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

Time-resolved photoelectron spectroscopy

Laser basics and mode locking

IntroductionWhy femtosecond laser pulses?

Example: photochemistry of vision

Generation of femtosecond laser pulses