1.55 μm direct bandgap electroluminescence from strained n-Ge quantum wells grown on Si substrates

Electroluminescence from strained n-Ge quantum well light emitting diodes grown on a silicon substrate are demonstrated at room temperature. Electroluminescence characterisation demonstrates two peaks around 1.55 μm and 1.8 μm, which correspond to recombination between the direct and indirect transitions, respectively. The emission wavelength can be tuned by around 4% through changing the current density through the device. The devices have potential applications in the fields of optical interconnects, gas sensing, and healthcare

In a nanoscience lab

The production, observation and manipulation of very small objects is a tour de force, but these objects, which could infiltrate anywhere without being seen, may arouse suspicion. To assess the situation at best, we describe the activity of a nanoscience research institution, some of the methods used there, the spirit of its researchers and their attitude towards risk.Comment: article accept\'e pour publication dans les C.R. Physique. El\'ement d'un dossier 'Nanosciences et nanotechnologies: esp\'erances et inqui\'etudes". D'autres articles de ce dossier, ainsi que la version fran\c{c}aise de cet articles, seront \'egalement soumis \`a HA

Distortion-corrected phase demodulation using phase-generated carrier with multitone mixing

We present a novel phase generated carrier (PGC) demodulation technique for homodyne interferometers which is robust to modulation depth variations and source intensity fluctuations. By digitally mixing the waveform with a multitone synthetic function (a linear combination of harmonics of the modulating signal), distortion can become negligible even in presence of large variations of the modulation depth. The technique only requires two mixers and can also provide the DC component of the phase in real time, without needing any previously recorded data or ellipse-fitting algorithms. We validate the technique with simulated waveforms and with experimental data from a wavelength metering experiment using an integrated unbalanced interferometer on-chip, showing that the technique corrects distortion without increasing the noise with respect to the standard PGC technique

Stimulated and spontaneous four-wave mixing in silicon-on-insulator coupled photonic wire nano-cavities

We report on four-wave mixing in coupled photonic crystal nano-cavities on a silicon-on-insulator platform. Three photonic wire cavities are side-coupled to obtain three modes equally separated in energy. The structure is designed to be self-filtering, and we show that the pump is rejected by almost two orders of magnitudes. We study both the stimulated and the spontaneous four-wave mixing processes: owing to the small modal volume, we find that signal and idler photons are generated with a hundred-fold increase in efficiency as compared to silicon micro-ring resonators

Ingénierie de mode en optique intégrée sur silicium sur isolant

This thesis enters upon the localization and the handling of the light at the wavelength scale. The work initially concerned microcavities with high quality factors (Q) integrated on ridge waveguides relying on a substrate (SOI). Thanks to a mode engineering based on modes profiles adaptation, the optimization of mirrors permits us to achieve Q factors up to 60000 for a modal volume of 0,6 (lambda/n)3. Optical confinement could be observed by near-field microscopy and we showed that it was possible to control these cavities by a SNOM tip. Lastly, we investigate the slow modes approach by tackling theoretically the issue of the injection of light in slow modes.Cette thèse porte sur la localisation et la manipulation de la lumière à l'échelle de la longueur d'onde. Les travaux ont d'abord porté sur des micro-cavités à grands facteurs de qualité (Q) intégrées sur des guides rubans reposant sur un substrat (SOI). L'optimisation des miroirs grâce à une ingénierie de mode basée sur l'adaptation des profils des modes, a permis d'obtenir des Q allant jusqu'à 60000 pour un volume modal de 0,6(lambda/n)3. Le confinement a pu être observé par microscopie en champ proche et nous avons montré qu'il était possible de contrôler ces cavités par une pointe SNOM. . Enfin, nous nous sommes intéressés aux modes lents en abordant de façon théorique l'injection efficace de la lumière dans un mode lent

Ohmic contacts to n-type germanium with low specific contact resistivity

A low temperature nickel process has been developed that produces Ohmic contacts to n-type germanium with specific contact resistivities down to (2.3 ± 1.8) x10<sup>-7</sup> Ω-cm<sup>2</sup> for anneal temperatures of 340 degC. The low contact resistivity is attributed to the low resistivity NiGe phase which was identified using electron diffraction in a transmission electron microscope. Electrical results indicate that the linear Ohmic behaviour of the contact is attributed to quantum mechanical tunnelling through the Schottky barrier formed between the NiGe alloy and the heavily doped n-Ge.<p></p&gt

