GaAs/AlOx Nonlinear Waveguides for Infrared Tunable Generation

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Magnetic polaron formation and exciton spin relaxation in single CdMnTe quantum dots

We study the formation dynamics of a spontaneous ferromagnetic order in single self-assembled CdMnTe quantum dots. By measuring time-resolved photoluminescence, we determine the formation times for QDs with Mn ion contents x varying from 0.01 to 0.2. At low x these times are orders of magnitude longer than exciton spin relaxation times evaluated from the decay of photoluminescence circular polarization. This allows us to conclude that the direction of the spontaneous magnetization is determined by a momentary Mn spin fluctuation rather than resulting from an optical orientation. At higher x, the formation times are of the same order of magnitude as found in previous studies on higher dimensional systems. We also find that the exciton spin relaxation accelerates with increasing Mn concentration.Comment: sent to Physical Review

Characterizing temporary hydrological regimes at a European scale

Monthly duration curves have been constructed from climate data across Europe to help address the relative frequency of ecologically critical low flow stages in temporary rivers, when flow persists only in disconnected pools in the river bed. The hydrological model is 5 based on a partitioning of precipitation to estimate water available for evapotranspiration and plant growth and for residual runoff. The duration curve for monthly flows has then been analysed to give an estimate of bankfull flow based on recurrence interval. The corresponding frequency for pools is then based on the ratio of bank full discharge to pool flow, arguing from observed ratios of cross-sectional areas at flood 10 and low flows to estimate pool flow as 0.1% of bankfull flow, and so estimate the frequency of the pool conditions that constrain survival of river-dwelling arthropods and fish. The methodology has been applied across Europe at 15 km resolution, and can equally be applied under future climatic scenarios

Silica masks for improved surface poling of lithium niobate

Surface periodic poling of congruent lithium niobate was performed with the aid of photolithographically defined silica masks. The latter helped improving the control of duty cycle in the periodic domain poling, with 50:50 mark-to-space ratios. The role of silica was ascertained by numerically solving the Poisson equation

Nonlinear AlGaAs waveguide for the generation of counterpropagating twin photons in the telecom range

We have designed and fabricated a set of AlGaAs multilayer waveguides, which can serve as a source of entangled photons at 1.55 mu m through parametric fluorescence. In our scheme two counterpropagating, orthogonally polarized signal/idler modes are nonlinearly generated by a pump wave impinging on the upper surface of the waveguide. To check the compliance with design specifications on phase-matching wavelength and parametric gain, we have systematically measured effective indices and surface-emitting second-harmonic generation, respectively. This characterization allowed us to single out a nominal sample with optimum performances, which we numerically modeled for counterpropagating parametric fluorescence. We predict a pair generation efficiency eta(PF)=4x10(-13) (signal photons per pump photon). For a 1 W (peak), 100 ns pump pulse at normal incidence, this corresponds to about 14 photons per dark count with state-of-the-art avalanche photodiodes

Visible emission from Er-doped SnO2 thin films deposited by sol-gel

Foi obtida emissão de filmes finos de SnO2 dopados com Er no intervalo 500-700 nm, com pico em 530 nm (verde). Esses filmes foram depositados pela técnica de molhamento via sol-gel. A geração de pares elétron-buraco na matriz de SnO2 é usada para promover a excitação do íon terra-rara. A avaliação do tamanho dos cristalitos por meio de resultados de difração de raios X indica dimensões nanoscópicas, o que pode ser relevante para a interpretação do espectro de emissão. O mecanismo de excitação elétron-buraco é também responsável pela excitação da transição no intervalo que inclui 1540 nm em pós obtidos da mesma solução precursora dos filmes. Filmes finos constituem um formato muito útil para aplicações tecnológicas, desde que permite integração em dispositivos ópticos e a aplicação de campos elétricos para operar dispositivos eletroluminescentes.Emission from Er-doped SnO2 thin film deposited via sol-gel by the dip coating technique is obtained in the range 500-700 nm with peak at 530 nm (green). Electron-hole generation in the tin dioxide matrix is used to promote the rare-earth ion excitation. Evaluation of crystallite dimensions through X-ray diffraction results leads to nanoscopic size, what could play a relevant role in the emission spectra. The electron-hole mechanism is also responsible for the excitation of the transition in the 1540 nm range in powders obtained from the same precursor solution of films. The thin film matrix presents a very useful shape for technological application, since it allows integration in optical devices and the application of electric fields to operate electroluminescent devices.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES