Alessandro Veltri - Profile on Academia.edu (original) (raw)
Papers by Alessandro Veltri
arXiv (Cornell University), Dec 20, 2023
We study theoretically the emission and lasing properties of a single nanoshell spaser nanopartic... more We study theoretically the emission and lasing properties of a single nanoshell spaser nanoparticle, or plasmonic nanolaser, made of an active core (gain material) and a plasmonic metal shell. Based on an analytical framework coupling together time-dependent equations for the gain and the metal, we calculate the lasing threshold with the help of an instability analysis. We characterize the regime under the threshold, where the nanoshell behaves as an optical amplifier when excited by an incident probe field. We then investigate in depth the non-linear lasing regime above the threshold, under autonomous conditions (free lasing without external drive), by computing the system's dynamics both in the transient state and in the final steady state. We show that at threshold, the lasing starts at one frequency only, usually one of the plasmon resonances of the nanoshell; then as the gain is further raised, the emission widens to other frequencies. This differs significantly from previous findings in the literature, which found only one emission wavelength above threshold. We proceed to calculate 1
Réversibilité du couplage fort entre nanoparticules d'argent et molécules photochromiques
HAL (Le Centre pour la Communication Scientifique Directe), Nov 1, 2012
International audienc
Exciton-Plasmon Strong Coupling Using Photochromic Molecules
HAL (Le Centre pour la Communication Scientifique Directe), Sep 1, 2012
International audienc
Theoretical Characterization of the Holographic Recording of Diffraction Grating in Multicomponent Media
Molecular Crystals and Liquid Crystals, Mar 26, 2007
ABSTRACT We report simulations of a theoretical model describing the formation of holographic dif... more ABSTRACT 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 realizing good holographic gratings. Along with the model, that is valid for any initial the multicomponent mixture, we report a complete characterization of the numerical solution of the model for two well known cases: H-PDLC and POLICRYPS grating. Obtained results are in good agreement with experimental observation, showing that our model represent a necessary reference for fabrication and the characterization of holographic diffraction gratings in multicomponent materials.
Nematic liquid crystals used to control photo-thermal effects in gold nanoparticles
Proceedings of SPIE, Mar 7, 2016
We report on photo-thermal effects observed in gold nanoparticles (GNPs) dispersed in Nematic Liqu... more 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, the system represents an ideal nano-source of heat, remotely controllable by light to adjust the temperature at the nanoscale. Photo-induced temperature variations in GNPs dispersed in NLCs have been investigated by implementing a theoretical model based on the thermal heating equation applied to an anisotropic medium; theoretical predictions have been compared with results of experiments carried out in a NLC medium hosting GNPs. Both theory and experiments represent a step forward to understand the physics of heat production at the nanoscale, with applications that range from photonics to nanomedicine.
Applied optics, Mar 21, 2008
We report on the realization and characterization of an active control system of the optical holo... more 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 depends not only on the applied voltage, but also on the history of the mirror motion. In an open-loop configuration, the hysteresis can be reduced by adjusting the delay time between the application of two different control voltage values; in a closed-loop operation, it is possible to eliminate the residual hysteresis. By testing the system in different conditions, it has been shown that residual fluctuations are comparable to the resolution of the piezomirror operation.
POLICRYPS Switchable Holographic Grating: A Promising Grating Electro-Optical Pixel for High Resolution Display Application
IEEE/OSA Journal of Display Technology, Mar 1, 2006
POLICRYPS, an acronym of POlymer LIquid CRYstal Polymer Slices, is a structure made of perfectly ... more 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 ...
Physical Review Letters, Apr 8, 2004
We investigate the dynamics of a realization of Fermi's relativistic acceleration mechanism that ... more 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 gained by the particle is bounded, and (2) the radiated energy represents a typical realization of an on-off intermittent process, due to numerous continuous encounters with a very small emission, interrupted by short and intense bursts of radiation. This intermittent radiative process exhibits non-Gaussian statistical features.
Applied optics, Jun 1, 2006
We report on the realization of what we believe to be a new holographic setup for the fabrication... more 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 configuration to keep constant the phase of the interferometer. The feedback system is driven by a proportional, integral, derivative control software, and the stability degree is controlled by the reference signal coming from a standard test grating. A preliminary experimental characterization indicates that good control and stabilization of parasitic fluctuations of the interference pattern are obtained.
Physical Chemistry Chemical Physics, 2015
Please note that technical editing may introduce minor changes to the text and/or graphics, which... more Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal's standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
Optics Letters, Jun 1, 2004
We present a new kind of UV-cured holographic grating that consists of polymer slices alternated ... more 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 increases diffraction efficiency.