Adaptable Pulse Compression in φ-OTDR With Direct Digital Synthesis of Probe Waveforms and Rigorously Defined Nonlinear Chirping

Recent research in Phase-Sensitive Optical Time Doman Reflectometry (φ-OTDR) has been focused, among others, on performing spatially resolved measurements with various methods including the use of frequency modulated probes. However, conventional schemes either rely on phase-coded sequences, involve inflexible generation of the probe frequency modulation or mostly employ simple linear frequency modulated (LFM) pulses which suffer from elevated sidelobes introducing degradation in range resolution. In this contribution, we propose and experimentally demonstrate a novel φ-OTDR scheme which employs a readily adaptable Direct Digital Synthesis (DDS) of pulses with custom frequency modulation formats and demonstrate advanced optical pulse compression with a nonlinear frequency modulated (NLFM) waveform containing a complex, rigorously defined modulation law optimized for bandwidth-limited synthesis and sidelobe suppression. The proposed method offers high fidelity chirped waveforms, and when employed in resolving a 50-cm event at ∼1.13 km using a 1.2-μs probe pulse, matched filtering with the DDS-generated NLFM waveform results in a significant reduction in range ambiguity owing to autocorrelation sidelobe suppression of ∼20 dB with no averages and windowing functions, for an improvement of ∼16 dB compared to conventional linear chirping. Experimental results also show that the contribution of autocorrelation sidelobes to the power in the compressed backscattering responses around localized events is suppressed by up to ∼18 dB when advanced pulse compression with an optical NLFM pulse is employed

Determinantes das diferenças entre o resultado contabilístico e o fiscal: evidência empírica de empresas privadas portuguesas

As diferenças entre o resultado contabilístico e o resultado fiscal, denominadas Book-Tax Differences (BTD), surgem quando os normativos contabilístico e fiscal aplicam regras diferentes na contabilização do mesmo item. Visto que existem diversos itens que podem originar estas diferenças, o principal objetivo deste estudo consiste em identificar os principais determinantes das BTD. Além disso, pretende-se também analisar qual o impacto nas BTD da reforma do IRC implementada em 2014 e das diversas alterações às taxas de imposto que se verificaram nos anos seguintes. Para realizar o estudo, são analisados os dados retirados da base de dados Bureau van Dijk’s Amadeus relativos a empresas privadas portuguesas entre 2012 e 2016, totalizando 327.050 observações. Os resultados dos modelos empíricos desenvolvidos permitiram concluir que as BTD apresentam uma relação positiva com o crescimento das vendas, accruals, nível de endividamento e taxa efetiva de imposto e uma relação negativa com a rentabilidade, crescimento do investimento em ativos, dimensão da empresa e variação anual das BTD. No que diz respeito ao impacto da reforma do IRC foi possível concluir que a mesma contribuiu para acentuar a relação positiva existente entre a taxa efetiva de imposto e as BTD. Algumas análises realizadas adicionalmente permitiram também encontrar evidências de que o tipo e a dimensão das BTD, bem como a rentabilidade das empresas têm influência na relação estabelecida entre as BTD das empresas e os seus determinantes.The differences between accounting income and taxable income, called Book-Tax Differences (BTD), arise when accounting and tax regulations apply different rules in accounting the same item. Since there are several items that originate these differences, the main goal of this study is to identify the main determinants of BTD. In addition, it is also intended to analyze the impact on the BTD of the IRC reform implemented in 2014 and the various changes to the tax rates that occurred in subsequent years. To carry out the study, the data taken from the Bureau van Dijk's Amadeus database for Portuguese private companies between 2012 and 2016 is analyzed, totaling 327,050 observations. The results of the empirical models developed made it possible to conclude that BTD have a positive relation with the growth of sales, accruals, indebtedness level and effective tax rate and a negative relation with profitability, growth in asset investment, company size and variation of BTD. Regarding the impact of the IRC reform, it was possible to conclude that it contributed to accentuate the positive relationship between the effective tax rate and the BTD. Further analyses have also provided evidence that the type and the size of the BTD as well as the profitability of the companies influence the relationship established between the companies' BTD and their determinants