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The molecular orientation in polymer fibers is investigated for the purpose of enhancing their optical properties through nanoscale control by nanowires mixed in electrospun solutions. A prototypical system, consisting of a conjugated polymer blended with polyvinylpyrrolidone, mixed with WO3 nanowires, is analyzed. A critical strain rate of the electrospinning jet is determined by theoretical modeling at which point the polymer network undergoes a stretch transition in the fiber direction, resulting in a high molecular orientation that is partially retained after solidification. Nearing a nanowire boundary, local adsorption of the polymer and hydrodynamic drag further enhance the molecular orientation. These theoretical predictions are supported by polarized scanning near-field optical microscopy experiments, where the dichroic ratio of the light transmitted by the fiber provides evidence of increased orientation nearby nanowires. The addition of nanowires to enhance molecular alignment in polymer fibers might consequently enhance properties such as photoluminescence quantum yield, polarized emission, and tailored energy migration, exploitable in light-emitting photonic and optoelectronic devices and for sensing applications.

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The occurrence of plasmon resonances on metallic nanometer-scale structures is an intrinsically nanoscale phenomenon, given that the two resonance conditions (i.e., negative dielectric permittivity and large free-space wavelength in comparison with system dimensions) are realized at the same time on the nanoscale. Resonances on surface metallic nanostructures are often experimentally found by probing the structures under investigation with radiation of various frequencies following a trial-and-error method. A general technique for the tuning of these resonances is highly desirable. In this paper we address the issue of the role of local surface patterns in the tuning of these resonances as a function of wavelength and electric field polarization. The effect of nanoscale roughness on the surface plasmon polaritons of randomly patterned gold films is numerically investigated. The field enhancement and relation to specific roughness patterns is analyzed, producing many different realizations of rippled surfaces. We demonstrate that irregular patterns act as metal-dielectric-metal local nanogaps (cavities) for the resonant plasmonic field. In turn, the numerical results are compared to experimental data obtained via aperture scanning near-field optical microscopy.

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Polarization modulated scanning near-field optical microscopy (PM-SNOM) is effective in detecting circular and linear dichroism with sub-wavelength resolution. PM-SNOM investigation of the chiroptical properties of single ribbon-like nanosized J-aggregates formed by acid induced aggregation of tris-(4-sulfonatophenyl)phenylporphyrin is reported. Linear dichroism maps give evidence of well-organized chromophores packed in linear arrays within the structure of the nanoribbons. Circular dichroism maps indicate that the molecules forming the nanoribbon have an inherently chiral structure at the local scale.

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Polymer fibers are currently exploited in tremendously important technologies. Their innovative properties are mainly determined by the behavior of the polymer macromolecules under the elongation induced by external mechanical or electrostatic forces, characterizing the fiber drawing process. Although enhanced physical properties were observed in polymer fibers produced under strong stretching conditions, studies of the process-induced nanoscale organization of the polymer molecules are not available, and most of fiber properties are still obtained on an empirical basis. Here we reveal the orientational properties of semiflexible polymers in electrospun nanofibers, which allow the polarization properties of active fibers to be finely controlled. Modeling and simulations of the conformational evolution of the polymer chains during electrostatic elongation of semidilute solutions demonstrate that the molecules stretch almost fully within less than 1 mm from jet start, increasing polymer axial orientation at the jet center. The nanoscale mapping of the local dichroism of individual fibers by polarized near-field optical microscopy unveils for the first time the presence of an internal spatial variation of the molecular order, namely the presence of a core with axially aligned molecules and a sheath with almost radially oriented molecules. These results allow important and specific fiber properties to be manipulated and tailored, as here demonstrated for the polarization of emitted light.

