arXiv subjects

J. M. L. Figueiredo

Publications and source records attributed to J. M. L. Figueiredo.

10 recordsLinked to original sources

arXiv2017

Delay dynamics of neuromorphic optoelectronic nanoscale resonators: Perspectives and applications

With the recent exponential growth of applications using artificial intelligence (AI), the development of efficient and ultrafast brain-like (neuromorphic) systems is crucial for future information and communication technologies. While the implementation of AI systems using computer algorithms of neural networks is emerging rapidly, scientists are just taking the very first steps in the development of the hardware elements of an artificial brain, specifically neuromorphic microchips. In this review article, we present the current state of neuromorphic photonic circuits based on solid-state optoelectronic oscillators formed by nanoscale double barrier quantum well resonant tunneling diodes. We address, both experimentally and theoretically, the key dynamic properties of recently developed artificial solid-state neuron microchips with delayed perturbations and describe their role in the study of neural activity and regenerative memory. This review covers our recent research work on excitable and delay dynamic characteristics of both single and autaptic (delayed) artificial neurons including all-or-none response, spike-based data encoding, storage, signal regeneration and signal healing. Furthermore, the neural responses of these neuromorphic microchips display all the signatures of extended spatio-temporal localized structures (LSs) of light, which are reviewed here in detail. By taking advantage of the dissipative nature of LSs, we demonstrate potential applications in optical data reconfiguration and clock and timing at high-speeds and with short transients. The results reviewed in this article are a key enabler for the development of high-performance optoelectronic devices in future high-speed brain-inspired optical memories and neuromorphic computing.

nlin.PS↗

arXiv2005

Do we really need the weight force?

In 1901 the third Conference Generale des Poids et Mesures (CGPM) defined the weight of a body as ``the product of its mass and the acceleration due to gravity". In practical terms the weight force corresponds to the gravitational force. However, this gravitational definition of the weight lacks logic from the perspective of present knowledge and can be misleading. In a space traveling time, weight associated concepts such as "true weight", "apparent weight", "weightlessness", "zero gravity", "microgravity", are ambiguous and strong deceptive. The superfluousness or ambiguousness of these concept would be removed if an operational definition of the weight of a body as the force the body exerts on its support or suspender is adopted. In this definition the weight force does not act on the body and it is not necessary to describe the body's sate of motion. This avoids the imprecision of the concepts such as "true weight" and "apparent weight" of a body, for example. This paper discusses the need of the weight force of a body concept in Physics, and asks for a re-exame of the CGPM definition to improve the physics teaching and learning.

physics.ed-ph↗

arXiv2005

Advantages of adopting the operational definition of weight

With rare exceptions, in high school and college/university physics courses literature and in journals of physics, the weight is defined as a gravitational force or an exclusive consequence of it. These definitions lack logic from the perspective of present knowledge and can be misleading. The operational definition of weight of a body as the force the body exerts on its support or suspender can eliminate the ambiguities associated to "true weight", "apparent weight", "state of weightlessness", "zero weight", "zero gravity", "microgravity", "vertical", "up" and "down". Nevertheless, the concept of weight of a body is not fundamental in Physics and its exclusion from the physics vocabulary will eliminate some of former concepts or their ambiguousness with evident benefit for physics teaching and learning. This paper discusses weight of a body concepts and presents several examples that show the advantages of avoiding its use or adopting the above-mentioned operational definition. It is believed that this will eliminates the frequent identification with the body's mass and will boost student's understanding associated subjects.

physics.ed-ph↗

arXiv2005

Modeling of a Resonant Tunneling Diode Optical Modulator

The integration of a double barrier resonant tunneling diode within a unipolar optical waveguide provides electrical gain over a wide bandwidth. Due to the non-linearities introduced by the double barrier resonant tunneling diode an unipolar InGaAlAs/InP optical waveguide can be employed both as optical modulator and optical detector. The modeling results of a device operating as optical modulator agree with preliminary experimental data, foreseeing for an optimized device modulation depths up to 23 dB with chirp parameter between -1 and 0 in the wavelength range analyzed (1520 nm - 1600 nm).

physics.optics↗

arXiv2005

Optical modulation at around 1550 nm in a InGaAlAs optical waveguide containing a InGaAs/AlAs resonant tunnelling diode

