Francesco Tantussi

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. 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.

A cold cesium atomic beam produced out of a pyramidal funnel

Abstract We have built an atomic funnel which produces a slow and cold cesium atomic beam. The atomic funnel is based on a pyramidal Magneto Optical Trap (MOT) with a small hole at its apex. Characterization of the funnel operation has been carried out by fluorescence emission and absorption spectroscopy, and optical time of flight (TOF) methods. The atomic beam has a longitudinal velocity in the range 8–12 m/s and a spread less than 1.5 m/s. The transverse temperature is close to the Doppler limit. Typically, an atom flux ∼4×10 9 atoms / s is attained. These features, combined with the compactness and simplicity of the experimental arrangement, make this system an ideal source for experiments in atom lithography and atom optics.

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. 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.

Hot electrons in water: injection and ponderomotive acceleration by means of plasmonic nanoelectrodes

We present a theoretical and experimental study of a plasmonic nanoelectrode architecture that is able to inject bunches of hot electrons into an aqueous environment. In this approach, electrons are accelerated in water by ponderomotive forces up to energies capable of exciting or ionizing water molecules. This ability is enabled by the nanoelectrode structure (extruding out of a metal baseplate), which allows for the production of an intense plasmonic hot spot at the apex of the structure while maintaining the electrical connection to a virtually unlimited charge reservoir. The electron injection is experimentally monitored by recording the current transmitted through the water medium, whereas the electron acceleration is confirmed by observation of the bubble generation for a laser power exceeding a proper threshold. An understanding of the complex physics involved is obtained via a numerical approach that explicitly models the electromagnetic hot spot generation, electron-by-electron injection via multiphoton absorption, acceleration by ponderomotive forces and electron-water interaction through random elastic and inelastic scattering. The model predicts a critical electron density for bubble nucleation that nicely matches the experimental findings and reveals that the efficiency of energy transfer from the plasmonic hot spot to the free electron cloud is much more efficient (17 times higher) in water than in a vacuum. Because of their high kinetic energy and large reduction potential, these proposed wet hot electrons may provide new opportunities in photocatalysis, electrochemical processes and hot-electron driven chemistry.

Intracellular and Extracellular Recording of Spontaneous Action Potentials in Mammalian Neurons and Cardiac Cells with 3D Plasmonic Nanoelectrodes

Three-dimensional vertical micro- and nanostructures can enhance the signal quality of multielectrode arrays and promise to become the prime methodology for the investigation of large networks of electrogenic cells. So far, access to the intracellular environment has been obtained via spontaneous poration, electroporation, or by surface functionalization of the micro/nanostructures; however, these methods still suffer from some limitations due to their intrinsic characteristics that limit their widespread use. Here, we demonstrate the ability to continuously record both extracellular and intracellular-like action potentials at each electrode site in spontaneously active mammalian neurons and HL-1 cardiac-derived cells via the combination of vertical nanoelectrodes with plasmonic optoporation. We demonstrate long-term and stable recordings with a very good signal-to-noise ratio. Additionally, plasmonic optoporation does not perturb the spontaneous electrical activity; it permits continuous recording even during the poration process and can regulate extracellular and intracellular contributions by means of partial cellular poration.

Optical vortex beam generator at nanoscale level

Optical beams carrying orbital angular momentum (OAM) can find tremendous applications in several fields. In order to apply these particular beams in photonic integrated devices innovative optical elements have been proposed. Here we are interested in the generation of OAM-carrying beams at the nanoscale level. We design and experimentally demonstrate a plasmonic optical vortex emitter, based on a metal-insulator-metal holey plasmonic vortex lens. Our plasmonic element is shown to convert impinging circularly polarized light to an orbital angular momentum state capable of propagating to the far-field. Moreover, the emerging OAM can be externally adjusted by switching the handedness of the incident light polarization. The device has a radius of few micrometers and the OAM beam is generated from subwavelength aperture. The fabrication of integrated arrays of PVLs and the possible simultaneous emission of multiple optical vortices provide an easy way to the large-scale integration of optical vortex emitters for wide-ranging applications.

Beaming of Helical Light from Plasmonic Vortices via Adiabatically Tapered Nanotip

We demonstrate the generation of far-field propagating optical beams with a desired orbital angular momentum by using a smooth optical-mode transformation between a plasmonic vortex and free-space Laguerre–Gaussian modes. This is obtained by means of an adiabatically tapered gold tip surrounded by a spiral slit. The proposed physical model, backed up by the numerical study, brings about an optimized structure that is fabricated by using a highly reproducible secondary electron lithography technique. Optical measurements of the structure excellently agree with the theoretically predicted far-field distributions. This architecture provides a unique platform for a localized excitation of plasmonic vortices followed by its beaming.

Determinant role of the edges in defining surface plasmon propagation in stripe waveguides and tapered concentrators

In this paper, we experimentally show the effect of waveguide discontinuity on the propagation of the surface plasmon in metal stripes and tapered terminations. Dual-plane leakage microscopy and near-field microscopy were performed on Au stripes with varied widths to image the surface plasmon intensity distribution in real and reciprocal spaces. We unambiguously demonstrate that edge diffraction is the limiting process determining the cutoff conditions of the surface plasmon mode. Finally, we determine the optimal tapered geometry leading to the highest transmission.

A laser-cooled atom beam for nanolithography applications

We are developing an apparatus for atom lithography with the main objective of to push the space resolution of the technique towards its ultimate limit, expected in the 10 nm range. We exploit an original implementation of laser-cooling techniques to produce a brilliant and collimated cesium beam with low longitudinal velocity. Beam characterization, carried out with a variety of spectroscopic techniques, demonstrates the compatibility of the system with the strict requirements of nanolithography experiments.

Pure optical and reversible optically driven nanowriting of azobenzene block copolymers

We report on pure optical nanowriting in the near-field of spin-coated films of a specifically synthesized azobenzene-containing block copolymer. Writing is accomplished by linear polarized blue laser radiation, which is efficiently absorbed by the material, and readout is based on polarization-modulation of red laser radiation, which is practically unabsorbed. Purely optical, stable, and reversible modifications are observed at the ∼100 nm scale. Remarkably, they are not accompanied by any observable topographical change. We attribute the result to the use of a small azobenzene content (3% mol) and to the block architecture of the material.