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Mechanism of surface plasmon polarition propagation for nano-optics applications
Full text linkIn recent times a new emerging research field has become more and more popular in the optics community, which is plasmonics. This discipline studies the surprising optical properties of metals at the nano-scale, which are substantially different from those at the macro-scale.
This thesis presents several theoretical/numerical studies in the field of plasmonics, both with fundamental research and with applicative purposes in the fields of photovoltaics and sensing.
One basic problem in plasmonics is the study of optical Bloch modes of planar arrays of metallic nanostructures, namely periodic in one or two dimensions but not in the third, what are called plasmonic crystal slabs. We present here a finite-elements-based numerical method for the modal analysis of such structures, which allows to retrieve complex Bloch modes dispersions both of truly bound optical modes and of leaky modes.
We present then a thorough investigation of the optical properties of a well-known plasmonic crystal, which is the 1-D lamellar grating. Our main interest here is the possible use of this structure as a light trapping device for photovoltaics applications. We consider its integration on top of a silicon solar cell and within a thin film organic solar cell. In both cases the mechanisms at the basis of the observed enhancement are analyzed in detail. In the former case, experimental evidence of the enhancement predicted by simulations is provided as well.
For what concerns sensing applications, we carried out three fundamental studies. The first concerns metal-coated dielectric wedges for plasmonic nanofocusing. These structures allow at a time an efficient coupling of impinging light to surface plasmon polaritons and their nanofocusing at the ridge. Finite elements method (FEM) was used to design the structure, which has been fabricated by means of FIB milling combined with silicon anisotropic etching and replica molding. Near field, and Raman optical characterizations were used to verify the nanofocusing effect.
The second study concerned the individuation and optimization of a plasmonic nanostructure suitable for the implementation in an optoelectronic biosensor based on a high electron mobility phototransistor (HEMT). Three different nanostructures were studied, maximizing their optical response to a surface refractive index variation. The best structure turned out to be an array of triangular grooves on a gold thick film, which has been finally fabricated and characterized in collaboration with the IOM-TASC Laboratory in Trieste.
Finally we carried out a study of a class of nanostructures termed as plasmonic vortex lenses, constituted by spiral and circular grooves on a gold surface. The great interest in these structures stems from their ability to couple and focalize impinging circularly polarized light in the form of plasmonic vortices, impressing them an arbitrary orbital angular momentum. We focused in particular on the transmission of such a plasmonic vortices through a hole placed a the lens center, analyzing in detail the angular momentum properties of the transmitted field
Complex Bloch-modes calculation of plasmonic crystal slabs by means of finite elements method
Full text linkWe present a Finite Element Method (FEM) to calculate the complex valued k(omega) dispersion curves of a photonic crystal slab in presence of both dispersive and lossy materials. In particular the method can be exploited to study plasmonic crystal slabs. We adopt Perfectly Matched Layers (PMLs) in order to truncate the open boundaries of the model, including their related anisotropic permittivity and permeability tensors in the weak form of Helmholtz's eigenvalue equation. Results of the model are presented in the interesting case of a holey metal film enabling to study the observed extraordinary optical transmission properties in term of the plasmonic Bloch modes of the structure
Nanofocusing on circularly distributed tapered metallic waveguides by means of plasmonic vortex lenses
No full textWe report our experimental results on the nanofocusing effect at the apex of planar nanotips placed at the center of a plasmonic vortex lens (PVL). PVLs are helical gratings that are able to generate surface plasmon polaritons (SPPs) carrying orbital angular momentum. A specific design allows us to couple the PVL with nanostructures placed at its center. The proposed configuration allows a simultaneous nanofocusing
effect on four facing planar nanotips, showing efficient condensation of SPPs at the metal–air interface toward the end point of the tips. An optimized fabrication process allows us to prepare high-quality structures
with a sharp tip apex. Near-field scanning optical microscopy has been used to demonstrate the nanofocusing effect
