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We have determined the main parameters of the quasi-one-dimensional excitons confined in GaAs V-shaped quantum wires, namely exciton Bohr radius and binding energy, by two-photon absorption and magnetoluminescence experiments. The experimental results are in excellent agreement with our calculations, based on realistic wave functions for the actual wire geometry.

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Non-intrusive and real-time monitoring techniques are increasingly required by manufacturing industry in order to detect flaws in arc welding processes. In this work the development of an optical inspection system, for monitoring the manual gas tungsten arc welding (GTAW) process of steel pipes, is described. The arc plasma visible emission produced during the process was acquired and spectroscopically analysed. Measuring the intensities of selected argon emission lines allowed real time calculation and recording of the axial electron temperature of the plasma. Experimental results showed that the temperature signal varies greatly in the case of instabilities of the weld pool that cause weld defects. A suitable algorithm, based on a statistical analysis of the signal, was developed in order to real time flag defective joints. It is shown that several weld defects such as porosity, dropout, lack of fusion, solid inclusions and craters were successfully detected in a production environment. The performances of the optical sensor were compared with the results of state-of-the-art post-weld controls such as x-rays and penetrating dyes, showing good agreement and thus demonstrating the validity of this quality monitoring system.

We have investigated the polarization-dependent two-photon absorption in GaAs/AlGaAs quantum wires. The anisotropic selection rules of the multiphoton absorption process are exploited to study the one-dimensional 2p exciton states and the transitions between quantum wire subbands of different quantum numbers (An y^0 selection rule). The deviations from the selection rules derived for the strict onedimensional case are discussed, and depend on the actual quasi-one-dimensional character of the excitonic wave functions.PACS numbers: 78.65.Fa, The nonlinear optical properties of low-dimensional semiconductors have attracted great interest in the last few years. In particular, two-photon absorption (TPA) has been investigated in detail in two-dimensional (2D) semiconductor quantum wells, both experimentally and theoretically, to elucidate the impact of the reduced dimensionality on the nonlinear absorption processes [1]. Currently, interest has turned to systems with even lower dimensionality, like ID quantum wires and OD quantum dots (for a review see Ref.[2]). The optical properties of undoped low-dimensional systems are strongly governed by excitonic effects. An interesting aspect of TPA is that it gives direct access to excited excitonic states having a significantly larger excitonic radius (2p states). It is therefore expected that the influence of lateral confinement is stronger on these excited 2p states than on the excitonic ground state, i.e., the \s exciton. A special situation should occur when the width of the lateral confinement is in between the extension of the \s and 2p states. In this Letter we report an experimental study of TPA in GaAs/AlGaAs quantum wires. We find a strong quantum confinement effect on the 2p exciton states, significantly enhanced with respect to that on the \s states observed in conventional photoluminescence excitation spectroscopy (PLE). The TPA curves show rich excitation spectra with a strong polarization dependence, in agreement with theoretical predictions [3,4]. In fact, we find that optical transitions between quantum wire states with the same quantum numbers (An y = 0) are allowed when the polarization direction of the exciting beam (e) is perpendicular to the wire quantization direction (y), whereas An y = ± 1 transitions are dominant in the 6lly geometry. These peculiar selection rules allow us to discriminate between true one-dimensional and quasitwo-dimensional excitonic states which are involved as the final states of different linear and nonlinear absorption processes.The investigated quantum wire array has been fabricated by holographic patterning and subsequent reactive ion etching of a molecular-beam-epitaxy-grown multiple-quantum-well heterostructure. The quantum well structure consisted of 25 GaAs wells of width L z = 103 A and Alo.34Gao.66As barriers of width L^z = 148 A grown on top of a 1-jum-thick Alo.34Gao.66As cladding layer providing optical confinement of the luminescence (throughout the paper the indices z and y indicate the confinement directions of t...