Alessandro Veltri

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• Ph.D Physics
• Professor at Universidad San Francisco de Quito

We investigate gain-assisted optical forces on dye-enriched silver nanoshell in the quasi-static limit by means of a theoretical/numerical approach. We demonstrate the onset of nonlinear optical trapping of these resonant nanostructures in a counterpropagating Gaussian beam configuration. We study the optical forces and trapping behaviour as a func...

We study theoretically the emission and lasing properties of a single nanoshell spaser nanoparticle with an active core and a plasmonic metal shell. Using time-dependent equations for the gain medium and metal, we calculate the lasing threshold through an instability analysis. Below threshold, the nanoshell acts as an optical amplifier when excited...

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...

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...

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.

We investigate gain-assisted optical forces on dye-enriched silver nanoshell in the quasi-static limit. We demonstrate the onset of nonlinear optical trapping of these resonant nanostructures in a counter-propagating Gaussian beam configuration. We study the optical forces and trapping behaviour as a function of wavelength, particle gain level, and...

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.

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 laser beam through the waveguide structure as a function of beam polarization, laser intensity and sample temperature. It...

Active plasmonics is a key focus for the development of advanced plasmonic applications. By selectively exciting the localized surface plasmon resonance sustained by the short or the long axis of silver nanorods, we demonstrate a polarization-dependent strong coupling between the plasmonic resonance and the excited state of photochromic molecules....

We present a review of polymer-liquid crystal-based devices for optical applications. Starting from a particular fabrication technique, which enables to obtain the POLICRYPS (POlymer LIquid CRYstal Polymer Slices) structure, we illustrate different realizations, along with their working principle and main features and performances. The name POLICRY...

We present an experimental characterization and a comprehensive theoretical modeling of macroscopic plasmonic heat production that takes place in a single layer of small gold nanoparticles (GNPs), randomly distributed on a glass substrate, covered with different host media and acted on by a resonant radiation. We have performed a detailed experimen...

We present a detailed study on the resonant gain phenomena occurring in two nanostructures, in which the presence of dielectric singularities is used to reach a huge amplification of the emitted photons resonantly interacting with the system. The presence of gain molecules in the considered nano-resonator systems allows to obtain optical features a...

The plasmonic response of a metal nanoparticle in the presence of surrounding gain elements is studied, using a space and time-dependent model, which integrates a quantum formalism to describe the gain and a classical treatment for the metal. Our model fully takes into account the influence of the system geometry (nanosphere) and offers for the fir...

We report on photo-thermal effects observed in gold nanoparticles (GNPs) dispersed in Nematic Liquid Crystals (NLCs). Under a suitable optical radiation, GNPs exhibit a strong light absorption/scattering; the effect depends on the refractive index of the medium surrounding the nanoparticles, which can be electrically or optically tuned. In this way,...

Here, we demonstrate the nonlinear optical responses of Ag-Al2O3 one-dimensional photonic crystals (1D PCs). The linear transmission spectra of the proposed 1D PC show higher transmission at shorter wavelengths and lower transmission at longer wavelengths with “N” pairs of Ag-Al2O3 will support “N − 1” transmission peaks. By using 5 pairs of Ag-Al2...

The last years have seen a growing interest in the ability of metallic nanoparticles (MNPs) to control temperature at the nanoscale. Under a suitable optical radiation, MNPs feature an enhanced light absorption/scattering, thus turning into an ideal nano-source of heat, remotely controllable by means of light. In this framework, we report our recen...

We present a model for the photoinduced formation of composite materials by means of an inhomogeneous curing pattern. 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 alrea...

Traditionally, most synthetically developed materials are hardened by heating them to an elevated temperature, a process requiring large amounts of energy and space. Interest in photo cured materials using UV-light is growing due to simplifications in manufacturing and growing environmental concerns; it is expected photocuring could reduce electric...

Traditionally, most synthetically developed materials are hardened by heating them to an elevated temperature, a process requiring large amounts of energy and space. Interest in photo cured materials using UV-light is growing due to simplifications in manufacturing and growing environmental concerns; it is expected photocuring could reduce electric...

The performance of all metamaterial-based applications is significantly limited by the inherent and strong energy dissipation present in metals, especially in the visible range. We experimentally demonstrate that the incorporation of excitonic material (chromophores) in the high-local-field areas of plasmon nanostructures induce coherent resonant e...

Inherent optical losses of plasmonic materials represent a crucial issue for optoplasmonics, whereas the realization of hierarchical plasmonic nanostructures implemented with gain functionalities is a promising and valuable solution to the problem. Here we demonstrate that porous silica capsules embedding gold nanoparticles (Au NPs) and fabricated...

