Pasqualino Maddalena

Light-induced spiral mass transport in azo-polymer films under vortex-beam illumination

When an azobenzene-containing polymer film is exposed to non-uniform illumination, a light-induced mass migration process may be induced, leading to the formation of relief patterns on the polymer-free surface. Despite many years of research effort, several aspects of this phenomenon remain poorly understood. Here we report the appearance of spiral-shaped relief patterns on the polymer film under the illumination of focused Laguerre–Gauss beams with helical wavefronts and an optical vortex at their axis. The induced spiral reliefs are sensitive to the vortex topological charge and to the wavefront handedness. These findings are unexpected because the doughnut-shaped intensity profile of Laguerre–Gauss beams contains no information about the wavefront handedness. We propose a model that explains the main features of this phenomenon through the surface-mediated interference of the longitudinal and transverse components of the optical field. These results may find applications in optical nanolithography and optical-field nanoimaging.

Marine diatoms as optical chemical sensors

Complex micro- and nanostructured materials for optical sensing purposes are designed and fabricated using top technologies. A completely different approach to engineering systems at the nanoscale consists in recognizing the nanostructures and morphologies that nature has optimized during life’s history on earth. We have found that the photoluminescence emission from silica skeleton of marine diatoms Thalassiosira rotula Meunier is strongly dependent on the surrounding environment. Both the optical intensity and the peaks positions are affected by gases and organic vapors. Depending on the electronegativity and polarizing ability, some substances quench the luminescence, while others effectively enhance it. These phenomena allow the discrimination between different substances. These naturally occurring organisms are thus good candidates as optical sensing materials.

Evaluation of the thermal conductivity of porous silicon layers by an optical pump-probe method

Measurements of the thermal conductivity for free-standing porous silicon layers were performed by means of an optical pump-probe experimental set-up. By transient heating due to laser pumping, a refractive index modulation was induced in the sample and, solving the heat propagation equation for inside the solid film, it is shown that the time decay of the nonlinear transmittance can be related to the thermal conductivity. The optical technique demonstrated here is contactless, quite simple and does not require much effort in data analysis, and is therefore very useful for thin-film characterization. The thermal conductivities of the porous silicon samples, whose porosities lay in the range 60-70%, were taken into account, and we found good agreement with results obtained with different techniques.

Nonlinear optical refraction of free-standing porous silicon layers

Measurements of the nonlinear refractive index of free-standing porous silicon samples by means of the Z-scan technique are reported. A sensitive enhancement of the optical nonlinearity is found with respect to bulk silicon. The results are in agreement with a simple theoretical model which is also presented and discussed, that attributes the enhancement to quantum confinement of carriers. The negative sign of nonlinear refractive index suggests that optical Stark effect gives the dominating contribution to the nonlinearity. It is also found that the nonlinearity is mainly refractive, which is very promising in order to use porous silicon for nonlinear optical applications such as power limiting or optical switching.

Two-photon patterning of a polymer containing Y-shaped azochromophores

We report on the patterning of the free surface of azo-based polymer films by means of mass migration driven by one- or two-photon absorption. A symmetric donor-acceptor-donor structured Y-shaped azochromophore is specifically synthesized to enhance two-photon absorption in the polymer. The exposure of the polymer film to a focused laser beam results in light-driven mass migration for both one- and two-photon absorptions. Features with subdiffraction resolution (250 nm) are realized and the patterning dynamics is investigated as a function of the light dose. Furthermore, functional photonic structures, such as diffraction gratings with periods ranging between 0.5 and 2.0 μm, have been realized.

Hydrogenated amorphous silicon carbon alloys for solar cells

Hydrogenated amorphous silicon carbon films were grown by PECVD from silane/methane gas mixtures by fixing the methane ratio in the gas phase and by changing the rf power and pressure. The effects of the discharge parameters on the optical, electrical and structural properties were investigated. These effects were attributed to the variation of carbon content in the film. The analyses enabled us to determine the optimal growth conditions to produce a-SiC:H materials, suitable in solar cell applications, with a good photosensitivity and low defect density of states.

