Ultrafast Spectroscopy

Gabriela Schlau-CohenFleming Group

Why femtoseconds?

timescale = distance/velocity~~~~~~

distance ≈ 10 ÅE ≈ hν ≈ (6.626*10-34kg*m2/s)*(3*108m/s /6*10-7m) ≈ 3*10-19kg*m2/s2

E= ½mv2 v=√(2*E*/m) =√(2*E*/9*10-31kg) =√(2*3*10-19/(9*10-31 ) m2/s2)

v=8*105 m/s

~~~~~~timescale ≈ (10*10-10m)/(8*105m/s) ≈ 10-15

sec

Ultrafast examples:

• Photosynthesis: energy transfer in <200 fs (Fleming group)

• Vision: isomerization of retinal in 200 fs (Mathies group)

• Dynamics: ring opening reaction in ~100s fs (Leone group)

• Transition states: Fe(CO)5 ligand exchange in <1 ps (Harris group)

• High intensity: properties of liquid carbon (Falcone group)

How can we measure things this fast?

1960 1970 1980 1990 2000

10–6

10–9

10–12

10–15T

imes

cale

(se

cond

s)

Year

Electronics

Optics

Laser Basics

Level empties

fast!

Four-level system

Laser Transition

Pump Transition

Fast decay

Fast decay

•Population inversion

•Pump energy source

•Lasing transition

• Method of creating pulsed output

• Compressed output

• Broadband laser pulse

What we need for ultrashort pulse generation:

Ultrafast Laser Overview

Laser oscillato

r

Amplifier medium

pump

3 pieces of ultrafast laser system:

• Oscillator• Regenerative

Amplifier

• Tunable Parametric Amplifier

Oscillator generates short pulses with mode-locking

Ti:Sapphirelaser crystal

Cavity with partially reflective mirror

Pump laser

Prisms

Titanium: Sapphire

oxygenaluminum

Al2O3 lattice

• 4 state system

• Upper state lifetime of 3.2 μs for population inversion

• Broadband of states around lasing wavelength

• Kerr-Lens effect (non-linear index of refraction)

Ti:Sapphire spectral

properties(nm)

FLU

OR

ES

CE

NC

E

(au)

Inte

nsity

(a

u)

Mode-locking

Mechanism of Mode-locking: Kerr Lens Effect

)(20 xInnn

Compression

• Prism compression

• Gratings, chirped mirrors

t t

Chirped Pulse Amplification

Pulse compressor

t

t

Solid state amplifiers

t

Dispersive delay linet

Short pulse

oscillator

• Stretch

• Amplify

• Recompress

Regenerative Amplifier

• Pulsed seed• Ti: Sapph crystal

Faraday rotator

thin-film polarizerPockels cell

• Pulsed pump laser• Pockels cell

p-polarized light

s-polarized light

OPA/NOPA

• Parametric amplification• Non-linear process• Energy, momentum conserved

1

32

Optical Parametric Amplification (OPA)

1 "signal"

"idler"

“seed"

“pump"

Non-linear processes

Emitted-light frequency

(1) (2) 2 (3) 30 ... P E E E

(5) *0 1 2 3 4 5E E E E E P

sig

Time Resolution for P(3)

“Excitation pulses”

Variably delayed “Probe pulse”

“Signal pulse”Medium under study

Sig

nal

pul

se e

nerg

y

Delay

Two-Dimensional Electronic Spectroscopy can study:

• Electronic structure

• Energy transfer dynamics

• Coupling

• Coherence

• Correlation functions

2D Spectroscopy

• Excitation at one wavelength influences emission at other wavelengths

• Diagonal peaks are linear absorption

• Cross peaks are coupling and

energy transfer

Excited StateAbsorption

Inhomogeneous Linewidth

HomogeneousLinewidth

CrossPeak

ωτ (“absorption”)

ωt

(“em

iss

ion

”)

Dimer Model (Theory)

Electronic Coupling

1 2Dimer

E

g1

e1

g2

e2

1

2

J

E

J

Principles of 2D Spectroscopy

τ T t

t e i tt e 3ωg e

g

e

ρ t ABSORPTIONFREQUENCY

EMISSIONFREQUENCY

1 3

SIGNAL

Recoveredfrom Experiment

3 ( , , )S T t

Time

eegt ti ||)(| 3

1

2

3

4

delay 1delay 2

1 2

3 4

1&2

3&4

diffractiveoptic (DO)

sample

2 f

sphericalmirror

spectro-meter

1 2 3 sig4=LO

coh.time

pop.time

echotime

T t

OD3

2D Heterodyne Spectroscopy

Opt. Lett. 29 (8) 884 (2004)

Experimental Set-up

Fourier Transform

Future directions of ultrafast

• Faster: further compression into the attosecond regime

• More Powerful: higher energy transitions with coherent light in the x-ray regime

0j k

0j k

NegativelyCorrelated Spectral Motion

PositivelyCorrelated Spectral Motion

2D spectrum with cross-peaksA measurement at the amplitude level