functional source of high-dimensional, noise-resilient hyperentangled states encoded in time-frequency and vector-vortex structured modes, which in turn carry single-particle entanglement between polarisation and orbital angular momentum. Pairing nonlinearity-engineered parametric downconversion in an interferometric scheme with spin-to-orbital-angular-momentum conversion, we generate highly entangled photon pairs at telecom wavelength that we characterise via two-photon interference and quantum state tomography, achieving near-unity visibilities and fidelities. While hyperentanglement has been demonstrated before in photonic qubits, this is the first instance of such a rich entanglement structure involving spectrally and spatially structured light, where three different forms of entanglement coexist in the same biphoton state., 5 pages, 4 figures, Supplement: 6 pages, 2 figures

The origin of eumelanin optical properties remains a formidable conundrum preventing a detailed understanding of the complex photo-protective role of these widespread natural pigments and the rational design of innovative bioinspired materials for optoelectronic applications. Here we report the unusual kinetic and thickness-dependent evolution of the optical properties of black eumelanin polymers generated by spontaneous aerial polymerization of 5,6-dihydroxyindole (DHI) thin films (0.1–1 μm), consistent with peculiar solid state reorganization mechanisms governing broadband absorption. The complete reversal of eumelanin UV-visible transmittance spectrum curvature on passing from 0.2 to 0.5 μm thick films, the marked increase in visible extinction coefficients with increasing film thickness and the higher UV extinction coefficients in slowly vs. rapidly generated polymers concur to support distinct dynamic regimes of solid-state molecular reorganization at the nanoscale level and to do affect the development of broadband visible absorption. Solid state control of molecular reorganization disclosed herein may delineate new rational strategies for tuning optical properties in eumelanin thin films for optoelectronic applications.

There is a constant need of new researches on combustion-generated particulate and techniques able to monitor an extended size range down to the smallest particles in order to provide exhaustive monitoring of air quality and provide new guidelines to update regulatory standards for limiting particulate emission. The main objective of the research program was to extend the characterization of particulate matter formation and growth in flame and in the exhaust of combustion system. 1) Development and characterization of a sampling method able to collect nanometric particulate matter from laboratory and practical combustion systems. 2) Sampling and analysis of the nanometric particulate at the exhausts of practical (domestic appliance and engines) systems. 3) Chemico-physical analysis of the sampled nanometric particulate matter by "ex situ" techniques. 4) Investigation of specific properties of combustion nanoparticles, which are of particular interest for environmental issues. Specifically: interaction of particles with water, which is of interest for health issue and climate effect related to cloud formation, and optical properties like emissivity, which are of interest for climate effect related to radiative balance.

Topological insulators are fascinating states of matter exhibiting protected edge states and robust quantized features in their bulk. Here we propose and validate experimentally a method to detect topological properties in the bulk of one-dimensional chiral systems. We first introduce the mean chiral displacement, an observable that rapidly approaches a value proportional to the Zak phase during the free evolution of the system. Then we measure the Zak phase in a photonic quantum walk of twisted photons, by observing the mean chiral displacement in its bulk. Next, we measure the Zak phase in an alternative, inequivalent timeframe and combine the two windings to characterize the full phase diagram of this Floquet system. Finally, we prove the robustness of the measure by introducing dynamical disorder in the system. This detection method is extremely general and readily applicable to all present one-dimensional platforms simulating static or Floquet chiral systems., The detection of topological invariants in the bulk remains challenging even in state-of-the-art experiments. Here, Cardano et al. propose a method to read-out the Zak phases and topological invariants in one-dimensional chiral systems and detect those in a photonic quantum walk of twisted photons.

We find an unambiguous relationship between disorder-driven features in the temperature dependence of the resistance and the behavior, as functions of the temperature, of the parameters necessary to describe some of the relaxation processes in the photoinduced differential time-resolved reflectivity of three samples of ${\text{Nd}}_{1.83}{\text{Ce}}_{0.17}{\text{CuO}}_{4\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}}$. The latter, sharing the same $\text{Ce}$ content, have been fabricated and annealed ad hoc in order to differ only for the degree of disorder, mainly related to oxygen content and location, and, consequently, for the temperature dependence of the resistance: Two of them present a minimum in the resistance and behave as a superconductor and as a metal, respectively; the third behaves as an insulator. The systematic coherence between the resistance and the relaxation parameters behaviors in temperature for all three samples is absolutely remarkable and shows that pump-probe measurements can be extremely sensitive to disorder as it drives the emergence of new excitations and of the related relaxation channels as in this paradigmatic case.

