Light Sources

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Light Sources

• Ulrike Frühling
• Bad Honnef 2014

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Wave length range
VUV - Soft X-Ray 200nm - 0.1nm 6 eV 1.2 keV
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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

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Relevant time scales
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Relevant time scales
Pulse duration needs to be short compared to the
studied dynamics.
short pulse ?sharp
long pulse ?blured
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Pump Probe experiment
We need two short, well synchronized light pulses
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Brilliance
Brilliance Photons / (secmrad2mm20.1bw) 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
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Synchrotron radiaton
ESRF
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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

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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 gt100 ps

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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).
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fs Synchrotron Pulses - Slicing
S. Kahn et al., PRL 97, 074801 (2006).
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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).
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Free-electron laser

• Free-electron laser
• gt106 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

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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).
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VUV/Soft X-ray FELs

• SPring-8
• SCSS-TA
• l gt 40 nm
• SACLA
• l gt 0.1 nm

• Proposed facilities and facilities under
  construction not listed

• SLAC
• LCLS
• l gt 0.12 nm

• DESY
• FLASH
• l gt 7 nm

• Elettra
• FERMI
• l gt 40 nm

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Free-electron laser
Linear accelerator ?highly compressed, well
defined electron bunch
Long undulator several 10 m)
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Free-electron laser
SASE-self amplified spontaneous emission
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Free-electron laser
SASE-self amplified spontaneous emission
Energy modulation of electrons in the
copropagating light field
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Free-electron laser
SASE-self amplified spontaneous emission
Bunch period l ? coherent emission ? P ? Ne2
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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

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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 energy
FLASH single shot spectra
Average FWHM-width 1,7
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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
Average FWHM-duration 35 fs
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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
CCD
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Delayscan over temporal window of 2.3 ps
T. Maltezopoulos et al., New Journ. Phys. 10,
033026 (2008).
Alternative methods Electro-optical
sampling Sidebands
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Jitter-compensated ion signal
delay scan
Red curve expected results with nominal XUV and
laser parameters
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FEL Seeding schemes
Direct seeded FEL (amplifier mode)
e.g. High-Harmonic Generation (HHG)
Low seed power Difficult Synchronization
Wavelength record 38 nm (FLASH)
High-gain harmonic generation (HGHG)
HGHG-cascade
Wavelength record 4 nm (FERMI)
Wavelength record 20 nm (FERMI)
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FEL Seeding schemes
Self-Seeding
SASE

• no external seed difficulties
• no direct control over pulse length, chirp,
  synchronization, etc

Wavelength record 0.12 nm (LCLS)
Most seeding projects are still experimental User
operation only at Fermi (20-65 nm)
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High-harmonic generation
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High-harmonic generation
Three-step model Kheldysh et.al.
Gas atom
Femtosecond x-ray science, T. Pfeifer, C.
Spielmann and G. Gerber, Rep. Prog. Phys. 69
(2006) 443505
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High-harmonic generation
HHG-Spectrum

• Ecutoff Ip3Up
• Up e2E02/(4mew2) Il2
• Pulse-duration is determined by the driving laser
  (fs to as).
• Pulse energy mJ (VUV)
• nJ (lt100 nm)
• Perfect XUV/laser synchronization
• Laser like XUV pulses

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HHG setup
B. Schütte PhD-Thesis (2012)
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Generation of as-pulses
Carrier envelope phase (CEP)
A. Baltuska et al., Nature 421, 611 (2003).
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Light field driven streak-camera
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Light field driven streak-camera
R. Kienberger et al., Nature 427, 817 (2004).
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Streaking with visible light
E. Goulielmakis et al., Science 305, 1267
(2004). Kienberger et al., Nature 427, 817 821
(2004).
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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 gt 300 10 10 000 106-1012
Synchrotron gt 10 000 0 100 000 1010-1013
Synchrotron slicing 100 200 500 8000 108
Free-electron laser 10-300 10 10 000 1016-1018
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Thank you