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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.
- 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.
- Subwavelength structures for taper waveguidesPublication . Lourenço, Paulo; Fantoni, Alessandro; Costa, João; Fernandes, Miguel; Vieira, ManuelaIn Photonic Integrated Circuits (PICs) it is often necessary some sort of mismatch adaptation between waveguides of different cross-sections. There are several instances of such a designing constraint, being the vertical coupling between the PIC and an optical fibre probably the most representative of all examples. Here, the beam of electromagnetic energy inside the PIC must be inserted/extracted through/to an optical fibre. Typical core diameters are approximately 10 µm and 5 µm, for single mode optical fibres operating in the near infrared and visible wavelengths, respectively. On the other hand, the optical interconnects linking individual structures in PICs are usually single mode waveguides, 400 to 500 nm wide and a few hundreds of nanometres thick. This presents a bidimensional mismatch between the optical fibre and the single mode waveguide within the PIC, that requires both lateral and longitudinal beam expansions. In this work, we have approached the lateral expansion of the fundamental mode propagating in a single mode waveguide, at the operating wavelength of 1550 nm and being coupled out into an optical fibre, through a grating structure 14.27 µm wide. To this end, we have designed and simulated a subwavelength metamaterial planar structure, which is able to expand laterally the fundamental mode’s profile from 450 nm to 14.27 µm, within 11.1 µm. Furthermore, we will be presenting the results obtained when comparing this structure with several linear inverted taper waveguides, regarding coupling and propagation efficiencies. Namely, we compared the coupling efficiencies of the modes propagating in an 100 µm long waveguide, when being excited by the analytically calculated fundamental mode and the fields obtained at the end of the designed structure. The results obtained for the designed structure 11.1 µm long and the calculated fundamental mode showed a coupling efficiency of -1.53 dB and -1.20 dB, respectively.
- Metamaterials in waveguide to fiber couplersPublication . Lourenço, Paulo; Fantoni, Alessandro; Costa, João; Fernandes, Miguel; Vieira, ManuelaCoupling light into or out of a photonic integrated circuit is often accomplished by establishing a vertical link between a single mode optical fibre and a resonant waveguide grating, which is then followed by a tapered and a single mode waveguides. For a chip to fibre coupler, the period of the diffraction grating is often apodized to achieve an optimal beam profile at the input of the optical fibre. The tapered waveguide operates as a spot-size converter, expanding laterally the light beam in the single mode waveguide, to match the profile of the fundamental mode of the resonant waveguide grating. In this work, we propose using subwavelength structures to modulate the refractive index of the tapered waveguide for the lateral expansion of the light beam, when operating at the 1550 nm wavelength. The engineered graded index structure is simulated through adequate numerical methods and its performance is analysed in terms of efficiency and mode profile matching. With our proposed inverted taper waveguide, we were able to obtain an adiabatic power transfer and coupling efficiency with the TE fundamental mode of -0.26 dB and -0.92 dB, respectively. This performance has been achieved in a structure 11.1 µm long and 14.27 µm wide. Furthermore, the obtained fields were fed into a resonant waveguide grating to evaluate the coupling efficiency into the fundamental mode of an optical fibre, resulting in an expected performance decrease of 0.1 dB and ~0.6 dB by comparing respectively with the power transfer and coupling efficiency of the resonant waveguide grating when propagating the calculated TE0 mode.
- 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.
- Design and optimization of a waveguide/fibre coupler in the visible rangePublication . Lourenço, Paulo; Fantoni, Alessandro; Costa, João; Fernandes, Miguel; Vieira, ManuelaWhen engineering photonic integrated structures, there will be a time that one must consider coupling out the electromagnetic field to an external device. Often, this coupling is made through a single mode optical fibre. Due to the mismatch in mode field diameters between waveguide and fibre modes, the propagating mode inside the dielectric waveguide must undertake a spot-size conversion. It requires to be radially expanded, often laterally by a tapered waveguide and longitudinally through other means, to match the radial profile of the optical fibre mode. Then, the energy must be coupled out of its propagating path into the plane of the optical fibre, through a structure that possesses such functional purpose. In this work, we describe the design steps and optimization of a silicon nitride waveguide/fibre coupler operating in the visible range. To this end, we start by designing an optimized 3D taper waveguide, using Beam Propagation method, that performs as the spot-size converter. Next, through the Eigen Mode Expansion method, a 2D subwavelength grating is designed and optimized regarding substrate leakage and propagating plane energy coupling out, thus vertically validating the energy distribution of the outgoing profile. The required subwavelength grating apodization is accomplished, once more through the Eigen Mode Expansion method, and by carefully engineering a metamaterial that performs accordingly. The obtained diffraction grating is then expanded horizontally to create a 3D structure and laterally validated through Beam Propagation method. Finally, the whole 3D structure is optimized and validated through Finite Differences Time Domain simulations regarding energy profile coupling out, and overlap integral matching is established with the fibre mode profile.