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 Analysis: What to look for in the pictures Results / Conclusions: Coherence Function Aside: Absorption Experiment

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

A. Schawlow, Sci. Am. 219, 120 (1968)

source

Motivation: Use on LCLSUltrafast Lensless Imaging Motivation: Use on LCLSUltrafast Lensless Imaging Motivation: Use on LCLS LCLS will produce single-shot pulses with LCLS will produce single-shot pulses with

unprecedented intensity and transverse coherenceunprecedented intensity and transverse coherence Need to be able to measure transverse coherence Need to be able to measure transverse coherence

function within timescale of pulsefunction within timescale of pulse Double pinhole method requires multiple Double pinhole method requires multiple

measurements measurements not feasible on LCLS not feasible on LCLS Non-Redundant Array (NRA) method potentially Non-Redundant Array (NRA) method potentially

solves this problemsolves this problem

Double Pinhole Standard method of measuring coherence –

based on Young’s double slit experiment Produces visible interference assuming coherence

and uniform intensity in regionsource

Beamline Optics

AreaDetector

CCD

7 μm

Photon Energy: 420 eV

Double Pinhole Images

Double Pinhole Images

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

NRA Images

100% Relative Intensity

12.7 % Relative Intensity

5.0 % Relative Intensity

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:

minmax

minmax

II

IIV

dtCItCI )()())(*(

ResultsVisibility vs. Distance at three aperture settings

(Percentage of Maximum Intensity Setting)

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8

Pinhole Seperation (μm)

Fri

ng

e V

isib

ilit

y

5.0% Io

12.7% Io

2.4 % Io

100% Io

ResultsVisibility vs Distance at Undulator Peak and Blue

Edge

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8

Pinhole speration (μm)

Vis

ibil

ity

Blue Edge

Undulator Peak

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

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