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XDL2011 Workshop 1

Diffraction Microscopy, Holography and Ptychography using Coherent Beams
Monday, June 6th - Tuesday, June 7th, 2011

Organizers: Janos Kirz (Lawrence Berkeley National Lab), Qun Shen (National Synchrotron Light Source II), & Darren Dale (Cornell University)

Workshop Agenda (html)
Workshop Poster (pdf)

Purpose: The purpose of the workshop is to assess the state-of-the-art in the use of coherent hard x-ray beams for high-resolution imaging. Diffraction microscopy, Fourier transform holography and ptychography are making rapid strides, but have yet to realize their full potential due to current limitations of spatial and temporal coherence of hard x-ray beams. We are especially interested in exploring what might be most feasible with a Energy Recovery Linac (ERL) and Ultimate Storage Ring (USR) x-ray sources.

Description: Coherence-based imaging experiments are limited mainly by the lack of coherent X-ray sources. Diffraction microscopy and related techniques have started to extend the resolution in X-ray microscopy beyond the limitations of X-ray focusing elements such as zone plates or multilayer Laue Lenses. Calculations indicate that these techniques should be limited only by radiation damage, and/or the diffraction limit. While using short pulse “flash” illumination at X-ray FELs can yield sufficient scattered intensity to image a sample with a single shot, the power in that shot vaporizes the specimen. Hence FELs are not ideal for imaging techniques requiring multiple exposures, for example ptychography and tomography. ERL and USR sources are being conceived to deliver intense, spatially coherent, hard X-ray beams with a quasi-continuous time structure. Sufficient temporal coherence may be provided by the undulator source directly, possibly further filtered using a monochromator. The ideal X-ray energy will depend on the sample, but in general soft X-rays offer higher scattering cross sections and higher coherent flux, but higher energies provide greater penetration through the specimen, and hence the Born approximation (generally assumed in the reconstruction process) is better satisfied. The increased penetration of harder x-rays will also allow greater emphasis on studies of samples in complex environments. The expected high coherent flux may provide the opportunity to study time-sequence series of dynamic phenomena.

For materials where the resolution is not limited by radiation damage, these techniques will provide new capabilities to analyze and visualize porosity, inclusions, defects, and other buried structures. These new imaging techniques can be used in combination with spectroscopy to create a sensitive tool highlighting elemental constituents. Work by Ian Robinson’s group has provided a unique new tool to investigate the morphology, strain fields and defects in nanocrystals. This approach makes use of the fine speckles around Bragg peaks, and requires the short wavelength and penetrating power of high-energy X-rays.

For biological samples, such as cells, tissue sections, etc., one can use cryogenic freezing to preserve the morphology of the sample while imaging with a resolution of about 10 nm. High-pressure cryogenic freezing techniques are under development at Cornell to prepare such specimens while minimizing structural perturbations due to the expansion of water on freezing.

Fourier transform holography techniques provide a quick and easy route to a reconstruction at a resolution given by the size of the source of the reference beam. Multiple reference beams, or uniformly redundant arrays have been used to increase the strength of the reference beam. Can this technique be extended to 3D?

Ptychography has been shown to provide a robust “engine” for 2D image reconstruction without the need for finite support. The first 3D reconstruction using this technique was recently demonstrated; the potential impact and limitations of extending this technique to 3D should be explored.

The premise of the workshop is that it is technologically feasible to extend the resolution in X-ray microscopy well beyond the limitations in X-ray optics, and optics-based instruments. The workshop will examine this premise from the standpoint of what might be needed to bring these techniques to routine use.

The workshop will bring together both practitioners of the various coherence-based imaging techniques and scientists from various fields using alternative techniques to analyze samples, to assess the scientific needs, to discuss the technical challenges, distill the best approaches, and to establish the requirements for a successful program.

Some of the technical challenges that need to be addressed could include:
  • Suppression of harmonics form the source
  • Dynamic range in the detector
  • Precision positioning, alignment and rotation of the sample
  • Efficient data handling, assembly and reconstruction algorithms
  • Pre-and post-exposure assessment of sample quality, suitability, and radiation damage
  • Understanding the effects of background from Compton scattering and other sources
  • Effects due to the missing data where either a beam-stop is used, or where the substrate interferes with complete 360 degree tomographic data collection
  • In the case of diffraction microscopy, determining the finite support, and minimizing the effect of the specimen mount

Workshop contributions: Presentations will focus on the issues of imaging using coherent hard X-ray beams, and on the advantages and disadvantages of the ERL and USR hard X-ray sources for such experiments.

Venue and Program: The workshop will occur at Cornell’s Robert Purcell Center on June 6-7, 2011 and is open to anyone who may be interested. The workshop will start with a short overview of ERL and USR specifications and capabilities for frontier science with coherent hard x-ray beams (see accompanying attachment for some ERL/USR background). The program, outlined below, will then continue with a core group of invited world-leaders to give short talks and lead the discussion in the relevant science areas. In order to allow time for discussion after each talk, speakers are asked to rigorously adhere to the 20 minute time limit. There will be ample additional time for open discussion and poster sessions. Workshop participants are especially encouraged to be inventive and explore unorthodox ideas.

Poster Session: There will be space to put up poster of up to 4’(height) x 6’(width) [1.2 x 1.8 meters]. If you plan to have a poster, please submit a ˝ page abstract to Laura Houghton at and she will try to compile them into the program. There is a specific poster session time shown in the program, but they can remain up for the entire duration of the workshop.

Workshop Results: Our goal is to communicate the possibilities for science with an ERL or USR, as well as with other sources, and engage the community in developing ideas for the science case. Imaginative thinking will be required -- that's why we are assembling this workshop group! Documenting the results of the workshop is essential. We hope to explore compelling science that is uniquely enabled by an ERL or USR. Accordingly, each invited participant will be asked to present at least one novel experiment of interest (described by a 1-3 paragraphs of text, an appropriate graphic, and relevant references) that would be very difficult to perform without the capabilities of an ERL or USR. Time will be provided towards the end of the workshop to summarize results and conclusions.

Of particular importance is the group discussion at the end of the meeting. It is therefore very important that all invited workshop attendees stay through to the end of the workshop. It is in this period that our discussion leaders will try to pull all the good ideas generated into a summary and we need your very active participation to do this well. As a motivator to stay through to the end and to further the development of “community” for this brief several day period, we have arranged a series of excursions. Depending on the workshop these include, for example, Cayuga Lake Boat dinner cruises, local winery tours, visits to Ithaca’s local spectacular gorge parks, etc.