Plasmonic nanoparticles, gain media and loss compensation
The resonant behaviour of single plasmonic nanoparticles in contact with gain media is studied th... more The resonant behaviour of single plasmonic nanoparticles in contact with gain media is studied theoretically, with a focus on the issue of loss compensation, resonance amplification and modification. Two approaches are presented. The first description is macroscopic and based on a simple, quasi-static continuum description of the gain medium, and shows that gain-assisted particle display unusual plasmonic responses, with features depending notably on the nature of the metal used (gold or silver). Results are discussed in two geometries, metallic nanoparticles in an infinite gain medium, and metal core-gain shell nanoparticles. The second model is microscopic and describes the situation of a metallic gold nanoparticle, surrounded with a corona of dye molecules or nanocrystals. A full analytical electromagnetic model is proposed, taking into account interactions, through a Green function formalism, between dye and particle, and also between dye and dye, both directly and mediated via the nanoparticle. The modes of the particle-and-dye system are studied, demonstrating that some modes, either emissive or absorptive, can be strongly amplified and dominate the response of the ensemble.
Non-Linear Effects in NLC Media Undergoing Two Beams Irradiation
Molecular Crystals and Liquid Crystals, Mar 26, 2007
A Nematic Liquid Crystal crossed by the overlap of multiple laser beams gives rise to different p... more 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 theoretical estimation of the re-orientational effect to
Model for Molecular Director Configuration in a Liquid Crystal Cell with Multiple Interfaces
Journal of Nonlinear Optical Physics & Materials, Jun 1, 2007
The study of optical solitons and light filaments steering in liquid crystal requires the utiliza... more 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 additional interface is estimated only by experimental observations. We report a simple model describing the distribution of the director orientation inside a liquid crystal sample under the anchoring action of multiple interfaces. The model is based on the elastic continuum theory, and strong anchoring is taken into account for boundary conditions. Results are in good agreement with experimental observations.
POLICRYPS structures as switchable optical phase modulators
Optics Express, May 12, 2008
We report on the electrically controlled optical phase modulator behavior of light sculptured per... more 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 crystalline material and on the geometrical cell parameters. The sample behaves as a retardation plate in good agreement with the Jones matrices formalism. Birefringence tuning is obtained by applying a suitable voltage, while a negligible birefringence variation is detected by increasing the incidence power. This makes POLICRYPS structures suitable as switchable phase retarders for high power laser beams.
All-optical switching of holographic gratings made of polymer-liquid-crystal-polymer slices containing azo-compounds
Applied Physics Letters, Nov 3, 2008
We present the observation of an all-optical switching effect that takes place when a light beam ... more 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 holographic UV curing technique. The observed switching effect takes place in samples that exhibit diffraction efficiency as high as 85% and is induced in less than 0.5 s by an unfocused pump beam of 245 mW/cm2 power density.
ACS Nano, Dec 30, 2016
We present a detailed study on the resonant gain phenomena occurring in two nanostructures, in wh... more 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 able to unlock several applications. Two noticeable cases have been investigated: a 1D nano-resonator based on hyperbolic metamaterials and a 3D metal/dielectric spherical multi-shell. The former has been designed in the framework of the effective medium theory, in order to behave as an Epsilon Near Zero and Pole metamaterial, showing extraordinary light confinement and collimation. Such a peculiarity represents the key to lead to a resonant gain behaviour, a condition in which the system is demonstrated to behave as a self amplifying perfect lens. Very high enhancement and spectral sharpness of 1 nm of the emitted light are demonstrated by means of a transfer matrix method simulation. The latter system consists in a metal/doped dielectric multishell. A dedicated theoretical approach has been set up to finely engineer its double tunable resonant nature. The resonant gain condition has been demonstrated also in this case. Finite element method based simulations, as well as an analytical model, clarify the electric field 1 Page 1 of 38 ACS Paragon Plus Environment ACS Nano distribution inside the multishell and suggest the opportunity to use this device as a self enhanced loss compensated multishell, being a favorable scenario for low threshold SPASER action. Counter-intuitively, by exceeding the resonant amount of gain molecules in both systems causes a significant drop in the amplitude of the resonance.
Different reorientational regimes in a liquid crystalline medium undergoing multiple irradiation
Optics Express, Feb 19, 2007
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.
