Browsing by Author "De Luca, Antonio"

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Across Scales Approach Based on Exciton-Plasmon Coupling for Low Loss Optical Metamaterials
(2015-12-15) Dhama, Rakesh; Bartolino, Roberto; Versace, Carlo; De Luca, Antonio
Dielectric and resonant Gain singularities in multilayered nanostructures
(2017-05-03) Caligiuri, Vincenzo; Carbone, Vincenzo; De Luca, Antonio
In this thesis work, the dielectric and gain singularity regimes in Hyperbolic Metamaterials (HMM) have been conceived and both theoretically and experimentally studied. For the first one it has been demonstrated how, in order to induce a dielectric singularity in the dielectric permittivities of an HMM specific conditions on both the geometry and optical properties of the fundamental metal/dielectric components have to be fulfilled. An HMM respecting these constrains is named Epsilon- Near-Zero-and-Pole (eNZP). Such a system manifests both the so-called Type I and Type II within the visible range and, noticeably, allows to cancel the usually found effective dielectric (or metallic) frequency gap betwen them, showing a inversion point of these two coexisting anisotropies, called Canalization Wavelength or Transition Wavelength. It has been demonstrated how a light wave propagating inside the eNZP HMM, remains perfectly subwavelength collimated, proceeding as a straight soliton for more than 100 Rayleigh lengths. Many fascinating new properties are unlocked in such regime, among which the supercollimation and the perfect lensing have been theoretically studied as well as experimentally demonstrated. Due to the specific stringent conditions to be respected, it has been demonstrated that with a classic two-component HMM it is not possible to tune the eNZP wavelength and a new configuration has to be adopted based on three components: a high index dielectric, a low index dielectric and a metal. By means of this new configuration, a full visible range design range of the eNZP wavelength has been demonstrated, keeping the same three fundamental materials and only acting on their thickness. The possibility of introducing thermal tunability of the optical features of a classic HMM has been demonstrated, thus overcoming the well known lack of tunability such structures usually are affected by. Basing on a sol-gel TiO2 matrix, a new material has been conceived, embedding a low index dielectric (Polyvinylpyrrolidone, PVP) and an organic fluorescent medium (Coumarin C500). It has been found that the unsintered sol-gel TiO2 remains extremely sensitive to any temperature change, endowing the HMM embedding such new mixture with thermally tunable features. The possibility to thermally reversibly reconfigure the most significant properties of an HMM embedding such a new dielectric has been both theoretically and experimentally demonstrated as well those one of a complete reconfiguration of the system, irreversibly switching from an effective metal to an effective dielectric, when exposed to high temperatures. In the end, it has been possible to theorize and study a new propagation regime called Resonant Gain, occurring in specifically modified eNZP HMMs. In order the resonant gain singularity to occur in the perpendicular dielectric permittivity of the eNZP HMM, a fluorescent medium has to embedded in the dielectric layers. Conditions to fulfil are very stringent but, once reached, it has been demonstrated that light propagating in such a regime is extremely subwavelength confined and amplificated inside the HMM, giving rise to a self-amplifying perfect lens and leading this system to configure as a promising candidate for LASER effect at the nanoscale. The same phenomenon has been verified in the framework of newly conceived system consisting in dye embedding metal/dielectric multishell nanoparticle, configuring as promising candidates for SPASER effect.
