Laura Cattaneo

Strong Thermo-Induced Single And Two-Photon Green Luminescence In Self-Organized Peptide Microtubes

Diphenylalanine peptide nano- and microtubes formed by self-assembly demonstrate strongly enhanced and tunable single-photon and two-photon luminescence in the visible range, which appears after heat- or laser treatment of these self-organized peptide microtubes. This process significantly extends the functionality of these microstructures and can trigger a new interest in the optical properties of structures based on short peptides.

Ultrafast all-optical response of a nematic liquid crystal

Liquid crystals are superior optical materials for large area displays, but it is considered that their collective and slow-millisecond response makes them useless for ultrafast optical applications. In contrast to that, we here demonstrate an ultrafast optical response of a nematic liquid crystal, which is induced by an intense femtosecond optical impulse. We show that the refractive index of the nematic liquid crystal pentyl-cyanobiphenyl can be modulated at a time scale as fast as 500 fs via a coherently excited optical Kerr effect. The change in the refractive index is in the order of 10-4 at a fluence of 4 mJ/cm2 and is strongly polarization dependent. This unprecedented result opens new ways towards ultrafast all-optical modulation in liquid crystal-based devices.

Engineered fabrication of ordered arrays of Au–NiO–Au nanowires

In the present paper, a novel method to fabricate ordered arrays of Au/NiO/Au nanowires is described, with the aim of filling the gap between the fundamental study of the electrical properties of scattered single nanowires and the engineered fabrication of nanowire arrays. This approach mainly consists of the following steps: (a) electrodeposition of Au/Ni/Au nanowires into an ordered porous anodic aluminum oxide template; (b) mechanical polishing of the sample to expose the gold tips of Au/Ni/Au nanowires to the template surface; (c) in situ annealing of the Au/Ni/Au nanowires without removing the template. The resulting structure consists in an ordered array of Au/NiO/Au nanowires slightly protruding out of a flat aluminum oxide template. Unlike current approaches, with the described method it is not necessary to remove the template in order to oxidize the middle metal, thus allowing the availability of an entire set of metal/oxide/metal nanowires ordered in a two-dimensional matrix and where single heterojunctions can be accessed individually.

Comparison of attosecond streaking and RABBITT

Recent progress in the generation of ultra-short laser pulses has enabled the measurement of photoionization time delays with attosecond precision. For single photoemission time delays the most common techniques are based on attosecond streaking and the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT). These are pump-probe techniques employing an extreme-ultraviolet (XUV) single attosecond pump pulse for streaking or an attosecond pump pulse train for RABBITT, and a phase-locked infrared (IR) probe pulse. These techniques can only extract relative timing information between electrons originating from different initial states within the same atom or different atoms. Here we address the question whether the two techniques give identical timing information. We present a complete study, supported by both experiments and simulations, comparing these two techniques for the measurement of the photoemission time delay difference between valence electrons emitted from the Ne 2p and Ar 3p ground states. We highlight not only the differences and similarities between the two techniques, but also critically investigate the reliability of the methods used to extract the timing information.

Gaining control through frustration: Two-fold approach for liquid crystal three-dimensional command layers

The alignment of Liquid Crystal (LC) molecules, essential for their applications in optical devices such as displays, is usually controlled by functionalizing their confining surfaces by either patterning or by specific surfactants that induce either parallel or perpendicular molecular arrangement. Inducing a bistable alignment, such as in the new zenithal bistable displays, offers new opportunities in terms of new functionalities and lower energy consumption but a full understanding of such bistable alignment appears still complicated. Here we present a simple phenomenological model that includes surface topography and chemistry. The predicted orientational transitions and bistable states are in excellent agreement with experiments, thus making this a proper tool to design multistable 3D command layers.

Sub-millisecond nematic liquid crystal switches using patterned command layer

Switching speeds of nematic liquid crystal (LC) cells in the sub-millisecond range are observed for 3-D patterned self assembled monolayers (SAMs) on indium-tin-oxide (ITO) substrates. To achieve such fast LC response and relaxation times, thermal treatments of the patterned SAM-on-ITO are required, increasing the relative anchoring energy experienced by the interacting LC molecules.

Electric field generation of Skyrmion-like structures in a nematic liquid crystal.

We report the generation of stable Skyrmion-like structures in a thin nematic liquid crystal film on chemically patterned patchy surfaces.

Orientation-dependent stereo Wigner time delay and electron localization in a small molecule

Time and place of electron exit Until about a decade ago, laser-induced ionization was considered instantaneous. Since then, applications of attosecond laser pulses have shown multiple subtle and complex factors that influence the precise timing of electron ejection from atoms and surfaces. Vos et al. measured the corresponding attosecond dynamics of dissociative photoionization in a diatomic molecule, carbon monoxide. By imaging the charged fragments, the timing could be correlated with the specific spatial portion of the molecule from which the electron wave packet emerged. Science, this issue p. 1326 The precise timing of ionization in CO varies with respect to the portion of the molecule from which the electron emerges. Attosecond metrology of atoms has accessed the time scale of the most fundamental processes in quantum mechanics. Transferring the time-resolved photoelectric effect from atoms to molecules considerably increases experimental and theoretical challenges. Here we show that orientation- and energy-resolved measurements characterize the molecular stereo Wigner time delay. This observable provides direct information on the localization of the excited electron wave packet within the molecular potential. Furthermore, we demonstrate that photoelectrons resulting from the dissociative ionization process of the CO molecule are preferentially emitted from the carbon end for dissociative 2Σ states and from the center and oxygen end for the 2Π states of the molecular ion. Supported by comprehensive theoretical calculations, this work constitutes a complete spatially and temporally resolved reconstruction of the molecular photoelectric effect.

Fast and ultrafast all-optical control of light in nematic and smectic-A liquid crystals

We review recent experiments on the fast and ultrafast all-optical control of light in bulk nematic and smectic-A liquid crystals. Ultrafast optical control at sub-picosecond time scalecan be achieved via the optical Kerr response of a nematic liquid crystal. We show that the refractive index changes are of the order of 10-4 in 5CB nematic liquid crystal and can be optically induced by applying 100 fs pulses of 4 mJ/cm2 fluence. We discuss stimulated emission depletion of fluorescence in a smectic-A liquid crystal and demonstrate nanosecond light control of fluorescent pulse shaping. Both methods could be applied to control light by light in future photonic devices based on liquid crystals.

Asymmetric wigner time delay in CO photoionization

We present photoionization experiments on CO molecules excited by an extreme-ultraviolet attosecond pulse train using RABBITT technique. We found a non-zero difference in Wigner delays between photoelectron wavepackets escaping towards the C-side and the O-side.