Light Sources

Soft X-ray light sources

Light Sources

Ulrike Frühling

Bad Honnef 2014


Wave length range

VUV - Soft X-Ray200nm - 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

• access to deeply bound electron shells

• high photo-absorption cross section

• high temporal resolution


Relevant time scales


Relevant time scales

Pulse duration needs to be short compared to the studied dynamics.

long pulse blured

short pulse sharp


Variable delay

Probe pulse

Pump pulse

t Sample

DetectorPump Probe experiment

M. Drescher Z. Phys. Chem. 218, 1147-1168 (2004).We need two short, well synchronized light pulses


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


Synchrotron radiatonESRF


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


Synchrotron radiatonSynchrotron 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


• Superimpose ps electron bunch with fs laser pulse to modulate the electron energy.

• Use only the modulated electrons for synchrotron radiation

fs Synchrotron Pulses - Slicing

S. Kahn et al., PRL 97, 074801 (2006).



S. Kahn et al., PRL 97, 074801 (2006).



Intensity is reduced by 10-4

Pulse duration: 100 fsPhoton energy: 300 – 1400 eVSources available at Bessy, PSI

S. Kahn et al., PRL 97, 074801 (2006).

Energy modulation


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 durationTime resolved experiments

•Repetition rate: few Hz to kHz


FEL Experiments

A.A Sorokin et al., PRL 99, 213002 (2007).

= 13.3 nm (93 eV)focus: 2.6 m (f =200 mm)E = 1012 – 10 16 W cm-2

Xe21+57 photons

Photoeffect at ultra high intensities


• Proposed facilities and facilities under construction not listed

DESYFLASH > 7 nm

SLACLCLS > 0.12 nm

SPring-8SCSS-TA > 40 nmSACLA > 0.1 nmElettra

FERMI > 40 nm

VUV/Soft X-ray FELs


Free-electron laser

Linear accelerator highly compressed, well defined electron bunch

Long undulator several 10 m)


Free-electron laserSASE-self amplified spontaneous emission

Spontaneous undulator emission



Energy modulation of electrons in the copropagating light field



Energy modulation leads to increasing density modulation of the electron bunch (microbunching)

Bunch period: coherent emission

P Ne2


SASE FEL propertiesSASE-self amplified spontaneous emission

No oscillator fluctuation of spectrum, pulse shape, pulse-energySolution: single shot measurement of all beam parameters + sorting of

experimental data




experimental data

FLASH single shot spectra

Average FWHM-width: 1,7%

FLASH Pulse energy




experimental data

FLASH pulse duration

Average FWHM-duration: 35 fs

FLASH Pulse shape (simulated)

0 200 400 600 8000

10

20

30

shot number

rms

puls

e du

ratio

n (f

s) = 13.7 nm

(fs)10 20 30 40 50

0

2

4

6

8

10

12

(GW

)P

t


200 µm

Optical laser: 400 nm, 130 fs

FLASH: 28 nm, 25 fs

CCD

GaAs

Single shot time delay measurement Intense XUV radiation changes reflectivity for optical laser

Synchronization


t (ps)

0

1

2

3

4

5

6

Nominal delay stage setting (ps)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 1.8 2.2 2.31.7

Delayscan over temporal window of 2.3 ps

Alternative methods:Electro-optical samplingSidebands

T. Maltezopoulos et al., New Journ. Phys. 10, 033026 (2008).


Jitter-compensated ion signal

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Xe3+ ion yield

Delay time (ps)-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Delay time (ps)

Xe4+ ion yield

Red curve – expected results with nominal XUV and laser parameters

sorted with timingexperiment

delay scan


FEL Seeding schemes

e.g. High-Harmonic Generation (HHG)

Wavelength record: 38 nm (FLASH)

Low seed powerDifficult Synchronization

Direct seeded FEL (amplifier mode)

High-gain harmonic generation (HGHG) HGHG-cascade

Wavelength record: 20 nm (FERMI)

Wavelength record: 4 nm (FERMI)


FEL Seeding schemesSelf-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 experimentalUser operation only at Fermi (20-65 nm)


High-harmonic generation

atomic gas target

Sphericalmirror

fs nir-laser


High-harmonic generation

tunnel ionization acceleration inthe laser field

E~ Ip

h

recombinationand photoemission

E

t1

2

3x

x

x

step 1 step 2

step 3

kin + E

“Three-step model” Kheldysh et.al.

Gasatom

“Femtosecond x-ray science”, T. Pfeifer, C. Spielmann and G. Gerber, Rep. Prog. Phys. 69 (2006) 443–505


High-harmonic generationHHG-Spectrum

• Ecutoff= Ip+3Up

Up = e2E02/(4me2)~ I

• Pulse-duration is determined by the driving laser (fs to as).

• Pulse energy: J (VUV) nJ (<100 nm)

• Perfect XUV/laser synchronization

• Laser like XUV pulses


HHG setup

Laser: 800 nm, 25 fs, 2 mJ/pulse XUV: 13.5 nm (higher harmonics generation)

B. Schütte PhD-Thesis (2012)


Generation of as-pulsesCarrier envelope phase (CEP)

A. Baltuska et al., Nature 421, 611 (2003).


Light field driven streak-camera

R. Kienberger et al., Nature 427, 817 (2004).

resolution: < 100 as

IXUV(t) Ie(p) Ie(E)IXUV(t) Ie(p) Ie(E)

XUV pulse

Atoms

Electrons

Electron energy detector

IR light field


Light field driven streak-camera

R. Kienberger et al., Nature 427, 817 (2004).

time

el. fi

eld

stre

ngth

/ v

ecto

r pot

entia

l A electron m

omentum

change

electron-momentumdistribution

I(Ekin)

p(t) = e A(t)

XUV wave packet|(t)|2

XUV pulse

Atoms

Electrons

Electron energy detector

IR light field


Streaking with visible light

E. Goulielmakis et al., Science 305, 1267 (2004).Kienberger et al., Nature 427, 817 – 821 (2004).


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


Thank you

Light Sources

Documents