Intérêts de recherche
Research Interests
Diffraction microscopy, the most important member of the "CDI family", is a two-step imaging technique involving (1) the measurment of the diffraction pattern of a specimen and (2) the reconstruction of the specimen's image from the diffracted intensities. As such, it can be seen as sitting in between the two traditional fields of x-ray microscopy and x-ray crystallography.
Up to now, the most successful applications of this method have occured using the highly coherent x-rays from synchrotrons (so-called 3rd generation x-ray sources). It is now possible to get images and 3D densities of inorganic specimens. In the recent past, I have been involved in one of the first reconstruction of a whole biological cell image (2D). High-resolution 3D maps of biological specimens is an objective that many are pursuing, and in which I am personally invoved.
La microscopie par diffraction ("diffraction microscopy"), membre important de la "famille CDI", est une technique d'imagerie à deux composantes distinctes: l'une, "réelle", consiste à mesurer la figure de diffraction d'un spécimen. La seconde composante, "virtuelle", est l'algorithme de reconstruction qui permet de générer l'image du spécimen à partir des données. Cette méthode se situe donc en quelque sorte entre la microscopie traditionnelle et la cristallographie.
Jusqu'à maintenant, les succès de cette méthode se rencontrent principalement dans le champ des rayons X. Les sources de rayonnement synchrotron modernes produisent des rayons X de grande brillance et hautement cohérents. Il est maintenant possible d'obtenir l'image et même la densité tri-dimensionnelle de structures inorganiques à l'aide de cette méthode. Il y a quelques années, j'ai participé à l'une des premières reconstructions d'un spécimen biologique. Reconstruire, grâce à la microscopie par diffraction, la densité tri-dimensionelle d'une cellule complète est un objectif poursuivi par plusieurs scientifiques et dans lequel je suis personnellement impliqué.
Introduced in electron microscopy by Hegerl and Hoppe in the early 1970s, "ptychography" is an imaging method that combines diffraction data from multiple datasets obtained by scanning a finite illumination on an extended specimen. It was recognized early on that this approach provides sufficient overdetermination to solve the phase problem. Yet efficient algorithms to do so appeared only a few years ago. In the last few years, my colleagues and I have developed the method further, and devised an improved reconstruction method that extracts from the data both the image of the specimen and the illumination function. An experimental demonstration with hard X-rays of this improved technique emphasized the ease and convenience of this "scanning diffraction microscopy" approach.
Iterative algorithms have proven very efficient tools to solve the difficult inverse problems that occur naturally in coherent diffractive imaging. A part of my work involves using the
L'utilisation d'algorithmes itératifs s'avère très efficace pour résoudre les problèmes inverses fréquemment rencontrés en CDI. Une partie de mes recherches est basée sur l'algorithme