Optical singularities manifesting at the center of vector vortex beams are unstable, since their topological charge is higher than the lowest value permitted by Maxwell’s equations. Inspired by conceptually similar phenomena occurring in the polarization pattern characterizing the skylight, we show how perturbations that break the symmetry of radially symmetric vector beams lead to the formation of a pair of fundamental and stable singularities, i.e. points of circular polarization. We prepare a superposition of a radial (or azimuthal) vector beam and a uniformly linearly polarized Gaussian beam; by varying the amplitudes of the two fields, we control the formation of pairs of these singular points and their spatial separation. We complete this study by applying the same analysis to vector vortex beams with higher topological charges, and by investigating the features that arise when increasing the intensity of the Gaussian term. Our results can find application in the context of singularimetry, where weak fields are measured by considering them as perturbations of unstable optical beams.

Many phenomena in solid-state physics can be understood in terms of their topological properties. Recently, controlled protocols of quantum walk (QW) are proving to be effective simulators of such phenomena. Here we report the realization of a photonic QW showing both the trivial and the non-trivial topologies associated with chiral symmetry in one-dimensional (1D) periodic systems. We find that the probability distribution moments of the walker position after many steps can be used as direct indicators of the topological quantum transition: while varying a control parameter that defines the system phase, these moments exhibit a slope discontinuity at the transition point. Numerical simulations strongly support the conjecture that these features are general of 1D topological systems. Extending this approach to higher dimensions, different topological classes, and other typologies of quantum phases may offer general instruments for investigating and experimentally detecting quantum transitions in such complex systems., Topological phases play a fundamental role in a variety of physical systems, yet there is a lack of efficient tools for revealing the occurrence of associated quantum transitions. Here, Cardano et al. report that such transitions can be identified in the statistics of quantum-walk dynamics and validate this idea in a photonic experimental implementation.

The "quantum walk" has emerged recently as a paradigmatic process for the dynamic simulation of complex quantum systems, entanglement production and quantum computation. Hitherto, photonic implementations of quantum walks have mainly been based on multi-path interferometric schemes in real space. Here, we report the experimental realization of a discrete quantum walk taking place in the orbital angular momentum space of light, both for a single photon and for two simultaneous photons. In contrast to previous implementations, the whole process develops in a single light beam, with no need of interferometers; it requires optical resources scaling linearly with the number of steps; and it allows flexible control of input and output superposition states. Exploiting the latter property, we explored the system band structure in momentum space and the associated spin-orbit topological features by simulating the quantum dynamics of Gaussian wavepackets. Our demonstration introduces a novel versatile photonic platform for quantum simulations., This revised version has a new title; few errors have been corrected and some images have been modified in order to present the data clearly. arXiv admin note: substantial text overlap with arXiv:1403.4857

The "quantum walk" has emerged recently as a paradigmatic process for the dynamic simulation of complex quantum systems, entanglement production and quantum computation. Hitherto, photonic implementations of quantum walks have mainly been based on multipath interferometric schemes in real space. We report the experimental realization of a discrete quantum walk taking place in the orbital angular momentum space of light, both for a single photon and for two simultaneous photons. In contrast to previous implementations, the whole process develops in a single light beam, with no need of interferometers; it requires optical resources scaling linearly with the number of steps; and it allows flexible control of input and output superposition states. Exploiting the latter property, we explored the system band structure in momentum space and the associated spin-orbit topological features by simulating the quantum dynamics of Gaussian wavepackets. Our demonstration introduces a novel versatile photonic platform for quantum simulations.

We report on femtosecond optical pump-probe studies of proximized ferromagnet/superconductor (F/S) hybrids, consisting of Ni0.5Cu0.5 layers deposited on top of Nb films. The weak ferromagnetic nature of the completely proximised Ni0.5Cu0.5 film makes possible to observe the dynamics of the nonequilibrium superconductivity through the near-surface optical reflectivity change measurements. The time-resolved photoresponse transient of the NiCu(21 nm)/Nb bilayer in the superconducting state shows strongly suppressed slow bolometric component. The fast relaxation time is also discussed accordingly to current theories on S/F heterostructures. The proposed S/F nanobilayers represent a new, artificially designed superconductor with the features (sub-picosecond photoresponse with suppressed bolometric component) very desirable for superconducting photodetectors and photon counters. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The photophysics of free base hemiporphyrazine has been studied by steady-state fluorescence and theoretical computations. The fluorescence spectrum shows two bands peaked at 420 and 675 nm, which, with the aid of theoretical computations, have been assigned to the emissions of two tautomers, differing for the position of the two central protons.