Hyper resolute laser writing mediated by tailored ENZ matematerials: the specifc case of all-dielectric broadband metalenses
(Università della Calabria, 2021-04-02) Lio, Giuseppe Emanuele; Cipparrone, Gabriella; Caputo, Roberto; Giocondo, Michele; De Luca, Antonio
Metamaterials are part of an emerging research field with a broad range of useful potential applications in cross-disciplinary fields spanning material science, optics, industrial applications, and last but not least, sensing from environmental hazards to cancer cells. Metamaterials present particular features especially when they are fabricated as multi-stack layered systems or optical nano-cavities. In fact, due to the particular features presented by this kind of materials as strong self-collimation and canalization effects, extraordinary transmittance and plasmonic behavior, they open a very wide scenario of nano-technological applications. The application that has been addressed in this thesis exploits the interesting and intriguing features of metal/ insulator/ metal/insulator systems, so-called MIMIs, in optical nano-cavities configuration tailored to drastically improve the resolution of a generic Two Photon Direct Laser Writing (TP-DLW) lithography process. The enhanced technique covers an important role in nanotechnology and especially in new nanomaterials frontiers for the possible realization of polymeric, thus completely dielectric, metasurfaces. For these reasons, the driving concept of the work presented in this research activity is to carry out the entire cycle of realization of MIMI devices, passing from their design, optimization, fabrication and characterization. Following their realization, the optimized MIMI are used to enhance the TP-DLW process in order to fabricate hyper-resolute test samples as 1D gratings, 2D metasurfaces and 3D complex objects. Given its self-collimation optical features, the MIMI metamaterial is used for the characterization of the realized structures as well. A specific, noticeable case that has been addressed in this thesis is the realization of ultra-flat all-dielectric apochromatic broadband metalenses assisted, during the design, by a Deep Machine Learning algorithm and, for their fabrication, by the above mentioned enhanced TP-DLW process. Finally, the realized metalenses have been optically characterized in the visible spectrum (300 − 1000 nm) confirming (as designed) fascinating features if compared with the already realized metalenses like the numerical aperture, extended focal length and depth of focus. Chapter 1 introduces the main aspects of metamaterials, as well as, the isofrequency surface describing the dispersion relations of hyperbolic metamaterials, and different geometrical configurations that allow exploiting particular physical effects and behaviors in light-matter interaction. Then, stacked multi-layer materials and a particular family of those, #psilon Near Zero, are presented. These metamaterials are a particular class of artificial optical structures consisting of a periodic arrangement of metallic and dielectric layers able to self-collimate and canalize light inside themselves. Finally, the chapter concludes with an overview of the plasmonic behavior in a simple metal/insulator interface that produces surface plasmon polaritons. Then it considers the bulk plasmon polaritons in multi-stacked metamaterials and the gap surface plasmon in Fabry-Perot nano-cavities. In Chapter 2, a simple and fast, yet robust, way to design metamaterials is evaluated as a function of their optical response and behavior. In fact, the first topic addressed in this chapter is ellipsometry and related advantages, to characterize nano-structures by retrieving the ellipsometric parameters Y and D, reflectance and transmittance and the complex refractive index n − ik. Then, the Transfer Matrix Method (TMM) has been detailed and used to code a homebuilt Matlab tool to predict the optical behavior as a function of the metamaterials design. On the same way, by using COMSOL Multiphysics, a Numerical Ellipsometer Analysis (NEA) has been realized. NEA covers the role of a robust tool to predict the optical response in much complex systems such as multi-layered materials with/without superstructures (gratings, holes, helices) placed above them. Some numerical simulations predicted by the NEA are experimentally validated by different cases with increasing system complexity. The plasmonic dispersion relations and the modal analysis have been addressed for dielectric cavities that support multi-spectral modes in the visible. Finally, in the last section, particular effects produced by MIMI cavities have been studied. Two key aspects related to the optical cavities are presented below. The first concerns the way they show hues / shades of color as a function of the cavity thickness and the involved material; the second one is a fast and effective way to identify the plasmons propagating inside these structures through the pseudo-dielectric function < ˜# >. These designed cavities present also particular effects like a large de-phasing well-known as Goos-Hänchen shift, that it is exploitable for extremely accurate sensing. Chapter 3 begins by introducing the main concepts of one and two-photon lithography and describing the state of the art of the Two-Photon Direct Laser Writing (TP-DLW) process. Then, as reported in the previous chapter, it shows detailed aspects of optical-nano cavities and leverages on the MIMI properties and features to design an embedded device able to work at the two photon lithography process wavelength (l = 780nm). The fabricated prototype is tested in terms of the incident beam waist modification by the evaluation of the Point Spread Function (PSF) reduction, measured by an homebuilt confocal setup equipped with a beam profiler. After its characterization, the MIMI device is used to realize 1D gratings compared with the ones fabricated through standard glass substrates. The reduction of 89% in height and 50% in width challenges our research product to reproduce the portrait "The Lady with an Ermine" by Leonardo Da Vinci that exhibits an high resolution level in terms of details and the nanoscale slicing in the 3D fabrication Chapter 4. The results obtained by the enhanced TP-DLW technique are exploited, in this chapter, to realize all-dielectric apochromatic broadband "flatland" metalenses with overall thickness less than 50nm. For their de facto two-dimensional nature, we call them "flatland" metalenses with the obvious reference to the famous Abbott’s novel ("Flatland: A Romance of Many Dimensions"). Next generation optics follow the trendsetting of miniaturized devices with extraordinary features as extended focal length and Depth of Focus (DOF), high Numerical Aperture and, last but not least, fast and easy way to produce them. In fact, this extremely flat design is the result of the novel two-photon direct laser writing (TP-DLW) process enhanced to hyper resolution performance by leveraging on the peculiar optical properties of our designed and developed ENZ metamaterials. Once fabricated, the characterization of the metalenses follows by means of a homebuilt setup equipped with beam-profiler and spectrometer. This measurement provided the characteristic values for these features like focal length f = 1.14mm, DOF in the range |50 − 150|μm and the numerical aperture NA = 0.087. In summary, the improved resolution of TP-DLWprocess presented in Chapter 3 is extremely significant for industrial applications in several fields such as anti-counterfeiting and flat optics, as shown in the last two Chapters of this Thesis work.
