LANS Informal Seminar: Stefano Marchesini

Seminar Title: Phase retrieval in high dimensions
Speaker: Stefano Marchesini, Staff Scientist, Advanced Light Source Experimental System Group, Berkeley National Laboratory

Date/Time: 2015-08-26 15:00
Location: Building 240, Room 1404/1405

Description:
The ability to reconstruct the structure of a sample from its
diffraction pattern has been a standing issue for over 100 year.
Experiments in x-ray science typically involve a scattering process,
whereby x-rays are scattered, absorbed or slowed down as they encounter
an object, and we only measure their distribution on a two-dimensional
detector. Knowing the direction (or the phase of the wavefront for
coherent light) from which every x-ray came would enable us to propagate
back to the sample and provide an image, but measuring this property is
a difficult task. The phase retrieval problem can be made tractable by
recording multiple diffraction patterns from the same region of the
object, compensating phase-less information with a redundant set of
measurements. One practical way (called ptychography) is to combine
diffraction measurements with microscopy nanoscale position control.
Diffraction measurements from neighbouring regions are related to each
other by the illumination geometry. This motivates us to consider
augmented projection methods and synchronization strategies aimed at
organizing local information in a global way to handle the big imaging
data in the coming new light source era.