120 research outputs found
Initiation expérimentale à la mécanique : le plan incliné de Galilée
No full textNational audienceComment faire de la mécanique ? Comment prendre des mesures, analyser le mouvement d'un objet de façon quantitative ? Comment trouver la loi horaire de ce mouvement ? Comment faire tout cela avant le lycée, avant l'apprentissage des rudiments du calcul différentiel tel que la dérivation et l'intégration ? Cet article tente de répondre à ces questions. En effet, son objet est de proposer un déroulement d'atelier afin d'initier les élèves des écoles primaires ou des collèges à la physique et plus particulièrement à la mécanique sans recourir au langage mathématique des dérivés, des primitives ou des équations différentielles. Ce travail s'est beaucoup appuyé sur diverses expériences de collègues ayant mis en place des ateliers de physique dans des classes de la maternelle au lycée en passant par l'élémentaire et le collège [1-4]. Dans ces ateliers mis en place pour des animations pour des scolaires âgés de 8 à 18 ans, il s'agissait simplement de présenter en quoi consiste la mécanique, quels sont les outils, les grandeurs physiques… dont il faut se doter pour pratiquer des expériences de mécanique et de les mettre en pratique. La première partie montrera comment nous proposons de présenter et définir la mécanique aux élèves. Nous y introduirons les notions de mesure, d'étalon, d'horloge, de repère spatial, de vitesse et d'accélération. Nous étudierons qualitativement le mouvement « la chute des corps » dans la deuxième partie. Ce mouvement sera étudié de façon semi-quantitative dans la dernière partie de cet article dans le cas de l'expérience historique de Galilée du « plan incliné », expérience qui ouvrira la voie à la physique moderne illustrant les propos du savant italien : « la nature est un livre écrit en langage mathématique »
Granularité laser et interférences de speckles
No full textInternational audienceIn this paper we introduce experimentally the phenomenon of speckle and its interferometric applications. With the popularization of CCD sensors and webcams, it is now easy to acquire speckles patterns, to reduce them and exploit them. The material used here is what we could find easily in high schools. All Image processing mentioned in this article could be done using the free software called IRIS. Keywords are essentially diffraction, interference phenomena and Fourier optics. After presenting the characteristics of speckles we discuss the phenomenon of speckles interferences by analogy with the conventional 2 and N waves interferences. Finally, we apply interferometry to measure the angular separation between the components of a double star in drawing heavily on the historical experience of Antoine Labeyrie
Transport of Indirect Excitons in Polar GaN/AlGaN Quantum Well Structures Grown on Sapphire and GaN Substrates.
No full textInternational audienceAn indirect exciton (IX) is a quasiparticle consisting of an electron and a hole spatially separated in two different planes of a quantum nanostructure, thus exhibiting a strongly dipolar character. Current research on transport properties of IXs is opening a pathway to the development of novel optoelectronic devices, which have already been demonstrated in GaAs-based heterostructures [1-3]. Applying the same ideas to IXs in wide-band gap polar quantum wells (QWs) is particularly promising because of much larger exciton binding energies and natural dipoles induced by strong built-in electric fields. We have recently studied the transport of IXs in GaN/AlGaN QWs grown on sapphire substrates, at temperatures up to 80 K [4], by mapping the micro-photoluminescence (µ-PL) signal obtained under intense, point excitation. The low-temperature PL recorded at long distances from the excitation spot (30 < r < 100µm) turned out to be a secondary PL, excited by the light emitted at the central spot, guided along the plane, due to the refractive index contrast between the layer and the substrate. At higher temperatures, this signal is rapidly quenched and the distance reached by the measurable PL is limited by recombination of excitons at non-radiative defects. Using GaN substrates instead of sapphire should both suppress the secondary emission and the nonradiative recombination, by reducing dislocation densities by 3-4 orders of magnitude. In this work, we compare exciton propagations in two GaN/Al0.19Ga0.81N QWs of identical structures, except for the substrates, respectively of GaN and sapphire. For the GaN substrate, we indeed observe the mere propagation of excitons up to 35 µm away from the excitation spot and up to 250 K (see below). We propose a drift/diffusion modelling of exciton transport, accounting for dipole-dipole repulsion in high-density regions and for disorder along the sample plane.[1]Y. Y. Kuznetsova, M. Remeika, A. A. High, A. T. Hammack, L. V. Butov, M. Hanson, and A. C. Gossard. Optics Letters 35 (10), 1587 (2010).[2]A.A. High, E.E. Novitskaya, L.V. Butov, and A.C. Gossard. Science 321, 229 (2008).[3]A.A. High, A.T. Hammack, L.V. Butov, and A.C. Gossard. Optics Letters 32, 2466 (2007).[4]F. Fedichkin, P. Andreakou, B. Jouault, M. Vladimirova, T. Guillet, C. Brimont, P. Valvin, T. Bretagnon, A. Dussaigne, N. Grandjean, P. Lefebvre. Phys. Rev. B 91, 205424 (2015)
Homoepitaxy of boron nitride on exfoliated hexagonal boron nitride flakes