Materials and processes for the optical Additive Manufacturing of advanced organic/inorganic nanocomposites for the mask-less plating of insulator and semiconductor substrates, and microfluidic devices
(Università della Calabria, 2020) Di Cianni, Wera; Cipparrone, Gabriella; Giocondo, Michele; De Luca, Antonio; De Leon, Alberto Sanz
The research presented in this doctoral thesis is carried out in the nanotechnology and soft matter frameworks, under the 4.0 Industry paradigm, inspired by the need to find new strategies for the Additive Manufacturing (AM) and to obtain new processable nanocomposites with enhanced properties. The AM technologies allow to build 3D objects with complex geometries by adding layer-upon-layer of material without any mold and permits to fabricate structured objects and microfluidic systems with particular optical and mechanical properties which cannot be easily made with classical Subtractive Manufacturing (SM) techniques. This paves the way to large improvements in optoelectronics, biotechnology, diagnostic or medicine. Moreover, the combined employment of bottom-up and top-down fabrication approaches could lead to important advances in the field of nanotechnology, widening further the possible applications field, permitting high resolution repeatable nanofabrication of 3D complex objects with the possibility of immediate industrial applications. The first AM technique used in this work is Stereolithography (SL), a vat photopolymerization technique that uses UV light to produce objects with resolution in the range 10-100 μm. Here, the novelty consists in adding a metallic precursor (KAuCl4) to a typical photosensitive resin to produce nanocomposites with gold nanoparticles synthesized in situ via photo- and thermal reduction. Nanocomposites produced are rich in gold nanoparticles and have interesting optical and plasmonic properties. Moreover, a fine tuning of the concentration of the gold salt allows the resin polymerization without suffering any inhibition of the gold precursor. A similar approach, taking advantage of the combination with photoreduction of a gold precursor (HAuCl4), can be achieved using a different technique belonging to the vat photopolymerization category, namely the Two Photon Direct Laser Writing (TP-DLW). This technique exploits the optical, nonlinear multiphoton absorption process and allows for the fabrication of 3D objects featuring details below the diffraction limit, down to 100 nm or even less. Here, this multiphoton absorption process is exploited to trigger the photo-reduction of the gold precursor. The use of a transparent hydrogel matrix allows for a fine control of the nanoparticles’ growth on either transparent or opaque substrates, such as glass or silicon, without the need of using masks or molds. An in-depth study on the diffusive process underlying the nanoparticles growth and a control of the ionic concentration are done to prove the importance of having a polymeric network to hold the created nanoparticles at their place, which enhances the quality of the created nanostructures. The nanofabrication of fiber reinforced polymer nanocomposites by TP-DLW was also demonstrated. For these experiments, the classical glass or silicon substrates were replaced with a silicon substrate on which silica nanowires (SiO2 NWs) have been previously grown. This research allowed to achieve the best resolution offered by the TP-DLW technique, even with high loads of fillers of SiO2 NWs, up to 70 wt%. This was achieved by matching the refractive indices of the SiO2 NWs and of the photoresist used as polymeric matrix. These nanocomposite materials presented a noticeable improvement of nano-hardness and elastic modulus when compared to the pristine photoresist, indicating how the proposed technique is well-suited for nano-applications with higher structural requirements, as in advanced microfluidics. A final comparison of the AM technologies used in the thesis is done to elucidate the advantages and disadvantages of each one of these techniques to choose the most efficient, easiest and fastest, depending on the materials to be used or the required resolution.