ultrafast laser excitation of solidswolf/femtoweb/... · ultrafast x-ray spectroscopy femtosecond...
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electron-elecron-scattering:Te-h >> TL
electron-phononscattering:Te-h↓, TL↑
phonon-phononscattering
Te-h ≈ TL
time [s]
10-15
10-14
10-13
10-12
10-11
10-10
10-9
non-thermal
thermalheat conduction
(thermal) melting
ablation
sample
fs-laser pluse
VB
CB
Ultrafast laser excitation of solids
semiconductor
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Characteristic time scales:
• Electronic excitation: τL• Thermalization: τR• Energy transport: τE• Mass transport: τM
τL << τR << τE < τM
• Spatial and temporal localization of energy deposition:
• Non-equilibrium• Impulsive heating at constant volume• High temperature and high pressure phases of matter:
„warm dense matter“
Ultrafast laser excitation of solids II
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Ultrafast x-ray spectroscopy
Femtosecond time scale:
Changes of nuclear configuration• chemical reactions• vibrations• structural changes
x-ray radiation:
Spatial structure ofatomic configuration
• structure of matter
Ultrafast x-ray spectroscopy: atomic resolution - temporal and spatial
Δx
Δt
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Sources for ultrafast x-ray pulses(how to realize ultrafast x-ray diffraction…)1. accelerator based sources:
slicing source, SPPS, LCLS, TESLA X-FEL, LUX, …
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Laser: Ti:Sa, 120 fs, 150 mJ Target: moving Titanium wire
wire
wheel
plasma
2. fs-laser plasma: “femtosecond x-ray tube”
fs-laser: ~100fs, 10 … 100 mJΙ ≈ 1016 … 1018 W/cm2
Ti wire
Rousse et al., Phys. Rev. E 50, 2200 (1994)
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Time-resolved x-ray diffraction (TXRD)
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Diffraction and focussing of keV x-rax radiation
Si (111)
50 … 200 nmGe, Bi (111)
Ge Si
80μm
≈ 3 × 104 photons/pulse
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x-ray diffraction data
Ge Si
CCD camera imageIntegration
Rocking Curve
different lattice constants of Ge and Si
integratedreflectivity
Dispersion
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x-ray diffraction experiment…
Titanime wire
lead shielding
x-ray mirror sample onx-y-Θ manipulator
CCD
Debrie protection
fs pump pulse
fs pulsefor plasma
kα radiation
AG von der Linde, University Essen
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Ultrafast non-thermal melting:
Laser
SampleSilicon
K. Sokolowski-Tinten et al., PRB 51, 14186 (1995), ibid. 58, R11805 (1998),
optical spectroscopyelectronic excitation
non-thermal melting
ablation
0.47 J/cm2
R(λ) of solid Si
liquid
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Ultrafast non-thermal melting:
> 10 % of all valence electrons!
K. Sokolowski-Tinten et al., PRB 61, 2643 (2001)
Intense electronic excitation
P. Stampfli et al., PRB 49, 7299 (1994)
lattice instability
Use ultrafast melting as test case for time-resolved x-ray diffraction
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Non-thermal und thermal melting and subsequent re-crystallization
≈ 300 fs
K.Sokolowski-Tinten et al., PRL 87, 225701 (2001)
170 nm Ge on Si; (111)-diffraction spot
800 m/s
X-ray diffraction: Ultrafast melting of Ge
Non-thermal meltingNon-thermal und thermal melting
Si (111)
170 nmGe, Bi (111)
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Analysis of x-ray pulse duration
0.8
0.9
1.0
-0.4 0.0 0.4 0.8
X-ray diffraction
Delay Time [ps]
Ge 0.2 J/cm2
0.8
0.9
1.0
-0.4 0.0 0.4 0.8
X-ray diffraction"melting"
Delay Time [ps]
Ge 0.2 J/cm2
0.8
0.9
1.0
-0.4 0.0 0.4 0.8
X-ray diffraction"melting"250 fs
Delay Time [ps]
Ge 0.2 J/cm2
0.8
0.9
1.0
-0.4 0.0 0.4 0.8
X-ray diffraction"melting"250 fs350 fs
Delay Time [ps]
Ge 0.2 J/cm2
τX = (300 ± 50) fs
for analysis phase transition is assumed as “instantaneous”
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• Bi is a semimetal
• rhombohedral structure:- small displacement from fcc lattice- two atom basis
Lattice dynamics in Bismut
a
Bi-Bi distance a (0.468 × diagonal c)stabilized by Peierls-Jones mechanism
A1g-Phonon (V = const.)
• Bi is a semimetal
• rhombohedral structure:- small displacement from fcc lattice- two atom basis
• Excitation of coherent optical phonons(A1g-mode) with fs-laser pulses- Zeiger et al., PRB 45, 768 (1992)- DeCamp et al., PRB 64, 92301 (2001)- Hase et al., PRL 88, 67401 (2002)
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Displacive Excitation of Coherent Phonons
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geometric structur factor of Bi
→ decrease & oscillationof (111)-diffraction spot
→ increase & oscillationog (222)-diffraction spot
⏐S(hkl)⏐2
a
initialequilibrium position a0
displacedquasi equilibrium position a0‘
a/c = 0.5
(222)
(111)
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A1g optical mode:νobs = 2.14 THz (470 fs)ν0 = 2.92 THz (342 fs)
Bi 50nm on Si, F ≈ 6 mJ/cm2
Coherent optical phonons
A1g optical mode:νobs = 2.14 THz (470 fs) softening & anharmonicity
Δa ~ 0,15 – 0,25 Å
DECP
K. Sokolowski-Tinten et al., Nature 422, 287 (2003)
⏐S(hkl)⏐2
a
(222)
(111)
a0‘a0
7%
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High excitation density: Bi (111) and (222)
aperiodic but coherentgowth Bi-Bi distance: 0.4 Å
followed bymelting
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Coherent phonons ⇔ phase transitionChange of atomic vibrations: periodic ⇒ aperiodic ?