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The properties of polymeric nanofibers can be tailored and enhanced by properly managing the structure of the polymer molecules at the nanoscale. Although electrospun polymer fibers are increasingly exploited in many technological applications, their internal nanostructure, determining their improved physical properties, is still poorly investigated and understood. Here, we unravel the internal structure of electrospun functional nanofibers made by prototype conjugated polymers. The unique features of near-field optical measurements are exploited to investigate the nanoscale spatial variation of the polymer density, evidencing the presence of a dense internal core embedded in a less dense polymeric shell. Interestingly, nanoscale mapping the fiber Young's modulus demonstrates that the dense core is stiffer than the polymeric, less dense shell. These findings are rationalized by developing a theoretical model and simulations of the polymer molecular structural evolution during the electrospinning process. This model predicts that the stretching of the polymer network induces a contraction of the network toward the jet center with a local increase of the polymer density, as observed in the solid structure. The found complex internal structure opens an interesting perspective for improving and tailoring the molecular morphology and multifunctional electronic and optical properties of polymer fibers.

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We explore the effect of re-radiation in surface-enhanced Raman scattering (SERS) through polarization-sensitive experiments on self-organized gold nanowires on which randomly oriented Methylene Blue molecules are adsorbed. We provide the exact laws ruling the polarized, unpolarized, and parallel- and cross-polarized SERS intensity as a function of the field polarizations. We show that SERS is polarized along the wire-to-wire nanocavity axis, independently from the excitation polarization. This proves the selective enhancement of the Raman dipole component parallel to the nanocavity at the single molecule level. Introducing a field enhancement tensor to account for the anisotropic polarization response of the nanowires, we work out a model that correctly predicts the experimental results for any excitation/detection polarization and goes beyond the E(4) approximation. We also show how polarization-sensitive SERS experiments permit one to evaluate independently the excitation and the re-radiation enhancement factors accessing the orientation-averaged non-diagonal components of the molecular Raman polarizability tensor.

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Knowledge of accuracy and precision rates is particularly important for long-term studies. Vegetation assessments include many sources of error related to overlooking and misidentification, that are usually influenced by some factors, such as cover estimate subjectivity, observer biased species lists and experience of the botanist. The vegetation assessment protocol adopted in the Italian forest monitoring programme (CONECOFOR) contains a Quality Assurance programme. The paper presents the different phases of QA, separates the 5 main critical points of the whole protocol as sources of random or systematic errors. Examples of Measurement Quality Objectives (MQOs) expressed as Data Quality Limits (DQLs) are given for vascular plant cover estimates, in order to establish the reproducibility of the data. Quality control activities were used to determine the "distance" between the surveyor teams and the control team. Selected data were acquired during the training and inter-calibration courses. In particular, an index of average cover by species groups was used to evaluate the random error (CV 4%) as the dispersion around the "true values" of the control team. The systematic error in the evaluation of species composition, caused by overlooking or misidentification of species, was calculated following the pseudo-turnover rate; detailed species censuses on smaller sampling units were accepted as the pseudo-turnover which always fell below the 25% established threshold; species density scores recorded at community level (100 m(2) surface) rarely exceeded that limit.

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Nanoindentation experiments carried out with atomic force microscopes (AFMs) open the way to understand size-related mechanical effects that are not present at the macro- or micro-scale. Several issues, currently the subject of a wide and open debate, must be carefully considered in order to measure quantities and retrieve trends genuinely associated with the material behaviour. The shape of the nanoindenter (the AFM tip) is crucial for a correct data analysis; we have recently developed a simple geometrical model to properly describe the tip effect in the nanoindentation process. Here, we demonstrate that this model is valid in indentation of both soft and hard, or relatively hard, materials carried out by two distinct, commercially available, AFM probes. Moreover, we implement the model with a data interpretation approach aimed at preventing underestimation of the tip penetration into the material. Experiments on soft polymeric materials (poly(methyl methacrylate) and polystyrene) and hard or relatively hard (Si, Au, Al) materials are reported. The results demonstrate that true hardness data can be attained also in shallow indentations and that the appearance of size effects strongly depends on data interpretation issues. In addition, we report on stiffness data measured on the considered materials during their nanoindentation.

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We describe an aperture scanning near-field optical microscope (SNOM) using cantilevered hollow pyramid probes coupled to femtosecond laser pulses. Such probes, with respect to tapered optical fibers, present higher throughput and laser power damage threshold, as well as greater mechanical robustness. In addition, they preserve pulse duration and polarization in the near field. The instrument can operate in two configurations: illumination mode, in which the SNOM probe is used to excite the nonlinear response in the near field, and collection mode, where it collects the nonlinear emission following far-field excitation. We present application examples highlighting the capability of the system to observe the nonlinear optical response of nanostructured metal surfaces (gold projection patterns and gold nanorods) with sub-100-nm spatial resolution.