We test experimentally the quantum ``paradox'' proposed by Lucien Hardy in 1993 [Phys. Rev. Lett. 71, 1665 (1993)] by using single photons instead of photon pairs. This is achieved by addressing two compatible degrees of freedom of the same particle, namely its spin angular momentum, determined by the photon polarization, and its orbital angular momentum, a property related to the optical transverse mode. Because our experiment involves a single particle, we cannot use locality to logically enforce non-contextuality, which must therefore be assumed based only on the observables' compatibility. On the other hand, our single-particle experiment can be implemented more simply and allows larger detection efficiencies than typical two-particle ones, with a potential future advantage in terms of closing the detection loopholes., 7 pages, 5 figures and 1 table

Electrical transport measurements and transient optical pump-probe experiments have been performed on epitaxial films of the electron doped Nd 1.83Ce0.17Cu4-δ compound for studying the non-equilibrium carrier dynamics in this material. Samples have been grown on (001)-oriented SrTiO3 substrates by dc sputtering in a mixed atmosphere of both Ar and O2. X-ray diffraction analysis and scanning electron microscope equipped with a wavelength dispersive spectroscopy detector have been used to characterize the structure and the composition of the thin films. Time-resolved femtosecond pump-and-probe spectroscopy has been also carried out on our samples in the temperature range 4.2K-300 K.

Transport properties of NbN/NiCu superconductor/ferromagnet (S/F) nanostripes fabricated in both in single-wire and series-parallel, meander-type configurations are presented down to T = 4.2 K. In particular, the enhancement of the superconducting critical current has been observed at smaller widths, apparently, due to an extra pinning mechanism, arising from clustering of ferromagnetic atoms inside the thin S layer. Moreover, we observed a number of characteristic voltage steps on the nanostripe current-voltage characteristics and their nature was investigated as a function of temperature. An explanation in terms of active phase-slip phenomena has been proposed based of the time-dependent Ginzburg-Landau theory and led to an estimation of the inelastic electron-phonon relaxation time τe-ph ~ 1 ps, in agreement with the τopt = 1.2±0.3 ps value, measured by the femtosecond transient optical reflectivity spectroscopy method on the same bilayer. Transient optical properties of our superconducting S/F nano-bilayers have been also investigated and compared to those obtained for pure NbN nanostripe reference samples. Finally, electrical photoresponse signals of S/F heterostructures exposed to ultraweak pulsed (width 400 ps, repetition rate ~100 MHz) laser radiation at 850 nm wavelength exhibited the falling time of voltage responses directly dependent on the NiCu overlayer. We have also noticed that the presence of the top F layer and the resulting proximity effect reduced frequency of dark counts in our samples.

We report the experimental violation of Leggett-type inequalities for a hybrid entangled state of spin and orbital angular momentum of a single photon. These inequalities give a physical criterion to verify the possible validity of a class of hidden-variable theories, originally named "crypto non-local", that are not excluded by the violation of Bell-type inequalities. In our case, the tested theories assume the existence of hidden variables associated with independent degrees of freedom of the same particle, while admitting the possibility of an influence between the two measurements, i.e. the so-called contextuality of observables. We observe a violation the Leggett inequalities for a range of experimental inputs, with a maximum violation of seven standard deviations, thus ruling out this class of hidden variable models with a high confidence., 5 pages, 4 figures

We present a convenient method to generate vector beams of light having polarization singularities on their axis, via partial spin-to-orbital angular momentum conversion in a suitably patterned liquid crystal cell. The resulting polarization patterns exhibit a C-point on the beam axis and an L-line loop around it, and may have different geometrical structures such as \qo{lemon}, \qo{star}, and \qo{spiral}. Our generation method allows us to control the radius of L-line loop around the central C-point. Moreover, we investigate the free-air propagation of these fields across a Rayleigh range., 6 pages, 4 figures, appears on Optics Express!

