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- Surface plasmon resonance sensing structurePublication . Lourenço, Paulo; Fantoni, Alessandro; Louro, Paula; Costa, João; Vieira, ManuelaSurface Plasmon Resonance occurs when a polarized electromagnetic field strikes a metallic surface at the separation interface between metal and an insulator. This phenomenon is characterized by the conduction electrons resonant oscillation at the interface, resulting on propagating plasmon waves on the metallic surface. Since this wave is generated at the boundary between the metallic surface and the external medium, these structures are highly sensitive to alterations on the surrounding environment, namely the refractive index, and may be used in sensing structures. The large majority of these devices use noble metals, namely gold or silver, as the active material. These metals present low resistivity, which leads to low optical losses in the visible and near infrared spectrum ranges. Gold shows high environmental stability, which is essential for long-term operation, and silver’s lower stability can be overcome through the deposition of an alumina layer. However, their high cost is a limiting factor if the intended target is large scale manufacturing. In this work, we performed Finite Differences Time Domain simulations on a Surface Plasmon Resonance based sensing structure, considering cost-effective materials such as aluminium for the active metal and hydrogenated amorphous silicon for the waveguide supporting elements, and verified that these structures are able to detect refractive index variations of the surrounding environment at the 1550 µm operating wavelength. This sensing architecture has also been modelled with dispersive materials, losses included, to reflect as much as possible physical reality, revealing good performance capabilities when compared to similar noble metals based devices.
- Multichannel detector system for surface plasmon resonance biosensorsPublication . Fernandes, Miguel; Fantoni, Alessandro; Soares, Paulo; Lourenço, Paulo; Vieira, ManuelaPhotonic systems are gaining an important role in the field of medical diagnosis due to the achievable high sensitivity and selectivity and low cost, enabling the fabrication of disposable point of care diagnosis systems for multiple pathologies. In this work we present the detector subsystem developed for a multi-channel surface plasmon resonance (SPR) based sensor. The core of the system is a multimode interferometer splitter, fabricated in amorphous silicon, followed by multiple sensitive SPR structures with a functionalized gold layer that modulate the transmitted light waves, in the presence of the biomarker, which are then detected by infrared detectors. For this purpose a highly adaptable detection system based on a InGaAs line CCD device was developed. The IR sensor used in the prototype has 128 (50 x 250 μm) pixels but other formats are supported. To adapt to different light guiding structures, the CCD pixels can be combined forming multiple detection channels. Optical sensor configuration and readout operations are performed trough a USB connection using the SCPI standard. The system includes an analog front end with a programmable gain amplifier and offset adjustment followed by a fast analog to digital converter feeding the data to a STM32 family processor. A computer application was also developed for system configuration and signal readout and storage. The testing results from the complete system are presented. Documentation of the developed system is provided for third party use, all the material generated within this work is available online in a repository.
- Finite-difference time-domain analysis of hydrogenated amorphous silicon and aluminum surface plasmon waveguidesPublication . Lourenço, Paulo; Fantoni, Alessandro; Fernandes, Miguel; Vygranenko, Yuri; Vieira, ManuelaThe large majority of surface plasmon resonance (SPR)-based devices use noble metals, namely gold or silver, in their manufacturing process. These metals present low resistivity, which leads to low optical losses in the visible and near-infrared spectrum ranges. Gold shows high environmental stability, which is essential for long-term operation, and the lower stability of silver can be overcome through the deposition of an alumina layer, for instance. However, their high cost is a limiting factor if the intended target is large-scale manufacturing. This work considers a cost-effective approach through the selection of aluminum as the plasmonic material and hydrogenated amorphous silicon instead of its crystalline counterpart. This SPR structure relies on Fano resonance to improve its response to refractive index deviations of the surrounding environment. Fano resonance is highly sensitive to slight changes of the medium, hence the reason we incorporated this interference phenomenon in the proposed sensing structure. We report the results obtained when conducting finite-difference timedomain algorithm-based simulations on this metal–dielectric–metal structure when the active metal is aluminum, gold, and silver. Then, we evaluate their sensitivity, detection accuracy, and resolution. The obtained results for our proposed sensing structure show good linearity and similar parameter performance as the ones obtained when using gold or silver as plasmonic materials.
