Alessandro Vaccari

Plasmonic Scattering by Metal Nanoparticles for Solar Cells

We investigate on absorption and scattering from metal nanoparticles in view of possible applications to photovoltaic cells. The analysis, accounting for most of the parameters involved in the physical mechanism of scattering, is split into two parts. In the first part, scattering from a metallic sphere is treated analytically to investigate the dependence on sphere size, sphere metal, and surrounding medium. In the second part, scattering from a metallic particle is investigated as a function of particle shape (spheroids, hemispheres, and cylinders) via numerical simulations based on the finite-difference time-domain method. The aim of the work is to provide a systematic study on scattering and absorption by metal nanoparticles, exploring several combinations of material and geometrical parameters in order to identify those combinations that could play a key role in solar cell efficiency improvement.

Parallel Implementation of a 3D Subgridding FDTD Algorithm for Large Simulations

In a previous paper, we proposed and tested a robust and e-cient three-dimensional (3-D) subgridding algorithm for the FDTD solution method of the Maxwells curl PDEs system. Its characteristic feature is the straight, non-recursive, embedding of Yee grids | reflned by factors of 3, 5, 7 and even larger | within coarser ones. There, the algorithms implementation was described with the traditional serial programming approach. In the present paper, we propose and test its parallel programming implementation. The goal is to make it suitable and e-cient for large scale electromagnetic simulations.

Light-opals interaction modeling by direct numerical solution of Maxwell’s equations

This work describes a 3-D Finite-Difference Time-Domain (FDTD) computational approach for the optical characterization of an opal photonic crystal. To fully validate the approach we compare the computed transmittance of a crystal model with the transmittance of an actual crystal sample, as measured over the 400 ÷ 750 nm wavelength range. The opal photonic crystal considered has a face-centered cubic (FCC) lattice structure of spherical particles made of polystyrene (a non-absorptive material with constant relative dielectric permittivity). Light-matter interaction is described by numerically solving Maxwells equations via a parallelized FDTD code. Periodic boundary conditions (PBCs) at the outer edges of the crystal are used to effectively enforce an infinite lateral extension of the sample. A method to study the propagating Bloch modes inside the crystal bulk is also proposed, which allows the reconstruction of the ω-k dispersion curve for k sweeping discretely the Brillouin zone of the crystal.

Modeling and Characterization of Antireflection Coatings with Embedded Silver Nanoparticles for Silicon Solar Cells

Plasmonics applied to solar cells is a widely investigated research field. Its main purpose is to include plasmonic structures in the cell design, in order to increase light trapping in the cell and, consequently, its energy conversion efficiency. Light scattering by plasmonic structures has been extensively studied by depositing metal nanoparticles on both sides of the cell, in order to enhance the transmission into the cell and/or the path length of the transmitted radiation. The effects due to the nanoparticles were studied also in the presence of dielectric layers covering the cell and working as anti-reflective coatings (ARC), although a complete discussion on the possible optimization of this setup is lacking. In this work, we provide a joint computational and experimental investigation of the optical properties of silver nanoparticles embedded in a SiO 2 ARC located on top of a crystalline silicon wafer. The effect of the particle size, particle position within the ARC layer, and surface coverage on the light transmitted to the silicon crystal are simulated by a finite-difference time-domain (FDTD) in-house software. On the experimental side, a composite anti-reflective structure, made of a silica layer with embedded silver nanoparticles, is deposited on top of silicon wafers. Samples differing in the size and position of the embedded metal particles are produced. For each configuration, the total reflectance is optically measured by means of a photo spectrometer coupled to an integrating sphere. We provide direct comparison of experimental and simulation results, along with an exhaustive discussion about the transmission efficiency of the investigated systems. We also discuss how our analysis might be extended to different configurations and cell design.

A Novel RC-FDTD Algorithm for the Drude Dispersion Analysis

One of the main techniques for the Finite-Difierence Time-Domain (FDTD) analysis of dispersive media is the Recursive Convolution (RC) method. The idea here proposed for calculating the updating FDTD equation is based on the Laplace transform and is applied to the Drude dispersion case. A novel RC-FDTD algorithm, that we call modifled, is then deduced. We test our algorithm by simulating gold and silver nanospheres exposed to an optical plane wave and by comparing the results with the analytical solution. The modifled algorithm guarantees a better overall accuracy of the solution, in particular at the plasmonic resonance frequencies.