We present a dynamical model for a classically treated metal spherical inclusion in a gain assisted medium described using quantum formalism. The model integrates geometry via a proper set of boundary conditions and it is shown to be consistent with the statical formula for polarizability, which can be obtained in the low amplitude linear approxima...

We study theoretically the polarizability of a single metallic nanoparticle immersed into an externally pumped, active gain medium able to couple to the plasmon resonance. Within the frame of a simple long-wavelength, macroscopic description, and under steady-state conditions, we show that localized plasmons can be strongly amplified, until becomin...

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...

In this letter, we demonstrate a reversible strong coupling regime between a dipolar surface plasmon resonance and a molecular excited state. This reversible state is experimentally observed on silver nanoparticle arrays embedded in a polymer film containing photochromic molecules. Extinction measurements reveal a clear Rabi splitting of 294 meV, c...

We present a review of a series of nano/micro, soft-matter-based, periodic, composite structures, along with a theoretical model, which explains their main physical and optical features as determined by a suitable choice of the values of given physical and geometrical parameters. The historical development is illustrated by starting from the fabric...

We experimentally demonstrate that gain materials properly encapsulated into the shell surrounding metal nanoparticles (NPs) are responsible for the modification of the overall plasmon response of engineered nanostructures. A comparison between designed systems based on functionalized core–shell NPs having different encapsulated dye molecules is pr...

We present a simple model which enables the derivation of light propagation in a POLICRYPS structure, utilized as an array of planar waveguides realized in liquid crystalline composite materials. Discrete diffraction and soliton solutions are first numerically derived, without the need of any approximation, for a sinusoidal periodic profile; result...

DOI:https://doi.org/10.1103/PhysRevE.83.059902

Random laser action in organic materials is of great topical interest that is fueled by the rapid development of active compounds and new dye molecules. We propose a pure-diffusive model to describe the strong connection established between a dye-host interaction and the scattering when considering an anisotropic complex fluid. The model considers...

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...

(Figure Presented) Polymeric microstructures, produced in a multi-step chemico-physical process, confine and stabilize a well-aligned nematic liquid crystal (NLC) film, which is doped with a high-performance mesogenic azobenzene dye, sensitive in the visible range.

Short movie showing ray tracing on a colaked generic shape.

Short movie showing ray tracing on a colaked generic shape.

The design of electromagnetic cloaks based on the coordinate transformation requires a suitable geometrical definition of both the internal and the external surfaces of the cloak itself. We describe a straightforward method to design the electromagnetic cloak of a 3d-generic star domain whose surface is defined just by a set of points distributed o...

POLICRYPS (an acronym of polymer liquid crystal polymer slices) is a nano/microcomposite structure made of slices of almost pure polymer alternated with films of well aligned nematic liquid crystal (NLC). The structure is obtained by irradiating a homogeneous syrup of NLC, monomer and curing agent molecules with an interference pattern of UV/visibl...

We present the observation of an all-optical switching effect that takes place when a light beam of suitable wavelength irradiates a “layered” structure made of polymer slices alternated to films of well aligned liquid crystal (LC) which contain also a small percentage of azo-LC molecules. The simple and reliable fabrication procedure exploits a ho...

Phenomena associated to lasing in random or quasi-random media are of growing interest both from theoretical and experimental points of view. Recent studies show that nematic liquid crystals are good candidates as scattering host for these lasing systems. We present theoretical analysis and Monte Carlo simulations for different modellizations of sc...

We report on the electrically controlled optical phase modulator behavior of light sculptured periodic structures made of polymer slices alternated to films of well aligned Liquid Crystals (POLICRYPS). Arbitrarily polarized light normally incident on the structure experiences a birefringence that depends on the anisotropy of the composite liquid cr...

We report on the realization and characterization of an active control system of the optical holographic setup used for fabrication of holographic gratings in liquid-crystalline composite materials. The system exploits a reference diffraction grating and a piezomirror in closed-loop feedback. The piezoelectric mirror exhibits a hysteresis that depe...

We present a theoretical analisys based on scattering in nematics and on two-level system for the dye action. A simple description of light diffusion is shown to be inadequate to describe processes at the basis of this lasing; different physical phenomena have to be considered.

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. A new set of experimental measures are also presented.

System of local control and stabilization of a device for the writing of holografic gratings in composite liquid crystal materials by means of interference figure generated by two components, defracted and reflected, of a UV laser beam. This beam falls on a semireflecting mirror (5), which two components are reflected from two mirrors (6,7) one of...

The study of optical solitons and light filaments steering in liquid crystal requires the utilization of particular cells designed for top view investigation and realized with an input interface which enables the control of the molecular director configuration, preventing light scattering. Actually, the director orientation imposed by this addition...

