Loading...
13 results
Search Results
Now showing 1 - 10 of 13
- Analysis of metallic nanoparticles embedded in thin film semiconductors for optoelectronic applicationsPublication . Fantoni, Alessandro; Fernandes, Miguel; Vygranenko, Yuri; Louro, Paula; Vieira, Manuela; Silva, R. P. O.; Teixeira, D.; Da Costa Ribeiro, Ana Paula; Prazeres, Duarte; Alegria, ElisabeteThis paper reports about a study of the local plasmonic resonance (LSPR) produced by metal nanoparticles embedded in a dielectric or semiconductor matrix. It is presented an analysis of the LSPR for different nanoparticle metals, shapes, and embedding media composition. Metals of interest for nanoparticle composition are Aluminum and Gold. Shapes of interest are nanospheres and nanotriangles. We study in this work the optical properties of metal nanoparticles diluted in water or embedded in amorphous silicon, ITO and ZnO as a function of size, aspect-ratio and metal type. Following the analysis based on the exact solution of the Mie theory and DDSCAT numerical simulations, it is presented a comparison with experimental measurements realized with arrays of metal nanospheres. Simulations are also compared with the LSPR produced by gold nanotriangles (Au NTs) that were chemically produced and characterized by microscope and optical measurements.
- 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.
- 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.
- 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.
- A model for the refractive index of amorphous silicon for FDTD simulation of photonics waveguidesPublication . Fantoni, Alessandro; Lourenço, P.; Vieira, ManuelaThis paper presents an analysis of the material quality influence for amorphous silicon waveguides for microphotonic applications. Material quality is taken into account by a model based on the absorption coefficient data obtained by Constant Photocurrent Measurement (CPM) in the near infrared region. The GUTL (Gauss-Urbach-Tauc-Lorentz) model has been presented as an extension of the standard Urbach-Tauc-Lorentz model and proposed as a predictor for the wavelength dependent optical constants of amorphous silicon in the near infrared spectra. Values produced for the GUTL model have been used as input for a set of FDTD simulations, taking in consideration different material qualities and waveguide dimensions directed to study the characteristics of amorphous silicon waveguides embedded in a SiO2 cladding.
- Measurement of photo capacitance in amorphous silicon photodiodesPublication . Gonçalves, Dora; Fernandes, Luís Miguel; Louro, Paula; Vieira, Manuela; Fantoni, AlessandroThis paper discusses the photodiode capacitance dependence on imposed light and applied voltage using different devices. The first device is a double amorphous silicon pin-pin photodiode; the second one a crystalline pin diode and the last one a single pin amorphous silicon diode. Double amorphous silicon diodes can be used as (de)multiplexer devices for optical communications. For short range applications, using plastic optical fibres, the WDM (wavelength-division multiplexing) technique can be used in the visible light range to encode multiple signals. Experimental results consist on measurements of the photodiode capacitance under different conditions of imposed light and applied voltage. The relation between the capacitive effects of the double diode and the quality of the semiconductor internal junction will be analysed. The dynamics of charge accumulations will be measured when the photodiode is illuminated by a pulsed monochromatic light.
- Grating coupler design for low-cost fabrication in amorphous silicon photonic integrated circuitsPublication . Almeida, Daniel; Lourenço, Paulo; Fantoni, Alessandro; Costa, João; Vieira, ManuelaPhotonic circuits find applications in biomedicine, manufacturing, quantum computing and communications. Photonic waveguides are crucial components, typically having cross-section orders of magnitude inferior when compared with other photonic components (e.g., optical fibers, light sources and photodetectors). Several light-coupling methods exist, consisting of either on-plane (e.g., adiabatic and end-fire coupling) or off-plane methods (e.g., grating and vertical couplers). The grating coupler is a versatile light-transference technique which can be tested at wafer level, not requiring specific fiber terminations or additional optical components, like lenses, polarizers or prisms. This study focuses on fully-etched grating couplers without a bottom reflector, made from hydrogenated amorphous silicon (a-Si:H), deposited over a silica substrate. Different coupler designs were tested, and of these we highlight two: the superimposition of two lithographic masks with different periods and an offset between them to create a random distribution and a technique based on the quadratic refractive-index variation along the device’s length. Results were obtained by 2D-FDTD simulation. The designed grating couplers achieve coupling efficiencies for the TE-like mode over −8 dB (mask overlap) and −3 dB (quadratic variation), at a wavelength of 1550 nm. The coupling scheme considers a 220 nm a-Si:H waveguide and an SMF-28 optical fiber.
