Soft X-ray light sources
Light Sources
Ulrike Frühling
Bad Honnef 2014
Soft X-ray light sources
Wave length range
VUV - Soft X-Ray
200nm - 0.1nm
6 eV – 1.2 keV
Soft X-ray light sources
Wave length range
Advantages of VUV – Soft X-ray radiation
•
selective single photon ionization/excitation
•
weak fields  perturbation of molecular orbitals avoided
•
access to deeply bound electron shells
•
high photo-absorption cross section
•
high temporal resolution
Soft X-ray light sources
Relevant time scales
Soft X-ray light sources
Relevant time scales
Pulse duration needs to be short compared to the studied dynamics.
long pulse
blured
short pulse
sharp
Soft X-ray light sources
Pump Probe experiment
Pump pulse
Dt
Sample
Variable delay
Probe pulse
M. Drescher
We need two short, well synchronized light
pulses Z. Phys. Chem. 218, 1147-1168 (2004).
Soft X-ray light sources
Brilliance
Brilliance: Photons / (sec·mrad2·mm2·0.1%bw)
•Peak brightness: within a pulse
•Often used to compare light sources, but need
to consider the requirements of specific
experiments.
• Can take data over many pulses? average
brightness
•Nonlinear experiments, or experiments where
the target is destroyed by each pulse “peak”
brilliance
HHG
Soft X-ray light sources
Synchrotron radiaton
ESRF
Soft X-ray light sources
Synchrotron radiaton
Petra III Undulator
-
Sinusoidal electron trajectory in the undulator
Emission of Radiation at every bend
Coherent superposition of light pulses emitted at consecutive bends leads to
highly brilliant beam
Wavelength tunable by changing the undulator gap
Soft X-ray light sources
Synchrotron radiaton
Synchrotron radiaton sources
•
•
•
•
Photonenergy: VUV to hard X-Rays (few eV to 100 keV)
High repetition rate (MHz)
Tuneable wavelenght, good spectral resolution (with monochromator)
Pulseduration: tens to >100 ps
Soft X-ray light sources
fs Synchrotron Pulses - Slicing
• Superimpose ps electron bunch with fs laser pulse to
modulate the electron energy.
• Use only the modulated electrons for synchrotron radiation
S. Kahn et al., PRL 97, 074801 (2006).
Soft X-ray light sources
fs Synchrotron Pulses - Slicing
S. Kahn et al., PRL 97, 074801 (2006).
Soft X-ray light sources
fs Synchrotron Pulses - Slicing
Energy modulation
Intensity is reduced by 10-4
Pulse duration: 100 fs
Photon energy: 300 – 1400 eV
Sources available at Bessy, PSI
S. Kahn et al., PRL 97, 074801 (2006).
Soft X-ray light sources
Free-electron laser
Free-electron laser
• >106 higher irradiance than
synchrotrons
• XUV: Emax ~ 1016Wcm-2 (FLASH)
• X-ray: Emax ~ 1018Wcm-2 (LCLS)
 Sources for multi-photon processes in
the XUV/X-ray range
• fs pulse duration
 Time resolved experiments
• Repetition rate: few Hz to kHz
Soft X-ray light sources
FEL Experiments
Photoeffect at ultra high intensities
l = 13.3 nm (93 eV)
focus: 2.6 mm (f =200 mm)
E = 1012 – 10 16 W cm-2
Xe21+57 photons
A.A Sorokin et al., PRL 99, 213002 (2007).
Soft X-ray light sources
VUV/Soft X-ray FELs
SLAC
LCLS
l > 0.12 nm
DESY
FLASH
l > 7 nm
Elettra
FERMI
l > 40 nm
SPring-8
SCSS-TA
l > 40 nm
SACLA
l > 0.1 nm
Soft X-ray light sources
Free-electron laser
Linear accelerator highly compressed,
well defined electron bunch
Long undulator
several 10 m)
Soft X-ray light sources
Free-electron laser
SASE-self amplified spontaneous emission
Spontaneous
undulator
emission
Soft X-ray light sources
Free-electron laser
SASE-self amplified spontaneous emission
Energy modulation of
electrons in the
copropagating light
field
Soft X-ray light sources
Free-electron laser
SASE-self amplified spontaneous emission
Energy modulation leads
to increasing density
modulation of the
electron bunch
(microbunching)
Bunch period: l
 coherent emission
 P  Ne2
Soft X-ray light sources
SASE FEL properties
SASE-self amplified spontaneous emission
No oscillator  fluctuation of spectrum, pulse shape, pulse-energy
Solution: single shot measurement of all beam parameters + sorting of
experimental data
Soft X-ray light sources
SASE FEL properties
SASE-self amplified spontaneous emission
No oscillator  fluctuation of spectrum, pulse shape, pulse-energy
Solution: single shot measurement of all beam parameters + sorting of
experimental data
FLASH single shot spectra
Average FWHM-width: 1,7%
FLASH Pulse energy
Soft X-ray light sources
SASE FEL properties
SASE-self amplified spontaneous emission
No oscillator  fluctuation of spectrum, pulse shape, pulse-energy
Solution: single shot measurement of all beam parameters + sorting of
experimental data
FLASH Pulse shape (simulated)
FLASH pulse duration
l = 13.7 nm
12
10
20
8
P (GW)
rms pulse duration (fs)
30
10
6
4
2
0
0
0
200
400
600
shot number
Average FWHM-duration: 35 fs
800
10
20
30
t (fs)
40
50
Soft X-ray light sources
Synchronization
Single shot time delay measurement
Intense XUV radiation
changes reflectivity for
optical laser
200 µm
GaAs
FLASH:
28 nm, 25 fs
Optical laser:
400 nm, 130 fs
Soft X-ray light sources
Delayscan over temporal window of 2.3 ps
0
1
2
3
4
5
6
0.0 0.1 0.2 0.3 0.6 0.7 0.8 0.9 1.1 1.2 1.3 1.4 1.5 1.7 1.8 2.2 2.3
t (ps)
Nominal delay stage setting (ps)
