Manuel Augusto Vieira

Optical Filter Design Using Background Wavelength Processing Techniques

Amorphous SiC tandem heterostructures are used to filter a specific band, in the visible range. Experimental and simulated results are compared to validate the use of SiC multilayered structures in applications where gain compensation is needed or to attenuate unwanted wavelengths. Spectral response data acquired under different frequencies, optical wavelength control and side irradiations are analyzed. Transfer function characteristics are discussed. Color pulsed communication channels are transmitted together and the output signal analyzed under different background conditions. Results show that under controlled wavelength backgrounds, the device sensitivity is enhanced in a precise wavelength range and quenched in the others, tuning or suppressing a specific band. Depending on the background wavelength and irradiation side, the device acts either as a long-, a short-, or a band-rejection pass filter. An optoelectronic model supports the experimental results and gives insight on the physics of the device.

Modeling of magnetically controlled Si-based optoelectronic devices

Abstract We present a theoretical analysis and results of modeling of a new integrated device for spintronics application, which is based on a hybrid metal–semiconductor structure. The proposed device consists of a Si-based p–i–n photodetector sandwiched between two layers of a ferromagnetic metal (3d ferromagnet or half-metallic compound). Electron–hole pairs are created in the semiconductor part of the structure by light illumination. The photocurrent flowing in such a system is shown to depend on its magnetic configuration. This is due to a difference in the specular reflection (as well as in the diffuse scattering) of spin-up and spin-down electrons and holes from magnetically polarized layers—similar to giant magnetoresistance effect in magnetic multilayers. This, in turn, allows controlling the device performance by an externally applied magnetic field. We have estimated magnitude of the effect and also determined the role of relevant material parameters.

A Novel Vehicular Mobility Model for Wireless Networks

In this paper a novel realistic vehicular mobility model is introduced. It captures the moving-in-groups, conscious traveling, and introduces the concept of smart traveling while following drivers’ social behavior extracted from inquiries and experimental traffic measurements. Under the model, a routing algorithm is considered. The routing algorithm minimizes the distance to a target on a step by step form, in every street crossing. This is done under a hierarchic street level structure that optimizes travel speed and quality. The mobility model was simulated and validated in a real mobility scenario using, as case study, the city of Lisbon. The correlation values (between simulated data and theoretical distributions), arrival to destination success rate and directional statistics produced satisfactory results. The model concept is perfectly generic and applicable to other locations, provided that the corresponding street database and vehicular traffic information are available. The model outputs several parameters, including sinuosity indicator and traffic load. The sinuosity indicator is a powerful parameter on the characterization of urban radio environments when mobility is used in wireless networks simulation. Vehicular traffic load analysis is also presented. It can be used in further work over wireless traffic modeling, on a street segment basis or on a AU (Analysis Unit) perspective.

Simple and Complex Logical Functions in a SiC Tandem Device

In this study we demonstrate the basic AND, OR and NOT logical functions based on SiC technology. The device consists of a p-i’(a-SiC:H)-n/p-i(a-Si:H)-n heterostructure with low conductivity doped layers. Experimental optoelectronic characterization of the fabricated device shows the feasibility of tailoring channel bandwidth and wavelength by optical bias through illumination at the back and front sides. Results show that, front background enhances the light-to-dark sensitivity of the long and medium wavelength range and strongly quenches the others. Back violet background has the opposite behavior; it enhances the magnitude in short wavelength range and reduces it in the long ones. This nonlinearity provides the possibility for selective removal or addition of wavelengths.

Photodetector with integrated optical thin film filters

This paper reports on optical filters based on a-SiC:H tandem pin/pin heterostructures. The spectral sensitivity is analyzed. Steady state optical bias with different wavelengths are applied from each front and back sides and the photocurrent is measured. Results show that it is possible to control the sensitivity of the device and to tune a specific wavelength range by combining radiations with complementary light penetration depths. The transfer characteristics effects due to changes in the front and back optical bias wavelength are discussed. Depending on the wavelength of the external background and irradiation side, the device acts either as a short-or a long-pass band filter or as a band-stop filter. The output waveform presents a nonlinear amplitude-dependent response to the wavelengths of the input channels.

