Paulo Lourenço

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Paulo Lourenço is a member of the Centre of Technology and Systems - UNINOVA, a research centre of NOVA School of Sciences and Technology, and collaborates with Instituto Superior de Engenharia de Lisboa as an Assistant Professor in lecturing Electronics and Physics Applied to Electronics.He has been publishing the work related to his research area of interest both in journals and conferences. His research interests encompass propagation of electromagnetic energy in dielectric materials, modal propagation, resonant waveguide grating couplers and the phenomenon of surface plasmon resonance, and its applications.

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Steven Pinker

Harvard University

Nicola Jane Holt

University of the West of England

Naim Kapucu

University of Central Florida

Benjamin Isakhan

Deakin University

Eric S . Kim (UBC)

University of British Columbia

David Seamon

Kansas State University

John Johnson

Pennsylvania State University

Louis de Saussure

University of Neuchâtel

Armando Marques-Guedes

UNL - New University of Lisbon

Noe Cornago

University of the Basque Country, Euskal Herriko Unibertsitatea

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Papers by Paulo Lourenço

Silicon Nitride Interferometers for Optical Sensing with Multi-micron Dimensions

Journal of Physics: Conference Series

Increasing the size of the smallest features of Photonic Integrated Circuits (PICs) to multi-micr... more Increasing the size of the smallest features of Photonic Integrated Circuits (PICs) to multi-micron dimensions can be advantageous to avoid expensive and complex lithographic steps in the fabrication process. In applications where extremely reduced chip size is not a requirement, the design of devices with multi-micron dimensions is potential interesting to avoid the need for e-beam lithography. Another benefit is that making the dimensions larger reduces the effect of lithographic imperfections such as waveguide surface roughness. However, the benefits do not come without limitations. Coupling the light in and out of the circuit is more challenging since diffraction gratings are not available when designing for such large dimensions. Circuit bends must have a larger radius of curvature and the existence of multimode propagation conditions can have detrimental impact in the performance of several devices, such as interferometers. In this study we perform simulations of the coupling ...

Computer simulation study about the dependence of amorphous silicon photonic waveguides efficiency on the material quality

The European Physical Journal Applied Physics

Amorphous silicon PECVD photonic integrated devices are promising candidates for low cost sensing... more Amorphous 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 th...

Subwavelength structures for taper waveguides

Journal of Physics: Conference Series

In Photonic Integrated Circuits (PICs) it is often necessary some sort of mismatch adaptation bet... more In 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 th...

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Simulation analysis of a thin film semiconductor MMI 3dB splitter operating in the visible range

Fourth International Conference on Applications of Optics and Photonics

Este trabalho foi financiado pelo Concurso Anual para Projetos de Investigação, Desenvolvimento, ... more Este trabalho foi financiado pelo Concurso Anual para Projetos de Investigação, Desenvolvimento, Inovação e Criação Artística (IDI&CA) 2018 do Instituto Politécnico de Lisboa. Código de referência IPL/2018/aSiPhoto_ISELIn 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.info:eu-repo/semantics/publishedVersio

