Bernardo Dias

Magneto-optic surface plasmon resonances (MOSPRs) rely on the interaction of magnetic fields with surface plasmon polaritons (SPP) to modulate plasmonic bands with magnetic fields and enhance magneto-optical activity. In the present work, a study on the magnetoplasmonic behavior of Ag/Fe bilayers is carried out by VIS-NIR spectroscopy and backed with SQUID measurements, determining the thickness-dependent magnetization of thin-film samples. The MOSPR sensing properties of Ag/Fe planar bilayers are simulated using Berreman’s matrix formalism, from which an optimized structure composed of 15 nm of Ag and 12.5 nm of Fe is obtained. The selected structure is fabricated and characterized for refractive index (RI) sensitivity, reaching 4946 RIU⁻¹ and returning an effective enhancement of refractometric sensitivity after magneto-optical modulation. A new optimized and cobalt-free magnetoplasmonic Ag/Fe bilayer structure is studied, fabricated, and characterized for the first time towards refractometric sensing, to the best of our knowledge. This configuration exhibits potential for enhancing refractometric sensitivity via magneto-optical modulation, thus paving the way towards a simpler, more accessible, and safe type of RI sensor with potential applications in chemical sensors and biosensors.

An optical fiber pH sensor based on a long-period fiber grating (LPFG) is reported. Two oppositely charged polymers, polyethylenimine (PEI) and polyacrylic acid (PAA), were alternately deposited on the sensing structure through a layer-by-layer (LbL) electrostatic self-assembly technique. Since the polymers are pH sensitive, their refractive index (RI) varies when the pH of the solution changes due to swelling/deswelling phenomena. The fabricated multilayer coating retained a similar property, enabling its use in pH-sensing applications. The pH of the PAA dipping solution was tuned so that a coated LPFG achieved a pH sensitivity of (6.3 ± 0.2) nm/pH in the 5.92–9.23 pH range. Only two bilayers of PEI/PAA were used as an overlay, which reduces the fabrication time and increases the reproducibility of the sensor, and its reversibility and repeatability were demonstrated by tracking the resonance band position throughout multiple cycles between different pH solutions. With simulation work and experimental results from a low-finesse Fabry–Perot (FP) cavity on a fiber tip, the coating properties were estimated. When saturated at low pH, it has a thickness of 200 nm and 1.53 ± 0.01 RI, expanding up to 310 nm with a 1.35 ± 0.01 RI at higher pH values, mostly due to the structural changes in the PAA.

The excitation of two different electromagnetic surface waves - Surface Plasmon Polaritons (SPPs) and Bloch Surface Waves (BSWs) - is demonstrated in a one-dimensional metal dielectric photonic crystal with numerical and experimental studies. The discussed structure consists of an Ag - TiO ₂ thin film stack forming a metal-insulator-metal-insulator device. The thickness of the TiO ₂ layer placed between the metals is tested for two different values (50 and 300nm), which also allows the excitation of guided-mode resonances. It is observed that BSWs in this metal-dielectric structure behave similarly 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 of the external dielectric is characterized. For the case of the plasmonic modes, a maximum sensitivity of (7.2 ± 0.3) × 10 ³ nm/RIU was measured, while for the BSW the maximum sensitivity was (1.20 ± 0.05) × 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.

The interrogation of optical fiber sensors (OFS) often relies on complex devices such as Optical Spectrum Analyzers (OSA) which are expensive with low portability and mainly suited to laboratory measurements or dedicated interrogation systems with limited spectral range. An interrogation unit was designed and fabricated using a photodetector combined with a Micro Electro-mechanical System and a Fabry-Pérot Interferometer (MEMS-FPI) working as a tunable filter with a response in the range 1350 to 1650nm. Deconvolution techniques were applied to mitigate the effect of the broadband response of the tunable filter on the measured signal. The performance of the unit was validated with the interrogation of Long Period Fiber Gratings (LPFG) as temperature, refractive index (RI) and relative humidity (RH) sensors. For the temperature a sensitivity of 0.135 ± 0.007nm/°C was obtained, which showed a 4.9% relative error when compared to the same measurement with an OSA. For the RI a sensitivity of 147 ± 11 nm/RIU was obtained which showed a relative error lower than 1% when compared to the OSA. For the humidity, sensitivities of 0.742 ± 0.005 nm/%RH and 0.056 ± 0.006 nm/%RH were obtained, with errors of 2.75% and 6.67% respectively, when compared to a commercial dedicated interrogation system. The low relative error obtained when compared to commercial alternatives, shows the potential of the system to be used in real-time applications that require portability, low-cost, energy efficiency and capacity for integration in dedicated systems.