- Simulation and analysis of surface plasmon resonance based sensorPublication . Lourenço, Paulo; Vieira, Manuela; Fantoni, AlessandroIn this paper, we will be presenting the results obtained through Finite-Difference Time Domain simulations on a photonic sensing architecture. This device consists on a dielectric/metal/dielectric sensing structure. Under adequate conditions, when electromagnetic energy strikes the different dielectrics interface, these devices develop surface plasmon resonances which are extremely sensitive to refractive index variations, thus being able to be used as sensing structures. Considering their minute dimensions, monolithic integration is attainable and by incorporating cost-effective materials in their manufacture, devices' mass production may be efficient and information and communication technological systems' resiliency will be greatly facilitated. Next, this architecture is analysed under amplitude and refractive index sensitivity perspectives, its performance is analysed and considerations about its use as a sensing device are contemplated. Finally, conclusions of our work are presented and future development directions are described.
- Simulation of an early warning fire systemPublication . Lourenço, Paulo; Fantoni, Alessandro; Vieira, ManuelaIn this paper, we will be using separate software tools (wireless network and Finite Differences Time Domain based simulators) to simulate the implementation of a wireless sensor network model based on low-rate/power transmission technology. The system operates in an unlicensed frequency range and the sensing nodes rely on surface plasmon resonance phenomenon for the detection of combustion by-products. More specifically, our simulations contemplate a system for early detection of fire in densely forested areas, which will then issue a warning in an automated way. As late detection of these events usually leads to severe flora, terrain, wild life and societal impact, an early warning system will provide better event assessment conditions, thus enabling efficient resources allocation, adequate response and would certainly be a promising improvement in minimizing such disruptive impairments.
- FDTD analysis of Aluminum/a-Si:H surface plasmon waveguidesPublication . Lourenço, Paulo; Fantoni, Alessandro; Fernandes, Miguel; Vygranenko, Yuri; Vieira, ManuelaThe large majority of surface plasmon resonance based devices use noble metals, namely gold or silver, in their manufacturing process. These metals present low resistivity, which leads to low optical losses in the visible and near infrared spectrum ranges. Gold shows high environmental stability, which is essential for long-term operation, and silver's lower stability can be overcome through the deposition of an alumina layer, for instance. However, their high cost is a limiting factor if the intended target is large scale manufacturing. In this work, it is considered a cost-effective approach through the selection of aluminum as the plasmonic material and hydrogenated amorphous silicon instead of its crystalline counterpart. This surface plasmon resonance device relies on Fano resonance to improve its response to refractive index deviations of the surrounding environment. Fano resonance is highly sensitive to slight changes of the medium, hence the reason we incorporated this interference phenomenon in the proposed device. We report the results obtained when conducting Finite-Difference Time Domain algorithm based simulations on this metal-dielectric-metal structure when the active metal is aluminum, gold and silver. Then, we evaluate their sensitivity, detection accuracy and resolution, and the obtained results for our proposed device show good linearity and similar parameter performance as the ones obtained when using gold or silver as plasmonic materials.
- Simulation analysis of a thin film semiconductor MMI 3dB splitter operating in the visible rangePublication . Lourenço, Paulo; Fantoni, Alessandro; Vieira, ManuelaIn this paper we present a simulation study that intends to characterize the influence of defects introduced by manufacturing processes on the geometry of a semiconductor structure suitable to be used as a multimode interference (MMI) 3 dB power splitter. Consequently, these defects will represent refractive index fluctuations which, on their turn, will drastically affect the propagation conditions within the structure. Our simulations were conducted on a software platform that implements both Beam Propagation and FDTD numerical methods. This work supports the development of a biomedical plasmonic sensor, which is based on the coupling between the propagating modes in a dielectric waveguide and the surface plasmon mode that is generated on an overlaid metallic thin film, and where the output readout is achieved through an a-Si:H photodiode. By using a multimode interference 1×2 power splitter, this sensor device can utilize the non-sensing arm as a reference one, greatly facilitating its calibration and enhanced performance. Amorphous silicon can be deposited by PECVD processes at temperatures lower than 300°C, an attractive characteristic which makes it back-end compatible to CMOS fabrication processes. As the spectral sensitivity of amorphous silicon is restricted to the visible range, this sensing device should be operating on a wavelength not higher than 700 nm, thus a-SiNx has been the material hereby proposed for both waveguides and MMI power splitter.