Metamaterials in waveguide to fiber couplers

Physics and Simulation of Optoelectronic Devices XXX, 2022

Lithographic mask defects mitigation on a multimode interference structure

Optica Pura y Aplicada, 2020

Over the last decades, the lithographic technology has greatly contributed for the confirmation o... more Over the last decades, the lithographic technology has greatly contributed for the confirmation of Moore's law in the semiconductor industry. Key developments in lithography such as the operational wavelength decreasing, together with a performance increase in lens and imaging technology, enabled the reduction of cost per function in integrated circuits technology. In this work, the impact of lithographic defects introduced by the manufacturing process is analyzed through simulations and two mitigation techniques are presented. These perturbations are a consequence of the limited lithographic mask resolution reflected on deviations from the geometry of the ideal device. For this purpose, the Beam Propagation and Finite Differences Time Domain methods have been used to simulate a multi-mode interference structure based on silicon nitride. The structure is affected by random perturbations and the obtained results revealed a strong dependence between mask resolution, and imbalance and power loss. Two strategies have been followed concerning the mitigation of power loss and imbalance:-Access waveguides tapering and adjustable power splitting ratios through the electro-optic effect. Both strategies revealed results that indicate an improvement on device's performance. However, once built, the former is a static design that favors indiscriminately all propagating modes in the multimode section. In the latter, finer tuning capabilities targeting different propagating modes may be enabled by dynamic compensation of power loss and imbalance, when in a closed loop control architecture. Such a control architecture may operate by sampling the output waveguides, extracting the error signal and, finally, negatively feeding it back to the electrooptic effect system, hence improving imbalance and power loss.

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Cross-section mismatch: metamaterials to the rescue

Photonics and Nanostructures - Fundamentals and Applications

Simulation of a parallel waveguide array structure suitable for interrogation scheme in a plasmonic biosensor

Physics and Simulation of Optoelectronic Devices XXVIII

Surface plasmon resonance sensors have emerged has one of the most suitable approaches for biosen... more Surface 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.

Surface plasmon resonance sensing structure

Physics and Simulation of Optoelectronic Devices XXVIII, 2020

Surface Plasmon Resonance occurs when a polarized electromagnetic field strikes a metallic surfac... more Surface 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.

Colour and image processing for output extraction of a LSPR sensor

Optics and Biophotonics in Low-Resource Settings VIII, 2022

Sensors based on the Local Surface plasmon Resonance (LSPR) are attractive due to their simple st... more Sensors based on the Local Surface plasmon Resonance (LSPR) are attractive due to their simple structure and good sensitivity, but the expensive optoelectronic part of the device is limiting the practical applications. There is a need for new strategies to bring the excellent detection properties of LSPR sensors to the playground of low-cost devices and materials. In this work, it is proposed a novel approach to the output extraction of from LSPR sensor whose sensing element is composed by metal nanoparticles (MNPs). Illuminated with an incident broad light source, the sensor produces a spectral transmission output where the MNPs act like a band-stop optical filter for a specific wavelength. An alteration of the refractive index in the surrounding medium corresponds directly to a shift of the filtering rejection band, which corresponds to a slight change in the colour of the light transmitted by the sensor elements. This colour change can be captured by a CMOS photo-camera, used as an image sensor. It is proposed in this paper an approach based on an automatized image processing algorithm for colour change detection, yielding to a system capable of detecting refractive index variations, avoiding the use of expensive spectrometers. The algorithm comprises three stages: (1) Region of interest detection: images are first cropped using the Otsu threshold binary image to remove the uninteresting areas in the image. (2) Image segmentation: using the watershed algorithm, the sensor elements (sample) area is detected automatically in the cropped image. The segmentation is done using the gradient image, where the watershed markers are the regions of low gradient and barriers are the areas of high values inside the image. (3) The resulted sample region is then processed to find its average or dominant LAB colour and then compare it to its corresponding sample image immersed in different mediums using the colour difference measurement CIEDE2000.

Multichannel detector system for surface plasmon resonance biosensors

Optical Fibers and Sensors for Medical Diagnostics, Treatment and Environmental Applications XXII, 2022

Photonic systems are gaining an important role in the field of medical diagnosis due to the achie... more Photonic 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.