Parallel finite-difference time-domain modeling of an opal photonic crystal

Abstract. This work describes a computational approach for the optical characterization of an opal photonic crystal (PC). We intend, in particular, to validate our approach by comparing the transmittance of a crystal model, as obtained by numerical simulation, with the transmittance of the same crystal, as measured over 400- to 700-nm wavelength range. We consider an opal PC with a face-centered cubic lattice structure of spherical particles made of polystyrene (a nonabsorptive material with constant relative dielectric permittivity). Light-crystal interaction is simulated by numerically solving Maxwell’s equations via the finite-difference time-domain method and by using the Kirchhoff formula to calculate the far field. A method to study the propagating Bloch modes inside the crystal bulk is also sketched.

Finite difference analysis and experimental validation of 3D photonic crystals for structural health monitoring

In this work, we validate the behavior of 3D Photonic Crystals for Structural Health Monitoring applications. A Finite Difference Time Domain (FDTD) analysis has been performed and compared to experimental data. We demonstrate that the photonic properties of a crystal (comprised of sub-micrometric polystyrene colloidal spheres embedded in a PDMS matrix) change as a function of the axial strain applied to a rubber substrate. The change in the reflected wavelength, detected through our laboratory experiments and equivalent to a visible change in crystal color, is assumed to be caused by changes in the interplanar spacing of the polystyrene beads. This behavior is captured by our full wave 3D FDTD model. This contains different wavelengths in the visible spectrum and the wave amplitudes of the reflected and transmitted secondary beams are then computed. A change in the reflectance or transmittance is observed at every programmed step in which we vary the distance between the spheres. These investigations are an important tool to predict, study and validate our understanding of the behavior of this highly complex physical system. In this context, we have developed a versatile and robust parallelized code, able to numerically model the interaction of light with matter, by directly solving Maxwells equations in their strong form. The ability to describe the physical behavior of such systems is an important and fundamental capability which will aid the design and validation of innovative photonic sensors.

Design of an innovative proximity detection embedded-system for safety application in industrial machinery

Safety of machine operation is an increasingly important matter in industrial applications. In this context, embedded systems have successfully been employed to build active barriers that react in real time to prevent injuries and accidents. In this paper, we present a novel safety barrier, based on the capacitive coupling effect, to detect the proximity of the hands to a dangerous zone. Our study focuses on the safety design phases of the system, according to rule IEC 62061, including safety hazard analysis, SIL allocation, and hardware design applied to a real industrial machine for “stone cutting” purpose. Compliance checking of reliability and safe failure fraction was performed through FMEA methods ensuring that the system can satisfy the SIL 2 safety level constraints.

Photonic crystal slab strain sensors: A viable tool for structural health monitoring

We discuss the possible use of photonic crystals as strain sensors. We demonstrate the feasible fabrication of a crystal having sub-micrometric polystyrene colloidal spheres in a PDMS matrix on a rubber substrate, and we demonstrate that the photonic properties change with substrate elongation according to theoretical prediction. The crystals sensitivity to strain depends directly on interplanar spacing and on Poissons ratio. To enhance the crystal strain resolution, we propose to fabricate photonic crystal slabs, which exhibit a high geometrical Poissons ratio, and we investigate their mechanical behavior through non-linear Finite Element analysis and then, through an approximate optomechanical approach, the optical response.

Glass-ceramics for photonics: Laser material processing

Transparent glass-ceramics, activated by luminescent species, present an important class of photonic materials because their specific optical, spectroscopic and structural properties. Several top-down and bottom-up techniques have been developed for transparent glass ceramic fabrication. Among them, laser material processing plays an important role and many significant results have been obtained in the field of waveguide glass ceramics fabrication. Here, after a short description of the state of art regarding laser material processing for glass ceramics, we report on the specific use of CO2 laser for the fabrication of transparent glass ceramic waveguides.