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Soft X-ray Absorption and Coherence Analysis

Soft X-ray Absorption and Coherence Analysis. Gabriel Weil Stanford Synchrotron Radiation Laboratory Northwestern University. Outline. Intro to Coherence and LCLS Experiment Overview: Testing Non-Redundant Array Method Against Double Pinhole Data Raw Data: Images from CCD camera

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Soft X-ray Absorption and Coherence Analysis

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  1. Soft X-ray Absorption and Coherence Analysis Gabriel Weil Stanford Synchrotron Radiation Laboratory Northwestern University

  2. Outline • Intro to Coherence and LCLS • Experiment Overview: Testing Non-Redundant Array Method Against Double Pinhole Data • Raw Data: Images from CCD camera • Analysis: What to look for in the pictures • Results / Conclusions: Coherence Function • Aside: Absorption Experiment

  3. What is Coherence? • Temporal: how uniform is the frequency? – measured with monochromator • Spatial: measured / detected with Young’s double slit experiment – visibility of interference fringes source A. Schawlow, Sci. Am. 219, 120 (1968)

  4. Ultrafast Lensless Imaging Ultrafast Lensless Imaging Motivation: Use on LCLS Motivation: Use on LCLS Motivation: Use on LCLS • LCLS will produce single-shot pulses with unprecedented intensity and transverse coherence • Need to be able to measure transverse coherence function within timescale of pulse • Double pinhole method requires multiple measurements  not feasible on LCLS • Non-Redundant Array (NRA) method potentially solves this problem

  5. source Area Detector CCD Beamline Optics Double Pinhole • Standard method of measuring coherence – based on Young’s double slit experiment • Produces visible interference assuming coherence and uniform intensity in region 7 μm Photon Energy: 420 eV

  6. Double Pinhole Images

  7. Double Pinhole Images

  8. Non-Redundant Array (NRA) • Designed to provide complete coherence picture with a single measurement • Produces an observed interference pattern that represents the convolution ideal interference pattern with the spatial coherence function NRA Au mask with aperture separation distances from 0.5 to 12 µm and 900 nm thickness

  9. 100% Relative Intensity 12.7 % Relative Intensity 5.0 % Relative Intensity NRA Images

  10. Analysis • Double pinhole: • NRA: Image is convolution of ideal diffraction pattern with spatial coherence function – need to deconvolve to extract coherence • Convolution is multiplication in Fourier space:

  11. Results

  12. Results

  13. Absorption Experiment • Purpose was to determine cause of ‘leaking’ radiation • Samples were sputtered Au films of thickness typical to other 5-2 experiments • Tested against control evaporated Au films of comparable thickness

  14. Results • Some dependence on gold microstructure, but small compared to deviation from theoretical values • Due to size of deviation there are probably higher harmonics being detected – this needs further study

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