Photoconductivity in defective carbon nanotube sheets under ultraviolet–visible–near infrared radiation

Multiwalled carbon nanotube sheets of relatively large area have been grown on a sapphire substrate by chemical vapor deposition at the substrate temperature of 500 and 750°C. The photoconductivity measurements, performed under white light and monochromatic radiation in the ultraviolet–visible–near infrared region, show that the highly defective sample grown at 500°C has a higher photosensitivity, thus revealing the crucial role of structural defects in determining the overall photoresponse of the nanotube’s sheets. The spectral photoresponse of these nanostructured films increases with the increase in photon energy, and is strongly correlated to the absorbance. The photoconductivity properties of these materials are favorable in potential development of large area light sensors as well as optoelectronic nanodevices.

Controlling spontaneous surface structuring of azobenzene-containing polymers for large-scale nano-lithography of functional substrates

In this work, we investigate the effect of illumination parameters which is light polarization, wavelength, and beam focalization, on the large-scale patterning of the surface of azobenzene-containing polymer films by means of spontaneous surface structuring. This is a phenomenon due to the interference at the sample surface between different light modes originated by scattering from the primary illuminating beam. In particular, the surface patterning in regions of a few squared millimeters with a spatial resolution down to 180 nm is achieved by means of a single beam illumination. The realized topographical structures are both preferentially oriented gratings and isotropically distributed topographical protrusions (dots), with sub-wavelength features.

Optical, structural and electrical properties of μc-Si:H films deposited by SiH4+H2

Abstract Hydrogenated microcrystalline silicon (μc-Si:H) films were prepared by Plasma Enhanced Chemical Vapor Deposition from a mixture of silane highly diluted in hydrogen. The effect of the silane concentration on the deposition rate and on the optical, electrical and structural properties were investigated. The silane concentration appears to control orientation and grain size. Highly conductive μc-Si:H films were grown with high deposition rate at silane concentration of 3%. These films show an enhancement of the optical absorption in the near infrared region. In the visible region the absorption is lower than a-Si:H, however the transient PC signal, induced by 532 nm laser pulses (6 ns time duration), shows an high amplitude and a width comparable with the optical pulse one. μc-Si:H materials can be used for fast photodetectors of pulsed visible light.

Instability-induced pattern formation of photoactivated functional polymers

Significance When azobenzene-containing polymer films are exposed to UV or visible light complex Turing patterns form on the polymers surface. But despite the large number of applications reported, a physical explanation for the pattern formation in this important class of materials and its dependence on both the lights intensity and polarization is still lacking. In this study, we present a general explanation for the pattern formation on these photoactivated azopolymer films. We believe that our findings are an innovative contribution to the field of pattern formation of functional polymers and photoactivated thin film engineering, which have the potential to boost the development of photoresponsive systems, such as molecular electronic devices. Since the pioneering work of Turing on the formation principles of animal coat patterns [Turing AM (1952) Phil Trans R Soc Lond B 237(641):37–72], such as the stripes of a tiger, great effort has been made to understand and explain various phenomena of self-assembly and pattern formation. Prominent examples are the spontaneous demixing in emulsions, such as mixtures of water and oil [Herzig EM, et al. (2007) Nat Mater 6:966–971]; the distribution of matter in the universe [Kibble TWB (1976) J Phys A: Math Gen 9(8):1387]; surface reconstruction in ionic crystals [Clark KW, et al. (2012) Nanotechnol 23(18):185306]; and the pattern formation caused by phase transitions in metal alloys, polymer mixtures and binary Bose–Einstein condensates [Sabbatini J, et al. (2011) Phys Rev Lett 107:230402]. Photoactivated pattern formation in functional polymers has attracted major interest due to its potential applications in molecular electronics and photoresponsive systems. Here we demonstrate that photoactivated pattern formation on azobenzene-containing polymer films can be entirely explained by the physical concept of phase separation. Using experiments and simulations, we show that phase separation is caused by an instability created by the photoactivated transitions between two immiscible states of the polymer. In addition, we have shown in accordance with theory, that polarized light has a striking effect on pattern formation indicated by enhanced phase separation.