Alessandro Veltri - Academia.edu (original) (raw)

Papers by Alessandro Veltri

$Research paper thumbnail of Model for the photoinduced formation of diffraction gratings in liquid-crystalline composite materials$

Applied Physics Letters, May 3, 2004

We modelize the photoinduced formation of switchable diffraction gratings in polymer-based liquid... more We modelize the photoinduced formation of switchable diffraction gratings in polymer-based liquid-crystalline composite materials. The model assumes that redistribution of molecules is due to mass diffusion and incorporates a realistic kinetic description of polymerization processes. Numerical simulations predict two different kinds of structure that have already been experimentally observed. Two parameters governing the phenomenon are pointed out, which determine the structure that will be formed after the curing process has been completed.

$Research paper thumbnail of Characterization of the diffraction efficiency of new holographic gratings with a nematic film–polymer-slice sequence structure$

Journal of The Optical Society of America B-optical Physics, Nov 1, 2004

Beilstein Journal of Nanotechnology, Oct 8, 2018

Optics Express, Oct 23, 2009

Molecular Crystals and Liquid Crystals, Nov 1, 2005

In this paper we report a detailed characterization of a new structure (POLICRYPS) in which liqui... more In this paper we report a detailed characterization of a new structure (POLICRYPS) in which liquid crystal films are completely separated by polymer slices. This structure behaves as a Bragg diffraction grating and exhibits efficiencies that can become as high as 96–98%. The temperature dependence of the diffraction efficiency can be explained in terms of a Kogelnik-like model. A numerical simulation has been implemented which makes use only of real values of physical quantities and accounts for the experimental results with good accuracy. An applied electric field is able to switch off the grating, with a characteristic switching time which is lower than 1 ms. Threshold values of the field vary in the range 3–5 V/μm for gratings with a 1.39 µm fringe spacing.

Applied Physics Letters, Sep 28, 2005

We present a model accounting for the main features of the wave coupling process that takes place... more We present a model accounting for the main features of the wave coupling process that takes place during the UV curing of diffraction gratings made of liquid crystal films separated by slices of polymeric material. The effect depends on the phase shift between the curing interference pattern and the grating being cured. For a unit ratio of the impinging beam intensities, the model confirms the absence of energy transfer, as experimentally observed. When the ratio is not at its unit value, the beam coupling tends to equalize the intensities of the two beams as they propagate inside the sample.

$Research paper thumbnail of Observation of two-wave coupling during the formation of POLICRYPS diffraction gratings$

Optics Letters, Jul 15, 2005

We report on the experimental observation of two-wave mixing that occurs inside a sample between ... more We report on the experimental observation of two-wave mixing that occurs inside a sample between the beams used for the fabrication of polymer-liquid-crystal-polymer slices (POLICRYPS) diffraction gratings. The effect depends on the phase shift between the curing interference pattern and the grating being cured. This shift can be mechanically induced by accidental vibrations of the experimental setup; thus a high setup stability is needed. We devised a mechanism that enables us to control setup vibrations in situ and used it to monitor the experiment. When the mechanically induced shift was significantly small, wave mixing was observed only if the initial intensities of the two curing beams were different from each other.

$Research paper thumbnail of Kogelnik-like model for the diffraction efficiency of POLICRYPS gratings$

Journal of The Optical Society of America B-optical Physics, Apr 1, 2005

HAL (Le Centre pour la Communication Scientifique Directe), Sep 1, 2013

International audienc

We present a numerical approach to the nemato-elasticity differential equation in a nematic liqui... more We present a numerical approach to the nemato-elasticity differential equation in a nematic liquid crystal cell when irradiated with multiple gaussian beams. Solutions have been carried out on a configuration with two coplanar beams illuminating the sample in order to compare it with particular nonlinear phenomena experimentally studied in the past. A new set of experimental measures were realized confirming the validity of the numerical model. Solutions for an instable case showing nonlocal effects are also presented as an example of the broader class of systems this approach can describe.

Journal of Nonlinear Optical Physics & Materials, Mar 1, 2011

We report on the design of a particular holographic grating to be exploited for the stability con... more We report on the design of a particular holographic grating to be exploited for the stability control of an active optical holographic setup. The low-cost transmission grating made of alternating slices of pure polymer slices and pure nematic liquid crystal is used as a reference grating for the active feedback system that enables the interference patterns stabilization. The stability test of the interference pattern confirms the effectiveness of the proposed technique.