No full textThe experimental data presented is related to the article: Binder, J., Dabrowska, A. K., Tokarczyk, M., Rousseau, A., Valvin, P., Bozek, R., Nogajewski, K., Kowalski, G., Pacuski, W., Gil, B., Cassabois, G., Stepniewski, R., & Wysmolek, A. (2024). Homoepitaxy of boron nitride on exfoliated hexagonal boron nitride flakes. Nano Letters, https://doi.org/10.1021/acs.nanolett.4c01310.Data in FigS4.zip presents PL spectra for different regions on the sample: on a homepitaxial triangular grain, on the bulk hBN flake and next to the flake on the sapphire substrate.Although large efforts have been made to improve the growth of hexagonal boron nitride (hBN) by heteroepitaxy, the non-native substrates remain a fundamental factor that limits the quality. This problem can be solved by homoepitaxy, which is the growth of hBN on hBN substrates. In this report, we demonstrate the homoepitaxial growth of triangular BN grains on exfoliated hBN flakes by Metal-Organic Vapour Phase Epitaxy and show by atomic force microscopy and photoluminescence that the stacking of these triangular islands can deviate from the AA’ stacking of hBN. We show that the stacking order is enforced by the crystallographic direction of the edge of the exfoliated hBN flakes, with armchair edges allowing for centrosymmetric stacking, whereas zigzag edges lead to the growth of non-centrosymmetric BN polytypes. Our results indicate pathways to grow homoepitaxial BN with tuneable layer stacking, which is required to induce piezoelectricity or ferroelectricity.</p
Optical properties of hexagonal boron nitride
No full textInternational audienceIn this paper, I will review our recent results demonstrating that hBN has an indirect bandgap at 5.9 eV. I will show that the optical properties of hBN are profoundly determined by phonon-assisted transitions with a mirror symmetry between emission and absorption around the indirect exciton at 5.9 eV (Figure 1). I will provide a comprehensive analysis of the emission spectrum in the deep ultraviolet in terms of phonon-assisted transitions involving either virtual or real excitonic states, the latter being provided by structural defects. I will finally point out the complex relaxation dynamics of the quantum gas formed by the reservoir of indirect excitons
Optical properties of hexagonal boron nitride
No full textInternational audienceIn this paper, I will review our recent results demonstrating that hBN has an indirect bandgap at 5.9 eV. I will show that the optical properties of hBN are profoundly determined by phonon-assisted transitions with a mirror symmetry between emission and absorption around the indirect exciton at 5.9 eV (Figure 1). I will provide a comprehensive analysis of the emission spectrum in the deep ultraviolet in terms of phonon-assisted transitions involving either virtual or real excitonic states, the latter being provided by structural defects. I will finally point out the complex relaxation dynamics of the quantum gas formed by the reservoir of indirect excitons
Optical properties of hexagonal boron nitride
No full textInternational audienceIn this paper, I will review our recent results demonstrating that hBN has an indirect bandgap at 5.9 eV. I will show that the optical properties of hBN are profoundly determined by phonon-assisted transitions with a mirror symmetry between emission and absorption around the indirect exciton at 5.9 eV (Figure 1). I will provide a comprehensive analysis of the emission spectrum in the deep ultraviolet in terms of phonon-assisted transitions involving either virtual or real excitonic states, the latter being provided by structural defects. I will finally point out the complex relaxation dynamics of the quantum gas formed by the reservoir of indirect excitons
Optical properties of hexagonal boron nitride
No full textInternational audienceIn this paper, I will review our recent results demonstrating that hBN has an indirect bandgap at 5.9 eV. I will show that the optical properties of hBN are profoundly determined by phonon-assisted transitions with a mirror symmetry between emission and absorption around the indirect exciton at 5.9 eV (Figure 1). I will provide a comprehensive analysis of the emission spectrum in the deep ultraviolet in terms of phonon-assisted transitions involving either virtual or real excitonic states, the latter being provided by structural defects. I will finally point out the complex relaxation dynamics of the quantum gas formed by the reservoir of indirect excitons
Time resolved photoluminescence of ZnO / (Zn,Mg)O quantum wells. Internal electric field effects.
No full textInternational audienc
Transient photoluminescence of aluminum‐rich (Al,Ga)N low‐dimensional structures
No full textInternational audienceLight-emitting devices based on high-Al content (Al,Ga)N suffer from high densities of dislocations or other point defects, counterbalanced by the carrier localization arising from disorder in the ternary alloy. One way to improve the internal quantum efficiency is the controlled Si-doping of AlxGa1-xN/AlyGa1-yN multi-quantum wells, which was assigned to the reduction of point defects by reduction of internal strains, for some ideal concentration of Si. Time-resolved photoluminescence (TR-PL) can be used [S. F. Chichibu et al., Appl. Phys. Lett. 99, 051902 (2011)] to try and correlate the observed PL time-decays with the density of defects, in relation with the nonradiative recombination probability of photoexcited carriers. We present TR-PL studies of MOVPE-grown Si-doped AlxGa1-xN/AlyGa1-yN multi-quantum wells with typical values of x=0.6 and y=0.7 and of AlxGa1-xN epilayers with x up to 0.86. High-Al content MQWs and epilayers exhibit similar bi-exponential PL decay dynamics, with the slower component rapidly quenched when T is increased. The fast decay component remains in the nanosecond range at all temperatures and the PL intensity loss is limited by carrier localization
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