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- Performance of an a-Si:H MMI multichannel beam splitter analyzed by computer simulationPublication . Costa, João; Almeida, Daniel; Fantoni, Alessandro; Lourenço, Paulo; Fernandes, Miguel; Vieira, ManuelaOptical power splitters are widely used in many applications and di_erent typologies have been developed for devices dedicated to this function. Among them, the multimode interference design is especially attractive for its simplicity and performance making it a strong candidate for low-cost applications, such as photonics lab-on-chips for biomedical point of care systems. Within this context, splitting the optical beam equally into multiple channels is of fundamental importance to provide reference arms, parallel sensing of di_erent biomarkers and allowing multiplexed reading schemes. From a theoretical point of view, the multimode structure allows implementation of the power splitting function for an arbitrary number of channels, but in practice its performance is limited by lithographic mask imperfections and waveguide width. In this work we analyze multimode waveguide structures, based on amorphous silicon (a-Si:H) over insulator (SiO2), which can be produced by the PECVD deposition technique. The study compares the performance of several 1 to N designs optimized to provide division of the fundamental quasi-TM mode as a function of input polarization and lithographic roughness. The performance is analyzed in terms of output power uniformity and attenuation and is based on numerical simulations using the Beam Propagation Method and Eigenmode Expansion Propagation Methods.
- Simulation of a parallel waveguide array structure suitable for interrogation scheme in a plasmonic biossensorPublication . Costa, João; Fantoni, Alessandro; Lourenço, Paulo; Vieira, ManuelaSurface plasmon resonance sensors have emerged has one of the most suitable approaches for biosensing. A common approach consists of exciting the plasmons at the interface between a functionalized metal film and a sample medium containing the analyte. The propagation of the surface plasmon is highly dependent on changes of the refractive index of the surrounding environment thus providing a mechanism for sensing. The typical interrogation schemes are based on scanning over the wavelength or the incident angle to search for the resonance condition. These solutions require additional motor-driven rotation stages, prisms or other bulky components, introducing complexity which prevents the fabrication of fully on-chip devices. This work reports a simulation study of an amorphous silicon waveguide structure consisting of an array of parallel surface plasmon interferometers with different propagation lengths, each one comprising a thin layer of gold embedded into a-Si:H waveguide. The surface plasmon modes at the end of the plasmonic structure can interfere constructively or destructively depending on the refractive index of the analyte and the interferometer’s length. The variation of the output intensity at the end of each element of the array provides a convenient interrogation scheme that is suitable for on-chip integration. In this paper we investigate this setup and analyze the output power at the end of the array as a function of the refractive index of the sampling medium. The setup is simulated and characterized by the eigenmode expansion method.
- Optical signal processing for data error detection and correction using a-SiCH technologyPublication . Vieira, Manuel; Vieira, Manuela; Silva, V.; Louro, Paula; Costa, JoãoIn this paper, we explore the nonlinear properties of SiC multilayer devices under UV irradiation to design an optical processor for error detection and correction, that enables reliable delivery of spectral data of four-wave mixing over unreliable communication channels. The SiC optical processor for error detection and correction is realized by using a SiC pin/pin photodetector with UV biased optical gating elements. The relationship between the optical inputs and the corresponding digital output levels is established. Data shows that the optical bias act as a selector that picks one or more states by splitting portions of the input multi optical signals across the front and back photodiodes. Boolean operations are demonstrated optically, additional parity logic operations are performed and checked for errors together. As an example we describe an all-optical processor for error detection and correction and then, provide an experimental demonstration of this fault tolerant reversible system. An intuitive representation with a 4 bit original string coloured message and the transmitted 7 bit string, the parity matrix, the encoding and decoding processes and the design of SiC syndrome generators are presented.
- Optimisation of a plasmonic parallel waveguide sensor based on amorphous silicon compoundsPublication . Costa, João; Fantoni, Alessandro; Lourenço, Paulo; Vieira, ManuelaThis work reports the simulation of a plasmonic waveguide sensor working in the visible range based on amorphous silicon compounds. Typical plasmonic sensor interrogation schemes are based on scanning over the wavelength or the incident angle to search for the resonance condition. These solutions usually require expensive or bulky components, such as prisms, motor-driven rotation stages or tunable lasers. In this work we propose an amorphous silicon nitride waveguide structure consisting of an array of parallel surface plasmon interferometers of different lengths, each one comprising a thin layer of aluminium embedded into the waveguide. Using modal decomposition simulations, we show that the variation of the output power at the end of each waveguide array element provides a convenient interrogation scheme. By exploring amorphous silicon compounds that can be deposited by Pressure Enhanced Chemical Vapor Deposition (PECVD) at low temperatures, we aim to achieve a low-cost fabrication process that is compatible with backend CMOS processing and wavelengths in the visible range.
- Stacked photo-sensing devices based on SiC alloys A non-pixelled architecture for imagers and demultiplexing devicesPublication . Vieira, Manuela; Louro, Paula; Fernandes, Miguel; Fantoni, Alessandro; Vieira, Manuel Augusto; Costa, JoãoIn this review paper different designs based on stacked p-i'-n-p-i-n heterojunctions are presented and compared with the single p-i-n sensing structures. The imagers utilise self-field induced depletion layers for light detection and a modulated laser beam for sequential readout. The effect of the sensing element structure, cell configurations (single or tandem), and light source properties (intensity and wavelength) are correlated with the sensor output characteristics (light-to-dark sensivity, spatial resolution, linearity and S/N ratio). The readout frequency is optimized showing that scans speeds up to 104 lines per second can be achieved without degradation in the resolution. Multilayered p-i'-n-p-i-n heterostructures can also be used as wavelength-division multiplexing /demultiplexing devices in the visible range. Here the sensor element faces the modulated light from different input colour channels, each one with a specific wavelength and bit rate. By reading out the photocurrent at appropriated applied bias, the information is multiplexed or demultiplexed and can be transmitted or recovered again. Electrical models are present to support the sensing methodologies.