$Research paper thumbnail of Two-Wave Coupling during the Formation of POLICRYPS Diffraction Gratings: Experimental Results Theoretical Model$

Molecular Crystals and Liquid Crystals, Sep 1, 2006

Liquid crystals reviews, Jun 1, 2013

ABSTRACT The acronym POLICRYPS indicates a nano/micro composite structure made of films of well a... more ABSTRACT The acronym POLICRYPS indicates a nano/micro composite structure made of films of well aligned nematic liquid crystal (NLC) alternated to slices of almost pure polymer. Structures are fabricated by curing a homogeneous mixture of mesogenic material, monomer and a curing agent with UV radiation, under suitable physical and geometrical conditions. In particular, geometrical conditions determine (in the range 0.2 ÷ 15μm) the spatial periodicity of the realized sample, which can be utilized for transmitting, diffracting or reflecting an impinging light beam, with negligible scattering losses. The spatial modulation of the refractive index (from polymer to NLC) can be switched ON and OFF both by applying an electric field of few V/μm or, in some cases, by irradiating the sample with a light beam of suitable wavelength. This electrical/optical tuneability is responsible for a series of distinctive characteristics of the optical effects produced by the structure, and determines the range of possible applications. In fact, in different geometries, the POLICRYPS can be exploited as a switchable holographic grating, a switchable optical phase modulator, a switchable beam splitter, a tuneable Bragg filter or it can be exploited as an electro-optical edge filter in an optical interrogation system.

Biophotonics Congress 2021, 2021

We study theoretically the optomechanics of a dyed dielectric/metallic nanoshell in Optical Tweez... more We study theoretically the optomechanics of a dyed dielectric/metallic nanoshell in Optical Tweezers in a counter-propagating configuration. In addition, we consider the thermophoretic effects due to the interaction between the incident radiation and the nanoparticle metallic component.

$Research paper thumbnail of Plasmon-mediated Discrete Diffraction Behaviour of an Array of Responsive Waveguides$

Nanoscale, 2019

We investigate the discrete diffraction phenomenon in a Polymer-Liquid Crystal-Polymer Slices (PO... more We investigate the discrete diffraction phenomenon in a Polymer-Liquid Crystal-Polymer Slices (POLICRYPS) overlaying a random distribution of gold nanoparticles (AuNPs, plasmonic elements). We study the propagation of a CW green...

Journal of the Optical Society of America B, 2021

Within the frame of a simple, long-wavelength, quasi-static description, we present a theoretical... more Within the frame of a simple, long-wavelength, quasi-static description, we present a theoretical characterization of the optical response of metal nanoparticles doped with active gain elements in a core-shell (metallic core within an active dielectric shell) and nano-shell (active dielectric core within a metallic shell) configurations. The common feature of these structures is that, adding gain to the system produces an increase of the quality of the plasmon resonance, which becomes sharper and sharper until a singular point, after which, the system switches from absorptive to emissive (nanolaser). We use this aforementioned simple model to develop a general method allowing us to calculate both the expected singular plasmon frequency and the gain level needed to realize it and to discuss the spectral deformation occurring before and after this singular point. Finally, we propose a way to calculate if the singular behavior is reachable using realistic amounts of gain.

Materials

Coupling externally pumped gain materials with plasmonic spherical particles, even in the simples... more Coupling externally pumped gain materials with plasmonic spherical particles, even in the simplest case of a single spherical nanoparticle in a uniform gain medium, generates an incredibly rich variety of electrodynamic phenomena. The appropriate theoretical description of these systems is dictated by the quantity of the included gain and the size of the nano-particle. On the one hand, when the gain level is below the threshold separating the absorption and the emission regime, a steady-state approach is a rather adequate depiction, yet a time dynamic approach becomes fundamental when this threshold is exceeded. On the other hand, while a quasi-static approximation can be used to model nanoparticles when they are much smaller than the exciting wavelength, a more complete scattering theory is necessary to discuss larger nanoparticles. In this paper, we describe a novel method including a time-dynamical approach to the Mie scattering theory, which is able to account for all the most e...

Frontiers in Optics / Laser Science, 2020

We investigate theoretically the non-linear optical trapping of dye enriched metal/dielectric nan... more We investigate theoretically the non-linear optical trapping of dye enriched metal/dielectric nanoshells in extended optical potentials. Moreover, we show that both trapping and channelling is achieved by changing the light wavelength at high gains.

Le Centre pour la Communication Scientifique Directe - HAL - Diderot, Sep 1, 2013