Ph.D. Candidate: Yanwen Sun
Research Advisor: Mike Dunne
Date: Wednesday, June 5, 2019
Time: 10 am
Location: SLAC B48 Redwood C/D Conference Room
Title: Development of two-pulse X-ray photon correlation spectroscopy at LCLS: new window of opportunity to probe atomic scale dynamics in condensed matter
Speckle patterns typically originate from the phase shifts introduced to the scattering of coherent illumination by the randomly distributed scatterers of a material. X-ray photon correlation spectroscopy (XPCS) directly measures the time correlation function of material inhomogeneity at nanometer and Angstrom length scale by studying the intensity fluctuations in X-ray speckle patterns. This technique is routinely performed with synchrotron light sources and has had wide applications in studying the dynamics of amorphous materials and disordered system. It typically probes dynamics on timescales from milliseconds to seconds or longer, limited at by the available coherent flux and/or, in many cases, by the performance of suitable X-ray imaging detectors.
The advent of X-ray free electron facilities (FELs) opens up a new window of opportunity in extending X-ray photon correlation spectroscopy (XPCS) to the atomic time scale with its unprecedented brightness, nearly full transverse coherence, and the femtosecond pulse durations. Two-pulse XPCS was proposed decades ago as a corresponding technique at X-ray FELs to extract dynamics information from the sample of interest by examine the speckle visibility of a summed-speckle pattern as a function of the temporal separation of the two coherent femtosecond pulses. This relies on the generation of two near-identical X-ray pulses with a well controlled time separation, i.e femto- to nanosecond. Depending on the timescale needed, the two X-ray pulses can either originate (i) from accelerator-based techniques using two electron bunches in adjacent RF buckets, or (ii) X-ray beam splitters and delay lines that split a single X-ray FEL pulse into two, introduce a path length difference in between, recombine the two pulses colinearly: the so-called X-ray split-delay optics. My thesis focuses on the development of two pulse XPCS techniques at the Linac Coherent Light Source. I will first talk about my work in building X-ray characterization tools, designing and performing a proof-of-principle experiment using the accelerator-based two-pulse method. In the second part, I will talk about the challenges of designing and constructing split-delay optics for X-ray FEL facilities, and show you my work in designing, building, and testing an ultrastable split-delay system based on a novel self-error-compensating optical layout. Finally, I will introduce my efforts towards building a robust analysis framework for extracting dynamics from very low intensity speckle patterns, as well as future experimental applications.