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Light Sources

Light Sources. Ulrike Frühling Bad Honnef 2014. Wave length range. VUV - Soft X-Ray 200nm - 0.1nm 6 eV – 1.2 keV. Wave length range. Advantages of VUV – Soft X-ray radiation selective single photon ionization/excitation weak fields  perturbation of molecular orbitals avoided

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Light Sources

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  1. Light Sources Ulrike Frühling Bad Honnef 2014

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

  3. 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

  4. Relevant time scales

  5. Relevant time scales Pulse duration needs to be short compared to the studied dynamics. short pulse sharp long pulse blured

  6. Pump Probe experiment Detector Pump pulse Dt Sample Variable delay Probe pulse M. Drescher Z. Phys. Chem. 218, 1147-1168 (2004). We need two short, well synchronized light pulses

  7. 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

  8. Synchrotron radiaton ESRF

  9. 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

  10. 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

  11. 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).

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

  13. 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).

  14. 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

  15. 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).

  16. VUV/Soft X-ray FELs SPring-8 SCSS-TA l > 40 nm SACLA l > 0.1 nm • Proposed facilities and facilities under construction not listed SLAC LCLS l > 0.12 nm DESY FLASH l > 7 nm Elettra FERMI l > 40 nm

  17. Free-electron laser Linear accelerator highly compressed, well defined electron bunch Long undulator several 10 m)

  18. Free-electron laser SASE-self amplified spontaneous emission Spontaneous undulator emission

  19. Free-electron laser SASE-self amplified spontaneous emission Energy modulation of electrons in the copropagating light field

  20. 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

  21. 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

  22. 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%

  23. 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 l = 13.7 nm FLASH Pulse shape (simulated) FLASH pulse duration Average FWHM-duration: 35 fs

  24. 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

  25. 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 Nominal delay stage setting (ps) T. Maltezopoulos et al., New Journ. Phys. 10, 033026 (2008). t (ps) Alternative methods: Electro-optical sampling Sidebands

  26. sorted with timing experiment Jitter-compensated ion signal delay scan Red curve – expected results with nominal XUV and laser parameters

  27. 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)

  28. 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)

  29. High-harmonic generation Spherical mirror fs nir-laser atomic gas target

  30. 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) 443–505

  31. 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

  32. Laser: 800 nm, 25 fs, 2 mJ/pulse XUV: 13.5 nm (higher harmonics generation) HHG setup B. Schütte PhD-Thesis (2012)

  33. Generation of as-pulses Carrier envelope phase (CEP) A. Baltuska et al., Nature 421, 611 (2003).

  34. 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).

  35. Light field driven streak-camera Dp(t) = e A(t) Electron energy detector IR light field electron momentum change XUV pulse Electrons el. field strength / vector potential A Atoms electron-momentum distribution I(Ekin) time XUV wave packet |Y(t)|2 R. Kienberger et al., Nature 427, 817 (2004).

  36. Streaking with visible light E. Goulielmakis et al., Science 305, 1267 (2004). Kienberger et al., Nature 427, 817 – 821 (2004).

  37. Sources for ultra short XUV pulses

  38. Thank you

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