We present an optical setup for generating a sequence of light pulses in which the orbital angular momentum (OAM) degree of freedom is correlated with the temporal one. The setup is based on a single $q$-plate within a ring optical resonator. By this approach, we demonstrate the generation of a train of pulses carrying increasing values of OAM, or, alternatively, of a controlled temporal sequence of pulses having prescribed OAM superposition states. Finally, we exhibit an "OAM-to-time conversion" apparatus dividing different input OAM states into different time-bins. The latter application provides a simple approach to digital spiral spectroscopy of pulsed light., 3 pages and 2 figures

Understanding the formation mechanism of particulate matter due to incomplete combustion is fundamental to controlling the atmospheric impact of combustion systems. Toxicological studies have demonstrated that ultrafine aerosols can penetrate deep into the pulmonary alveoli, and affect human health. Different studies have identified in rich premixed flames a variety of carbon nanoparticles, ranging from those smaller than 3 nm (defined as nanoparticles of organic carbon, NOC) to larger soot particles of 20 nm, and more for aggregates. Optical methods represent the most desirable methods for nanoparticles studies, due to their accuracy, flexibility, and nonintrusive nature. Two principal uncertainties limit standard optical diagnostics in combustion systems: the spectroscopic properties of the NOC are similar to those of polycyclic aromatic hydrocarbons; and when light scattering is weak and comparable to that from gas-phase compounds, size measurement is affected by large uncertainties. Thus, it is necessary to exploit certain properties of fluorescence, which will depend unambiguously upon the size of the nanoparticles. This goal is achieved by using time-resolved fluorescence anisotropy (TRFA), which leads to both size and spectral characterization of the investigated particles. TRFA shows much promise, and is highly sensitive for detecting nanoparticles, even in a flame hostile environment. It also differs from other sizing methods such as light scattering, as it does not depend on the refractive index. Moreover, exploiting spectral selectivity in light excitation and emission information on chemical functionalities of nanoparticles can be deduced. TRFA opens up new possibilities for combustion and environmental monitoring of nanoparticles. In this chapter, the theory of TRFA ultrafast technique is derived for different operating conditions. Experimental realizations, results and the analysis of nanoparticles produced in flames in two different configurations (directly in flames and for samples collected from the flame) are then presented and discussed. Keywords: Nanoparticles; nano-organic compounds; ultrafast spectroscopy; nanosizing; fluorescence; time-resolved; in-situ diagnostics; ex-situ analysis

We have experimentally investigated the use of diffraction-free beams to generate low-order harmonics in gas jets. Bessel–Gauss beams have been generated from a Ti:sapphire, 100-fs laser by use of annular apertures and then by focusing of the ring-type–produced radiation. The conversion efficiencies for these beams for the third- and fifth-order harmonic generation in xenon have been measured and compared with those of pure Gaussian beams of the same energy content. We have thus found that, by matching the gas medium length to the range of the Bessel–Gauss beams, conversion efficiencies considerably higher than those of Gaussian beams can be obtained. The experimental results have also been discussed and interpreted in terms of a simple numerical model that emphasizes the effects of phase matching in the two different geometries.

The V–T relaxation from Freon‐22 molecules has been studied by using an interferometric technique. By raising the excitation energy from 500 cm−1 up to 3000 cm−1 (about three photons) a decrease of the effective relaxation time followed by a leveling off, has been observed. In agreement with this behavior, the average energy 〈ΔE〉 transferred per collision was found to depend linearly on 〈E〉 for extremal values of the energy absorbed, and stronger than linearly for intermediate energy values. This behavior parallels that observed in SF6 and other polyatomic molecules, and confirms the role played by the sparse and quasicontinuum regions of the vibrational spectrum.

The size of nanometric carbonaceous particles produced in various combustion systems is determined by means of time resolved fluorescence polarization anisotropy (TRFPA). We also compare the performances of two different experimental implementations of the technique, which are complementary in terms of cost, simplicity and resolution. Both methods are first employed on standard molecules to demonstrate the reliability of the results. A study of the sizes of nanometric particles collected at the exhaust of diesel and gasoline vehicle engine, as well as from controlled laminar flames is presented. The high sensitivity (0.04 nm) achieved with the use of a streak camera as detector makes the TRFPA technique particularly suitable for characterizing nanometric particles.

The mean size of fluorescent nanoparticles produced in a propane flame has been measured with an in-situ technique employing a femtosecond laser to excite the sample and a streak camera for time-resolved detection of the fluorescence. The time profile of the fluorescence anisotropy showed a Gaussian behaviour, typical of free rotor reorientation. By measuring its width, we estimated an average carbon particle diameter of 3.3 nm, thus confirming the existence of combustion produced nanoparticles. The technique proves to be applicable to the study of gas-phase nanoparticles, both in combustion and environmental studies.