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. Motivation: Use on LCLS
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. Double Pinhole
Standard method of measuring coherence – based on Young’s double slit experiment
Produces visible interference assuming coherence and uniform intensity in region
source
Beamline
Optics
Area
Detector
CCD
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. NRA Images 100% Relative Intensity 12.7 % Relative Intensity

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

15. 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