We report electro-absorption modulation of light at around 1550 nm in a unipolar InGaAlAs optical waveguide containing a InGaAs/AlAs double-barrier resonant tunneling diode (DB-RTD). The RTD peak-to-valley transition increases the electric field across the waveguide, which shifts the core material absorption band-edge to longer wavelengths via the Franz-Keldysh effect, thus changing the light-guiding characteristics of the waveguide. Low-frequency characterisation of a device shows modulation up to 28 dB at 1565 nm. When dc biased close to the negative differential conductance (NDC) region, the RTD optical waveguide behaves as an electro-absorption modulator integrated with a wide bandwidth electrical amplifier, offering a potential advantage over conventional pn modulators.

physics.optics↗

arXiv2005

Optical modulation in a resonant tunneling relaxation oscillator

We report high speed optical modulation in a resonant tunneling relaxation oscillator consisting of a resonant tunneling diode (RTD) integrated with a unipolar optical waveguide and incorporated in a package with a coplanar waveguide transmission line. When appropriately biased, the RTD can provide wide-bandwidth electrical gain. For wavelengths near the material band-edge, small changes of the applied voltage give rise to large, high-speed electro-absorption modulation of the light. We have observed optical modulation at frequencies up to 14 GHz, associated with sub harmonic injection locking of the RTD oscillation at the fundamental mode of the coplanar transmission line, as well as generation of 33 ps optical pulses due to relaxation oscillation.

physics.optics↗

arXiv2005

Electric field switching in a resonant tunneling diode electroabsorption modulator

The basic mechanism underlying electric field switching produced by a resonant tunnelling diode (RTD) is analysed and the theory compared with experimental results; agreement to within 12% is achieved. The electro-absorption modulator (EAM) device potential of this effect is explored in an optical waveguide configuration. It is shown that a RTD-EAM can provide significant absorption coefficient change, via the Franz-Keldysh effect, at appropriate optical communication wavelengths around 1550 nm and can achieve up to 28 dB optical modulation in a 200 um active length device. The advantage of the RTD-EAM, over the conventional reversed biased pn junction EAM, is that the RTD-EAM has in essence an integrated electronic amplifier and therefore requires considerably less switching power.

cond-mat.mes-hall↗

arXiv2005

Ultra-low voltage resonant tunnelling diode electroabsorption modulator

Embedding a double barrier resonant tunnelling diode (RTD) in an unipolar InGaAlAs optical waveguide gives rise to a very low driving voltage electroabsorption modulator (EAM) at optical wavelengths around 1550 nm. The presence of the RTD within the waveguide core introduces high non-linearity and negative differential resistance in the current-voltage (I-V) characteristic of the waveguide. This makes the electric field distribution across the waveguide core strongly dependent on the bias voltage: when the current decreases from the peak to the valley there is an increase of the electric field across the depleted core. The electric field enhancement in the core-depleted layer causes the Franz-Keldysh absorption band-edge to red shift, which is responsible for the electroabsorption effect. High frequency ac signals as low as 100 mV can induce electric field high speed switching, producing substantial light modulation (up to 15 dB) at photon energies slightly lower than the waveguide core band-gap energy. The key difference between this device and conventional p-i-n EAMs is that the tunnelling characteristics of the RTD are employed to switch the electric field across the core depleted region; the RTDEAM has in essence an integrated electronic amplifier and therefore requires considerably less switching power.

physics.optics↗

arXiv2005

It is about time to say "eureka!" in teaching physics

In the spirit of the International Year of Physics 2005 the author advices the replacement of the teaching methods based on verbal explanations and to perform a number of tasks following a given recipe by procedures that actively involve the students in the physics discovery process.

physics.ed-ph↗

arXiv2003

A quantum coherent approach to transport and noise in double barrier resonant diodes: shot noise a way to distinguish coherent from sequential tunneling

We implement a quantum approach which includes long range Coulomb interaction and investigate current voltage characteristics and shot noise in double barrier resonant diodes. Theory applies to the region of low applied voltages up to the region of the current peak and considers the wide temperature range from zero to room temperature. The shape of the current voltage characteristic is well reproduced and we confirm that even in the presence of Coulomb interaction shot noise can be suppressed with a Fano factor well below the value of 0.5. This feature is a signature of coherent tunnelling since the standard sequential tunnelling predicts in general a Fano factor equal to or greater than the value 0.5. This giant suppression is a consequence of Pauli principle as well as long range Coulomb interaction. The theory generalizes previous findings and is compared with experiments.

cond-mat.mes-hall↗