Double Pin Photodiodes with Two Optical Gate Connections for Light Triggering

Light-activated multiplexer/demultiplexer silicon-carbon devices are analyzed. An electrical model for the device operation is presented and used to compare output signals with experimental data. An algorithm that takes into accounts the voltage and the optical bias controlled sensitivities are developed. The device is a double pi’n/pin a-SiC:H heterostructure with two optical gate connections for light triggering in different spectral regions. Multiple monochromatic pulsed communication channels were transmitted together, each one with a specific bit sequence. The combined optical signal was analyzed by reading out, under different applied voltages and optical bias, the generated photocurrent across the device. Experimental and simulated results show that the output multiplexed signal has a strong nonlinear dependence on the light absorption profile, i.e., on the incident light wavelength, bit rate and intensity under unbalanced light generation of carriers. By switching between positive and negative voltages the input channels can be recovered or removed.

Multilayered a-SiC:H device for Wavelength-Division (de)Multiplexing applications in the visible spectrum

A multiplexer is a device that combines two or more signals onto a single output without losing their specificity. In this paper we present results on the use of multilayered a-SiC:H heterostructures either as wavelength-division multiplexing or demultiplexing device (WDM). The WDM is a glass/ITO/a-SiC:H (p-i-n)/ a-SiC:H(-p) /Si:H(-i)/SiC:H (-n)/ITO double heterostructure which faces the modulated light incoming together from different beams, each one with a specific wavelength and period. By reading out, at different applied bias, the photocurrent generated by all the incoming optical carriers, the information is multiplexed or demultiplexed and can be transmitted and recovered again. The devices were characterized through spectral response measurements, under different electrical bias and frequencies. Results show that in the multiplexing mode the output signal is balanced by the wavelength of each incoming optical carrier and modulated by their frequencies. In the demultiplexing mode the photocurrent is controlled by the applied voltage and optical bias allowing to regain the transmitted information. An electrical model is presented to explain the device operation.

Optical signal processing for indoor positioning using a-SiCH technology

Abstract. We use the nonlinear property of silicon carbon (SiC) multilayer devices under UV irradiation to design an optical processor for indoor positioning. The transducer combines the simultaneous demultiplexing operation with photodetection and self-amplification. The proposed coding is based on SiC technology. Based on that, we present a way to achieve indoor localization using the parity bits and a navigation syndrome. A representation with a 4-bit original string color message and the transmitted 7-bit string, the encoding and decoding of accurate positional information processes, and the design of SiC navigation syndrome generators are discussed and tested. A visible multilateration method estimates the position of the device using the decoded information received from several noncollinear transmitters. The location and motion information is found by mapping position and estimates the location areas. Since the indoor position of the light-emitting diode light source is known from building floor plans and lighting plans, the corresponding indoor position and travel direction of a mobile device can be determined.

Ferromagnetic ordering in diluted magnetic semiconductors

A.H.Slobodskyy 1,2,3 , V.K.Dugaev 1,4 , M.Vieira 4 1 Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 5 Vilde Str., 58001 Chernivtsi, Ukraine 2 Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, 02-668 Warszawa, Poland 3 Physikalisches Institut, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany 4 Department of Electronics and Communications, Instituto Superior de Engenharia de Lisboa, 1949-014 Lisbon, Portugal

Optical signal processing for a smart vehicle lighting system using a-SiCH technology

We propose the use of Visible Light Communication (VLC) for vehicle safety applications, creating a smart vehicle lighting system that combines the functions of illumination and signaling, communications, and positioning. The feasibility of VLC is demonstrated by employing trichromatic Red-Green-Blue (RGB) LEDs as transmitters, since they offer the possibility of Wavelength Division Multiplexing (WDM), which can greatly increase the transmission data rate, when using SiC double p-i-n receivers to encode/decode the information. Trichromatic RGB Light Emitting Diodes (LED)s (RGB-LED) are used together for illumination proposes (headlamps) and individually, each chip, to transmit the driving range distance and data information. An on-off code is used to transmit the data. Free space is the transmission medium. The receivers consist of two stacked amorphous a-H:SiC cells. They combine the simultaneous demultiplexing operation with the photodetection and self-amplification. The proposed coding is based on SiC technology. Multiple Input Multi Output (MIMO) architecture is used. For data transmission, we propose the use of two headlights based on commercially available modulated white RGB-LEDs. For data receiving and decoding we use three a-SiC:H double pin/pin optical processors symmetrically distributed at the vehicle tail Moreover, we present a way to achieve vehicular communication using the parity bits. A representation with a 4 bit original string color message and the transmitted 7 bit string, the encoding and decoding accurate positional information processes and the design of SiC navigation system are discussed and tested. A visible multilateration method estimates the drive distance range by using the decoded information received from several non-collinear transmitters.