Optimisation of a plasmonic parallel waveguide sensor based on amorphous silicon compounds

Optical Sensing and Detection VI, 2020

This work reports the optimisation of a plasmonic waveguide sensor based on amorphous silicon com... more This work reports the optimisation of a plasmonic waveguide sensor based on amorphous silicon compounds (a-SiC:H, a-SiN:H or a-SiCN:H) using the FDTD method and modal decomposition. The sensor consists of an array of parallel surface plasmon interferometers with different propagation lengths, each one comprising a thin layer of metal embedded into an amorphous silicon waveguide. In order to reduce the complexity and hardware, we have proposed a structure consisting of an array of parallel surface plasmon interferometers with different propagation lengths, such that at the end of the plasmonic structure the modes can interfere constructively or destructively depending on the refractive index of the sampling medium and the propagation length. The variation of the output intensity at the end of each waveguide element provides a convenient interrogation scheme. In this work we analyse different solutions for splitting the input fundamental mode into the different parallel waveguides, including multi-mode interference structures and directional coupler splitters. 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 process that is compatible with back-end CMOS processing and wavelengths in the visible to near infrared range.

Simulation and Analysis of Surface Plasmon Resonance Based Sensor

In this paper, we will be presenting the results obtained through Finite-Difference Time Domain s... more In 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 pre...

Applications for a-Si:H TFTs: Modelling and Simulation

Hydrogenated amorphous silicon thin film transistors have been used as switching elements in liqu... more Hydrogenated amorphous silicon thin film transistors have been used as switching elements in liquid crystal displays and large area matrix addressed sensor arrays. Later, these devices have also been used as analogue active elements in organic light emitting diode displays. However, this technology suffers from bias induced meta-stability. This issue introduces both threshold voltage and subthreshold slope shifts over time when gate bias is applied. Such instabilities jeopardize long term performance of circuits that rely on these components. Nevertheless, hydrogenated amorphous silicon thin film transistors present an exponential transfer characteristic when operating on subthreshold region and their typical power consumption is under 1 µW. This low power characteristic makes these devices ideally suited for low power electronic design. This work demonstrates, through transient analysis of a well-established simulation model for hydrogenated amorphous silicon, the viability of thin...

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Simulation of an Early Warning Fire System

In this paper, we will be using separate software tools (wireless network and Finite Differences ... more In 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.

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$Research paper thumbnail of Thin Film Refractive Index and Thickness$

Thin Film Refractive Index and Thickness

Integrated optics are a contemporaneous reality in which thin-film technology and methods utilize... more Integrated optics are a contemporaneous reality in which thin-film technology and methods utilized in the development of integrated circuitry, are applied to both optical circuits and devices. This provides systems that show improved characteristics when compared to their electronic counterparts. Optical systems enable wider bandwidth operation, less power consumption, more immunity to interference and higher cost-efficiency. These features definitely represent a huge improvement in our daily lives when completely embedded in Information and Communications Technologies, replacing a large percentage of contemporaneous electronic based systems. The building blocks of these optical systems consist on waveguides and structures formed by deposited thin films. Two characteristics of utmost importance for these structures are the height and refractive index of the deposited film. In this work and by using a prism coupler, we will be presenting an optical setup and the experimental method t...

Lithographic Mask Defects Analysis on an MMI 3 dB Splitter

Photonics, 2019

In this paper, we present a simulation study that intends to characterize the influence of defect... more In 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 the Beam Propagation numerical method. This work supports the development of a biomedical plasmonic sensor, which is based on the coupling between 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 calibr...

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Silicon nitride based devices: lithographic mask roughness mitigation

Lithographic technology has been one of the main upholders to Moore's law in the semiconducto... more Lithographic 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 ...

Finite-difference time-domain analysis of hydrogenated amorphous silicon and aluminum surface plasmon waveguides

Optical Engineering, 2018

Abstract. The large majority of surface plasmon resonance (SPR)-based devices use noble metals, n... more Abstract. The 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 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. 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.

Performance of an a-Si:H MMI multichannel beam splitter analyzed by computer simulation

Silicon Photonics XVI

Optical power splitters are widely used in many applications and different typologies have been d... more Optical power splitters are widely used in many applications and different 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 different 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.

Silicon Nitride Interferometers for Optical Sensing with Multi-micron Dimensions

Journal of Physics: Conference Series