- Theory and FDTD simulations of an amorphous silicon planar waveguide structure suitable to be used as a surface plasmon resonance biosensorPublication . Fantoni, Alessandro; Costa, João; Fernandes, Miguel; Vygranenko, Yury; Vieira, ManuelaIn this paper we present our work concerning the design of a semiconductor waveguide structure to be used as a 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 aluminium are the metals considered for the plasmonic coating, amorphous silicon and a reduced graphene oxide layer are considered for the waveguide structure. FDTD simulations of the proposed structure show a clear attenuation peak in the output power at the wavelength where the plasmonic resonance is excited.
- DEMUX devices based on a-SiC:HPublication . Fantoni, Alessandro; Louro, Paula; Vieira, Manuel Augusto; Silva, T.; Vieira, ManuelaIn this paper we present results about the functioning of a multilayered a-SiC:H heterostructure as a device for wavelength-division demultiplexing of optical signals. The device is composed of two stacked p-i-n photodiodes, both optimized for the selective collection of photogenerated carriers. Band gap engineering was used to adjust the photogeneration and recombination rates profiles of the intrinsic absorber regions of each photodiode to short and long wavelength absorption and carrier collection in the visible spectrum. The photocurrent signal using different input optical channels was analyzed at reverse and forward bias and under steady state illumination. This photocurrent is used as an input for a demux algorithm based on the voltage controlled sensitivity of the device. The device functioning is explained with results obtained by numerical simulation of the device, which permit an insight to the internal electric configuration of the double heterojunction.These results address the explanation of the device functioning in the frequency domain to a wavelength tunable photocapacitance due to the accumulation of space charge localized at the internal junction. The existence of a direct relation between the experimentally observed capacitive effects of the double diode and the quality of the semiconductor materials used to form the internal junction is highlighted.
- Optical properties of metal nanoparticles embedded in amorphous silicon analysed using discrete dipole approximationPublication . Fantoni, Alessandro; Fernandes, Miguel; Vygranenko, Yuri; Vieira, Manuela; Oliveira-Silva, Rui; Prazeres, Duarte; Da Costa Ribeiro, Ana Paula; Alegria, ElisabeteLocalized surface plasmons (LSP) can be excited in metal nanoparticles (NP) by UV, visible or NIR light and are described as coherent oscillation of conduction electrons. Taking advantage of the tunable optical properties of NPs, we propose the realization of a plasmonic structure, based on the LSP interaction of NP with an embedding matrix of amorphous silicon. This study is directed to define the characteristics of NP and substrate necessary to the development of a LSP proteomics sensor that, once provided immobilized antibodies on its surface, will screen the concentration of selected antigens through the determination of LSPR spectra and peaks of light absorption. Metals of interest for NP composition are: Aluminium and Gold. Recent advances in nanoparticle production techniques allow almost full control over shapes and size, permitting full control over their optical and plasmonic properties and, above all, over their responsive spectra. Analytical solution is only possible for simple NP geometries, therefore our analysis, is realized recurring to computer simulation using the Discrete Dipole Approximation method (DDA). In this work we use the free software DDSCAT to study the optical properties of metal nanoparticles embedded in an amorphous silicon matrix, as a function of size, shape, aspect-ratio and metal type. Experimental measurements realized with arrays of metal nanoparticles are compared with the simulations.