Browsing by Author "Niehus, Manfred"
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- Halting the fuse discharge propagation using optical fiber microwiresPublication . Rocha, A. M.; Fernandes, G.; Domingues, F.; Niehus, Manfred; Pinto, A. N.; Facão, M.; André, P. S.We report and analyze the halting of the fuse effect propagation in optical fiber microwires. The increase of the mode field diameter in the tapered region decreases the optical intensity resulting in the extinction of the fuse effect. This fiber element presents a low insertion loss and can be introduced in the optical network in order to protect the active equipment from the damage caused by the fuse effect.
- Non-linear optical spontaneous photoluminescence emission enhancement effect in wide gap gallium nitride thin filmsPublication . Niehus, Manfred; Schwarz, ReinhardWith two interfering pulses from the 4th harmonic of a Nd-YAG laser we burnt a periodic lattice structure into the surface of GaN thin films. The lattice period of this permanent grating could be controlled between less than one and several tens of microns. Above the decomposition threshold, nitrogen evades from the sample surface, and the residual metallic gallium accumulates in the form of tiny droplets at the surfaces. The patterned structure shows structural similarities with microcavities. The question arises if the residual metallic gallium may act as a partially reflecting mirror. To test this hypothesis, we studied the steady-state and transient photoluminescence through the modulation of light emerging from the ubiquitous broad “yellow” photoluminescence band. The microlattice shows up by energy-equidistant spontaneous emission enhancement peaks in the steady-state photoluminescence spectra. We suggest that the partial reflection due to the residual metallic gallium leads to the observed enhancement effect.
- Optical properties and transport in PLD-GaNPublication . Niehus, Manfred; Sanguino, P.; Monteiro, T.; Soares, M. J.; Pereira, E.; Vieira, Manuela; Koynov, S.; Schwarz, R.We present structural, optical and transport data on GaN samples grown by hybrid, two-step low temperature pulsed laser deposition. The band gap of samples with good crystallinity has been deduced from optical spectra. Large below gap band tails were observed. In samples with the lowest crystalline quality the PL spectra are quite dependent on spot laser incidence. The most intense PL lines can be attributed to excitons bounded to stacking faults. When the crystalline quality of the samples is increased the ubiquitous yellow emission band can be detected following a quenching process described by a similar activation energy to that one found in MOCVD grown samples. The samples with the highest quality present, besides the yellow band, show a large near band edge emission which peaked at 3.47 eV and could be observed up to room temperature. The large width of the NBE is attributed to effect of a wide distribution of band tail states on the excitons. Photoconductivity data supports this interpretation.
- Propriedades ópticas e transporte electrónico transiente, relacionados com a desordem, em semicondutores de grande hiatoPublication . Niehus, ManfredNeste trabalho estudamos as propriedades ópticas e o transporte electrónico em semicondutores de grande hiato, usando uma abordagem relacionada com a desordem estrutural frequentemente encontrada nestes materiais. Observamos uma variedade de características da fotocorrente transiente (TPC) e fotoluminescência transiente (TPL) que não podem ser explicados no âmbito dos mecanismos “canónicos” de relaxação electrónica, que são, para TPC, o modelo de Captura Múltipla (MT) de portadores livres e, para TPL, o modelo de recombinação entre pares de dadores e aceitadores (DAP) localizados. Em consequência, desenvolvemos um novo modelo analítico (Modelo TR) que integra a evidência experimental complementar de dispersão espacial (DAP) e dispersão energética (caudas das bandas). O modelo baseia-se na competição entre a termalização e recombinação (TR) de portadores minoritários capturados em estados energéticos, distribuídos exponencialmente em energia, e localizados no espaço. O modelo descreve a dependência da TPL da energia e do tempo, em excelente concordância com os dados experimentais, e revela a correlação entre TPL e TPC.
- The contribution of submarine optical fiber telecom cables to the monitoring of earthquakes and tsunamis in the NE AtlanticPublication . Matias, Luis; Carrilho, Fernando; Sá, Vasco; Omira, Rachid; Niehus, Manfred; Corela, Carlos; Barros, José; Omar, YasserRecent developments in optical fiber cable technology allows the use of existing and future submarine telecommunication cables to provide seismic and sea-level information. In this work we study the impact of three different technologies, 1) SMART, Science Monitoring and Reliable Telecommunications; 2) DAS, Distributed Acoustic Sensing, and; 3) LI, Laser Interferometry, for effective earthquake and tsunami monitoring capabilities on the NE Atlantic. The SW Iberia is the source area of the largest destructive earthquake that struck Europe since the year 1000, the November 1, 1755 event. This earthquake generated also a destructive tsunami affecting the whole basin. This tectonically active area is crossed by the CAM (Continent-Azores-Madeira) submarine cable on a ring configuration. Due to the end of cable lifetime the current cables need to be replaced by 2024 and the technical requirements must be defined in mid-2021. The Azores archipelago is the focus of frequent seismic crizes and occasionally destructive earthquakes. A common feature of these seismic events is that they take place offshore, an area that is difficult to monitor from land-based instruments. In this work we evaluate the contribution of SMART cables to the earthquake monitoring and tsunami early warning system in SW Iberia and show how DAS and LI can improve earthquake monitoring on two active domains of the Azores. For tsunami early warning, we show how the offshore sea-level measurements provide clean offshore tsunami records when compared to coastal observations by tide gauges, which greatly improves the efficiency of the system. For earthquake monitoring, the data processing operational routine is examined using Monte-Carlo simulations. These take into consideration the errors in phase picking and the uncertainty on the 1D velocity model used for earthquake location. Quality of earthquake location is examined using the difference between the true location and the centroid of the computed epicenters and by the overall ellipse of uncertainty obtained from 100 runs. The added value provided by instrumented submarine telecommunication cables to mitigate earthquake and tsunami risk demonstrated in this work will help authorities and the society in general to take the political decisions required for its full implementation worldwide.
- Theoretical analysis of multimodal four-wave mixing in optical microwiresPublication . Fernandes, Gil M.; Almeida, Alvaro J.; Niehus, Manfred; Pinto, Armando NolascoOptical fiber microwires (OFMs) are nonlinear optical waveguides that support several spatial modes. The multimodal generalized nonlinear Schrodinger equation (MM-GNLSE) is deduced taking into account the linear and nonlinear modal coupling. A detailed theoretical description of four-wave mixing (FWM) considering the modal coupling is developed. Both, the intramode and the intermode phase-matching conditions is calculated for an optical microwire in a strong guiding regime. Finally, the FWM dynamics is studied and the amplitude evolution of the pump beams, the signal and the idler are analyzed.
- Towards a hardware-in-the-loop quantum optical ground station simulator and testbedPublication . Niehus, Manfred; Simão, José; Silva, João Castanheira da; Serrador, António; Carvalho, João M.; Gonçalves Cavaco Mendes, Mário JoséA single pass Bennett-Brassard 1984 protocol quantum key distribution downlink from a small quantum satellite in low earth orbit to an optical ground station is studied with the objective to develop the framework for a hardware-in-the-loop quantum optical ground station simulator and testbed.