T. Maltezopoulos et al., New Journ.
Phys. 10, 033026 (2008).
Alternative methods:
Electro-optical sampling
Sidebands
Soft X-ray light sources
Jitter-compensated ion signal
1.2
1.2
3+
Xe ion yield
1.0
1.0
0.8
0.8
0.6
0.4
0.2
sorted
with timing
experiment
0.6
0.4
delay scan
0.0
-0.2
-1.5
4+
Xe ion yield
0.2
0.0
-1.0
-0.5
0.0
0.5
1.0
Delay time (ps)
1.5
-0.2
2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Delay time (ps)
Red curve – expected results with nominal XUV and laser parameters
1.5
2.0
Soft X-ray light sources
FEL Seeding schemes
Direct seeded FEL (amplifier mode)
e.g. High-Harmonic Generation (HHG)
Wavelength record: 38 nm (FLASH)
Low seed power
Difficult Synchronization
High-gain harmonic generation (HGHG)
HGHG-cascade
Wavelength record: 4 nm (FERMI
Wavelength record: 20 nm (FERMI)
Soft X-ray light sources
FEL Seeding schemes
Self-Seeding
SASE
Wavelength record: 0.12 nm (LCLS)
- no external seed difficulties
- no direct control over pulse length, chirp,
synchronization, etc…
Most seeding projects are still experimental
User operation only at Fermi (20-65 nm)
Soft X-ray light sources
High-harmonic generation
Spherical
mirror
fs nir-laser
atomic gas target
Soft X-ray light sources
High-harmonic generation
step 1
“Three-step model”
Kheldysh et.al.
step 2
ac c eleration in
the laser field
tunnel ionization
h
Gas
atom
step 3
E
x2
E ~ Ip + Ekin
rec ombination
and photoem ission
x3
x1
“Femtosecond x-ray science”, T. Pfeifer, C. Spielmann and G. Gerber, Rep. Prog. Phys. 69 (2006) 443–505
t
Soft X-ray light sources
High-harmonic generation
HHG-Spectrum
• Ecutoff= Ip+3Up
Up = e2E02/(4mew2)~ Il2
• Pulse-duration is determined by
the driving laser (fs to as).
• Pulse energy: mJ (VUV)
nJ (<100 nm)
• Perfect XUV/laser synchronization
• Laser like XUV pulses
Soft X-ray light sources
HHG setup
Laser: 800 nm, 25 fs, 2 mJ/pulse
XUV: 13.5 nm (higher harmonics generation)
B. Schütte PhD-Thesis (2012)
Soft X-ray light sources
Generation of as-pulses
Carrier envelope phase (CEP)
A. Baltuska et al., Nature 421, 611 (2003).
Soft X-ray light sources
Light field driven streak-camera
Electron energy
detector
IR light field
XUV pulse
Electrons
Atoms
IXUV(t)  Ie(p)  Ie(E)
resolution: < 100 as
R. Kienberger et al., Nature 427, 817 (2004).
Soft X-ray light sources
IR light field
XUV pulse
Electrons
Atoms
Dp(t) = e A(t)
electron
momentum change
Electron
energy
detector
el. field strength / vector potential A
Light field driven streak-camera
electron-momentum
distribution
I(Ekin)
XUV wave packet
|Y(t)|2
R. Kienberger et al., Nature 427, 817 (2004).
time
Soft X-ray light sources
Streaking with visible light
E. Goulielmakis et al., Science 305, 1267 (2004).
Kienberger et al., Nature 427, 817 – 821 (2004).
Soft X-ray light sources
Sources for ultra short XUV pulses
Pulse duration (fs)
Photon energy (eV)
Light flux
(photons/s)
High harmonics
0.2 – 100
10 – 500
108-1011
Laser plasma
> 300
10 – 10 000
106-1012
Synchrotron
> 10 000
0 – 100 000
1010-1013
Synchrotron +
slicing
100 – 200
500 – 8000
108
Free-electron
laser
10-300
10 – 10 000
1016-1018
Soft X-ray light sources
Thank you