Biochemical–chemical sensing with plasmonic sensors is widely performed by tracking the responses of surface plasmonic resonance peaks to changes in the medium. Interestingly, consistent sensitivity and resolution improvements have been demonstrated for gold nanoparticles by analyzing other spectral features, such as spectral inflection points or peak curvatures. Nevertheless, such studies were only conducted on planar platforms and were restricted to gold nanoparticles. In this work, such methodologies are explored and expanded to plasmonic optical fibers. Thus, we study—experimentally and theoretically—the optical responses of optical fiber-doped gold or silver nanospheres and optical fibers coated with continuous gold or silver thin films. Both experimental and numerical results are analyzed with differentiation methods, using total variation regularization to effectively minimize noise amplification propagation. Consistent resolution improvements of up to 2.2× for both types of plasmonic fibers are found, demonstrating that deploying such analysis with any plasmonic optical fiber sensors can lead to sensing resolution improvements.

The sensitivity of one-dimensional Bloch surface wave (BSW) sensors to external refractive index variations using Kretschmann’s configuration is calculated analytically by employing first-order perturbation theory for both TE and TM modes. This approach is then validated by comparison with both transfer matrix method simulations and experimental results for a chosen photonic crystal structure. Experimental sensitivities of (8.4±0.2)×10² and (8.4±0.4)×10² nm/RIU were obtained for the TE and TM BSW modes, corresponding to errors of 0.02% and 4%, respectively, when comparing with the perturbation theory approach. These results provide interesting insights into photonic crystal design for Bloch surface wave sensing by casting light into the important parameters related with sensor performance.

Bloch Surface Waves (BSW) consist of electromagnetic modes generated at the interface between a photonic crystal and an isotropic dielectric. This type of surface mode displays sharp resonances and high sensitivity to external refractive index variations, and thus appears to be an ideal candidate for usage in optical sensors. Nevertheless, design and optimization of photonic crystals is not a trivial task and constitutes an ongoing field of research. The sensitivity of BSW in both refractometric and adsorption sensing is calculated analytically using first-order perturbation theory for TE modes, allowing the understanding of how several physical parameters of the photonic crystal influence the sensitivity. Preliminary experimental results are presented, which aim to use the analytical calculations to allow for both refractometric and adsorption sensing in a single photonic crystal structure.

The sensing performance of two types of electromagnetic surface waves are compared, a Surface Plasmon Polariton, where a gold thin film is used, being a standard material in biosensing applications; and a Bloch Surface Wave, using a photonic crystal made of a stack of silica and titanium dioxide layers. It is verified that the sensing performance (as measured by the Figure of Merit) of the gold film is higher, even though the Bloch Surface Waves can serve specific applications due to its narrow bandwidth. At the same time, it is concluded that further research must be made in order to choose the right set of parameters that maximize the Bloch Surface Wave performance.

Biogenic amines (BA) are compounds found in a vast range of food products. In recent years, has been a crescent awareness towards food safety, followed by an increase in food regulations. Long period fiber gratings (LPFG) coated with titanium dioxide (TiO ₂ ) were used to monitor the optical properties of a layer of poly(ethylene-co-vinyl acetate) (PEVA) doped with maleic anhydride (MA) with was polymerized on top of TiO ₂ . This hydrophobic polymeric structure is permeable to BA which cause a steady increase of its effective refractive index causing a wavelength shift in the coated LPFG attenuation band. LPFG wavelength shift was observed and measured for the monoamine tyramine (TYR), to the diamines, putrescine (PUT), cadaverine (CAD), histamine (HIS) and tryptamine (TRYP) and to the polyamines, spermidine (SPED) and spermine (SPEM). It was determined that while PEVA coated devices present a residual sensitivity to BA, the maleic anhydride greatly increases it. In fact, for PEVA only coated LPFGs the sensitivities of 1.45±0.11, 0.97±0.05, 0.46±0.08 and 0.94±0.09 nmM ^-1 , for PUT, CAD, HIS and TYR, respectively, were measured. However, for PEVA doped maleic anhydride coated LPFGs the sensitivities are 3.34±0.13, 3.06±0.11, 2.62±0.14 and 3.65±0.23 nmM ^-1 , for PUT, CAD, HIS and TYR, respectively. Thus, the refractive index of PEVA increase with BAs in-diffusion and MA doping further enhance PEVA sensitivity to BA. The proposed sensor is expected to play a part in further development of a biosensor for the quantification of BA in real foodstuff, providing a methodology for quality control.