POLICRYPS: a liquid-crystalline composed nano/micro structure with a wide range of optical and electro-optical applications
InTech eBooks, Sep 9, 2011
arXiv (Cornell University), Dec 20, 2023
We study theoretically the emission and lasing properties of a single nanoshell spaser nanopartic... more We study theoretically the emission and lasing properties of a single nanoshell spaser nanoparticle, or plasmonic nanolaser, made of an active core (gain material) and a plasmonic metal shell. Based on an analytical framework coupling together time-dependent equations for the gain and the metal, we calculate the lasing threshold with the help of an instability analysis. We characterize the regime under the threshold, where the nanoshell behaves as an optical amplifier when excited by an incident probe field. We then investigate in depth the non-linear lasing regime above the threshold, under autonomous conditions (free lasing without external drive), by computing the system's dynamics both in the transient state and in the final steady state. We show that at threshold, the lasing starts at one frequency only, usually one of the plasmon resonances of the nanoshell; then as the gain is further raised, the emission widens to other frequencies. This differs significantly from previous findings in the literature, which found only one emission wavelength above threshold. We proceed to calculate 1
Réversibilité du couplage fort entre nanoparticules d'argent et molécules photochromiques
HAL (Le Centre pour la Communication Scientifique Directe), Nov 1, 2012
International audienc
Exciton-Plasmon Strong Coupling Using Photochromic Molecules
HAL (Le Centre pour la Communication Scientifique Directe), Sep 1, 2012
International audienc
Theoretical Characterization of the Holographic Recording of Diffraction Grating in Multicomponent Media
Molecular Crystals and Liquid Crystals, Mar 26, 2007
ABSTRACT We report simulations of a theoretical model describing the formation of holographic dif... more ABSTRACT 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 realizing good holographic gratings. Along with the model, that is valid for any initial the multicomponent mixture, we report a complete characterization of the numerical solution of the model for two well known cases: H-PDLC and POLICRYPS grating. Obtained results are in good agreement with experimental observation, showing that our model represent a necessary reference for fabrication and the characterization of holographic diffraction gratings in multicomponent materials.
Nematic liquid crystals used to control photo-thermal effects in gold nanoparticles
Proceedings of SPIE, Mar 7, 2016
We report on photo-thermal effects observed in gold nanoparticles (GNPs) dispersed in Nematic Liqu... more 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, the system represents an ideal nano-source of heat, remotely controllable by light to adjust the temperature at the nanoscale. Photo-induced temperature variations in GNPs dispersed in NLCs have been investigated by implementing a theoretical model based on the thermal heating equation applied to an anisotropic medium; theoretical predictions have been compared with results of experiments carried out in a NLC medium hosting GNPs. Both theory and experiments represent a step forward to understand the physics of heat production at the nanoscale, with applications that range from photonics to nanomedicine.
Applied optics, Mar 21, 2008
We report on the realization and characterization of an active control system of the optical holo... more 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 depends not only on the applied voltage, but also on the history of the mirror motion. In an open-loop configuration, the hysteresis can be reduced by adjusting the delay time between the application of two different control voltage values; in a closed-loop operation, it is possible to eliminate the residual hysteresis. By testing the system in different conditions, it has been shown that residual fluctuations are comparable to the resolution of the piezomirror operation.
POLICRYPS Switchable Holographic Grating: A Promising Grating Electro-Optical Pixel for High Resolution Display Application
IEEE/OSA Journal of Display Technology, Mar 1, 2006
POLICRYPS, an acronym of POlymer LIquid CRYstal Polymer Slices, is a structure made of perfectly ... more 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 ...
Physical Review Letters, Apr 8, 2004
We investigate the dynamics of a realization of Fermi's relativistic acceleration mechanism that ... more 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 gained by the particle is bounded, and (2) the radiated energy represents a typical realization of an on-off intermittent process, due to numerous continuous encounters with a very small emission, interrupted by short and intense bursts of radiation. This intermittent radiative process exhibits non-Gaussian statistical features.
Applied optics, Jun 1, 2006
We report on the realization of what we believe to be a new holographic setup for the fabrication... more 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 configuration to keep constant the phase of the interferometer. The feedback system is driven by a proportional, integral, derivative control software, and the stability degree is controlled by the reference signal coming from a standard test grating. A preliminary experimental characterization indicates that good control and stabilization of parasitic fluctuations of the interference pattern are obtained.
Physical Chemistry Chemical Physics, 2015
Please note that technical editing may introduce minor changes to the text and/or graphics, which... more Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal's standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
Optics Letters, Jun 1, 2004
We present a new kind of UV-cured holographic grating that consists of polymer slices alternated ... more 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 increases diffraction efficiency.
