arXiv subjects

Roberto Caputo

Publications and source records attributed to Roberto Caputo.

11 recordsLinked to original sources

arXiv2025

Near-visible low power tuning of nematic-liquid crystal integrated silicon nitride ring resonator

The development of compact, low-power, and high-performance integrated photonic phase shifters is critical for advancing emerging technologies such as light detection and ranging (LiDAR), optical information processing and quantum applications. Liquid crystal (LC)-based phase shifters offer a promising solution thanks to their large refractive index contrast and their low power consumption. However, it remains challenging to incorporate LCs into integrated photonics and the operating wavelength has been limited to near infrared. Here, we demonstrate a liquid-crystal-based phase shifter operating at 780 nm, a relevant wavelength for cold atom-based quantum applications, by incorporating nematic LCs (E7) into a silicon nitride (SiN) ring resonator. Our device achieves 2pi phase modulation with very low power of 2.1 nW and low driving voltages of 7 V with exceptionally low Vpi times L (half wave voltage times length) value of 0.014 V-cm, enabling precise control over light propagation in a compact footprint. This work marks a significant step toward realizing low-power, compact, and efficient LC integrated photonic circuits based on SiN platform for next-generation LiDAR and quantum optical systems.

physics.optics↗

arXiv2025

Anti-counterfeiting tags with camouflaged QR codes on nanocavities, using polymer-dispersed-liquid-crystals

Counterfeiting poses an evergrowing challenge, driving the need for innovative and sophisticated anti-counterfeiting strategies and technologies. Many solutions focus on tags characterized by optical features that are partially or completely camouflaged to the human eye, thus discouraging scammers. In this paper, a QR code is laser printed on a thin plastic foil previously coated by a specific nanocavity consisting of a metal/insulator/metal/insulator (MIMI) multilayer. This metamaterial possesses unique features in terms of light transmission that are due to the specific design. A thin layer of polymer dispersed liquid crystals, fabricated incorporating specific nematic liquid crystals in a polymer matrix, is able to camouflage the QR code that becomes, then, readable only under specific thermal conditions. Three anti-counterfeiting tags were fabricated, each using a distinct LC with its own nematic-isotropic transition temperature. The peculiar combination of the unique optical properties of nematic liquid crystals and optical nanocavities results in the creation of a novel type of tags showing two different encoding levels. Stress tests including water immersion, bending test, and prolonged heating have been performed ensuring the long-term stability of the tags. The realized two security-level anti-counterfeiting tags are cost-effective, straightforward to manufacture and, thanks to their flexibility, can be easily integrated into packaging and products.

cs.CR↗

arXiv2024

Fluorescence Profile of Calabrian Opuntia ficus-indica Cladodes Extract: a Promising Low-cost Material for Technological Applications

The autofluorescence of calabrian Opuntia ficus-indica bioactive extract upon UV illumination is explored by fluorescence spectroscopy enabling to investigate the typology and distribution of responsible molecules within green cladodes. The spectroscopic analysis of the extract shows a significative red emission, suggesting an abundance of chlorophylls in the sample. Such molecules show pronounced fluorescence in the visible range (400-800 nm) with a very large Stokes shift, when excited with UV light source. The fluorescence profiling is performed also in the case of polymers, such as poly(methyl methacrylate) (PMMA), poly(vinylpyrrolidone) (PVP) and polyvinyl alcohol (PVA), enriched with OFI extract with a signal improvement up to 40 times greater than the extract alone. This by-product fluorescent molecule can replace commercial dyes into several applications spanning from nano-optics to anti-counterfeiting and bioimaging.

physics.optics↗

arXiv2023

Flexible Physical Unclonable Functions based on non-deterministically distributed Dye-Doped Fibers and Droplets

The development of new anti-counterfeiting solutions is a constant challenge and involves several research fields. Much interest is devoted to systems that are impossible to clone, based on the Physical Unclonable Function (PUF) paradigm. In this work, new strategies based on electrospinning and electrospraying of dye-doped polymeric materials are presented for the manufacturing of flexible free-standing films that embed different PUF keys. Films can be used to fabricate anticounterfeiting labels having three encryption levels: i) a map of fluorescent polymer droplets, with non deterministic positions on a dense yarn of polymer nanofibers; ii) a characteristic fluorescence spectrum for each label; iii) a challenge-response pairs (CRPs) identification protocol based on the strong nature of the physical unclonable function. The intrinsic uniqueness introduced by the deposition techniques encodes enough complexity into the optical anti-counterfeiting tag to generate thousands of cryptographic keys. The simple and cheap fabrication process as well as the multilevel authentication makes such colored polymeric unclonable tags a practical solution in the secure protection of merchandise in our daily life.

cond-mat.mtrl-sci↗

arXiv2023

Unlocking optical coupling tunability in epsilon-near-zero metamaterials through liquid crystal nanocavities

Epsilon-near-zero (ENZ) metamaterials represent a powerful toolkit for selectively transmitting and localizing light through cavity resonances, enabling the study of mesoscopic phenomena and facilitating the design of photonic devices. In this experimental study, we demonstrate the feasibility of engineering and actively controlling cavity modes, as well as tuning their mutual coupling, in an ENZ multilayer structure. Specifically, by employing a high-birefringence liquid crystal film as a tunable nanocavity, the polarization-dependent coupling of resonant modes with narrow spectral width and spatial extent was achieved. Surface forces aparatus (SFA) allowed us to continuously and precisely control the thickness of the liquid crystal film contained between the nanocavities and thus vary the detuning between the cavity modes. Hence, we were able to manipulate nanocavities anti-crossing behaviors. The suggested methodology unlocks the full potential of tunable optical coupling in epsilon-near-zero metamaterials and provides a versatile approach to the creation of tunable photonic devices, including bio-photonic sensors, and/or tunable planar metamaterials for on-chip spectrometers.