- 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.
- Computer simulation study about the dependence of amorphous silicon photonic waveguides efficiency on the material qualityPublication . Fantoni, Alessandro; Costa, João; Lourenço, Paulo; Vieira, ManuelaAmorphous silicon PECVD photonic integrated devices are promising candidates for low cost sensing applications. This manuscript reports a simulation analysis about the impact on the overall efficiency caused by the lithography imperfections in the deposition process. The tolerance to the fabrication defects of a photonic sensor based on surface plasmonic resonance is analysed. The simulations are performed with FDTD and BPM algorithms. The device is a plasmonic interferometer composed by an a-Si:H waveguide covered by a thin gold layer. The sensing analysis is performed by equally splitting the input light into two arms, allowing the sensor to be calibrated by its reference arm. Two different 1 × 2 power splitter configurations are presented: a directional coupler and a multimode interference splitter. The waveguide sidewall roughness is considered as the major negative effect caused by deposition imperfections. The simulation results show that plasmonic effects can be excited in the interferometric waveguide structure, allowing a sensing device with enough sensitivity to support the functioning of a bio sensor for high throughput screening. In addition, the good tolerance to the waveguide wall roughness, points out the PECVD deposition technique as reliable method for the overall sensor system to be produced in a low-cost system. The large area deposition of photonics structures, allowed by the PECVD method, can be explored to design a multiplexed system for analysis of multiple biomarkers to further increase the tolerance to fabrication defects.
- Silicon nitride based devices: lithographic mask roughness mitigationPublication . Lourenço, Paulo; Fantoni, Alessandro; Costa, João; Vieira, ManuelaLithographic technology has been one of the main upholders to Moore's law in the semiconductor industry for the last decades. The underlying reason that enabled the evolution in semiconductor industry has been a steady silicon wafer printing cost, while being able to dramatically increase the number of nodes that can be printed per chip. Key developments in lithography such as wavelength decreasing, together with performance increase in lens and imaging technology, should be accounted for almost all the reduction of cost per function in integrated circuits technology. In this work, we will be presenting the simulation of two mitigation techniques for the impact of defects introduced by manufacturing processes. Namely, the lithographic mask limited resolution on the geometry of the representative device. These perturbations are a consequence of the lithographic mask limited resolution on the geometry of the representative device. For this purpose, the Beam Propagation and Finite Differences Time Domain methods will be used to simulate a multimode interference structure based on silicon nitride. The structure will be affected by previously mentioned perturbations and we expect results revealing a strong dependence between mask resolution, and imbalance and power loss. Two strategies will be followed concerning the mitigation of power loss and imbalance introduced by the limited resolution of lithographic mask: - Access waveguides tapering; - Adjustable power splitting ratios through the electro-optic effect. Through both strategies we aim to achieve an improvement on device’s performance but, in the latter are expected finer tuning capabilities, being enabled by dynamic compensation of power loss and imbalance when in a closed loop control architecture.
- ECG simulator with configurable skin-electrode impedance and artifacts emulationPublication . Almeida, Daniel; Costa, João; Lourenço, AndréElectrocardiograms (ECG) recorded from everyday objects, such as wearables, fitness machines or smart steering wheels are becoming increasingly common. Applications are diverse and include health monitoring, athletic performance optimization, identification, authentication, and entertainment. In this study we report the design and implementation of an innovative ECG simulator, providing simulation of signal related artifacts and a dynamically adjustable skin-electrode interface model. The ECG simulator includes a unique combination of features: emulation of time dependent skinelectrode impedance, adjustable differential and common-mode interference, generation of lead-off events and analog front-end output digitalization. The skin-electrode capacitance range is 1 nF-255 nF and the resistance span is 4 kΩ-996 kΩ. System’s functionality is demonstrated using a commercially available ECG front-end. The simulated SNR degradation introduced by the ECG simulator is under 0.1 dB. Results show that the skin-electrode interface can have a significant impact in the acquired waveforms. Impedance electrode imbalance, specifically of the resistive component, can generate artifacts which can be misinterpreted has arrhythmias. The proposed device can be useful for hardware and software ECG development and for training physicians and nurses to readily recognize skin-electrode impedance related artifacts.
- A simulation analysis for dimensioning of an amorphous silicon planar waveguide structure suitable to be used as a surface plasmon resonance biossensorPublication . Fantoni, Alessandro; Costa, João; Fernandes, Miguel; Vygranenko, Yury; Vieira, ManuelaIn this work we present a simulation study about the characteristics of a semiconductor structure suitable to be used as a guided wave optical biosensor, based on Surface Plasmonic Resonance effects (SPR). The proposed structure is a planar metal-dielectric waveguide where the sensor operation is based on the coupling between the fundamental propagation TM mode and the surface plasmon excited at the outer boundary of the metal, which interfaces the sample medium. Gold and aluminum are the metals considered for the plasmonic coating, amorphous silicon and others PECVD materials are considered for the waveguide structure. The results are based on modal analysis of the waveguide and plasmonic modes. The results obtained point out the possibility of generating SPPs in the near infrared range by including a functionalized cover of reduced graphene oxide (rGO) over the metal layer.
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