Martin Wolf Department of Physics, Freie Universität Berlin Festkörperphysik II im SS 2006 Introduction to femtosecond laser spectroscopy and ultrafast x-ray diffraction from solids Application of femtosecond laser spectroscopy Goal: Microscopic understanding of ultrafast dynamics in materials structure kinetics dynamics E. Muybrigde 1887 Principle: Stroboscopic investigation of motion and structural changes
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Martin WolfDepartment of Physics, Freie Universität Berlin
Festkörperphysik II im SS 2006
Introduction to femtosecond laser spectroscopy and ultrafast x-ray diffraction from solids
Application of femtosecond laser spectroscopy
Goal: Microscopic understanding of ultrafast dynamics in materials
structure
kinetics
dynamics
E. Muybrigde 1887
Principle: Stroboscopic investigation of motion and structural changes
Laser basics and mode locking
Outline
IntroductionWhy femtosecond laser pulses?
Example: photochemistry of vision
Generation of femtosecond laser pulses
Femtochemistry
Vibrational wave packet dynamics
Bi
Time-resolved probe of structural dynamics
Time-resolved x-ray diffraction
Femtosecond laser spectroscopy
Non-thermal melting and coherent phonon excitation
Time-resolved photoelectron spectroscopy
Why femtosecond spectroscopy ?
Typical timescale: 10-100 fs
Temporal evolution from reactants to products:
Dynamics of the transition state
Need femtosecond laser pulses for direct observation in time domain
Time scales of chemical reactions:
Rhodopsin
Eye
Example: photochemistry of vision
Rod cells: A single rod contains 108 rhodopsins(renewed after each photo-absorption + isomerization)
cis-trans isomerization
Retinal
hν
11-cis all-trans
How do we see ?
Hirarchy of time scales
ultrafastconformationalchanges
therrmal relaxationde-protonation
Optical absorption changes during reaction cycle
Study time-resolved changes of optical absorption spectrum
pump
probe
signal
Δt
Time resolution is determined by pulse duration
Pump-probe spectroscopy
Basic concept to resolve processes on ultrafast time-scales
reflection transmission
Primary step of vision
How can this process be so fast ?
Spectral changes after excitation
Primary step (cis-trans isomerization) of Retinal must occur within 200 fs !
pump@ 500 nm
Δα
t ~ 0
t = 200 fs
“Vibrational coherent wave packet motion”
excited stateabsorption
(t ~ 0)
probe
History of time resolution
Year
time
reso
lutio
n(s
)
Break through by development of pulsed laser sources
A.H. Zewail
Nobel prize 1999
Development of short laser pulses
Year
puls
e du
ratio
n(s
)
„mode locking“
„Titanium Sapphire“
normal dispersion(e.g. glass)
Properties of ultrashort laser pulsesShort pulse: Electrical field E(t) = ε(t) cos(ω0t + ϕ) with envelope ε(t)
time bandwidth product
(transform limit)
ε(t) ε(ω)
Generation of fs-laser pulses
A generic ultrafast laser consits of a broadband gain medium,a pulse-shortening device, and a resonator:
Laser basics ILASER: Stimulated emission
Example: Four level laser (Nd:YAG)
Populationinversion
Titanium-sapphire laser
Laser basics II Resonator modes:
longitudinal modes:
Stability:g2 = 1-d/R2
g1 = 1-d/R1
R1R2d
g1 g2 = 1
0 ≤ g1 g2 ≤ 1
condition for a stable resonator:
Mode-locking
Synchronization of laser modes:One mode:
All modes:
Constant phase between modes
Frequency domain:
Temporal structure:
Random phaseswith number of modes M
(within gain profile)
Δt = 1/Δν
m
Mode-locking
„2 beam interference“
non-linear optical Kerr effectInduced polarization in non-linear optical medium
Linear optics: n0 = √ε = √ε0(1+4πΧ(1))
Non-linear optics:
Longitudinal and transversal Kerr effect
“Kerr lens”
1) self phase modulation
2) self focussing“Kerr lens”
Kerr lens mode-locking and dispersion
Pulse broadening due to dispersion
Example: Ti:Sa laser
Prism compressor (GVD compensation)
Measurement of pulse duration
auto correlation
interferrometric autocorrelation
I(τ) =
Bi
Laser basics and mode locking
Outline
IntroductionWhy femtosecond laser pulses?
Example: photochemistry of vision
Generation of femtosecond laser pulses
Femtochemistry
Vibrational wave packet dynamics
Time-resolved probe of structural dynamics
Time-resolved x-ray diffraction
Femtosecond laser spectroscopy
Non-thermal melting and coherent phonon excitation
Time-resolved photoelectron spectroscopy
Real-time probing of chemical reactions
Real-time probing of chemical reactions II
Example: photodissociation
Analysis of repulsive excited state PES
“Clocking of chemical reactions”
Real-time probing of chemical reactions III
“Coherent wave packet motion”
reaction product
transition state
Summary I
Femtochemistry
Vibrational wave packet dynamics
Bi
Time-resolved probe of structural dynamics
Time-resolved x-ray diffraction
Femtosecond laser spectroscopy
Non-thermal melting and coherent phonon excitation