physics.optics↗

arXiv2021

Tailoring of Plasmonic Functionalized Metastructures to Enhance Local Heating Release

Plasmonic nanoheaters are reported that produce a significant local heating when excited by a 532 nm wavelength focussed laser beam. A significant temperature increase derives from the strong confinement of electric field enabled by the specific arrangement of Au nanodisks constituting the nanoheater. The thermal response is much more sensitive when layering the gold nanoheaters by a thick layer of doped polymer, reaching a temperature variation of more than 250{\deg}C. The modulation of the excitation by a chopper enables the fine control of the thermal response with a measured maximum temperature variation of about 60{\deg}C in a single period. These intriguing features can be efficiently exploited for the design of novel systems finding application in nano medicine and nano chemistry

physics.optics↗

arXiv2020

Hyper Resolution Two Photon Direct Laser Writing using ENZ Nano-Cavity

A novel technique is reported to improve the resolution of two-photon direct laser writing lithography. Thanks to the high collimation enabled by extraordinary $\varepsilon_{NZ}$ (near-zero) metamaterial features, ultra-thin dielectric hyper resolute nanostructures are within reach. With respect to the standard direct laser writing approach, a size reduction of $89\%$ and $50\%$ , in height and width respectively, is achieved with the height of the structures adjustable between 5nm and 50nm. The retrieved 2D fabrication parameters are exploited for fabricating hyper resolute 3D structures. In particular, a highly detailed dielectric bas-relief (500 nm of full height) of Da Vinci's \textit{"Lady with an Ermine"} has been realized. The proof-of-concept result shows intriguing cues for the current and trendsetting research scenario in anti-counterfeiting applications, flat optics and photonics.

physics.app-ph↗

arXiv2020

Color Tuning by Spontaneous Propagation of Gap Surface Plasmons in Epsilon Near Zero Nano-Cavity

This work presents numerical and experimental results of plasmonic Metal-Insulator optical nano-cavities. The systems are composed of Silver as metal, polyvinylpyrrolidone or indium tin oxide or zinc oxide as insulator. The proposed nano-cavities exhibit extraordinary effects as colors changing in function of the incident/view angles, enhancement of the experimental real and imaginary parts of the pseudo dielectric constant, an extraordinary transmission of 50$\%$ and zero reflection for both polarizations at the resonant wavelengths. These results lead to the formation and propagation of surface plasmon polaritons and their hybridization in gap surface plasmons. The concurrence of these effects allow studying the Goos-H\"anchen shift by exploiting the very narrow $\Delta$ and $\Psi$ ellipsometer parameters. Thank to their optical properties, the proposed nano-cavities could be applied in several fields such tunable color filters, optics, photonics and sensing.

physics.optics↗

arXiv2020

Numerical modeling of active thermo-plasmonics experiments

In this paper, we present a simple and robust numerical method able to predict, with high accuracy, the photo-thermal effects occurring for a gold nanoparticles arrangement under externally applied strain. The physical system is numerically implemented in the COMSOL Multiphysics simulation platform. The gold nanoparticles distributions are excited by linearly polarized light. By considering the system at rest and under the action of a mechanical stress, we analyze the extinction cross section, and we observe the production of heat at the nanoscale. The purpose of this work is to describe how sensitive the local temperature of the gold nanoparticles arrangement is to the formation of localized photo-thermal hot spots.

physics.app-ph↗

arXiv2019

A numerical ellipsometric analysis (NEA) of nanoscale layered systems

A simple and robust method able to predict, with high accuracy, the optical properties of single and multi-layer nanostructures is presented. The method exploits a COMSOL Multiphysics simulation platform and it has been validated by three case studies with increasing numerical complexity: i) a single thin layer (20 nm) of Ag deposited on a glass substrate; ii) a metamaterial composed of five bi-layers of Ag/ITO (Indium Tin Oxide), with a thickness of 20 nm each; iii) a system based on a three-materials unit cell (AZO/ITO/Ag), but without any thickness periodicity (AZO stands for Al$_2$O$_3$/Zinc Oxide). Numerical results have been compared with experimental data provided by real ellipsometric measurements performed on the above mentioned nanostructures ad-hoc fabricated. The obtained agreement is excellent suggesting this research as a valid approach to design materials able to work in a broad spectrum range.

physics.app-ph↗

arXiv2015

Polar POLICRYPS Diffractive Structures Generate Cylindrical Vector Beams

Local shaping of the polarization state of a light beam is appealing for a number of applications. This can be achieved by employing devices containing birefringent materials. In this article, we present one such device that permits one to convert a uniformly circularly polarized beam into a cylindrical vector beam (CVB). This device has been fabricated by exploiting the POLICRYPS photocuring technique. It is a liquid-crystal-based optical diffraction grating featuring polar symmetry of the director alignment. We have characterized the resulting CVB profile and polarization for the cases of left and right circularly polarized incoming beams.

physics.optics↗