Bilayer holey plasmonic vortex lenses for the far field transmission of pure orbital angular momentum light states
No full textWe report the design of a holey plasmonic vortex lens (PVL) structure able to couple circularly polarized impinging light to a plasmonic vortex in the form of the fundamental TM mode of a metal–insulator–metal plasmonic waveguide. The field transmitted through the hole milled at the center of the second metal layer of the structure is characterized by a well-defined spiral harmonic, entirely determined by the spin of impinging light and by the chirality of the PVL structure. Scattering finite elements simulations are presented for single layer standard PVLs and for bilayer ones, comparing the spiral spectra of the transmitted field and the efficiencies of the architectures
Sub-wavelength confinement of the orbital angular momentum of light probed by plasmonic nanorods resonances
Full text linkWe discuss how the topological charge of an OAM-carrying plasmon (Plasmonic Vortex) can be probed by monitoring the near-field response of plasmonic nanostructures suitably arranged inside a Plasmonic Vortex Lens. The turning "on" or "off" of four gold nanorods, detected by a Scanning Near field Optical Microscope (SNOM), acts as a fingerprint of the OAM state of the PV at the nanoscale. Different configurations are studied numerically, the integrated structure is fabricated and near field characterization is performed for a particularly meaningful case
Angular momentum properties of electromagnetic field transmitted through holey plasmonic vortex lenses
No full textWe performed three-dimensional finite elements simulations of the optical response of holey plasmonic vortex lenses, i.e., spiral grooves milled on a thin gold film with a hole at the center. We focus in particular on the properties of the wave transmitted in the underlying half-space, which is shown to be a relevant part of the transmitted field. We find out that the angular momentum selection rule for this part of the field is different from the one for the transmitted plasmonic vortex, although closely related to the plasmonic interaction of the impinging wave with the chiral geometry
Polarization independence of extraordinary transmission trough 1D metallic gratings
Full text linkExtraordinary optical transmission of 1D metallic gratings is studied. Experimental samples are fabricated by means of Electron Beam Lithography. The optical characterization is focused on far field transmission properties and in particular on polarization dependence of the incident light. A peculiar symmetry in transmission spectra at different polarization angles is shown; this symmetry is studied both experimentally, and numerically with FEM method. A comparison between numerical and experimental data is provided. (C) 2011 Optical Society of Americ
Design and fabrication of a light trapping method for photovoltaic devices based on plasmonic gratings
No full textThe integration of silver 1D gratings as light trapping method on both wafer-based and thin-film flat silicon solar cells has been investigated by EM field numerical simulations. The far field scattering properties or the near field enhancement and coupling to guided modes have been respectively studied in combination with thick (200 μm) and thin (100 nm) absorber layers. The optimization of the layout shows that arrays of metallic nanowires placed on the front surface improve light harvesting in thin film devices but provide absorption enhancement only for TM polarized light in wafer based devices. The designed gratings have been fabricated with a process based on Laser Interference Lithography suitable to nanopattern large area substrates and able to control the duty cycle. Optical characterizations of the samples validate the modelling results
Focusing dynamics on circular distributed tapered metallic waveguides by means of plasmonic vortex lenses
No full textWe investigate the focusing effect on circularly distributed planar tapered plasmonic waveguides by means of three-dimensional (3D) finite elements simulations. The proposed configuration allows nanofocusing on four faced planar nanotips, showing efficient condensation of surface plasmons polaritons (SPPs) at the silver/air interface toward the endpoint of the tips. By means of a plasmonic vortex lens it is possible to illuminate the tips with SPP waves carrying orbital angular momentum (OAM), namely plasmonic vortices. Our 3D simulations show that by acting on the topological charge of the plasmonic vortex the electric field charge distribution at the tips apex can be controlled accordingly to the input electric field phase distribution. The results for three particular OAM values are shown, along with a generalization for arbitrary plasmonic vortex angular momentum values
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