Luís Carlos Coelho

Abstract
A novel technique based on multiple amplitude wavelength modulation spectroscopy (MA-WMS) for simultaneous measurement of CH4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {CH}_4$$\end{document} gas concentration and pressure was developed and validated both through simulation and experiment, showing good agreement. To capture the spectrum broadening caused by increasing pressure and concomitantly obtain the concentration at the sensor's location, a laser centered at 1650.9 nm was subjected to multiple amplitude modulation depths while the 2fm\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$2f_{m}$$\end{document} signal, normalized by the DC component (an invariant quantity under optical loss), was recorded. While the use of a single and fixed modulation can introduce an ambiguity, as different pairs of pressure and concentration can yield the same value, this ambiguity is eliminated by employing multiple amplitude modulations. In this approach, the intersection point of the three level curves can provide the local pressure and concentration. The proposed system was able to measure concentrations from 5% up to 45% and pressures from 0.25 atm up to 1.75 atm, with a maximum error of 2% in concentration and 0.06 atm in pressure, respectively. The system was also tested for attenuation insensitivity, demonstrating that measurements were not significantly affected for up to 10 dB applied optical loss.

Abstract
Hydrogen (H2) detection has become extremely important in recent years due to the increasing need for sustainable alternative energy sources. In this field, optical sensors can contribute significantly due to remote interrogation capabilities and the absence of ignition sources. Among the different H2 optical sensors, plasmonic sensors appear to be a very sensitive technology; however, they require expensive plasmonic materials like gold or silver, which, together with a palladium-sensitive layer, can increase the sensor cost. In addition, plasmonic bands are usually outside the ideal infrared range for remote interrogation, between 1500 and 1600 nm. This work presents a polymer-protected Tamm Plasmon Resonance (TPR) sensor with a well-defined resonance band at 1572 nm composed of SiO2, TiO2 layers, and palladium as a sensitive layer. This architecture can reduce the production cost of sensing structures, replacing plasmonic films with dielectric materials, while offering improved resonance definition at longer wavelengths. First, numerical calculations were carried out using the Transfer-Matrix Method to study the impact of the thickness of each layer, incidence angle, and light polarization on the resonance band wavelength and H2 sensitivity. The optimized structure was then fabricated, exhibiting a wavelength shift of 9.5 nm to 4 vol % of H2, a response time of 30 s, and no cross-sensitivity to methane or ammonia. The sensor also demonstrated high stability and resistance to environmental degradation up to eight days. These results emphasize the advantages of TPR structures for gas sensing in the infrared spectral range, opening new avenues for remote plasmonic sensing.

Abstract
Detection of leaks in hydrogen (H2) infrastructure is required on a large scale to enable a safe widespread use of this clean energy source. Sensing solutions must be low-cost, use scalable fabrication methods and allow multiplexed detection while providing reliable safety alarms as fast as possible. Optical methods can make this possible while avoiding the risk of ignition due to electronics at the point of detection. Metal hydride-based micro-mirror configurations benefit from a simple interrogation scheme, as long as the sensitive element can produce a large optical response. Magnesium thin films undergo a drastic variation of properties when hydrogenated, making them suitable for this application. In this work, a micro-mirror device using single-mode fibers capable of detecting the presence of H2 with a loading t10 and t90 of 1.2 and 3.0 s, respectively, is demonstrated. A complete interrogation unit was developed, presenting a solution suited for widespread deployment using industry-standard optical components and equipment.

Abstract
Carbon dioxide (CO2) plays a crucial role in the biosphere, acting as an indicator of anthropogenic activity. Its monitoring is fundamental for controlling air and water quality, preserving the environment and optimizing industrial processes. The preparation of a bright fluorescent scaffold, named rhodol, was optimized by employing microwave heating as an alternative heating source, achieving shorter reaction times and higher yields. Structural characterization was performed by nuclear magnetic resonance (NMR) and high-resolution mass spectrometry (HRMS-ESI). Its application to produce a fluorescent optical membrane for monitoring CO2 in gas (gCO2) and in water (dCO2) was explored. Two different setups are used for this purpose, and in both, the same optical response is observed: the membrane's fluorescence intensity decreases as the CO2 concentration increases. The sensor's reliability for dCO2 is demonstrated through testing concentrations ranging from 1 ppm to 100 ppm with minimal photobleaching (0.0026 dB) over 7500 data points with an integration time of 200 ms each. The sensor performance for dCO2 evaluation exhibits an experimental error of +/- 1.81 ppm, a response time of 2 min, a limit of detection of 0.6 ppm and a Stokes-shift of 90 nm for concentrations between 1 and 100 ppm. Monitoring of gCO2 using this membrane is hindered by changes in relative humidity (RH), hence the results for concentration between 0.3 % and 100 % of gCO2 were achieved by maintaining a consistent high value of RH. Our findings highlight the effectiveness of the optimized rhodol synthesis and its application in an optical membrane for reliable monitoring of CO2 in various environmental conditions.

Abstract
The phase-matching conditions for exciting surface plasmon resonances (SPR) in plasmonic films are typically satisfied via prism, optical fibers or grating-assisted coupling. We recently showed that plasmonic nanospheres can act as local emitters, exciting SPR waves on thin films-termed nanoparticle-induced SPR (NPI-SPR). This structure holds promise for sensing, but the effects of optical fiber geometry and nanoparticle anisotropy on the response were unexplored. This study examines these factors, showing that an etched multimode fiber with a 200 mu m core diameter, taper ratio of 4, and etching angle of 20 degrees optimizes interaction with plasmonic nanoparticles. Tuning the nanoparticle aspect ratio from 1 to 3 shifts the NPI-SPR band from 780 to 1580 nm, with excitation highly dependent on the incident light angle. Notably, for light incident parallel to the film plane, a refractive index sensitivity exceeding 1000 nm/RIU is achieved. This efficient light emission combines the field locality enhancements of plasmonic nanoparticle-on-film structures with the emission efficiency of plasmonic nanoantennas, advancing plasmonic optical fiber chemical and biosensors.