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We present in this work the design of second harmonic generation process in χ(2) nonlinear waveguides by a new time domain simulator. The presented Hertzian potential formulation (HPF) is applied to quasi-phase-matched GaAs/AlGaAs nonlinear waveguide with dielectric discontinuities. With the introduction of the presented rigorous time domain method it is possible to represent the physical phenomena such as light propagation and second harmonic generation (SHG) process inside a nonlinear optical device with a good convergent solution and low computational cost. We first check the convergence of the HPF approach by comparing the numerical results with experimental ones for a GaAs/AlGaAs nonlinear waveguide with a fundamental mode at λFU=1.955 μm coupled to a coprapagating second-harmonic SH mode (λSH=9.775 μm) through an appropriate nonlinear susceptibility coefficient. Then we apply the modelling to a GaAs/AlGaAs ridge waveguide with λFU=1.55 μm and λSH=0.775 μm by evaluating the second harmonic conversion efficiency versus the waveguide length. The model is generic and can be applied also to periodic microwave waveguides.

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This paper focuses on the design and fabrication of piezoelectric cantilevers and on the analysis of their bending, due to residual stress, as a function of growth parameters and geometrical features. The multilayered cantilevers consist of two top and bottom Molybdenum metals and an Aluminium nitride piezoelectric layer deposited onto a Polysilicon elastic layer using DC magnetron sputtering. After having optimized the growth parameters we fabricate cantilever arrays with different lengths and thicknesses of the Molybdenum and Aluminium nitride layers. The curvature of the fabricated cantilevers is determined by the tip deflection measured by scanning electron microscopy.

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The authors report a detailed investigation of the structural and optical properties of vertically stacked InAs quantum dots embedded in an (In,Ga)As quantum well by means of transmission electron microscopy and time resolved photoluminescence based on the upconversion technique. By comparing the optical features of quantum dot samples of different barrier thicknesses (nominal values between 5 and 65 nm), they have found evidence for electronic coupling among the quantum dots, featured by an increase of radiative lifetime and a relatively blueshifted emission peak for the thinnest spacer layer sample. (C) 2008 American Institute of Physics.

International audience

We report the results of an extensive optical investigation by continuous-wave and time resolved photoluminescence experiments on double GaInAsSb/AlGaAsSb quantum wells emitting at 2.3 mu m at room temperature. We have found that, at low temperature (< 70 K), the recombination is dominated by excitons trapped in disorder and interface defects. Whereas, at higher temperature, free-exciton recombination occurs. The observed temperature dependent photoluminescence quenching is ascribed to the ionization of bound exciton at low temperatures, while thermoionic emission of the hole out of the quantum well dominates photoluminescence quenching at high temperatures. The experimental results are supported by theoretical calculations. (c) 2008 American Institute of Physics.

We report on the growth and characterization of low threshold 1.32-mu m quantum dots (QDs) laser diodes. The quantum dot active region was optimised to get the highest photoluminescence emission and the lowest Full Width at Half Maximum (FWHM). From samples containing multilayer QDs and using the Limited-Area Photoluminescence (LAPL) technique we have shown that the gain of an N-layer structure is higher than N times that of a single layer. This enhancement is attributed to the increase of the quantum dot density in the upper layers and also to the use of the high growth temperature spacer layer. Broad area laser diodes were processed from the grown samples containing three layers of InAs QDs grown directly on GaAs and capped with 4-nm-thick InxGa1-xAs layer. Than measurements were performed at room temperature under pulsed excitation. The laser diodes operate at room temperature and emit between 1.29 and 1.32-mu m which is beyond the strategic telecommunication wavelength. The characteristic temperature is around 80 K and very stable in the hole range of the operating temperature (from 0 to 90 degrees C). The internal quantum efficiency is 53% and the modal gain per QD layer was estimated to be similar to 6 cm(-1). For an infinite cavity length a threshold current density of 8 A/cm(2) per QD layer was obtained. From the calculation of the optical confinement of QDs, we have estimated a material gain of 1979 cm(-1).

We investigate the optical properties of quantum-dot (QD) microcavity light-emitting diodes (MCLED) operating at 1.3 μ m at room temperature. The active medium consists of a single layer of InGaAs quantum dots, directly grown in a GaAs matrix by metalorganic chemical vapor deposition. Electrical injected QD MCLEDs were fabricated by exploiting a hybrid technology, which employs epitaxial and oxide-based mirrors. Such technology allows us to preserve the QD properties and leads to a wider optical bandwidth of the hybrid top distributed Bragg reflector with respect to the fully epitaxial mirror, resulting in photon recycling of the overall QDs spectrum, thus improving the efficiency of the device. The devices exhibit bright electroluminescence peaked at 1.29 μ m, with a full width at half maximum of 10 meV. The room-temperature external quantum efficiency of such devices is 0.52%, higher than that reported in the literature for QD MCLEDs operating at 1.3 μ m at room temperature.

This work explores the conditions to obtain the extension of the PL emission beyond 1.3 mum in InGaAs quantum dot (QD) structures growth by MOCVD. We found that, by controlling the In incorporation in the barrier embedding the QDs, the wavelength emission can be continuously tuned from 1.25 mum up to 1.4 mum at room temperature. However, the increase in the overall strain of the structures limits the possibility to increase the maximum gain in the QD active device, where an optical density as high as possible is required. By exploring the kinetics of QD surface reconstruction during the GaAs overgrowth, we are able to obtain, for the first time, emission beyond 1.3 pin from InGaAs QDs grown on GaAs matrix. The wavelength is tuned from 1.26 mum up to 1.33 mum and significant improvements in terms of line shape narrowing and room temperature efficiency are obtained. The temperature-dependent quenching of the emission efficiency is reduced down to a factor of 3, the best value ever reported for QD structures emitting at 1.3 mum