Bi (111): higher excitation density
≈ 1.1 ps
softening of phonons precursor for melting
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Phonon dynamics
coher. optical phonons incoh. phonons:“heat”
coher. acoustic phonons: lattice expansion
Debey-Waller Eff.:
)/exp( 22234
hklduDW π−=
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Summary II
BiTime-resolved probe of structural dynamics
Time-resolved x-ray diffraction
Femtosecond laser spectroscopy
Non-thermal melting and coherent phonon excitation
Time-resolved photoelectron spectroscopy
Questions ?
Application to femtochemistry
IntroductionWhy femtosecond laser pulses?
Example: photochemistry of vision
Generation of femtosecond laser pulses
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+-
fs-laser pulse
metal surface
Electron thermalization dynamics in metals and the Two-Temperature Model
Electron dynamics in metals following optical excitation
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time-resolved photoemission spectroscopy
probe: 6 eV, 90 fs
E-EF = Ekin + Φ - hνprobe
pump: 1.5 eV, 55 fs
probe:6 eV, 90 fs
probe pulse polarization
p-polarized bulk + surface
probe pulse polarization
s-polarized bulk
Gd/W(110) film preparationAspelmeier et al.,
JMMM 132, 22 (1994)
dz2
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electronic structure of the Gd(0001) surface
T=80 K
Kurz et al., J. Phys. Cond. Mat. 14, 6353 (2002)
surface plane
[000
1]
LDOS
charge densitydifference plot
5dz2
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time-resolved photoemission of Gd(0001) bulk
Δt >100 fs: thermalized electron distribution function in Gd bulk
normal emission
T=100 K
s-polarized probe pulse
thermalizationin ~100 fs
non-thermal e-
unoccupied
occupied DOS
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electron thermalization and cooling in Gd(0001)
characteristic times transient changes of electron and phonon
temperature
• excitation: < 50 fs• relaxation: ≈ 1 ps
+∇(κph∇Tl)
Two Temperature Model (2TM):
)TlTe(Ht
Tl)Tl(Cl
)t,z(S)TlTe(H)Te(t
Te)T(Ce
,
,el
=∂
∂
+-∇κ∇=∂
∂
H(Te,Tl ): thermalized electron distributionPhonons: Debye Model
transport e-ph coupl. opt. exc.
Anisimov et.al., Sov. Phys. JETP 39, 375 (1974)
good agreement withtwo-temperature model
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TRPE of the Gd(0001) surface: p-pol. probe
time evolution of surface state binding energy
M. Lisowski et al., Phys. Rev. Lett. 95, 137402 (2005)
T=100 K
normal emission
p-polarized probe pulse
occupiedsurface state
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time-resolved binding energy of S
Transient binding energy of Gd(0001) surface state
3 THz
LO phonon frequency
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origin of the coherent mode at the surface ?
LO phonon derived mode:Surface vibrates with respectto underlying bulk layers
Köhler et al., PRL 24, 16 (1970)Rao and Menon, J. Phys. Chem. Solids 35, 425 (1974)
observedfrequency
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Estimation of phonon amplitude
LO phonon derived mode:Surface vibrates with respectto underlying bulk layers
estimated phonon amplitude
ΔEB ≈ 1 meV ⇔ Δd12 ≈ 0.2% · d12
(equilibrium value d12 = 2.8 Å)
DFT calculations of S↑ binding energyas function of interplane spacing d12
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Se
Ta
Charge density wave (CDW) in TaS2 or TaSe2
rearrangement of electronic structure:
UCO(upper cluster orbitals)6
6
1
b
c
a
quasi 2D crystal
strong e-ph coupling CDW at 300 K
1T-TaSe2
Colonna et al., Phys. Rev. Lett. 94, 036405 (2005)
~e/2 + -charge transfer
TaSe2: hexagonal layered structure
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metal-insulator transition in bulk TaS2
6
0.01
2
46
0.1
2
4
T e / W
1.81.61.41.21.0U/W
insulator
metal
Crossover
Mott transition controlled by CDW (overlap ⇔ bandwidth)
Perfetti et al., Phys. Rev. Lett. 90, 166401 (2003)Phys. Rev. B 71, 153101 (2005)
hν=6 eV
photoemission spectra
phase diagram
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time-resolved ARPES: T = 30 K
T=30 Khνpump=1.5 eVhνprobe=6.0 eV
normal emission
PE intensity
delay (ps)
pump-probe delay (ps)
UC
O p
eak
shift
(meV
)
2.82.52.2Frequency (THz)
bulksurface
see Demsar et al., PRB 66, R041101
beating between two modes
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+ - • In phase “breathing” of the metal clusters • Large coupling and low dampingExcitation of Mott Phase
Coherent CDW excitation
Nuclear coordinate X
Raman-like excitation