From single particle to superfuid excitations in a dissipative polariton gas

Using angle-resolved heterodyne four-wave-mixing technique, we probe the low momentum excitation spectrum of a coherent polariton gas. The experimental results are well captured by the Bogoliubov transformation which describes the transition from single particle excitations of a normal fluid to sound-wave-like excitations of a superfluid. In a dense coherent polariton gas, we find all the characteristics of a Bogoliubov transformation, i.e. the positive and negative energy branch with respect to the polariton gas energy at rest, sound-wave-like shapes for the excitations dispersion, intensity and linewidth ratio between the two branches in agreement with the theory. The influence of the non-equilibrium character of the polariton gas is shown by a careful analysis of its dispersion.Comment: 4 pages, 3 figure

Métodos de processamento na ensilagem e qualidade das silagens de capim-elefante (Pennisetum purpureum Schum. cv. Cameroon).

O Capim-Elefante foi ensilado aos 75 dias do estabelecimento da cultura e submetido a diferentes tratamentos: a) sem emurchamento; b) com emurchamento; c) sem emurchamento + 8% de grão de sorgo moído e d) com emurchamento + 8% de grão de sorgo moído. Os tratamentos foram analisados estatísticamente em um delineamento completamente casualizado, com cinco repetições.bitstream/item/110448/1/METODOS-DE-PROCESSAMENTO.pd

Direct measure of the exciton formation in quantum wells from time resolved interband luminescence

We present the results of a detailed time resolved luminescence study carried out on a very high quality InGaAs quantum well sample where the contributions at the energy of the exciton and at the band edge can be clearly separated. We perform this experiment with a spectral resolution and a sensitivity of the set-up allowing to keep the observation of these two separate contributions over a broad range of times and densities. This allows us to directly evidence the exciton formation time, which depends on the density as expected from theory. We also evidence the dominant contribution of a minority of excitons to the luminescence signal, and the absence of thermodynamical equilibrium at low densities

Feasibility and Environmental Sustainability of a 103.5 kWp floating Photovoltaic Electrical System with a Case Study in a Hydroelectric Power Plant, Santa Clara Hpp, Located in the South of Brazil Region

Typical environmental problems associated with the implementation of solar photovoltaic systems for the generation of peak electrical energy, on a larger scale, such as on the order of 1 MWp, is in the occupied area, usually more than 3 km2. This can be minimized by the use of water parks or water dam’s reservoir, small and large hydroelectric power plants dams. Both the terrestrial and aquatic systems can impact the site, the first one, for the need to promote earthworks, removal of extensive green areas in the surroundings, installation of new transmission line, among others; and the second, despite the fact that a flat surface is already used and that there is no need for new civil procedures for its installation and can normally take advantage of the existing power transmission line, may cause changes in the biota of the reservoir, depending on the shading areas on the surface of the lake. Due to these facts, this research was proposed to investigate, parameterize and tropicalize an electric power generation system based on floating silicon photovoltaic cell panels installed in the Santa Clara HPP reservoir, in terms of peak power, durability, aspects and environmental impacts, with the study of possible evolutionary improvements of the project such as "tracking" or solar tracking, as well as dynamism of the structure, allowing the shadow area to be shifted over time, minimizing its effects in the local biota

Coherent Optical polarization of Bulk GaAs Studied by Femtosecond Photon-Echo Spectroscopy

The nonlinear polarization close to the band gap of GaAs is studied by spectrally and temporally resolved four-wave mixing. Excitonic and free carrier contributions both excited within the bandwidth of the 100 fs pulses are distinguished for the first time. The excitonic part dominates at carrier densities below 1016 cm-3. At higher density, nonthermalized free carriers give rise to an additional component resonant to the pulse that shows a photon-echo-like time behavior. Monte Carlo simulations including the coherent polarization and the scattering dynamics of the carriers account for the data

Excitonic and free-carrier polarizations of bulk GaAs studied by femtosecond coherent spectroscopy

The transient third-order polarization at the band gap of undoped and p-doped GaAs is investigated by spectrally and temporally resolved four-wave mixing. Excitonic and free-carrier contributions simultaneously excited within the bandwidth of the 100-fs pulses are clearly distinguished by their different spectral envelopes. The excitonic part dominates at carrier densities below 1016 cm-3 and shows a time evolution governed by exciton-free-carrier scattering and by many-body effects. At higher density, the free-carrier polarization has a strength similar to the exciton contribution and exhibits a spectrum resonant to the femtosecond pulses with a photon-echo-like temporal behavior. The data are analyzed by a numerical solution of the semiconductor Bloch equations including an ensemble Monte Carlo simulation of the scattering dynamics of the carriers. The theoretical model is in good agreement with the experimental results