WO Nanowires Enhance Molecular Alignment and Optical Anisotropy in Electrospun Nanocomposite Fibers: Implications for Hybrid Light-Emitting Systems.

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 WO 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.

Near-field surface plasmon field enhancement induced by rippled surfaces.

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.

Linear and circular dichroism in porphyrin J-aggregates probed by polarization modulated scanning near-field optical microscopy.

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.

Conformational Evolution of Elongated Polymer Solutions Tailors the Polarization of Light-Emission from Organic Nanofibers.

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.

Local mechanical properties of electrospun fibers correlate to their internal nanostructure.

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.

Re-radiation enhancement in polarized surface-enhanced resonant Raman scattering of randomly oriented molecules on self-organized gold nanowires.

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.

Broadband lasers to detect and cool the vibration of cold molecules.

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.

Size effects in nanoindentation of hard and soft surfaces.

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.

Hollow-pyramid based scanning near-field optical microscope coupled to femtosecond pulses: a tool for nonlinear optics at the nanoscale.

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.

Defect-induced photoluminescence from dark excitonic states in individual single-walled carbon nanotubes.

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.

Optical pumping and vibrational cooling of molecules.

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.

An atomic force microscopy tip model for investigating the mechanical properties of materials at the nanoscale.

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.

A polarization-modulation method for the near-field mapping of laterally grown InGaN samples.

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.

Optical near-field Raman imaging with subdiffraction resolution.

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.