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We show that new low-energy photoluminescence (PL) bands can be created in the spectra of semiconducting single-walled carbon nanotubes by intense pulsed excitation. The new bands are attributed to PL from different nominally dark excitons that are "brightened" because of a defect-induced mixing of states with different parity and/or spin. Time-resolved PL studies on single nanotubes reveal a significant reduction of the bright exciton lifetime upon brightening of the dark excitons. The lowest-energy dark state has longer lifetimes and is not in thermal equilibrium with the bright state.

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By using broadband lasers, we demonstrate the possibilities for control of cold molecules formed via photoassociation. Firstly, we present a detection REMPI scheme (M. Viteau et al., Phys. Rev. A, 2009, 79, 021402) to systematically investigate the mechanisms of formation of ultracold Cs2 molecules in deeply bound levels of their electronic ground state X1sigma(g)+. This broadband detection scheme could be generalized to other molecular species. Then we report a vibrational cooling technique (M. Viteau et al., Science, 2008, 321, 232) through optical pumping obtained by using a shaped mode locked femtosecond laser. The broadband femtosecond laser excites the molecules electronically, leading to a redistribution of the vibrational population in the ground state via a few absorption-spontaneous emission cycles. By removing the laser frequencies corresponding to the excitation of the v = 0 level, we realize a dark state for the so-shaped femtosecond laser, leading, with the successive laser pulses, to an accumulation of the molecules in the v = 0 level, ie. a laser cooling of the vibration. The simulation of the vibrational laser cooling allows us to characterize the criteria to extend the mechanism to other molecular species (R. V. Krems, Int. Rev. Phys. Chem., 2005, 24, 99). We finally discuss the generalization of the technique to laser cooling of the rotation of the molecule.

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The methods producing cold molecules from cold atoms tend to leave molecular ensembles with substantial residual internal energy. For instance, cesium molecules initially formed via photoassociation of cold cesium atoms are in several vibrational levels nu of the electronic ground state. We applied a broadband femtosecond laser that redistributes the vibrational population in the ground state via a few electronic excitation/spontaneous emission cycles. The laser pulses are shaped to remove the excitation frequency band of the nu = 0 level, preventing re-excitation from that state. We observed a fast and efficient accumulation ( approximately 70% of the initially detected molecules) in the lowest vibrational level, nu = 0, of the singlet electronic state. The validity of this incoherent depopulation pumping method is very general and opens exciting prospects for laser cooling and manipulation of molecules.

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Investigation of the mechanical properties of materials at the nanoscale is often performed by atomic force microscopy nanoindentation. However, substrates with large surface roughness and heterogeneity demand careful data analysis. This requirement is even more stringent when surface indentations with a typical depth of a few nanometers are produced to test material hardness. Accordingly, we developed a geometrical model of the nanoindenter, which was first validated by measurements on a reference gold sample. Then we used this technique to investigate the mechanical properties of a coating layer made of Balinit C, a commercially available alloy with superior anti-wear features deposited on steel. The reported results support the feasibility of reliable hardness measurements with truly nanosized indents.

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Epitaxial Laterally overgrown (ELOG) InGaN materials are investigated using a polarization modulated scanning near-field optical microscope. The authors found that luminescence has spatial inhomogeneities and it is partially polarized. Near-field photoluminescence shows polarization phase fluctuation up to 45 degrees over adjacent domains. These results point toward the existence of asymmetries in carrier confinement due to structural anisotropic strain within the framework of the ELOG structure.

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We report optical near-field Raman imaging with subdiffraction resolution (approximately 120 nm) without field enhancement effects. Chemical discrimination on tetracyanoquinodimethane organic thin films showing localized salt complexes is accomplished by detailed Raman maps. Acquisition times that are much shorter than previously reported are due to the high Raman efficiency of the materials and to careful collection and detection of the optical signals in our near-field Raman spectrometer.