Plasmonic nanoparticles, gain media and loss compensation
The resonant behaviour of single plasmonic nanoparticles in contact with gain media is studied th... more The resonant behaviour of single plasmonic nanoparticles in contact with gain media is studied theoretically, with a focus on the issue of loss compensation, resonance amplification and modification. Two approaches are presented. The first description is macroscopic and based on a simple, quasi-static continuum description of the gain medium, and shows that gain-assisted particle display unusual plasmonic responses, with features depending notably on the nature of the metal used (gold or silver). Results are discussed in two geometries, metallic nanoparticles in an infinite gain medium, and metal core-gain shell nanoparticles. The second model is microscopic and describes the situation of a metallic gold nanoparticle, surrounded with a corona of dye molecules or nanocrystals. A full analytical electromagnetic model is proposed, taking into account interactions, through a Green function formalism, between dye and particle, and also between dye and dye, both directly and mediated via the nanoparticle. The modes of the particle-and-dye system are studied, demonstrating that some modes, either emissive or absorptive, can be strongly amplified and dominate the response of the ensemble.
Non-Linear Effects in NLC Media Undergoing Two Beams Irradiation
Molecular Crystals and Liquid Crystals, Mar 26, 2007
A Nematic Liquid Crystal crossed by the overlap of multiple laser beams gives rise to different p... more 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 theoretical estimation of the re-orientational effect to
Model for Molecular Director Configuration in a Liquid Crystal Cell with Multiple Interfaces
Journal of Nonlinear Optical Physics & Materials, Jun 1, 2007
The study of optical solitons and light filaments steering in liquid crystal requires the utiliza... more 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 additional interface is estimated only by experimental observations. We report a simple model describing the distribution of the director orientation inside a liquid crystal sample under the anchoring action of multiple interfaces. The model is based on the elastic continuum theory, and strong anchoring is taken into account for boundary conditions. Results are in good agreement with experimental observations.
POLICRYPS structures as switchable optical phase modulators
Optics Express, May 12, 2008
We report on the electrically controlled optical phase modulator behavior of light sculptured per... more 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 crystalline material and on the geometrical cell parameters. The sample behaves as a retardation plate in good agreement with the Jones matrices formalism. Birefringence tuning is obtained by applying a suitable voltage, while a negligible birefringence variation is detected by increasing the incidence power. This makes POLICRYPS structures suitable as switchable phase retarders for high power laser beams.
All-optical switching of holographic gratings made of polymer-liquid-crystal-polymer slices containing azo-compounds
Applied Physics Letters, Nov 3, 2008
We present the observation of an all-optical switching effect that takes place when a light beam ... more 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 holographic UV curing technique. The observed switching effect takes place in samples that exhibit diffraction efficiency as high as 85% and is induced in less than 0.5 s by an unfocused pump beam of 245 mW/cm2 power density.
ACS Nano, Dec 30, 2016
We present a detailed study on the resonant gain phenomena occurring in two nanostructures, in wh... more 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 able to unlock several applications. Two noticeable cases have been investigated: a 1D nano-resonator based on hyperbolic metamaterials and a 3D metal/dielectric spherical multi-shell. The former has been designed in the framework of the effective medium theory, in order to behave as an Epsilon Near Zero and Pole metamaterial, showing extraordinary light confinement and collimation. Such a peculiarity represents the key to lead to a resonant gain behaviour, a condition in which the system is demonstrated to behave as a self amplifying perfect lens. Very high enhancement and spectral sharpness of 1 nm of the emitted light are demonstrated by means of a transfer matrix method simulation. The latter system consists in a metal/doped dielectric multishell. A dedicated theoretical approach has been set up to finely engineer its double tunable resonant nature. The resonant gain condition has been demonstrated also in this case. Finite element method based simulations, as well as an analytical model, clarify the electric field 1 Page 1 of 38 ACS Paragon Plus Environment ACS Nano distribution inside the multishell and suggest the opportunity to use this device as a self enhanced loss compensated multishell, being a favorable scenario for low threshold SPASER action. Counter-intuitively, by exceeding the resonant amount of gain molecules in both systems causes a significant drop in the amplitude of the resonance.
Different reorientational regimes in a liquid crystalline medium undergoing multiple irradiation
Optics Express, Feb 19, 2007
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.
POLICRYPS: a liquid-crystalline composed nano/micro structure with a wide range of optical and electro-optical applications
InTech eBooks, Sep 9, 2011