A Nematic Liquid Crystal crossed by the overlap of multiple laser beams gives rise to different phenomena, such as the cancellation of reorientation and critical reorientation. A nonlinear 2D theoretical model is presented to describe these effects in the case of two laser beams impinging on a homeotropically aligned cell. By comparing the theoreti...

We report simulations of a theoretical model describing the formation of holographic diffraction gratings in multicomponent materials. The concentration of the chemical species in the initial mixture plays a fundamental role in determining the final features of the realized sample; our investigation is devoted to determine the best initial mixture...

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...

The study of optical solitons and light filaments steering in liquid crystals requires utilization of particular cells designed for top view investigation and realized with an input interface which enables both to control the molecular director configuration and to prevent light scattering. Up to now, the director orientation imposed by this additi...

In this paper we report the investigation of a beam coupling effect that occurs during the formation (by UV curing) of POLICRYPS diffraction gratings. Along with observations, we present a complete theoretical model which accounts for the main experimental features of the effect. Numerical solutions confirm the observed absence of any energy transf...

We report on the realization of what we believe to be a new holographic setup for the fabrication of polymer liquid-crystal polymer-slice diffraction gratings, which utilizes an optical-feedback-driven nanopositioning technique. We have increased the stability of the interference pattern by means of a simple piezomirror used in a feedback configura...

Composite polymer/liquid crystal material having a holographic grating structure formed by an alternating ordered succession of polymer layers and nematic liquid crystal layers. The nematic liquid crystal layers comprise an ordered nematic monophase region which is substantially free from liquid crystal droplets. Furthermore, the holographic gratin...

POLICRYPS, an acronym of POlymer LIquid CRYstal Polymer Slices, is a structure made of perfectly aligned liquid crystal films separated by slices of almost pure polymer. Under suitable experimental and geometrical conditions, the structure is obtained by curing a homogeneous syrup of liquid crystal, monomer and curing agent molecules with a spatial...

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 t...

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 impi...

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 induc...

We report on diffraction gratings consisting of continuous and well-aligned nematic films separated by polymer slices, which are originated in nematic-containing polymer composites when cured by a UV interference pattern. The diffraction efficiency of these gratings is experimentally investigated in detail; its temperature dependence reveals a rath...

We have performed a first characterization of the diffraction efficiency of gratings written in liquid-crystalline composite materials by the interference pattern of two curing beams. The grating fringes consist of polymer slices separated by films of continuous nematic phase. The dependence of the diffraction efficiency on temperature reveals a no...

In view of POLICRYPS (an acronym for polymer-liquid crystal-polymer slices) optimisation, this study analyses the dynamical response of POLICRYPS gratings and its relation with some geometrical and morphological factors. The final goal is to get overall response times in the sub-millisecond range. Results show that even for not-optimised grating sa...

We have studied the dynamical response of a new kind of holographic grating to a pulsed electric field. The structure, named POLICRYPS, consists of a sequence of homogeneous polymer slices separated by liquid-crystal films. We have found that both the rise and fall response times depend on the pulse duration and amplitude of the applied electric fi...

We report the diffraction properties at wavelengths of 632.8 and 1550 nm for volume transmission gratings made of a sequence of continuously aligned nematic liquid-crystal layers separated by isotropic polymer slices. The gratings are generated by holographically curing a solution of liquid crystal diluted in an isotropic prepolymer by means of a l...

We present a new kind of UV-cured holographic grating that consists of polymer slices alternated with pure nematic films. By preventing the appearance of the nematic phase during the curing process, it is possible to avoid the formation of liquid-crystal droplets and obtain a sharp and uniform morphology, which reduces scattering losses and increas...

We investigate the dynamics of a realization of Fermi's relativistic acceleration mechanism that is a charged test particle oscillating between two reflecting plates that move stochastically. By allowing the charge to radiate energy during each collision, we find that the main features of the system are (1) due to the radiation drag the energy gain...

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 th...

We have studied the dynamical response of a new polymer-liquid crystal composite, realised in a form of a thick grating. This structure, named POLICRYPS, consists in an alternate sequence of homogeneous polymer slides and liquid crystal stripes. Optimisation of response time is an important factor to propose POLICRYPS as basic element in different...

This study proposes a novel technique in fabricating permanent switchable gratings in composite materials which are characterized by both lower scattering losses and lower driving electric fields in comparison with usual PDLC gratings. Such a technique exploits the interference pattern produced by two UV (cw) laser beams. Acting on a solution of hi...

Optical characterization at the wavelengths of 632.8 nm and 1549 nm of an electrically switchable POLICRYPS grating sample is reported. The sample consists of a 10 µm thick cell made of ITO coated glass filled with a mixture of 30% nematic liquid crystal 5CB and pre-polymer NOA61. The grating with a period of 1.34 µm was written by laser curing at...