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Publications

Publications by CAP

2024

Observation of Surface Plasmon Polaritons and Bloch Surface Waves in a Metal-Dielectric Photonic Crystal

Authors
Dias, BS; de Almeida, JMMM; Coelho, LCC;

Publication
IEEE SENSORS JOURNAL

Abstract
The excitation of two different electromagnetic surface waves-surface plasmon polaritons (SPPs) and Bloch surface waves (BSWs)-is demonstrated in a 1-D metal-dielectric photonic crystal with numerical and experimental studies. The discussed structure consists of an Ag-TiO2 thin-film stack forming a metal-insulator-metal-insulator device. The thickness of the TiO2 layer placed between the metals is tested for two different values (50 and 300 nm), which also allows the excitation of guided-mode resonances. It is observed that BSWs in this metal-dielectric structure behave similar to the case of all-dielectric photonic crystals, whereas the SPP modes display similar properties to those excited in metal-insulator-metal cavities. The sensitivity of these surface states to variations in the refractive index (RI) of the external dielectric is characterized. For the case of the plasmonic modes, a maximum sensitivity of (7.2 +/- 0.3) x 10(3) nm/RIU was measured, while for the BSW the maximum sensitivity was (1.20 +/- 0.05) x 10(2) nm/RIU. Due to the large field enhancement and penetration on external media, these surface states display exceptional properties for application in optical sensors, and the presented results provide interesting possibilities in the design of novel sensing structures with a flexible selection of surface states for interrogation.

2024

High-visibility Fabry-Perot interferometer fabricated in ULE® glass through fs-laser machining

Authors
Maia, JM; Marques, PVS;

Publication
OPTICS AND LASER TECHNOLOGY

Abstract
Low-finesse Fabry-Perot interferometers (FPI) with a plano-convex geometry are fabricated in ULE (R) glass through ultrafast laser machining. With this geometry, it is possible to overcome beam divergence effects that contribute to the poor fringe visibility usually observed in 100-mu m or longer planar-planar FPIs. By replacing the planar surface with a spherical one, the diverging beam propagating through the cavity is re-focused back at the entrance of the lead-in fiber upon reflection at this curved interface, thereby balancing out the intensities of both interfering beams and enhancing the visibility. The design of a 3D shaped cavity with a spherical sidewall is only made possible through fs-laser direct writing followed by chemical etching. In this technique, the 3D volume is reduced to writing of uniformly vertically spaced 2D layers with unique geometry, which are then selectively removed during chemical etching with HF acid. The radius of curvature that maximizes fringe visibility is computed using a numerical tool that is experimentally validated. By choosing the optimal radius of curvature, uniform visibilities in the range of 0.98-1.00 are measured for interferometers produced with cavity lengths spanning from 100 to 1000 mu m.

2024

From sensor fusion to knowledge distillation in collaborative LIBS and hyperspectral imaging for mineral identification

Authors
Lopes T.; Capela D.; Guimarães D.; Ferreira M.F.S.; Jorge P.A.S.; Silva N.A.;

Publication
Scientific Reports

Abstract
Multimodal spectral imaging offers a unique approach to the enhancement of the analytical capabilities of standalone spectroscopy techniques by combining information gathered from distinct sources. In this manuscript, we explore such opportunities by focusing on two well-known spectral imaging techniques, namely laser-induced breakdown spectroscopy, and hyperspectral imaging, and explore the opportunities of collaborative sensing for a case study involving mineral identification. In specific, the work builds upon two distinct approaches: a traditional sensor fusion, where we strive to increase the information gathered by including information from the two modalities; and a knowledge distillation approach, where the Laser Induced Breakdown spectroscopy is used as an autonomous supervisor for hyperspectral imaging. Our results show the potential of both approaches in enhancing the performance over a single modality sensing system, highlighting, in particular, the advantages of the knowledge distillation framework in maximizing the potential benefits of using multiple techniques to build more interpretable models and paving for industrial applications.

2024

Linear Fiber Laser Configurations for Optical Concentration Sensing in Liquid Solutions

Authors
Soares, L; Perez-Herrera, RA; Novais, S; Ferreira, A; Silva, S; Frazao, O;

Publication
PHOTONICS

Abstract
In this study, different configurations based on linear fiber lasers were proposed and experimentally demonstrated to measure the concentration of liquid solutions. Samples of paracetamol liquid solutions with different concentrations, in the range from 52.61 to 201.33 g/kg, were used as a case-study. The optical gain was provided by a commercial bidirectional Erbium-Doped Fiber Amplifier (EDFA) and the linear cavity was obtained using two commercial Fiber Bragg Gratings (FBGs). The main difference of each configuration was the coupling ratio of the optical coupler used to extract the system signal. The sensing head corresponded to a Single-Mode Fiber (SMF) tip that worked as an intensity sensor. The results reveal that, despite the optical coupler used (50:50, 60:40, 70:30 or 80:20), all the configurations reached the laser condition, however, the concentration sensing was only possible using a laser drive current near to the threshold value. The configurations using a 70:30 and an 80:20 optical coupler allowed paracetamol concentration measurements with a higher sensitivity of (-3.00 +/- 0.24) pW/(g/kg) to be performed. In terms of resolution, the highest value obtained was 1.75 g/kg, when it was extracted at 20% of the output power to the linear cavity fiber laser configuration.

2023

Towards Safe Cooperative Autonomous Platoon systems using COTS Equipment

Authors
Kurunathan, H; Santos, J; Moreira, D; Santos, PM;

Publication
2023 IEEE 24TH INTERNATIONAL SYMPOSIUM ON A WORLD OF WIRELESS, MOBILE AND MULTIMEDIA NETWORKS, WOWMOM

Abstract
The domain of Intelligent Transportation Systems (ITS) is becoming a key candidate to enable safer and efficient mobility in IoT enabled smart cities. Several recent research in cooperative autonomous systems are conducted over simulation frameworks as real experiments are still too costly. In this paper, we present a platooning robotic test-bed platform with a 1/10 scale robotic vehicles that functions based on the input front commercially off the shelf technologies (COTS) such as Lidars and cameras. We also present an in-depth analysis of the functionalities and architecture of the proposed system. We also compare the performance of the aforementioned sensors in some real-life emulated scenarios. From our results, we were able to concur that the camera based platooning is able to perform well at partially observable scenarios than its counterpart.

2023

Open Source Solutions for Vulnerability Assessment: A Comparative Analysis

Authors
Cruz, DB; Almeida, JR; Oliveira, JL;

Publication
IEEE ACCESS

Abstract
As software applications continue to become more complex and attractive to cyber-attackers, enhancing resilience against cyber threats becomes essential. Aiming to provide more robust solutions, different approaches were proposed for vulnerability detection in different stages of the application life-cycle. This article explores three main approaches to application security: Static Application Security Testing (SAST), Dynamic Application Security Testing (DAST), and Software Composition Analysis (SCA). The analysis conducted in this work is focused on open-source solutions while considering commercial solutions to show contrast in the approaches taken and to better illustrate the different options available. It proposes a baseline comparison model to help evaluate and select the best solutions, using comparison criteria that are based on community standards. This work also identifies future opportunities for application security, highlighting some of the key challenges that still need to be addressed in order to fully protect against emerging threats, and proposes a workflow that combines the identified tools to be used for vulnerability assessments.

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