Emission of photon echoes in a strongly scattering medium

We observe the two- and three-pulse photon echo emission from a scattering powder, obtained by grinding a Pr$^{3+}$:Y$_2$SiO$_5$ rare earth doped single crystal. We show that the collective emission is coherently constructed over several grains. A well defined atomic coherence can therefore be created between randomly placed particles. Observation of photon echo on powders as opposed to bulk materials opens the way to faster material development. More generally, time-domain resonant four-wave mixing offers an attractive approach to investigate coherent propagation in scattering media

Experimental observation of second-harmonic generation and diffusion inside random media

We have experimentally measured the distribution of the second-harmonic intensity that is generated inside a highly-scattering slab of porous gallium phosphide. Two complementary techniques for determining the distribution are used. First, the spatial distribution of second-harmonic light intensity at the side of a cleaved slab has been recorded. Second, the total second-harmonic radiation at each side of the slab has been measured for several samples at various wavelengths. By combining these measurements with a diffusion model for second-harmonic generation that incorporates extrapolated boundary conditions, we present a consistent picture of the distribution of the second-harmonic intensity inside the slab. We find that the ratio $\ell_{2\omega}/L_c$ of the mean free path at the second-harmonic frequency to the coherence length, which was suggested by some earlier calculations, cannot describe the second-harmonic yield in our samples. For describing the total second-harmonic yield, our experiments show that the scattering parameter at the fundamental frequency \k_{1\omega}\ell_{1\omega} is the most relevant parameter in our type of samples.Comment: 10 pages, 7 figure

Mapping of periodically poled crystals via spontaneous parametric down-conversion

A new method for characterization of periodically poled crystals is developed based on spontaneous parametric down-conversion. The method is demonstrated on crystals of Y:LiNbO3, Mg:Y:LiNbO3 with non-uniform periodically poled structures, obtained directly under Czochralski growth procedure and designed for application of OPO in the mid infrared range. Infrared dispersion of refractive index, effective working periods and wavelengths of OPO were determined by special treatment of frequency-angular spectra of spontaneous parametric down-conversion in the visible range. Two-dimensional mapping via spontaneous parametric down-conversion is proposed for characterizing spatial distribution of bulk quasi-phase matching efficiency across the input window of a periodically poled sample.Comment: 19 pages, 6 figure

State of the art in construction aspects of deep mixing technology

The installation technique of deep mixing (DM) and its monitoring during construction affect the quality of the improved ground. Accordingly, the objective of this paper is to examine the construction-related issues including construction techniques and monitoring of DM technology. The paper discusses: the machinery required for construction of sea-based and land-based projects; rate of production; construction sequence and the construction methods based on dry mix, slurry-based, high-jet injection; and the methods required for special applications. For economic reasons the installation time for soil-mixed columns should be as short as possible, while complying with the construction specifications. To achieve this goal, various factors affecting the installation process are examined. These include the effect of the following on the quality of the improved ground: injection method, degree of mixing, configuration of the blades, rotational speed of the shafts, and penetration and withdrawal speeds. Finally, the details of construction monitoring and expected environmental impacts are presented, along with a summary of research needs. La technique d'installation DM (mixage profond) et son suivi tout au long des travaux de construction affecte la qualité du sol amélioré. Dans cette optique, l'objectif de cet exposé est d'examiner plusieurs sujets apparentés à la construction, notamment les techniques de construction et de suivi de la technologie DM. Cet exposé aborde donc les sujets suivants: machines nécessaires aux chantiers en mer et sur terre; vitesse de production; séquence de construction, méthodes de construction basées sur un mélange à sec, en boue, à injection puissante, méthodes requises pour les applications spéciales. Pour des raisons économiques, l'installation des colonnes de sol mixé doit se faire aussi rapidement que possible tout en restant en conformité avec les spécifications de construction. Dans ce but, nous examinons divers facteurs affectant le processus d'installation, en particulier les effets des facteurs suivants sur la qualité du sol amélioré: méthode d'injection, degré de mixage, configuration des lames, vitesse de rotation des arbres et vitesses de pénétration et de retrait. Enfin, nous présentons les détails du suivi tout au long de la construction ainsi que les impacts prévus sur l'environnement, en résumant aussi des besoins en recherche. </jats:p