Directional emission of stadium-shaped micro-lasers

The far-field emission of two dimensional (2D) stadium-shaped dielectric cavities is investigated. Micro-lasers with such shape present a highly directional emission. We provide experimental evidence of the dependance of the emission directionality on the shape of the stadium, in good agreement with ray numerical simulations. We develop a simple geometrical optics model which permits to explain analytically main observed features. Wave numerical calculations confirm the results.Comment: 4 pages, 8 figure

Anisotropic Decay Dynamics of Photoexcited Aligned Carbon Nanotube Bundles

We have performed polarization-dependent ultrafast pump-probe spectroscopy of a film of aligned single-walled carbon nanotube bundles. By taking into account imperfect nanotube alignment as well as anisotropic absorption cross sections, we quantitatively determined distinctly different photo-bleaching dynamics for polarizations parallel and perpendicular to the tube axis. For perpendicular polarization, we observe a slow (1.0-1.5 ps) relaxation process, previously unobserved in randomly-oriented nanotube bundles. We attribute this slower dynamics to the excitation and relaxation of surface plasmons in the radial direction of the nanotube bundles.Comment: 4 pages, 3 figure

Origin of the excitonic recombinations in hexagonal boron nitride by spatially resolved cathodoluminescence spectroscopy

The excitonic recombinations in hexagonal boron nitride (hBN) are investigated with spatially resolved cathodoluminescence spectroscopy in the UV range. Cathodoluminescence images of an individual hBN crystallite reveals that the 215 nm free excitonic line is quite homogeneously emitted along the crystallite whereas the 220 nm and 227 nm excitonic emissions are located in specific regions of the crystallite. Transmission electron microscopy images show that these regions contain a high density of crystalline defects. This suggests that both the 220 nm and 227 nm emissions are produced by the recombination of excitons bound to structural defects

Alignement experience in STAR

The STAR experiment at RHIC uses four layers of silicon strip and silicon drift detectors for secondary vertex reconstruction. An attempt for a direct charm meson measurement put stringent requirements on alignment and calibration. We report on recent alignment and drift velocity calibration work performed on the inner silicon tracking system

Hard and soft probe - medium interactions in a 3D hydro+micro approach at RHIC

We utilize a 3D hybrid hydro+micro model for a comprehensive and consistent description of soft and hard particle production in ultra-relativistic heavy-ion collisions at RHIC. In the soft sector we focus on the dynamics of (multi-)strange baryons, where a clear strangeness dependence of their collision rates and freeze-out is observed. In the hard sector we study the radiative energy loss of hard partons in a soft medium in the multiple soft scattering approximation. While the nuclear suppression factor $R_{AA}$ does not reflect the high quality of the medium description (except in a reduced systematic uncertainty in extracting the quenching power of the medium), the hydrodynamical model also allows to study different centralities and in particular the angular variation of $R_{AA}$ with respect to the reaction plane, allowing for a controlled variation of the in-medium path-length.Comment: 5 pages, 4 figures, Quark Matter 2006 proceedings, to appear in Journal of Physics

Inferring periodic orbits from spectra of simple shaped micro-lasers

Dielectric micro-cavities are widely used as laser resonators and characterizations of their spectra are of interest for various applications. We experimentally investigate micro-lasers of simple shapes (Fabry-Perot, square, pentagon, and disk). Their lasing spectra consist mainly of almost equidistant peaks and the distance between peaks reveals the length of a quantized periodic orbit. To measure this length with a good precision, it is necessary to take into account different sources of refractive index dispersion. Our experimental and numerical results agree with the superscar model describing the formation of long-lived states in polygonal cavities. The limitations of the two-dimensional approximation are briefly discussed in connection with micro-disks.Comment: 13 pages, 19 figures, accepted for publication in Physical Review